Tangier Island, Virginia. Photographer: Mary Hollinger. Photo courtesty of NOAA Central LIbrary.

NOAA Technical Report NOS CO-OPS 36

SEA LEVEL VARIATIONS OF THE UNITED STATES 1854-1999

Silver Spring, Maryland

July, 2001

noaa National Oceanic and Atmospheric Administration

U.S. DEPARTMENT OF COMMERCE

National Ocean Service

Center for Operational Oceanographic Products and Services

Products and Services Division
Center for Operational Oceanographic Products and Services

National Ocean Service

National Oceanic and Atmospheric Administration

U.S. Department of Commerce

The National Ocean Service (NOS) Center for Operational Oceanographic Products and

Services (CO-OPS) collects and distributes observations and predictions of water levels

and currents to ensure safe, efficient and environmentally sound maritime commerce.

The Center provides the set of water level and coastal current products required to

support NOS' Strategic Plan mission requirements, and to assist in providing operational

oceanographic data/products required by NOAA's other Strategic Plan themes. For

example, CO-OPS provides data and products required by the National Weather Service

to meet its flood and tsunami warning responsibilities. The Center manages the National

Water Level Observation Network (NWLON), and a national network of Physical

Oceanographic Real-Time Systems (PORTS) in major U.S. harbors. The Center:

establishes standards for the collection and processing of water level and current data;

collects and documents user requirements which serve as the foundation for all resulting

program activities; designs new and/or improved oceanographic observing systems;

designs software to improve CO-OPS' data processing capabilities; maintains and

operates oceanographic observing systems; performs operational data analysis/quality

control; and produces/disseminates oceanographic products.
NOAA Technical Report NOS CO-OPS

SEA LEVEL VARIATIONS OF THE UNITED STATES 1854-1999

Chris Zervas

July, 2001

noaa National Oceanic and Atmospheric Administration

U.S. DEPARTMENT OF COMMERCE National Oceanic and Atmospheric Administration

Don Evans, Secretary Scott B. Gudes, Acting Under Secretary for Oceans and

Atmosphere/Administrator and Deputy Under Secretary

National Ocean Service

Margaret A. Davidson, Acting Assistant Administrator for

Ocean Services and Coastal Zone Management

 

Center for Operational Oceanographic Products and Services

Michael Szabados, Acting Director

i

Table of Contents

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

List of Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

I. Introduction ................................................................1

II. Water Level Stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

III. Derivation of Mean Sea Level Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

IV. Linear Mean Sea Level Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

V. Average Seasonal Mean Sea Level Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

VI. Variability of Residual Monthly Mean Sea Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

VII. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

VIII. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Appendix I. National Water Level Observation Network Stations . . . . . . . . . . . . . . . . . . . . . 67

Appendix II. Time series of monthly mean sea level after removal of seasonal cycle showing the

derived linear trend . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

Appendix III. Average seasonal cycle of monthly mean sea level . . . . . . . . . . . . . . . . . . . . . . 109

Appendix IV. Time series of monthly mean sea level after removal of seasonal cycle and linear

trend . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

Appendix V. Linear trends for 50-year periods of mean sea level data . . . . . . . . . . . . . . . . . 181
ii
iii

List of Figures

Figure 1. NWLON stations on the U.S. east coast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 2. NWLON stations on the U.S. west coast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 3. NWLON stations in the eastern Gulf of Mexico and Caribbean . . . . . . . . . . . . . . . . . 9

Figure 4. NWLON stations in the western Gulf of Mexico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 5. NWLON stations in Alaska . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 6. NWLON stations in the eastern Pacific Ocean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 7. NWLON stations in the western Pacific Ocean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 8. MSL trend range for all data up to 1999 for U.S. east coast stations . . . . . . . . . . . . . 22

Figure 9. MSL trend range for all data up to 1999 for U.S. west coast and Alaska stations . . . 22

Figure 10. MSL trend range for all data up to 1999 for the tropical Pacific, Bermuda, Gulf of

Mexico, and Caribbean stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Figure 11. Monthly mean sea level at Honolulu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Figure 12. Monthly mean sea level at Washington, DC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 13. Monthly mean sea level at Unalaska . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 14. Autoregressive coefficient range for U.S. east coast stations . . . . . . . . . . . . . . . . . 25

Figure 15. Standard error of the linear regression with autoregressive residuals method for U.S.

east coast stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Figure 16. Autoregressive coefficient range for U.S. west coast and Alaska stations . . . . . . . 26

Figure 17. Standard error of the linear regression with autoregressive residuals method for U.S.

west coast and Alaska stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Figure 18. Autoregressive coefficient range for the tropical Pacific, Bermuda, Gulf of Mexico, and

Caribbean stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Figure 19. Standard error of the linear regression with autoregressive residuals method for the

tropical Pacific, Bermuda, Gulf of Mexico, and Caribbean stations . . . . . . . . . . . . 27

Figure 20. Average seasonal MSL cycle for Honolulu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Figure 21. Average seasonal MSL cycle for Eastport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Figure 22. Average seasonal MSL cycle for Baltimore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Figure 23. Average seasonal MSL cycle for Fernandina Beach . . . . . . . . . . . . . . . . . . . . . . . . 30

Figure 24. Average seasonal MSL cycle for Galveston Pier 21 . . . . . . . . . . . . . . . . . . . . . . . . 31

Figure 25. Average seasonal MSL cycle for San Diego . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Figure 26. Average seasonal MSL cycle for Neah Bay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Figure 27. Average seasonal MSL cycle for Yakutat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Figure 28. Monthly mean sea level residual at Honolulu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Figure 29. Standard error of linear MSL trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Figure 30. Standard error of linear MSL trends (log-log plot) . . . . . . . . . . . . . . . . . . . . . . . . . 46

Figure 31. 95% confidence intervals for linear MSL trends . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Figure 32. 95% confidence intervals for 1950-1999 linear trends at Atlantic NWLON stations50

Figure 33. 95% confidence intervals for 1950-1999 linear trends at Pacific NWLON stations 50

Figure 34. 95% confidence intervals for Honolulu MSL trends . . . . . . . . . . . . . . . . . . . . . . . . 52

Figure 35. 95% confidence intervals for Portland MSL trends . . . . . . . . . . . . . . . . . . . . . . . . . 52

Figure 36. 95% confidence intervals for Boston MSL trends . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Figure 37. 95% confidence intervals for The Battery MSL trends . . . . . . . . . . . . . . . . . . . . . . 53

Figure 38. 95% confidence intervals for Atlantic City MSL trends . . . . . . . . . . . . . . . . . . . . . 53
iv

Figure 39. 95% confidence intervals for Philadelphia MSL trends . . . . . . . . . . . . . . . . . . . . . . 53

Figure 40. 95% confidence intervals for Baltimore MSL trends . . . . . . . . . . . . . . . . . . . . . . . . 54

Figure 41. 95% confidence intervals for Charleston, SC MSL trends . . . . . . . . . . . . . . . . . . . . 54

Figure 42. 95% confidence intervals for Fernandina Beach MSL trends . . . . . . . . . . . . . . . . . 54

Figure 43. 95% confidence intervals for Key West MSL trends . . . . . . . . . . . . . . . . . . . . . . . . 55

Figure 44. 95% confidence intervals for Cedar Key MSL trends . . . . . . . . . . . . . . . . . . . . . . . 55

Figure 45. 95% confidence intervals for Galveston Pier 21 MSL trends . . . . . . . . . . . . . . . . . 55

Figure 46. 95% confidence intervals for San Diego MSL trends . . . . . . . . . . . . . . . . . . . . . . . 56

Figure 47. 95% confidence intervals for San Francisco MSL trends . . . . . . . . . . . . . . . . . . . . 56

Figure 48. 95% confidence intervals for Seattle MSL trends . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Figure 49. 95% confidence intervals for Ketchikan MSL trends . . . . . . . . . . . . . . . . . . . . . . . 57

Figure 50. Expected absolute value of sea level change in 10 years . . . . . . . . . . . . . . . . . . . . . 58

Figure 51. Expected absolute value of sea level change in 30 years . . . . . . . . . . . . . . . . . . . . . 58
v

List of Tables

Table 1. Major earthquakes near NWLON stations ...................................5

Table 2. Average standard error of mean sea level trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Table 3. Linear MSL trends for all monthly data up to 1999 . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Table 4. Periods of suspect data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Table 5. Number of months with extreme residual water levels for Atlantic stations . . . . . . . . 36

Table 6. Number of months with extreme residual water levels for Pacific stations . . . . . . . . . 40

Table 7. Linear MSL trends for 1950-1999 monthly data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Table 8. 50-year periods when linear MSL trend differs significantly from overall trend . . . . 51

Table A. NWLON Stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Table B. Average seasonal mean sea level cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

Table C. Linear trends for 50-year periods of MSL data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
vi
vii

List of Acronyms

CO-OPS Center for Operational Oceanographic Products and Services

ENSO El Nino/Southern Oscillation

MSL Mean Sea Level

NOAA National Oceanic and Atmospheric Administration

NOS National Ocean Service

NTDE National Tidal Datum Epoch

NWLON National Water Level Observation Network
viii
ix

EXECUTIVE SUMMARY

In this report, monthly mean sea level (MSL) variations are analyzed for 117 stations of the National

Ocean Service’s (NOS) National Water Level Observation Network (NWLON) having between 25

and 146 years of data. Monthly MSL data up to the end of 1999 are used to calculate linear trends,

and to obtain the average seasonal cycle, the residual time series, and the autoregressive coefficient

of the residual with accurate estimates of standard errors. Months with extreme high or low residuals

are defined and periods of broad regional correlations between station anomalies are observed. 

Since the derived trends include the local vertical land motion, they are spatially variable.

Calculated MSL trends range from 9.85 mm/yr for Grand Isle, LA to -16.68 mm/yr for Skagway,

AK and are consistent with previous station trends published by NOS. The appendices of this report

include time series plots for each station of the monthly MSL with the seasonal cycle removed, the

seasonal cycle, and the MSL residual after both the seasonal cycle and the trend are removed. The

location and timing of any major earthquakes near stations in tectonically-active areas are noted,

since an associated vertical offset or a change in MSL trend is possible.

An inverse power relationship is derived empirically, relating the standard error for linear trends

to the year range of MSL data. An estimated 50 to 60 years of data are required for obtaining linear

MSL trends having a 1 mm/yr precision with a 95% statistical confidence interval. For a given

length of data, the standard errors for trends at Pacific Ocean and western Gulf of Mexico stations

tend to be greater than standard errors for trends at Atlantic coastal stations. 

MSL trends for the most recent 50-year period of 1950-1999 are compared with trends obtained

from each station’s entire data set. The trend for the past 50 years is significantly lower at only three

out of sixty stations (Eastport, Portland, and Boston). At no station is the 1950-1999 trend

significantly higher than the trend obtained from the station’s entire data set.

In an examination of 50-year MSL trends at sixteen of the longest term stations, it was found that

for six Atlantic stations, the periods centered on years from 1930 to 1955 tend to have significantly

higher trends than periods centered on years from 1965 to 1975. For San Francisco, trends for all

50-year periods centered from 1890 to 1915 are significantly lower than the overall trend and the

trend since the 1906 San Francisco earthquake.
x
1

I. INTRODUCTION

The National Water Level Observation Network (NWLON) is operated by the Center for

Operational Oceanographic Products and Services (CO-OPS), a component of the National Ocean

Service (NOS). The NWLON is composed of approximately 175 long term, continuously-operating

stations located along the United States coast, including the Great Lakes and islands in the Atlantic

and Pacific Oceans. The NWLON provides the basis for establishing U.S. tidal datums for marine

and coastal boundaries and the NOS navigational chart datum for hydrographic surveys. In addition,

NWLON data are used for scientific studies of wetlands, coastal processes, tectonics, tsunamis,

storm surges, and climate change.

Most of the NWLON stations have been recording water levels for several decades, with two

stations (New York and San Francisco) having accurate records beginning in the 1850s. Water

levels have been recorded by a variety of instruments over the years with continuing improvements

in instrument accuracy (Bilham, 1991) and with occasional small shifts in instrument location. The

earliest water levels were observed with a tide staff and recorded by the U.S. Coast and Geodetic

Survey, a predecessor of the NOS. As a result of technological innovations, water levels have

subsequently been recorded automatically by float-driven analog gauges, gas-purged pressure or

bubbler recorders, analog-to-digital recording gauges, pressure sensors, and air acoustic ranging

sensors. 

The data recorded by water level instruments over the years can be combined into a single time

series for a location only if the vertical datum (or datum of tabulation) has been carefully

maintained. This is accomplished through frequent leveling to fixed benchmarks maintained in the

vicinity of the station (Smith, 1980; Gill and Schultz, 2001). 

The data to be examined in this report consist of monthly mean sea levels (MSL) for all NWLON

stations in operation for a span of at least 25 years. The monthly means are an average of hourly

water level heights for a complete month of data. The sea level variations to be determined in this

report are the linear secular trend, the average seasonal cycle, and the residual variability at each

station. Recorded water levels are a combination of changes in the sea level and the vertical land

motion at the location of the gauge. Therefore, the trends derived are relative MSL trends and can

be considered valid only for a region near the gauge with uniform vertical land motion. Calculation

of absolute MSL trends requires the accurate determination of vertical land motion at the gauges and

is beyond the scope of this report. 

The monthly MSLs used in this report are relative to the NOS-established MSL datum, which is the

arithmetic mean of hourly heights observed during the National Tidal Datum Epoch (NTDE), a

standard 19-year period presently defined by NOS as 1960-1978. The NTDE will be updated to the

1980-1998 period in the near future due to the effect of the linear secular trends at each station since

1978. At a limited number of stations, where the linear secular trends have been large, MSL datums

were very recently updated using a shorter 5-year period in the 1990s in an effort to keep the datum

current. 
2

The NOS has previously determined and published listings of linear secular trends at NWLON

stations derived from annual MSLs (an average of all the monthly MSLs for a complete year). Hicks

and Shofnos (1965) determined MSL trends up to the year 1962 at 44 stations. Hicks and Crosby

(1974) redetermined the MSL trends up to the year 1972 at 50 locations. Hicks et al. (1983)

recalculated MSL trends up to the year 1980 at 67 stations and included a determination of the

average seasonal MSL cycles. Lyles et al. (1988) updated the MSL trends and the seasonal cycles

up to the year 1986 at 78 stations.

Smith and Leffler (1980) published an analysis of long term sea level trends at NWLON stations

along the California coast. Mitchell et al. (2000) calculated sea level trends up to 1999 for Australia

and the Pacific Ocean including trends for twelve NWLON stations. An up-to-date compilation of

MSL secular trends at most long term water level stations around the world is produced by the

Permanent Service for Mean Sea Level (available at http://www.nbi.ac.uk/psmsl/datainfo/rlr.trends).

There have been a number of studies (Parker, 1992) in which MSL trends from stations around the

world are combined in order to determine the global MSL trend due to the thermal expansion of

seawater caused by global temperature changes and glacial melting. Although most coastal regions

of the world indicate a MSL rise, some coastlines show rapidly falling MSLs. This is a consequence

of water level gauges measuring relative MSL change which combines the effects of absolute MSL

change and any vertical land movement. Various averaging schemes and/or corrections for vertical

land motions have been devised, resulting in estimates of global MSL rise ranging from 1.0 to 2.4

mm/yr (Douglas et al., 2001). 

Continuing isostatic rebound of the lithosphere in response to melting of the ice sheets is the cause

of the largest vertical land motion at many northern hemisphere stations (Hicks, 1965). Viscoelastic

models of the earth have been developed based on radiocarbon dating of shoreline levels over the

past 18,000 years. The glacial isostatic adjustment effect is expected to be essentially linear for the

period of instrumentally-recorded water levels. The vertical trends derived from these models have

been used to correct individual sea level trends for the glacial isostatic rebound and to derive a

globally-averaged MSL trend (Peltier and Tushingham, 1989; Mitrovica and Davis, 1995; Davis and

Mitrovica, 1996). An additional significant cause of rapid vertical land motion is the tectonic

processes at plate boundaries. Douglas (1991) produced an estimated global MSL trend by

excluding all water level stations at convergent plate boundaries. 

Another use of MSL data is to estimate the acceleration of global MSL in order to identify possible

changes in the rate of sea level rise due to man-made contributions to global warming. Using data

up to the mid to late 1980s from long term stations, no consistent, statistically-significant nonlinear

parameters have been found (Woodworth, 1990; Douglas, 1992).

For scientific research in the fields of tectonics, sedimentary basin subsidence, seismology, and

volcanology, vertical land motion is the desired signal in the MSL record while absolute changes

in oceanic water levels are considered as noise to be removed (Vanicek, 1978; Chelton and Enfield,

1980). At plate boundary faults, strain is continuously accumulated and periodically released as

earthquakes, aseismic fault creep, and/or viscoelastic readjustment, resulting in time-dependent

horizontal and vertical deformation. The earthquake cycle at major plate boundaries can be divided
3

into four time periods: interseismic (strain accumulation), preseismic (incipient strain release),

coseismic (elastic rebound), and postseismic (transient strain release) (Mavko, 1981). Vertical land

movement at different rates may be expected during each period and consequently can cause

variations in relative MSL trends. 

Water level records in Japan show offsets and clear changes in MSL rates after major earthquakes

(Kato, 1983). These records have been used to model the fault mechanics and the viscoelastic

structure of subduction zones (Savage and Thatcher, 1992; Savage, 1995). The NWLON stations

in California, Oregon, Washington, and Alaska are also in tectonically active regions close to plate

boundaries with the potential for large magnitude earthquakes. 

Water level records along the south coast of Alaska have been used in studies of transient

postseismic deformation following the March 1964 Prince William Sound earthquake, one of the

strongest recorded seismic events (Hicks, 1972; Savage and Plafker, 1991; Cohen et al., 1995;

Cohen, 1996; Cohen and Freymueller, 1997). Water level records for the Cascadia subduction zone

along the coast of California, Oregon, and Washington have been examined in an effort to measure

tectonic strain accumulation and the potential for major earthquakes (Savage et al., 1991; Dragert

et al., 1994; Mitchell et al., 1994). 

The occurrence of a major earthquake near a water level station should be noted in evaluating MSL

trends, even if there is no obvious coseismic offset in the record. A change in MSL trend may not

become evident until several decades after an earthquake (Cohen and Freymueller, 2001).
4
5

II. WATER LEVEL STATIONS

NOS has collected water level data for a span of more than 25 years at 117 locations. These stations

are listed in Appendix I. Although there may be long data gaps at some of the locations, the existing

data can provide good estimates of linear MSL trends since the vertical datums have been carefully

maintained through periodic leveling to stable benchmarks with respect to the adjacent landmass.

Appendix I gives the NWLON station number, latitude, longitude, first year of data, last year of

data, range of years, and the station name and state or territory. 

At a few stations, a new NWLON station number was given when the gauge was moved to a nearby

location. A single time series can represent these locations, since the vertical station datums have

been leveled to some of the same benchmarks. These locations are Bermuda, Philadelphia,

Cambridge, Springmaid Pier, Grand Isle, Sabine Pass, and Kodiak Island.

Data collection at some stations has been discontinued, but a long enough span of data exists to

produce accurate linear MSL trends. These stations and their termination years are Chuuk (1995),

Seavey Island (1986), Port Jefferson (1992), New Rochelle (1981), Portsmouth (1987), Miami

Beach (1981), Eugene Island (1974), Padre Island (1994), Newport Beach (1993), Rincon Island

(1990), and Guantanamo Bay (1971). 

The locations of the stations in Appendix I are shown in Figures 1 to 7, with the size of the station

symbols indicating the year range of water level data. Also shown are the locations of major

earthquakes with magnitudes greater than 7.5. The dates and epicenter locations of these

earthquakes are listed in Table 1. All the earthquakes are in Alaska, except for one each in

California, Hawaii, and Guam. These earthquakes can result in slip along several hundred

kilometers of a tectonic plate boundary, immediately affecting the water levels at nearby NWLON

stations with an offset and/or a change in the MSL trend.

Table 1. Major earthquakes near NWLON stations

Date State or

Territory

Longitude Latitude Magnitude

04/18/1906 California 122 28.8 W 37 40.2 N 7.7

03/09/1957 SW Alaska 175 37.8 W 51 17.4 N 8.8

07/10/1958 SE Alaska 136 31.2 W 58 20.4 N 8.3

03/28/1964 South Alaska 147 43.8 W 61 2.4 N 9.2

02/04/1965 SW Alaska 178 33.0 W 51 17.4 N 8.7

07/30/1972 SE Alaska 135 41.4 W 56 49.2 N 7.6

11/29/1975 Hawaii 155 0.0 W 19 20.4 N 7.5

02/28/1979 South Alaska 141 36.0 W 60 38.4 N 7.6
Table 1. Major earthquakes near NWLON stations

Date State or

Territory

Longitude Latitude Magnitude

6

05/07/1986 SW Alaska 174 45.0 W 51 19.8 N 8.0

11/30/1987 SE Alaska 142 47.4 W 58 40.8 N 7.9

03/06/1988 SE Alaska 143 1.8 W 56 57.0 N 7.7

08/08/1993 Guam 144 48.1 E 12 58.9 N 8.0

06/10/1996 SW Alaska 177 37.8 W 51 33.6 N 7.9
7

30° N

45° N

75° W

Nantucket Island

Providence

New London

Portland

Eastport

Bar Harbor

Solomons Island

New Rochelle

Portsmouth

Fernandina Beach

Fort Pulaski

Boston

Philadelphia

Lewes Washington Cape May

Seavey Island

Bridgeport

Cambridge Colonial Beach

Lewisetta

Gloucester Point

Sewells Point Chesapeake Bay Bridge Tunnel

Montauk

Willets Point

Sandy Hook

Port Jefferson The Battery

Annapolis

Baltimore

Bermuda

Newport

Kiptopeke

Woods Hole

Atlantic City

Beaufort

Wilmington

Springmaid Pier

Charleston

Mayport

Range of Water Level Data

75 to 150 years

50 to 75 years

25 to 50 years

Figure 1. NWLON stations on the U.S. east coast.
8

45° N

120° W

South Beach

Monterey

Point Reyes

Los Angeles

Astoria

Charleston

San Diego

Santa Barbara

Friday Harbor

Port Angeles

Crescent City

San Francisco

Neah Bay

Toke Point

Port San Luis

La Jolla

Newport Beach

Santa Monica

Rincon Island

Seattle

Cherry Point

Port Townsend

7.7

Alameda

Range of Water Level Data

75 to 150 years

50 to 75 years

25 to 50 years

Figure 2. NWLON stations on the U.S. west coast with major earthquake indicated by diamond.
975° W

30° N

Mayport

Fernandina Beach

Clearwater Beach

Charlotte 

Amalie

San Juan

St. Petersburg

Guantanamo Bay

Fort Myers

Naples

Key West

Magueyes Island

Miami Beach

Vaca Beach

Cedar Key

Apalachicola

Panama City

Range of Water Level Data

75 to 150 years

50 to 75 years

25 to 50 years

Figure 3. NWLON stations in the eastern Gulf of Mexico and Caribbean. 90° W

30° N

Galveston Pier 21

Galveston Pleasure Pier

Eugene 

Island

Port Mansfield

Padre Island

Port Isabel

Pensacola

Panama City

Dauphin 

Island Apalachicola

Grand Isle

Sabine Pass

Rockport

Freeport

Range of Water Level Data

75 to 150 years

50 to 75 years

25 to 50 years

Figure 4. NWLON stations in the western Gulf of Mexico.
10

135° W

120° W

15° S

15° N

0°

165° W150° W

Kahului Honolulu

Johnston Atoll

Nawiliwili Mokuoloe

Sand Island

Pago Pago

7.5

Hilo

Range of Water Level Data

75 to 150 years

50 to 75 years

25 to 50 years

Figure 6. NWLON stations in the eastern Pacific Ocean with major earthquake

indicated by diamond.

150° W

60° N

135° W

165° W

Adak Island

Yakutat

Ketchikan

Cordova

Valdez

Unalaska

Anchorage

Seward

Sand Point

Kodiak Island

Seldovia

Nikiski

7.9 8.8 8.7

8

7.9

7.7 7.6

8.3

7.6 9.2

Sitka

Juneau

Skagway

Range of Water Level Data

75 to 150 years

50 to 75 years

25 to 50 years

Figure 5. NWLON stations in Alaska with major earthquakes indicated by diamonds.
11

15° N

30° N

135° E

0°

150° E165° E

Guam

8

Kwajalein

Chuuk

Wake Island

Range of Water Level Data

75 to 150 years

50 to 75 years

25 to 50 years

Figure 7. NWLON stations in the western Pacific Ocean with major

earthquake indicated by diamond.
12
13

b

x y

x

i i

i

i

i

=

∑

∑ 2 (1)

s

s

x

b

xy

i

i

=

∑ 2 (2)

s

y b xy

n xy

i

i

i i

i =

− 



 





−

∑ ∑ 2

2

(3)

y bx y bx ( ) ii i i i −= − + − − φ ε 11 1 (4)

III. DERIVATION OF MEAN SEA LEVEL TRENDS

Linear secular trends in annual MSL data are usually computed as the least squares linear regression

to a bivariate distribution of the data value versus year. If xi and yi represent the year and the

corresponding annual MSL after the subtraction of their respective means, the slope of the least

squares best-fit line is

The standard error of the slope is 

where

is the standard deviation of the MSL residual yi - bxi and n is the number of annual MSL values.

This method does not take into account the fact that the data are a consecutive time series rather than

being composed of independent data points. 

The estimated error of the slope sb is valid only if the residual values are serially uncorrelated (Box

and Jenkins, 1976; Box et al, 1978). The residual MSL for each year should be independent of the

residual MSL for any previous year or any following year (i.e. the residual series must be a random

stationary process). This condition is often untrue in time series analysis and, in particular, is

usually untrue for sea level time series. The predominant residual signal is characterized by periods

of 3-5 years and is driven by the propagation of ENSO (El Nino/Southern Oscillation) conditions

in the tropical Pacific throughout the global oceans and atmosphere (Chelton and Enfield, 1986;

Douglas et al., 2001).

In fact, the sea level residual variability can be characterized, to first order, as an autoregressive time

series of order 1. For an autoregressive process of order 1, the best estimate of the next data value

is not zero, but a fraction of the present value. For a residual water level yi

 - bxi,
14

y bx m y bx m i ij i i j i −−= − − + φ ε 11 1 1 ( − −− ) (5)

where i-1 indicates the previous data value, N1 is the autoregressive parameter of order 1, and 0i is

a stationary random time series representing the model’s error. 0i

 is essentially the unpredictable

part of the residual MSL.

Both the slope b of the least squares fitted line and the autoregressive parameter N1 must be

determined from the available MSL data. Although the slope b will be nearly identical to the slope

obtained from a simple linear regression, the standard error sb will be greater, due to the necessity

of also determining N1 from the same set of data. The main consequence of not considering the

autocorrelation of the residual is that the uncertainty in MSL trends is underestimated. Observation

of significant differences in MSL trends over time or between locations can require a longer data

series, if they are correctly modeled as linear trends with autoregressive residuals instead of as

simple linear trends.

MSL trends can be determined with higher precision from monthly MSL data even if twelve

additional parameters are introduced to describe the average seasonal cycle by month. The equation

to be solved for linear regression with autoregressive residuals of order 1 from monthly MSL data

is

where yi is now the monthly MSL, xi is the year plus a fractional year corresponding to the month

of yi

, and mj is a mean offset corresponding with the month of yi and is one of the twelve parameters

representing the average seasonal MSL cycle. The autoregressive coefficient N1 for each station

indicates the predictable part of the residual MSL. 0i is a stationary random time series representing

the model’s error which is the unpredictable part of the residual MSL.

When MSL trends were calculated for all 117 stations described in Section II, the average of all the

trend standard errors are shown in Table 2 for four different methods of calculation. The results

discussed in the following sections of this report will be those obtained by linear regression of

monthly MSL data with autoregressive residuals of order 1. In addition to producing the correct

standard error for the trend, the average seasonal MSL cycle and the autoregression coefficient are

also produced.

Table 2. Average standard error of mean sea level trends

Method of Calculation Annual MSL Data Monthly MSL Data

Linear Regression 0.57 mm/yr 0.27 mm/yr

Linear Regression with Autoregressive Residuals

of Order 1

0.75 mm/yr 0.47 mm/yr
15

IV. LINEAR MEAN SEA LEVEL TRENDS

Following the method outlined in Section III, linear MSL trends were calculated for the NWLON

water level stations listed in Appendix I using all available monthly data. Table 3 lists the slope of

the trend, the intercept at the year 2000, and the autoregressive coefficient along with the standard

errors of each value. The 95% confidence interval for each value is 1.96 times the standard error

above and below the calculated value (Box et al., 1978). Also shown in the last column of Table

3 is the standard error of the linear regression with autoregressive residual model. Values for the

average seasonal MSL cycles were obtained simultaneously and will be discussed in Section V of

this report.

The 95% confidence intervals of the MSL trends for all water level stations are displayed in Figures

8 to 10. The range of the confidence intervals are inversely proportional to the length of the data

series. The data time periods vary from a minimum of 25 years up to 146 years. The widest

confidence intervals are for the stations with the shortest time periods of data.

Since most of the U.S. east coast stations (Figure 8) have been in operation for many years, the

trends are well determined showing narrow confidence intervals. The stations south of The Battery

generally have a greater MSL trend than stations north of The Battery. New Rochelle appears to

have an anomalously low trend (0.54 mm/yr) based on 25 years of data (1957-1981) while

Chesapeake Bay Bridge Tunnel appears to have an anomalously high trend (7.01 mm/yr) based on

25 years of data (1975-1999). However, comparable trends were found at nearby stations over the

same 25-year periods. This indicates the danger in relying on only 25 years of data to determine a

long term MSL trend without considering the confidence interval. 

The U.S. west coast and Alaskan NOS stations (Figure 9) have a greater variability of trends due

to the variety of tectonic activity affecting vertical land motions. Often nearby stations can have

significantly different trends. Some of the newer west coast and Alaskan stations have wide

confidence intervals due to shorter data sets. The most negative trend for all the stations is at

Skagway where rapid vertical uplift due to local glacial rebound is occurring. Four stations (San

Francisco, Seward, Adak Island, and Unalaska) have separate trends determined for pre- and postearthquake time periods. Although pre-earthquake trends for Adak Island and Unalaska are based

on only 14 and 23 years of data, respectively, they are included for comparison with post-earthquake

trends.

There is somewhat less variability in the trends for the tropical Pacific, Bermuda, Gulf of Mexico,

and Caribbean stations (Figure 10), except for a group of stations in Louisiana and part of Texas

which show large positive trends. The stations in Louisiana and Texas are undergoing rapid land

subsidence due to sediment loading and compaction of the Mississippi River delta and/or oil and gas

extraction. In the Hawaiian Islands, the station at Hilo has a larger MSL trend than the other

Hawaiian stations. Hilo is on the volcanically active island of Hawaii, which is in the process of

loading the Pacific lithospheric plate. The MSL trend initially derived for Freeport was 10.83

mm/yr. However, a comparison of the data at Freeport with nearby stations showed an unexplained

offset from December 1971 to January 1972. Therefore, a parameter was introduced to account for

this offset, and the corrected MSL trend is 5.87 mm/yr with an offset of 0.160 m +/- 0.039.
16

Figure 11 shows an example of how well the MSL trend and its 95% confidence interval fit the

monthly MSL data for Honolulu after the removal of the average seasonal cycle. The plot also

shows a smoother 5-month running average. There are similar plots for all NWLON stations in

Appendix II. The value of the intercept at which the trend crosses the year 2000 axis (given in Table

3) indicates how far the MSL has changed since the last establishment of the MSL datum. For most

stations, the last MSL datum was established with data for the epoch 1960-1978, centered on the

year 1969. For most stations with significant trends, the trendline can be seen to cross zero close

to the year 1969 (Figure 11 and 12). Therefore, 31 years have elapsed by the year 2000 with

corresponding changes in MSL caused by the individual station trends. The MSL datums of the

stations with the most rapid trends have been recently revised, and their year 2000 MSL values are

now closer to zero. Sabine Pass, Galveston Pier 21, and Galveston Pleasure Pier MSL datums were

updated with 5 years of data from 1990-1994. Juneau, Skagway, Seldovia, Nikiski, Anchorage,

Kodiak Island, and Unalaska datums were updated with 5 years of data from 1994-1998. The rest

of the station datums are due for updating with data from the NOS-defined epoch 1980-1998.

In the plots in Appendix II, the times of major earthquakes occuring near NWLON stations (as listed

in Table 1) are shown as solid vertical lines. Several stations show clear offsets due to vertical land

motion caused by a major earthquake, sometimes accompanied by a change in the MSL trend after

the earthquake. At four stations, separate MSL trends were calculated for periods before and after

the event. Unalaska (Figure 13) and Adak Island show small offsets at the time of the March 9,

1957 Andreanof Islands earthquake. Seward had a substantial offset of about 1 m at the time of the

March 28, 1964 Prince William Sound earthquake. Although there are no clear offsets associated

with the other earthquakes in Table 1, there may have been a change in MSL trend at San Francisco

after the April 18, 1906 earthquake. The San Francisco station is only 6 km away from the San

Andreas fault which slipped horizontally by over 3 m at some locations. Therefore, three MSL

trends were determined for San Francisco using 1) the total data set, 2) the data before the event, and

3) the data after the event (Table 3 and Appendix II).

The 95% confidence intervals of the autoregressive coefficients of order 1 (Table 3) are plotted in

Figures 14, 16, and 18. A high coefficient indicates a high autocorrelation (or predictability) from

one monthly residual MSL to the next. For the U.S. east coast stations (Figure 14) and Alaskan

stations (Figure 16), the autoregressive coefficients cluster about 0.4. For the California stations

south of Point Reyes, autocorrelations are higher, with the autoregressive coefficients grouped about

0.7. Further north, the autoregressive coefficients drop to 0.4-0.5. The sparseness of the data at

Nikiski gives a poorly resolved autoregressive coefficient. The coefficients for the Gulf of Mexico,

Bermuda, Johnston Atoll, Sand Island, and Wake Island cluster about 0.5 (Figure 18). Higher values

around 0.7 are found for the Hawaiian and Caribbean stations. The highest values are above 0.8 and

occur at the four western Pacific stations at Guam, Pago Pago, Kwajalein, and Chuuk.

The standard errors (in mm) listed in the last column of Table 3 are plotted in Figures 15, 17, and

19. They comprise the part of the time series not fitted by the linear regression with autoregressive

residuals method. The standard errors indicate the level of the remaining variability which is the

unpredictable portion of the month-to month change in mean sea level. 
17

For the U.S. east coast (Figure 15), the standard errors rise gradually from a low of 40 mm in the

north to a high of 70 mm in the south with two exceptions at Philadelphia and Washington, DC.

These two river stations have high standard errors above 70 mm. Figure 12 shows the high monthto-month variability for Washington, DC which is greatly smoothed out by the 5-month running

average.

In contrast to the east coast, standard errors of the U.S. west coast and Alaska stations (Figure 17)

increase from south to north. The standard errors rise from 30 mm for southern California to over

90 mm in southwest Alaska. Standard errors rise to levels above 80 mm at Astoria and Toke Point

and fall to about 60 mm for stations in Puget Sound.

The Hawaiian stations and the western Pacific stations generally have low standard errors below 45

mm (Figure 19). Note the effect of low month-to-month variability for Honolulu in Figure 11

compared to the higher variability for Washington, DC in Figure 12. The central Pacific stations

of Johnston Atoll and Wake Island, and the central Atlantic station of Bermuda have high month-tomonth variability with standard errors above 60 mm. In the Gulf of Mexico, standard errors are

below 50 mm for the eastern Gulf rising above 60 mm in the western Gulf. The lowest standard

errors at any of the NWLON stations are below 30 mm for the Caribbean stations.
18

Table 3. Linear MSL trends for all monthly data up to 1999

Station Name First

Year

Year

Range

MSL Trend

and Standard

Error (mm/yr)

Year 2000 MSL

and Standard

Error (mm)

Autoregressive

Coefficient and

Standard Error

Standard

Error of

Model (mm)

Nawiliwili 1954 46 1.53 0.38 42.1 11.6 0.66 0.03 39.9

Honolulu 1905 95 1.50 0.14 48.2 8.6 0.75 0.02 33.2

Mokuoloe 1957 43 1.12 0.46 44.2 13.2 0.72 0.04 36.0

Kahului 1954 46 2.09 0.43 55.2 12.4 0.77 0.03 31.1

Hilo 1927 73 3.36 0.21 64.9 9.3 0.62 0.03 40.7

Johnston Atoll 1947 53 0.68 0.31 22.5 13.1 0.43 0.04 65.1

Sand Island 1947 53 0.09 0.31 8.9 12.4 0.55 0.03 54.7

Guam 1948 52 0.10 0.90 13.3 27.4 0.88 0.02 40.1

Pago Pago 1948 52 1.48 0.56 37.9 17.4 0.82 0.02 36.7

Kwajalein 1946 54 1.05 0.51 20.1 16.3 0.83 0.02 33.3

Chuuk 1947 49 0.68 0.90 11.3 29.8 0.85 0.02 42.3

Wake Island 1950 50 1.89 0.35 60.9 14.1 0.46 0.04 66.4

Bermuda 1932 68 1.83 0.30 49.0 14.7 0.47 0.03 77.0

Eastport 1929 71 2.12 0.13 30.3 7.2 0.45 0.03 40.7

Bar Harbor 1947 53 2.18 0.16 52.0 7.6 0.32 0.04 40.1

Portland 1912 88 1.91 0.09 45.8 6.6 0.44 0.03 43.1

Seavey Island 1926 61 1.75 0.17 64.0 9.6 0.38 0.04 45.0

Boston 1921 79 2.65 0.10 68.2 6.9 0.38 0.03 45.2

Woods Hole 1932 68 2.59 0.12 65.1 7.0 0.40 0.03 41.0

Nantucket Island 1965 35 3.00 0.32 87.7 9.5 0.34 0.05 42.6

Newport 1930 70 2.57 0.11 75.7 6.5 0.35 0.03 41.0

Providence 1938 62 1.88 0.17 54.9 7.8 0.47 0.04 39.3

New London 1938 62 2.13 0.15 64.2 7.8 0.40 0.03 44.4

Bridgeport 1964 36 2.58 0.41 73.5 11.7 0.43 0.05 49.2

Montauk 1947 53 2.58 0.19 80.0 8.7 0.36 0.04 46.2

Port Jefferson 1957 36 2.44 0.39 73.9 13.2 0.39 0.05 49.1

Willets Point 1931 69 2.41 0.15 68.6 8.6 0.33 0.03 55.0

New Rochelle 1957 25 0.54 0.85 11.0 28.6 0.48 0.05 51.6

The Battery 1856 144 2.77 0.05 76.9 6.3 0.34 0.02 56.1

Sandy Hook 1932 68 3.88 0.15 110.2 8.7 0.33 0.03 55.1

Atlantic City 1911 89 3.98 0.11 126.5 8.2 0.31 0.03 59.1

Cape May 1965 35 3.88 0.53 102.8 14.2 0.41 0.05 57.5

Philadelphia 1900 100 2.75 0.12 89.8 10.0 0.37 0.03 73.7

Lewes 1919 81 3.16 0.16 92.7 9.8 0.34 0.04 59.7

Cambridge 1943 57 3.52 0.24 95.5 11.0 0.44 0.04 51.2
Table 3. Linear MSL trends for all monthly data up to 1999

Station Name First

Year

Year

Range

MSL Trend

and Standard

Error (mm/yr)

Year 2000 MSL

and Standard

Error (mm)

Autoregressive

Coefficient and

Standard Error

Standard

Error of

Model (mm)

19

Baltimore 1902 98 3.12 0.08 90.8 7.1 0.32 0.03 54.8

Annapolis 1928 72 3.53 0.13 101.8 8.3 0.34 0.03 53.3

Solomons Island 1937 63 3.29 0.17 92.9 9.1 0.36 0.04 53.2

Washington 1931 69 3.13 0.21 94.6 12.1 0.36 0.03 76.2

Kiptopeke 1951 49 3.59 0.27 115.7 11.1 0.35 0.04 58.5

Colonial Beach 1972 28 5.27 0.72 133.1 16.4 0.40 0.05 61.6

Lewisetta 1974 26 4.85 0.79 110.4 16.6 0.42 0.05 60.4

Gloucester Point 1950 50 3.95 0.27 116.3 11.6 0.35 0.04 61.3

Sewells Point 1927 73 4.42 0.16 136.8 9.9 0.34 0.03 64.2

Portsmouth 1935 53 3.76 0.23 114.6 12.7 0.29 0.04 61.8

Chesapeake Bay

Bridge Tunnel 1975 25 7.01 0.86 163.9 18.0 0.38 0.06 66.6

Beaufort 1973 27 3.71 0.64 115.0 14.7 0.38 0.05 56.8

Wilmington 1935 65 2.22 0.25 70.8 12.3 0.49 0.03 65.2

Springmaid Pier 1957 43 5.17 0.49 110.8 15.9 0.50 0.04 64.2

Charleston, SC 1921 79 3.28 0.14 101.1 9.3 0.40 0.03 59.5

Fort Pulaski 1935 65 3.05 0.20 98.4 10.7 0.38 0.03 63.3

Fernandina Beach 1897 103 2.04 0.12 67.8 9.9 0.42 0.03 67.9

Mayport 1928 72 2.43 0.18 73.2 10.7 0.42 0.03 64.8

Miami Beach 1931 51 2.39 0.22 73.6 12.2 0.39 0.04 47.7

Vaca Key 1971 29 2.58 0.44 88.7 10.4 0.38 0.05 39.6

Key West 1913 87 2.27 0.09 77.8 6.4 0.47 0.03 40.1

Naples 1965 35 2.08 0.43 54.1 11.3 0.54 0.04 41.3

Fort Myers 1965 35 2.29 0.45 69.0 12.2 0.48 0.04 47.1

St. Petersburg 1947 53 2.40 0.18 78.4 7.9 0.40 0.04 41.9

Clearwater Beach 1973 27 2.76 0.65 72.7 13.8 0.50 0.05 42.4

Cedar Key 1914 86 1.87 0.11 61.3 7.6 0.42 0.03 46.3

Apalachicola 1967 33 1.53 0.58 56.2 14.6 0.50 0.05 53.6

Panama City 1973 27 0.30 0.64 40.9 13.5 0.45 0.05 46.4

Pensacola 1923 77 2.14 0.15 66.6 8.8 0.52 0.03 49.2

Dauphin Island 1966 32 2.93 0.59 81.5 15.2 0.47 0.05 52.8

Grand Isle 1947 53 9.85 0.35 157.3 13.1 0.62 0.03 51.0

Eugene Island 1939 36 9.74 0.63 315.0 29.8 0.58 0.04 52.8

Sabine Pass 1958 42 6.54 0.72 29.2* 21.7 0.62 0.04 72.6

Galveston Pier 21 1908 92 6.50 0.16 17.7* 11.1 0.53 0.03 67.2
Table 3. Linear MSL trends for all monthly data up to 1999

Station Name First

Year

Year

Range

MSL Trend

and Standard

Error (mm/yr)

Year 2000 MSL

and Standard

Error (mm)

Autoregressive

Coefficient and

Standard Error

Standard

Error of

Model (mm)

20

Galveston

Pleasure Pier 1957 43 7.39 0.53 26.8* 16.9 0.54 0.04 66.3

Freeport 1954 46 5.87 0.74 42.3 15.7 0.49 0.04 63.6

Rockport 1948 52 4.60 0.41 119.9 15.1 0.53 0.04 65.4

Port Mansfield 1963 35 2.05 0.75 86.7 20.0 0.56 0.04 65.3

Padre Island 1958 37 3.44 0.56 104.5 17.9 0.54 0.05 53.4

Port Isabel 1944 56 3.38 0.27 111.9 11.7 0.50 0.03 56.5

San Diego 1906 94 2.15 0.12 77.0 7.1 0.71 0.02 30.3

La Jolla 1924 76 2.22 0.17 83.6 8.5 0.72 0.02 31.8

Newport Beach 1955 39 2.22 0.53 81.6 15.5 0.76 0.03 30.5

Los Angeles 1923 77 0.84 0.16 39.4 8.0 0.71 0.02 30.9

Santa Monica 1933 67 1.59 0.25 52.2 11.0 0.74 0.03 35.6

Rincon Island 1962 29 3.22 0.85 97.2 22.3 0.75 0.04 34.1

Santa Barbara 1973 26 2.77 0.99 68.2 18.3 0.75 0.05 29.3

Port San Luis 1945 55 0.90 0.32 35.8 11.3 0.71 0.03 36.7

Monterey 1973 27 1.86 1.09 70.8 18.3 0.74 0.04 38.4

San Francisco 1854 146 1.41 0.08 46.0 8.0 0.69 0.02 44.9

San Francisco

(Pre EQ) 1854 52 1.12 0.35 23.6 43.5

0.66 0.02 44.5

San Francisco

(Post EQ) 1906 94 2.13 0.14 73.1 8.7

Alameda 1939 61 0.89 0.32 37.0 12.8 0.71 0.03 45.2

Point Reyes 1975 25 2.51 1.27 76.4 21.1 0.70 0.04 48.9

Crescent City 1933 67 -0.48 0.23 -5.1 11.6 0.49 0.03 61.2

Charleston, OR 1970 30 1.74 0.87 30.5 19.4 0.54 0.05 65.7

South Beach 1967 33 3.51 0.73 79.6 18.4 0.50 0.04 69.3

Astoria 1925 75 -0.16 0.24 7.1 14.0 0.46 0.03 83.4

Toke Point 1973 27 2.82 1.05 35.9 22.8 0.43 0.05 83.4

Neah Bay 1934 66 -1.41 0.22 -44.0 12.3 0.35 0.03 75.1

Port Angeles 1975 25 1.49 1.10 54.3 20.9 0.48 0.05 69.1

Port Townsend 1972 28 2.82 0.88 84.0 18.6 0.52 0.05 62.7

Seattle 1898 102 2.11 0.10 67.6 8.1 0.40 0.03 59.7

Cherry Point 1973 27 1.39 0.94 50.5 19.0 0.53 0.05 62.2

Friday Harbor 1934 66 1.24 0.20 48.8 10.6 0.43 0.03 61.0

Ketchikan 1919 81 -0.11 0.16 -4.1 10.8 0.36 0.03 73.2
Table 3. Linear MSL trends for all monthly data up to 1999

Station Name First

Year

Year

Range

MSL Trend

and Standard

Error (mm/yr)

Year 2000 MSL

and Standard

Error (mm)

Autoregressive

Coefficient and

Standard Error

Standard

Error of

Model (mm)

21

Sitka 1938 62 -2.17 0.21 -69.6 11.2 0.36 0.03 66.5

Juneau 1936 64 -12.69 0.26 -16.9** 13.2 0.40 0.03 75.3

Skagway 1944 56 -16.68 0.42 -18.3** 19.6 0.48 0.04 89.0

Yakutat 1940 60 -5.75 0.27 -75.1 13.4 0.40 0.04 75.3

Cordova 1964 36 6.97 0.60 148.6 17.6 0.36 0.05 77.4

Valdez 1973 27 -0.34 1.00 35.9 22.1 0.37 0.05 83.7

Seward (Pre EQ) 1925 39 -0.13 0.57 -1019.1 38.8 0.36 0.04 84.0

Seward (Post EQ) 1964 36 -1.46 0.61 -42.8 16.3

Seldovia 1964 36 -9.93 0.78 -29.5** 21.9 0.43 0.05 90.9

Nikiski 1973 27 -10.71 1.17 -25.5** 38.1 0.17 0.13 88.2

Anchorage 1972 28 2.76 1.16 26.9** 25.0 0.48 0.06 83.1

Kodiak 1975 25 -12.08 1.06 -56.1** 20.1 0.38 0.06 69.0

Sand Point 1972 28 0.07 0.93 -45.5 22.4 0.31 0.05 91.3

Adak Island 

(Pre EQ) 1943 14 2.48 1.84 116.7 93.0

0.29 0.04 66.0

Adak Island 

(Post EQ) 1957 43 -2.63 0.35 -38.9 12.2

Unalaska 

(Pre EQ) 1934 23 -0.57 1.11 183.8** 63.0

0.37 0.04 75.7

Unalaska 

(Post EQ) 1957 43 -6.44 0.44 -41.1** 14.7

Guantanamo Bay 1937 35 1.64 0.41 812.2 19.4 0.65 0.04 28.3

Charlotte Amalie 1975 25 0.50 0.74 17.5 11.8 0.73 0.04 24.7

San Juan 1962 38 1.43 0.36 58.4 9.3 0.68 0.04 27.2

Magueyes Island 1955 45 1.24 0.25 34.5 7.4 0.69 0.03 23.1

* Recently updated datum based on 1990-1994.

** Recently updated datum based on 1994-1998.
22

-20

-15

-10

-5

0

5

10

15

SAN DIEGOLA JOLLANEWPORT BEACHLOS ANGELESSANTA MONICARINCON ISLANDSANTA BARBARAPORT SAN LUISMONTEREYSAN FRANCISCOSAN FRANCISCO (PRE EQ)SAN FRANCISCO (POST EQ)ALAMEDAPOINT REYESCRESCENT CITYCHARLESTONSOUTH BEACHASTORIATOKE POINTNEAH BAYPORT ANGELESPORT TOWNSENDSEATTLECHERRY POINTFRIDAY HARBORKETCHIKANSITKAJUNEAUSKAGWAYYAKUTATCORDOVAVALDEZSEWARD (PRE EQ)SEWARD (POST EQ)SELDOVIANIKISKIANCHORAGEKODIAK ISLANDSAND POINTADAK ISLAND (PRE EQ)ADAK ISLAND (POST EQ)UNALASKA (PRE EQ)UNALASKA (POST EQ)

95% Confidence Interval of MSL Trend (mm/yr)

Figure 9. MSL trend range (mm/yr) for all monthly data up to 1999 for U.S. west coast and

Alaska stations.

-20

-15

-10

-5

0

5

10

15

EASTPORTBAR HARBORPORTLANDSEAVEY ISLANDBOSTONWOODS HOLENANTUCKET ISLANDNEWPORTPROVIDENCENEW LONDONBRIDGEPORTMONTAUKPORT JEFFERSONWILLETS POINTNEW ROCHELLETHE BATTERYSANDY HOOKATLANTIC CITYCAPE MAYPHILADELPHIALEWESCAMBRIDGEBALTIMOREANNAPOLISSOLOMONS ISLANDWASHINGTONKIPTOPEKECOLONIAL BEACHLEWISETTAGLOUCESTER POINTSEWELLS POINTPORTSMOUTHCHESAPEAKE BAY BR TNLBEAUFORTWILMINGTONSPRINGMAID PIERCHARLESTONFORT PULASKIFERNANDINA BEACHMAYPORT

95% Confidence Interval of MSL Trend (mm/yr)

Figure 8. MSL trend range (mm/yr) for all monthly data up to 1999 for U.S. east coast stations.
23

-20

-15

-10

-5

0

5

10

15

NAWILIWILIHONOLULUMOKUOLOEKAHULUIHILOJOHNSTON ATOLLSAND ISLANDGUAMPAGO PAGOKWAJALEINCHUUKWAKE ISLANDBERMUDAMIAMI BEACHVACA KEYKEY WESTNAPLESFORT MYERSST. PETERSBURGCLEARWATER BEACHCEDAR KEYAPALACHICOLAPANAMA CITYPENSACOLADAUPHIN ISLANDGRAND ISLEEUGENE ISLANDSABINE PASSGALVESTON PIER 21GALVESTON PLEASURE PIERFREEPORTROCKPORTPORT MANSFIELDPADRE ISLANDPORT ISABELGUANTANAMO BAYCHARLOTTE AMALIESAN JUANMAGUEYES ISLAND

95% Confidence Interval of MSL Trend (mm/yr)

Figure 10. MSL trend range (mm/yr) for all monthly data up to 1999 for the tropical Pacific,

Bermuda, Gulf of Mexico, and Caribbean stations.

Honolulu

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

meters

.6

.5

.4

.3

.2

.1

-.0

-.1

-.2

-.3

-.4

-.5

-.6

Figure 11. Monthly mean sea level at Honolulu with the average seasonal cycle removed

(dashed curve), 5-month running average (solid curve), and the calculated MSL trend with

95% confidence interval.
24

Washington

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

meters

.6

.5

.4

.3

.2

.1

-.0

-.1

-.2

-.3

-.4

-.5

-.6

Figure 12. Monthly mean sea level at Washington, DC with the average seasonal cycle

removed (dashed curve), 5-month running average (solid curve), and the calculated MSL

trend with 95% confidence interval.

Unalaska

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

meters

.6

.5

.4

.3

.2

.1

-.0

-.1

-.2

-.3

-.4

-.5

-.6

Figure 13. Monthly mean sea level at Unalaska with the average seasonal cycle removed

(dashed curve), 5-month running average (solid curve), and the calculated MSL trends for

the data before and after the 1957 earthquake with 95% confidence intervals.
25

-0.2

0

0.2

0.4

0.6

0.8

1

EASTPORTBAR HARBORPORTLANDSEAVEY ISLANDBOSTONWOODS HOLENANTUCKET ISLANDNEWPORTPROVIDENCENEW LONDONBRIDGEPORTMONTAUKPORT JEFFERSONWILLETS POINTNEW ROCHELLETHE BATTERYSANDY HOOKATLANTIC CITYCAPE MAYPHILADELPHIALEWESCAMBRIDGEBALTIMOREANNAPOLISSOLOMONS ISLANDWASHINGTONKIPTOPEKECOLONIAL BEACHLEWISETTAGLOUCESTER POINTSEWELLS POINTPORTSMOUTHCHESAPEAKE BAY BR TNLBEAUFORTWILMINGTONSPRINDMAID PIERCHARLESTONFORT PULASKIFERNANDINA BEACHMAYPORT

95% Confidence Interval of the Autoregression Coefficient

Figure 14. Autoregressive coefficient range for U.S. east coast stations.

0

10

20

30

40

50

60

70

80

90

100

EASTPORTBAR HARBORPORTLANDSEAVEY ISLANDBOSTONWOODS HOLENANTUCKET ISLANDNEWPORTPROVIDENCENEW LONDONBRIDGEPORTMONTAUKPORT JEFFERSONWILLETS POINTNEW ROCHELLETHE BATTERYSANDY HOOKATLANTIC CITYCAPE MAYPHILADELPHIALEWESCAMBRIDGEBALTIMOREANNAPOLISSOLOMONS ISLANDWASHINGTONKIPTOPEKECOLONIAL BEACHLEWISETTAGLOUCESTER POINTSEWELLS POINTPORTSMOUTHCHESAPEAKE BAY BR TNLBEAUFORTWILMINGTONSPRINGMAID PIERCHARLESTONFORT PULASKIFERNANDINA BEACHMAYPORT

Standard Error (mm)

Figure 15. Standard error of the linear regression with autoregressive residuals method for

U.S. east coast stations.
26

-0.2

0

0.2

0.4

0.6

0.8

1

SAN DIEGOLA JOLLANEWPORT BEACHLOS ANGELESSANTA MONICARINCON ISLANDSANTA BARBARAPORT SAN LUISMONTEREYSAN FRANCISCOALAMEDAPOINT REYESCRESCENT CITYCHARLESTONSOUTH BEACHASTORIATOKE POINTNEAH BAYPORT ANGELESPORT TOWNSENDSEATTLECHERRY POINTFRIDAY HARBORKETCHIKANSITKAJUNEAUSKAGWAYYAKUTATCORDOVAVALDEZSEWARDSELDOVIANIKISKIANCHORAGEKODIAK ISLANDSAND POINTADAK ISLANDUNALASKA

95% Confidence Interval of the Autoregression Coefficient

Figure 16. Autoregressive coefficient range for U.S. west coast and Alaska stations.

0

10

20

30

40

50

60

70

80

90

100

SAN DIEGOLA JOLLANEWPORT BEACHLOS ANGELESSANTA MONICARINCON ISLANDSANTA BARBARAPORT SAN LUISMONTEREYSAN FRANCISCOALAMEDAPOINT REYESCRESCENT CITYCHARLESTONSOUTH BEACHASTORIATOKE POINTNEAH BAYPORT ANGELESPORT TOWNSENDSEATTLECHERRY POINTFRIDAY HARBORKETCHIKANSITKAJUNEAUSKAGWAYYAKUTATCORDOVAVALDEZSEWARDSELDOVIANIKISKIANCHORAGEKODIAK ISLANDSAND POINTADAK ISLANDUNALASKA

Standard Error (mm)

Figure 17. Standard error of the linear regression with autoregressive residuals method for

U.S. west coast and Alaska stations.
27

-0.2

0

0.2

0.4

0.6

0.8

1

NAWILIWILIHONOLULUMOKUOLOEKAHULUIHILOJOHNSTON ATOLLSAND ISLANDGUAMPAGO PAGOKWAJALEINCHUUKWAKE ISLANDBERMUDAMIAMI BEACHVACA KEYKEY WESTNAPLESFORT MYERSST. PETERSBURGCLEARWATER BEACHCEDAR KEYAPALACHICOLAPANAMA CITYPENSACOLADAUPHIN ISLANDGRAND ISLEEUGENE ISLANDSABINE PASSGALVESTON PIER 21GALVESTON PLEASURE PIERFREEPORTROCKPORTPORT MANSFIELDPADRE ISLANDPORT ISABELGUANTANAMO BAYCHARLOTTE AMALIESAN JUANMAGUEYES ISLAND

95% Confidence Interval of the Autoregression Coefficient

Figure 18. Autoregressive coefficient range for the tropical Pacific, Bermuda, Gulf of

Mexico, and Caribbean stations.

0

10

20

30

40

50

60

70

80

90

100

NAWILIWILIHONOLULUMOKUOLOEKAHULUIHILOJOHNSTON ATOLLSAND ISLANDGUAMPAGO PAGOKWAJALEINCHUUKWAKE ISLANDBERMUDAMIAMI BEACHVACA KEYKEY WESTNAPLESFORT MYERSST. PETERSBURGCLEARWATER BEACHCEDAR KEYAPALACHICOLAPANAMA CITYPENSACOLADAUPHIN ISLANDGRAND ISLEEUGENE ISLANDSABINE PASSGALVESTON PIER 21GALVESTON PLEASURE PIERFREEPORTROCKPORTPORT MANSFIELDPADRE ISLANDPORT ISABELGUANTANAMO BAYCHARLOTTE AMALIESAN JUANMAGUEYES ISLAND

Standard Error (mm)

Figure 19. Standard error of the linear regression with autoregressive residuals method for

the tropical Pacific, Bermuda, Gulf of Mexico, and Caribbean stations.
28
29

V. AVERAGE SEASONAL MEAN SEA LEVEL CYCLE

As a result of using monthly values rather than annual values for the determination of linear MSL

trends, an average seasonal cycle is obtained for each station. The average seasonal cycles can show

wide variations depending on the water temperature, winds, and currents in the nearby coastal ocean.

The results for each NWLON station are given in Table B in Appendix III showing average MSL

values for January through December and the mean value for the twelve months. The corresponding

standard errors are given in the following line of the table. The mean values are, in effect, the

intercept at which the trend crosses the year 2000 axis (and are also listed in Table 3). 

The average seasonal cycles at eight selected representative stations are shown in Figures 20 to 27,

with their 95% confidence intervals. Two cycles are displayed for clarity. Similar plots for all

NWLON stations are found in Appendix III. 

At Eastport, there is very little month-to-month variation in average MSL, while Fernandina Beach

and Neah Bay have large seasonal variations of nearly 0.3 meters. At most stations, the water levels

are highest in the late summer-early fall and lowest in late winter-early spring (Honolulu, Baltimore,

and San Diego), reflecting the annual cycle of water temperature. However, at many U.S. west coast

and Alaskan stations, the seasonal cycle shows highest water levels in the winter and lowest water

levels in the summer (Neah Bay and Yakutat). Moreover, at some of the U.S. southeastern stations,

there are two high and two low water level periods each year (Fernandina Beach and Galveston Pier

21). These effects are likely due to dynamic seasonal variations in the California Current, Alaska

Current, and the Gulf Stream.

The level of the seasonal cycle reflects positive or negative trends since the establishment of the

MSL datum. The monthly MSL values used in the analyses are relative to datums of the present

NTDE (1960-1978), except for a few locations recently updated, as noted previously. The seasonal

cycle is completely positive for Honolulu, Eastport, and San Diego, and mostly positive for

Baltimore and Fernandina Beach, due to rising sea levels at these locations. The seasonal cycle is

mostly negative for Yakutat, due to vertical uplift of the land since the datum was established. The

cycles at Galveston Pier 21 and Neah Bay have relatively equal positive and negative months. At

Neah Bay, the trend is small (-1.41 mm/yr), while at Galveston Pier 21 the MSL datum was recently

updated with data from 1990-1994.
30

Figure 20. Average seasonal MSL cycle for Honolulu over a 2-year period with 95%

confidence intervals.

Figure 21. Average seasonal MSL cycle for Eastport over a 2-year period with 95%

confidence intervals.

Figure 22. Average seasonal MSL cycle for Baltimore over a 2-year period with 95%

confidence intervals.

Figure 23. Average seasonal MSL cycle for Fernandina Beach over a 2-year period with

95% confidence intervals.
31

Figure 24. Average seasonal MSL cycle for Galveston Pier 21 over a 2-year period with

95% confidence intervals.

Figure 25. Average seasonal MSL cycle for San Diego over a 2-year period with 95%

confidence intervals.

Figure 26. Average seasonal MSL cycle for Neah Bay over a 2-year period with 95%

confidence intervals.

Figure 27. Average seasonal MSL cycle for Yakutat over a 2-year period with 95%

confidence intervals.
32
33

Honolulu

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

meters

.3

.2

.1

-.0

-.1

-.2

-.3

Figure 28. Monthly mean sea level residual at Honolulu with the average seasonal cycle and

the linear trend removed (dashed curve), and the 5-month running average (solid curve).

VI. VARIABILITY OF RESIDUAL MONTHLY MEAN SEA LEVEL

After the linear trend and the average seasonal cycle are removed from monthly MSLs, a residual

time series is obtained. The residual represents periods when the MSL is anomalously high or low.

This often occurs when regional ocean temperatures are anomalously warm or cool or when there

are anomalies in oceanic winds or currents (i.e., the ocean’s “weather”). The MSL residuals for all

the NWLON stations are plotted in Appendix IV. A sample plot for Honolulu is shown in Figure

28. The 5-month running average is also plotted in order to smooth out any individual anomalous

months in order to focus more attention on anomalous seasons. The plots in Appendix IV are also

marked with solid vertical lines to show the times of any major earthquakes in the vicinity of each

station. 

The primary cause of anomalous ocean conditions is the El Nino/Southern Oscillation (ENSO) in

the equatorial Pacific Ocean. Westward winds normally maintain slightly higher water levels in the

western Pacific relative to the eastern Pacific. Every three to five years, in a non-periodic pattern,

the winds weaken and water levels in the western Pacific drop below normal. The southern

equatorial current is weakened and water temperatures in the eastern Pacific rise. This condition is

known as El Nino. The opposite condition, known as La Nina, occurs when westward equatorial

winds are unusually strong and water levels in the western Pacific become anomalously high. The

south equatorial current strengthens, accompanied by below normal water temperatures in the

eastern Pacific. Anomalous temperatures in the eastern Pacific can eventually affect ocean

temperatures, winds, and water levels in many regions of the world’s oceans and atmosphere.

In comparing MSL time series between nearby NWLON stations, it appears that a small proportion

of the data at a few stations are suspect because of unexplained differences of 0.1 to 0.2 m relative

to nearby stations. These periods, lasting from 4 months to 27 months, are easily isolated. Since

it appears that these short periods do not significantly affect the trends derived in Section IV, they

were not removed from the data sets. However, they are marked as suspect data in the plots in

Appendices II and IV and are listed in Table 4. 
34

Table 4. Periods of suspect data

Station Number Period Estimated Error (m)

Springmaid Pier 8661000 7/71-12/71 0.15

Sabine Pass 8770590 1/74-8/74 0.1

Galveston Pleasure Pier 8771510 1/67-6/67 0.1

Galveston Pleasure Pier 8771510 12/70-4/72 0.1

Santa Monica 9410840 8/83-6/85 -0.1

Santa Monica 9410840 1/92-4/92 0.1

Port San Luis 9412110 1/69-1/70 0.1

Astoria 9439040 6/48-5/49 0.2

Ketchikan 9450460 6/66-9/68 0.1

Skagway 9452400 10/69-3/70 0.2

Skagway 9452400 1/82-12/82 -0.1

Seldovia 9455500 8/77-6/78 0.2

The 5-month average time series in Appendix IV are highly correlated with time series at nearby

stations and are representative of the regional ocean “weather”. The months when the 5-month

average is above 0.1 m or below -0.1 m are defined as periods of extreme conditions. The number

of months with extreme levels in each year are listed in Tables 5 and 6 for all the Atlantic and

Pacific NWLON stations. These tables show the high regional correlations during extreme

conditions. Years with any month above 0.1 m are positive and shaded red. Years with any month

below -0.1 m are negative and shaded blue. Zeros indicate years when there were no months with

residuals above 0.1 m or below -0.1 m. Blank spaces indicate no data. The suspect data periods in

Table 4 are not included in Tables 5 and 6.

For the first quarter of the 20th century, there were few stations operating and little can be inferred

about regional residual water level conditions. The first clearly regional events are the low residuals

in 1929-1930 from Astoria to Ketchikan, and the low residuals in 1931 at Charleston, Mayport,

Pensacola, and Galveston Pier 21. Low residuals also occurred in 1936 from Astoria to Seattle and

in 1939 from San Francisco to Friday Harbor. 

There were high regional residuals in 1940-1941 from Los Angeles to Crescent City and from Friday

Harbor to Yakutat. A period of high residuals on the southeastern U.S. coast occurred in 1947-1948

from Wilmington to Grand Isle followed by high residuals in 1949-1950 from Grand Isle to Port

Isabel.

With the establishment of western Pacific NWLON stations at Guam, Pago Pago, Kwajalein, and

Chuuk in the late 1940s, the long distance effects of anomalous ENSO conditions can be observed.

High residuals at Pago Pago and Chuuk in 1955 were followed by low residuals in 1956-1957 from

Ketchikan to Yakutat. Low residuals at Pago Pago, Kwajalein and Chuuk in 1957-1958 were

followed in 1958 by high residuals from San Francisco to Ketchikan. 
35

In 1963-1964, low residuals occurred along the Gulf coast from Grand Isle to Port Isabel and in

1964 along the Pacific coast from Crescent City to Friday Harbor. After high residuals at Guam and

Pago Pago in 1971, low residuals appeared from Seward to Unalaska in 1971-1972.

The strong 1972 El Nino is represented by low residuals at Guam, Kwajalein, and Chuuk, and was

followed by high residuals in 1972-1973 along the Atlantic coast from New London to Mayport and

along the Gulf coast from Apalachicola to Rockport. High residuals did not occur on the west coast

at this time. There were high residuals in 1975 along the Gulf coast from Pensacola to Padre Island.

In 1976, high residuals were observed from Juneau to Unalaska.

In 1976-1977, low residuals occurred along the Atlantic coast from Providence to Beaufort.

Residuals were also low in 1980-1981 at Atlantic coast stations from Willets Point to Mayport. The

extremely strong El Nino of 1982-1983 resulted in low residuals at Guam, Pago Pago, Kwajalein,

and Chuuk and high residuals at all west coast stations from San Diego to Unalaska. High residuals

were also observed in 1983 along the Atlantic coast from Gloucester Point to Fernandina Beach.

High residuals in 1985 at Guam, Pago Pago and Chuuk were accompanied by low residuals from

Crescent City to Sand Point. Low residuals at Guam in 1987 were accompanied by high residuals

from Juneau to Sand Point. The strong La Nina in 1988-1989 is represented by high residuals at

Guam and Chuuk and by low residuals from Newport Beach to Sand Point.

Low residuals in 1991 at Guam and Kwajalein were followed in 1992 with high residuals from San

Diego to Crescent City and from Cherry Point to Anchorage. Low residuals were observed in 1996

along the Gulf coast from Sabine Pass to Port Mansfield and in Alaska from Juneau to Anchorage.

The extremely powerful El Nino of 1997-1998 is represented by low residuals for Guam, Pago Pago,

and Kwajalein and high residuals at all west coast stations from San Diego to Unalaska and, as in

1983, along the Atlantic coast from Providence to Wilmington.
36

Table 5. Number of months with extreme residual water levels for Atlantic stations

Station 1900190119021903190419051906190719081909191019111912191319141915191619171918191919201921192219231924

Bermuda 

Eastport

Bar Harbor

Portland 0 0 0 0 0 0 0 0 0 0 0 0 0 

Seavey Island

Boston 0 0 0 0 

Woods Hole

Nantucket Island

Newport

Providence

New London

Bridgeport

Montauk

Port Jefferson

Willets Point

New Rochelle

The Battery 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Sandy Hook 

Atlantic City 0 0 0 0 0 0 0 0 0 0 0 0 

Cape May

Philadelphia -1 -1 5 6 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Lewes 0 0 0 0 0 

Cambridge 

Baltimore 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Annapolis 

Solomons Island

Washington

Kiptopeke

Colonial Beach

Lewisetta

Gloucester Point

Sewells Point

Portsmouth

Ches. Bay Br Tnl

Beaufort

Wilmington

Springmaid Pier

Charleston, SC 0 0 0 0 

Fort Pulaski

Fernandina Beach 000 5 0 0 0 -2 2 0 0 0 0 0 0 0 0 -4 0 0 0 0 0 0 0 

Mayport 

Miami Beach

Vaca Key

Key West 0 0 0 0 0 0 0 0 0 0 0 0 

Naples

Fort Myers

St. Petersburg

Clearwater Beach

Cedar Key 0 0 0 0 0 0 0 0 0 0 0 

Apalachicola

Panama City

Pensacola 0 0 

Dauphin Island

Grand Isle

Eugene Island

Sabine Pass

Galveston Pier 21 2 0000 2 0 0 0 -4 -1 1 0 7 0 2 0 

Galv. Pleasure Pier 

Freeport

Rockport

Port Mansfield

Padre Island

Port Isabel

Guantanamo Bay

Charlotte Amalie

San Juan

Magueyes Island
37

Table 5. Number of months with extreme residual water levels for Atlantic stations

Station 1925192619271928192919301931193219331934193519361937193819391940194119421943194419451946194719481949

Bermuda 0 0 0 0 0 0 -2 -4 0 0 0 

Eastport 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Bar Harbor 0 0 0 

Portland 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Seavey Island 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Boston 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Woods Hole 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Nantucket Island 

Newport 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Providence 0 0 0 0 0 0 0 0 0 

New London 0 0 0 0 0 0 0 0 0 0 0 0 

Bridgeport 

Montauk 0 0 0 

Port Jefferson

Willets Point 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

New Rochelle

The Battery 0 0 0 -1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Sandy Hook 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Atlantic City 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Cape May 

Philadelphia 0 0 1 -1 0 -1 0 0 0 0 0 1 00000000000 1 0 

Lewes 0 0 0 0 0 0 0 

Cambridge 0 0 0 0 0 0 0 

Baltimore 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Annapolis 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Solomons Island 0 0 0 0 0 0 0 0 0 0 0 0 

Washington 00000 2 00000000000 2 0 

Kiptopeke 

Colonial Beach

Lewisetta

Gloucester Point

Sewells Point 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Portsmouth 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Ches. Bay Br Tnl 

Beaufort

Wilmington 00 1 0 0 0 -2 0 0 0 4136 0 

Springmaid Pier 

Charleston, SC 0 0 0 0 0 0 -4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 8 0 

Fort Pulaski 000000000000 4 5 0 

Fernandina Beach 00000000 3 4 0 

Mayport 0 0 0 -4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 7 0 

Miami Beach 0000000000000000 2 6 0 

Vaca Key 

Key West 0000000000000000000000 1 3 0 

Naples 

Fort Myers

St. Petersburg 0 4 0 

Clearwater Beach 

Cedar Key 0 0000000000 4 1 

Apalachicola 

Panama City

Pensacola 0000 1 0 -2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5 0 

Dauphin Island 

Grand Isle 0 4 1 

Eugene Island 0 -4 0 0 0 0 0 0 0 0 2 

Sabine Pass 

Galveston Pier 21 0 -1 0 0 3 0 -6 0 0 0 0 0 0 0 -2 -6 0 0 0 0 1 3 0 0 2 

Galv. Pleasure Pier 

Freeport

Rockport 0 1 

Port Mansfield 

Padre Island

Port Isabel 00 2 0 1 0 

Guantanamo Bay 0000 1 1 0 0 0 0 0 

Charlotte Amalie 

San Juan

Magueyes Island
38

Table 5. Number of months with extreme residual water levels for Atlantic stations

Station 1950195119521953195419551956195719581959196019611962196319641965196619671968196919701971197219731974

Bermuda 00000 4141 0 0 2 6 0000 1 0 -1 -6 -2 0 -2 3 

Eastport 00000000000000000000 2 0 0 0 0 

Bar Harbor 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Portland 00000000000000000000 3 2 0 0 0 

Seavey Island 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Boston 00000000 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Woods Hole 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Nantucket Island 0 0 0 0 0 0 0 0 0 0 

Newport 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Providence 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

New London 00000000000000000000000 1 0 

Bridgeport 0 000000 2 0 

Montauk 00000000 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Port Jefferson 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Willets Point 0000000000000000000000 1 3 0 

New Rochelle 000000000000000 1 2 0 

The Battery 00000000000000000000000 1 0 

Sandy Hook 0000000000000000000000 1 3 0 

Atlantic City 00000000000000000000 0 1 3 0 

Cape May 000 1 2 0 

Philadelphia 000 2 000000 00000000000 2 4 0 

Lewes 00000000000000000000 2 4 0 

Cambridge 00 00 1 0 

Baltimore 00000000000000000000000 2 0 

Annapolis 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Solomons Island 00000000000000000000000 1 0 

Washington -1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 6 3 0 

Kiptopeke 0000000000000000000000 1 0 

Colonial Beach 2 3 0 

Lewisetta 0 

Gloucester Point 0000000000000000000 00 1 2 0 

Sewells Point 0000000000000000000000 2 3 0 

Portsmouth 0000000000000000000000 1 2 0 

Ches. Bay Br Tnl 

Beaufort 1 0 

Wilmington 00000 1 0000000000000000 1 4 0 

Springmaid Pier 0000000 0000 0 1 0 

Charleston, SC 0000000000000000000000 1 3 0 

Fort Pulaski 000000000000000000000000 

Fernandina Beach 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -1 0 0 0 0 0 0 0 0 2 0 

Mayport 0000000000 2 000000000000 2 0 

Miami Beach 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Vaca Key 0 0 0 0 

Key West 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Naples 0 0 0 0 0 0 0 0 0 0 

Fort Myers 0 0 0 0 0 0 0 0 0 0 

St. Petersburg 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Clearwater Beach 0 0 

Cedar Key 00000000000 1 1 0 0 0 0 0 0 0 0 0 0 0 0 

Apalachicola 0000 0 4 0 

Panama City 0 

Pensacola 00000000000000000000000 2 0 

Dauphin Island 0 0 0 0 0 0 0 

Grand Isle 1 0 0 0 0 0 0 0 0 0 0 0 0 -2 -2 0 0 0 0 0 0 0 0 1 0 

Eugene Island 3 000000000000000000000 1 

Sabine Pass 000 2 0 -6 -4 -3 0 0 0 0 0 122 

Galveston Pier 21 3 0000000000 2 0 -4 -3 0 0 0 0 0 0 -2 0 1 0 

Galv. Pleasure Pier 0000 1 0 0 -9 0 0 2 0 0 0 0 

Freeport 0000000 2 0 0 -1 0 0 0 0 0 0 2 0 0 0 

Rockport 3 0 0 0 0 -3 -4 0 0 0 0 0 0 1 0 1 0 

Port Mansfield -1 0 -1 1 2 5 0 0 

Padre Island 0 0 0 0 0 -1 0 0 0 0 0 0 0 0 0 0 

Port Isabel 1 0000000000 1 0 0 -1 0 0 0 0 0 0 0 0 0 0 

Guantanamo Bay 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Charlotte Amalie 

San Juan 0 0 0 0 0 0 0 0 0 0 0 0 0 

Magueyes Island 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
39

Table 5. Number of months with extreme residual water levels for Atlantic stations

Station 1975197619771978197919801981198219831984198519861987198819891990199119921993199419951996199719981999

Bermuda 3 1 0 0 0 0 0 0 -2 0 -2 0 0 -1 0 0 0 0 -3 0 2 0 0 0 0 

Eastport 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Bar Harbor 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Portland 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Seavey Island 0 0 0 0 0 0 0 0 0 0 0 0 

Boston 00000000000000000000000 1 0 

Woods Hole 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -1 0 0 0 0 0 0 0 0 0 

Nantucket Island 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Newport 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Providence 0 -1 -2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 

New London 00 0000000000000000000 1 4 0 

Bridgeport 0 -2 -2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4 0 

Montauk 000000000000000000 0000 3 0 

Port Jefferson 0 -2 -2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Willets Point 0 -2 -2 0 0 -2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4 0 

New Rochelle 0 0 0 0 0 0 0 

The Battery 0 -2 -3 0 0 -1 -1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 224 0 

Sandy Hook 0 -2 -3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 4 0 

Atlantic City 0 0 0 0 0 -2 -1 0 1 0 0 0 0 0 0 0 0 0 0 0 -3 0 1 4 

Cape May 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -1 0 0 0 0 0 0 1 4 0 

Philadelphia 0 -2 -3 0 0 -3 -2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 214 0 

Lewes 0 -2 -4 0 0 0 0 0 1 000 1 0 0 -2 0 0 0 0 0 0 1 4 0 

Cambridge 00 0000000000000000000 1 4 0 

Baltimore 0 -2 -3 0 0 -2 -2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 1 4 0 

Annapolis 0 -2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 0 2 0 

Solomons Island 0 -2 0 0 -2 -2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 214 0 

Washington 0 -1 -4 0 0 -2 -5 0 0 2 0 0 0 0 0 -1 0 0 2 0 0 625 0 

Kiptopeke 0 -2 -4 0 0 0 0 0 0 0 0 0 0 0 0 -2 0 0 0 0 0 0 1 4 0 

Colonial Beach 0 -2 -4 0 0 0 -1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Lewisetta 0 0 0 0 0 0 -1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 4 0 

Gloucester Point 0 -2 -4 0 0 -1 -1 0 1 0000000000000 1 4 0 

Sewells Point 0 -2 -4 0 0 -2 -1 0 2 0 0 0 0 -3 0 0 0 0 0 0 0 0 1 5 0 

Portsmouth 0 -2 -4 0 0 -1 -1 0 4 0 0 0 0 

Ches. Bay Br Tnl 0 -1 -3 0 0 0 0 1 4 0000000000000 1 4 0 

Beaufort 0 -1 -1 0 0 0 0 0 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Wilmington 0 0 0 0 0 -4 -4 0 3 00000000000 1 2 0 4 3 

Springmaid Pier 0 0 0 -2 0 0 2 000 2 00 3 0 0 0 0 -2 0 -3 0 

Charleston, SC 00000000 2 0 0 0 0 0 -1 0 0 0 0 0 0 0 0 0 0 

Fort Pulaski 00 0000 1 2 000 1 0000000 5 0 0 0 0 

Fernandina Beach 0 0 0 0 0 -1 0 0 2 000 2 0 -1 0 1 0 0 0 0 0 0 

Mayport 0 0 0 0 0 -5 -1 0 0 0 0 0 0 0 -1 0 0 0 0 0 5 0 0 0 0 

Miami Beach 0 0 0 0 0 0 0 

Vaca Key 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Key West 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -1 0 0 0 0 0 0 0 0 0 0 

Naples 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Fort Myers 00000000 4 00000000000 2 0 0 0 0 

St. Petersburg 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Clearwater Beach 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Cedar Key 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Apalachicola 0000000 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Panama City 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Pensacola 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Dauphin Island -1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Grand Isle 2 000000 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Eugene Island 

Sabine Pass 8 0 0 0 0 0 0 0 -1 0 4 0 0 0 0 -7 0 0 0 

Galveston Pier 21 5 -3 0 0 0 0 0 0 1 0000000 2 0 0 0 0 -3 0 0 0 

Galv. Pleasure Pier 0 0 -8 -1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -3 0 0 0 

Freeport 4 -3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -1 0 0 0 

Rockport 5 0 0 0 0 -2 0 0 0 0 0 0 0 0 -2 0 0 0 0 0 0 0 4 0 

Port Mansfield 5 0 0 0 0 -1 0 0 0 2 0 0 0 0 0 0 0 0 0 0 0 -3 0 

Padre Island 5 -3 0 0 0 0 0 0 0 0 0 0 0 0 

Port Isabel 00000000000000000000 1 0 0 0 

Guantanamo Bay 

Charlotte Amalie 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

San Juan 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Magueyes Island 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 
40

Table 6. Number of months with extreme residual water levels for Pacific stations

Station 1900190119021903190419051906190719081909191019111912191319141915191619171918191919201921192219231924

Nawiliwili 

Honolulu 0 0 0 0 -6 -2 0 0 0 0 2 0000 7 0 0 0 0 

Mokuoloe 

Kahului

Hilo 

Johnston Atoll

Sand Island

Guam

Pago Pago

Kwajalein

Chuuk

Wake Island

San Diego 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

La Jolla

Newport Beach

Los Angeles 0 

Santa Monica

Rincon Island

Santa Barbara

Port San Luis

Monterey

San Francisco 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -2 

San Francisco w/EQ 00000000000000 1 2 0 0 0 0 0 0 0 0 0 

Alameda 

Point Reyes

Crescent City

Charleston, OR

South Beach

Astoria

Toke Point

Neah Bay

Port Angeles

Port Townsend

Seattle 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Cherry Point

Friday Harbor

Ketchikan -1 0 0 0 0 0 

Sitka

Juneau

Skagway

Yakutat

Cordova

Valdez

Seward

Seldovia

Nikiski

Anchorage

Kodiak Island

Sand Point

Unalaska

Adak Island
41

Table 6. Number of months with extreme residual water levels for Pacific stations

Station 1925192619271928192919301931193219331934193519361937193819391940194119421943194419451946194719481949

Nawiliwili 

Honolulu 000000000000000000 1 0 0 0 0 0 0 

Mokuoloe 

Kahului

Hilo 0 0 0 0 0 0 0 0 0 

Johnston Atoll 0 

Sand Island 0 0 0 

Guam 0 

Pago Pago 0 0 

Kwajalein 0 0 0 

Chuuk 

Wake Island

San Diego 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

La Jolla 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Newport Beach 

Los Angeles 0000000000000000 2 0 0 0 0 0 0 0 0 

Santa Monica 00000000 2 0 0 0 0 0 0 0 0 

Rincon Island 

Santa Barbara

Port San Luis 0 -1 0 0 0 

Monterey

San Francisco 0 0 0 0 0 0 0 0 0 0 0 0 0 0 -4 1 4 0 0 0 0 0 0 0 0 

San Francisco w/EQ 000000000000000 1 4 0 0 0 0 0 0 0 0 

Alameda 0 1 5 0 0 0 0 0 0 0 0 

Point Reyes 

Crescent City 0 0 0 0 0 0 -2 1 4 0 0 0 0 0 0 

Charleston,OR 

South Beach

Astoria 0 0 2 -1 -6 -1 0 0 1 0 0 -3 -1 -1 -2 0 0 0 0 -5 -1 0 0 

Toke Point 

Neah Bay 0 1 -3 -1 0 -2 0 0 0 0 0 0 0 0 0 0 

Port Angeles 

Port Townsend

Seattle 0 0 0 0 -4 -1 0 0 0 0 0 -2 0 0 -2 0 0 0 0 0 0 0 0 0 0 

Cherry Point

Friday Harbor 0 0 0 0 0 -1 0 1 0 0 0 0 0 0 0 0 

Ketchikan 0 2 0 0 -2 -1 3 00000000 2 1 0 0 0 0 0 0 0 0 

Sitka 00 2 1 0 0 0 0 -1 -1 0 0 

Juneau 000 3 3 0 0 0 -1 0 0 

Skagway 0 0 -1 0 0 0 

Yakutat 0 1 0 0 0 0 -1 -1 0 0 

Cordova 

Valdez

Seward 0 5 000 1 3 0 0 0 0 0 0 0 0 0 0 0 0 0 

Seldovia 

Nikiski

Anchorage

Kodiak Island

Sand Point

Unalaska 0 0 0 -2 0 0 0 0 0 0 

Adak Island 0 0 0 0 0 0 0 
42

Table 6. Number of months with extreme residual water levels for Pacific stations

Station 1950195119521953195419551956195719581959196019611962196319641965196619671968196919701971197219731974

Nawiliwili -1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Honolulu 0 0 0 0 0 -3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Mokuoloe 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Kahului 0 -2 0 0 0 0 0 2 0 0 0 0 0 0 0 0 0 0 0 0 0 

Hilo 00000000000 4 0 0 0 0 0 0 0 0 0 0 0 0 0 

Johnston Atoll 00000000000 4 00000000 1 0 0 0 0 

Sand Island 00000000000000 2 0 0 0 0 -1 0 0 0 0 

Guam 3 0 0 3 4 0 0 0 0 0 0 0 0 0 0 0 0 -1 -3 -3 5 4 -7 0 1 

Pago Pago 00000 2 0 -2 0 0 0 0 0 0 0 0 0 0 0 0 4 0 0 0 

Kwajalein 0 0 0 0 0 0 0 -3 -1 0 0 0 0 0 0 -1 0 0 0 0 0 0 -1 0 0 

Chuuk 00 00 3 1 -3 -2 0 0 0 0 -1 0 -4 -1 0 0 0 0 0 -7 2 0 

Wake Island 0 0 1 4 000000 2 0 0 0 0 0 -2 0 0 0 0 0 0 

San Diego 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

La Jolla 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Newport Beach 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Los Angeles 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Santa Monica 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Rincon Island 0 0 0 0 0 0 0 0 0 0 0 0 0 

Santa Barbara 0 

Port San Luis 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Monterey 0 

San Francisco 00000000 1 0000000000000 2 1 0 

San Francisco w/EQ 00000000 3 0000000000000 1 0 0 

Alameda 00000000 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Point Reyes 

Crescent City 0000000 1 4 0 0 0 0 0 -3 0 0 0 0 0 0 0 0 0 0 

Charleston, OR 0 0 0 0 0 

South Beach 0 1 1 0 0 0 0 0 

Astoria 1 0 0 0 0 -1 -2 -1 2 0 0 -2 -1 0 -3 0 0 0 1 1 -2 0 2 -4 2 

Toke Point 0 

Neah Bay 0 0 0 0 0 0 -1 0 3 0 0 -1 0 0 -1 2 1 0 1 0 0 0 0 0 0 

Port Angeles 

Port Townsend 0 0 0 

Seattle 00000000 1 0 0 0 0 0 -1 0 0 0 1 1 0 0 0 0 0 

Cherry Point 0 0 

Friday Harbor 00000000 3 0 0 0 0 -1 0 0 0 0 0 0 0 0 0 0 

Ketchikan 0 0 0 0 0 0 -1 -2 1 0000 1 0 -2 0 0 0 0 0 0 

Sitka 0 0 0 0 0 0 -2 -3 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 

Juneau 0 0 0 0 0 0 -2 -3 0 0 0 0 0 3 0 0 0 0 0 0 -2 -4 -1 0 0 

Skagway -1 -3 2 2 0 -2 -4 -2 0 0 1 1 -2 0 0 1 0 0 

Yakutat 0 0 0 0 0 0 -3 -3 0 0 0 -1 -4 1 0 0 0 0 0 0 0 0 0 0 0 

Cordova 0 0 0 0 0 0 0 0 0 

Valdez 0 

Seward 0 0 3 3 0 -1 -1 0 0 0 0 2 0 0 0 -3 0 0 -1 -1 -3 0 

Seldovia 0 0 0 0 0 -3 -5 -1 0 

Nikiski 0 

Anchorage -2 -2 -1 

Kodiak Island 

Sand Point -1 0 

Unalaska 0 0 2 3 00000 2 0 0 0 0 0 0 -1 0 0 1 2 -2 -3 0 0 

Adak Island 0000000 2 1 0 0 -1 0 0 0 0 0 0 0 0 0 0 0 
43

Table 6. Number of months with extreme residual water levels for Pacific stations

Station 1975197619771978 19719801981198219831984198519861987198819891990199119921993199419951996199719981999

Nawiliwili 0 0 0 0 0 0 0 0 0 0 0 0 -2 0 0 0 0 0 0 0 0 0 0 0 0 

Honolulu 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Mokuoloe 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Kahului 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Hilo 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Johnston Atoll 0 0 0 -1 -2 0 0 0 0 0 0 0 0 5 -1 0 -1 0 0 0 0 0 0 0 0 

Sand Island 0 1 0 -2 0 0 -2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

Guam 0 0000 1 -5 -6 7 1 0 -3 3 3 0 -2 0 0 0 1 9 -3 1 

Pago Pago 0 0 0 -1 0 0 0 0 -7 0 2 0000000000 2 0 -10 0 

Kwajalein 0 0 0 0 0 0 0 -6 -3 0 0 0 0 0 0 0 -2 -2 0 0 0 0 -6 -1 0 

Chuuk 1 0 0 0 0 0 0 -7 -2 0 2 0 0 5 2 0 -4 0 

Wake Island 000 00000 7 0 -1 0 0 -2 0 0 -2 0 0 0 0 0 0 0 

San Diego 0000000 2 6 00000000 2 0000 4 1 0 

La Jolla 0000000 2 3 00000000 2 0000 4 0 0 

Newport Beach 0000000 2 4 0 0 0 0 -2 -1 0 0 0 0 

Los Angeles 0000000 2 3 0 0 0 0 -2 -1 0 0 1 0000 4 1 0 

Santa Monica 0000000 1 0 0 0 0 -1 -1 0 0 0 0 0 4 1 0 

Rincon Island 0 0000 2 7 0 0 0 0 0 0 0 

Santa Barbara 000000 000000 2 

Port San Luis 0000000 2 4 0 0 0 0 -1 0 0 0 1 0000 4 1 0 

Monterey 0000000 2 9 00000000 3 0000 3 1 0 

San Francisco 0 0 -1 0 0 0 0 3 11 00000000 3 0 0 1 0 4 4 0 

San Francisco w/EQ 0 0 -3 0 0 0 0 3 11 0 0 0 0 -1 -1 0 0 0 0 0 0 0 3 4 0 

Alameda -2 -3 -2 0 0 0 0 3 11 0 0 0 0 -2 -2 0 0 0 0 0 1 0 4 4 0 

Point Reyes 0 -1 0 0 0 0 0 2 10 0 0 0 0 -2 -2 0 0 3 0000 3 2 0 

Crescent City 0 -2 0 1 000 2 6 0 -1 0 0 -2 -1 0 0 4 1 000 2 3 0 

Charleston,OR 0 0 1 2 000 3 4 0 -1 0 0 -1 0 -1 0 0 0 0 0 0 4 3 0 

South Beach 0 0 -3 0 0 0 0 2 6 0 -2 0 0 -1 0 0 0 0 0 0 0 0 5 3 0 

Astoria 0 -1 -6 -2 -1 0 0 3 5 0 -3 0 0 -1 0 0 0 0 0 0 1 3 8 3 1 

Toke Point 2 -1 0 0 0 3 5 0 -2 0 0 -1 -2 0 0 0 0 0 0 0 6 3 0 

Neah Bay 0 00000 2 6 0 -2 0 0 -1 0 0 0 0 0 0 0 0 6 3 0 

Port Angeles 0000000 2 5 0 -2 0 0 -1 0 0 0 0 0 0 0 0 5 3 0 

Port Townsend 0000000 2 6 0 -2 0 0 -1 0 0 0 0 0 0 0 0 4 2 0 

Seattle 0 0 -1 0 0 0 0 2 5 0 -1 0 0 0 0 0 0 0 0 0 0 0 4 3 0 

Cherry Point 0000000 3 6 0 -1 0 0 -1 -1 0 0 1 0000 4 3 0 

Friday Harbor 0000000 2 5 0 -1 0 0 -1 -1 0 0 0 0 0 0 0 5 3 0 

Ketchikan 0 00000 1 5 0 -1 0 0 0 0 0 1 3 -2 0 0 0 2 2 0 

Sitka 00000000 3 0 -2 0 0 0 -1 0 0 3 0000 2 3 0 

Juneau 0 4 00000 1 3 0 -2 0 1 000 1 4 0 0 0 -2 0 2 0 

Skagway 1 0 -2 0 6 3 0 0 1 4 0 0 -3 1 2 0 

Yakutat 0 2 2 000 114 1 -1 0 4 0 -3 -2 -2 1 -3 0 0 0 1 -1 0 

Cordova 0000000 1 3 0 -2 0 2 0000 3 0 0 0 -1 1 1 0 

Valdez -1 0 0 -1 -2 0 0 0 2 0 -2 0 3 3 000 3 0 0 0 -2 1 1 0 

Seward 0000 0 1 3 0 -2 142 -2 -1 0 2 0 0 0 -2 1 2 0 

Seldovia 0 4 0 0 0 1 3 0 -2 0 4 0 -4 -2 -1 2 0000 2 3 0 

Nikiski 00 0 2 2 0 

Anchorage 0 2 2 0 142 0 0 2 3 0 0 0 -2 1 2 0 

Kodiak Island 00 0 1 0 0 0 0 0 -3 0 -1 0 0 0 0 0 2 3 0 

Sand Point 0 2 0000 114 0 -1 1 3 0 -4 -2 -1 0 0 0 0 0 1 2 0 

Unalaska 0 1 0 00 1 0 4 000000000 1 000 1 3 0 

Adak Island 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 
44
45

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 20 40 60 80 100 120 140 160

Year Range of Data

Standard Error of Linear Trend (mm/yr)

Atlantic Ocean, Eastern Gulf of Mexico, and Caribbean Sea

Pacific Ocean, Western Gulf of Mexico, and Bermuda

Figure 29. Standard error of linear MSL trends (mm/yr) versus year range of data.

y

x x = =

±

±

10 1808 2 257 0 217

1 614 0 127 1 614

(. . )

(. . ) .

. (6)

VII. DISCUSSION

In Section IV, linear MSL trends were calculated for all NWLON stations with greater than 25 years

of data. As Figures 8-10 showed, there are a wide range of error bars associated with the trends.

Most of this variation can be attributed to the length of the data time series. Figure 29 is a plot of

the standard errors of the linear trends from Table 3 versus the year range of the MSL data. The

scatter implies an inverse power relationship. There are two stations that are clear outliers, Guam

and Chuuk. These stations have 52 and 49 years of data respectively, but their standard errors of

0.9 mm/yr are similar to those of stations with only 25-30 years of data. This greater uncertainty

in trend is due to the large magnitude of the ENSO events in the western Pacific Ocean.

The relationship between the trend standard error (y) and the year range of data (x) can be seen more

clearly in a log-log plot (Figure 30). The best-fit regression line to the points (with associated 95%

confidence intervals) is
46

0.01

0.1

1

10

10 100 1000

Year Range of Data

Standard Error of Linear Trend (mm/yr)

Atlantic Ocean, Eastern Gulf of Mexico, and Caribbean Sea

Pacific Ocean, W estern Gulf of Mexico, and Bermuda

Best Fit Line y=180.8/(x**1.614)

Figure 30. Standard error of linear MSL trends (mm/yr) versus year range of data. The least

squares fitted line is also shown.
47

-3

-2

-1

0

1

2

3

0 20 40 60 80 100 120 140 160 180 200

Year Range of Data

Estimated Trend Error with 95% Confidence (mm/yr)

Figure 31. 95% confidence intervals for linear MSL trends (mm/yr) versus year range of data

set based on equation 6.

It can be observed that, for a given length of data, the stations in the Pacific Ocean, the western Gulf

of Mexico, and Bermuda have greater standard errors than stations in the Atlantic Ocean, eastern

Mexico, and the Caribbean Sea. This is because ENSO events result in roughly twice the variability

for the first group of stations compared to the second group of stations. Furthermore, Guam and

Chuuk have roughly twice the standard errors of the second group of stations.

Figure 31 shows the expected width of the 95% confidence interval (+/- 1.96 times the standard

error) as a function of data length based on the relationship in equation 6. A confidence interval or

precision of 1 mm/yr should be obtainable at most stations with 50-60 years of data on average,

providing there is no acceleration in sea level change or abrupt shifts in trend due to tectonic events.

In order to compare linear MSL trends between NWLON stations without dependence on the lengths

of the time series, trends were obtained for the common period of 1950-1999. Sixty stations have

at least 50 years of monthly MSL data. The trends were obtained using the linear regression with

autoregressive residuals method described in Section III. The results are listed in Table 7 and the

95% confidence intervals of the trends are plotted in Figures 32 to 33. The differences between

stations are mostly due to differences in vertical land motion. Note the wider confidence intervals

of the western Pacific stations of Guam, Pago Pago, and Kwajalein which are greatly affected by

ENSO events.
48

Table 7. Linear MSL trends for 1950-1999 monthly data

Station

Number Station MSL Trend

(mm/yr)

Standard Error

(mm/yr)

1612340 Honolulu 1.44 0.34

1617760 Hilo 3.26 0.31

1619000 Johnston Atoll 0.58 0.34

1619910 Sand Island -0.01 0.34

1630000 Guam -0.09 0.95

1770000 Pago Pago 1.24 0.58

1820000 Kwajalein 1.15 0.58

1890000 Wake Island 1.90 0.36

2695535 Bermuda 0.98 0.42

8410140 Eastport 1.44 0.22

8413320 Bar Harbor 2.09 0.18

8418150 Portland 1.20 0.22

8443970 Boston 2.02 0.21

8447930 Woods Hole 2.39 0.21

8452660 Newport 2.35 0.20

8454000 Providence 1.82 0.28

8461490 New London 1.99 0.22

8510560 Montauk 2.60 0.21

8516990 Willets Point 2.05 0.25

8518750 The Battery 2.72 0.25

8531680 Sandy Hook 3.41 0.25

8534720 Atlantic City 4.47 0.27

8545530 Philadelphia 2.74 0.35

8557380 Lewes 3.04 0.29

8574680 Baltimore 2.73 0.24

8575512 Annapolis 3.18 0.23

8577330 Solomons Island 3.37 0.25

8594900 Washington 3.15 0.35

8637624 Gloucester Point 3.95 0.27

8638610 Sewells Point 4.48 0.30

8658120 Wilmington 2.76 0.34

8665530 Charleston, SC 3.05 0.27

8670870 Fort Pulaski 3.43 0.28

8720030 Fernandina Beach 2.66 0.31
Table 7. Linear MSL trends for 1950-1999 monthly data

Station

Number Station MSL Trend

(mm/yr)

Standard Error

(mm/yr)

49

8720220 Mayport 2.58 0.31

8724580 Key West 2.42 0.20

8726520 St. Petersburg 2.67 0.18

8727520 Cedar Key 1.72 0.24

8729840 Pensacola 2.00 0.27

8761720 Grand Isle 10.37 0.36

8771450 Galveston Pier 21 7.24 0.37

8774770 Rockport 4.98 0.45

8779770 Port Isabel 4.03 0.31

9410170 San Diego 2.45 0.36

9410230 La Jolla 2.66 0.37

9410660 Los Angeles 1.14 0.34

9410840 Santa Monica 1.27 0.42

9412110 Port San Luis 0.71 0.37

9414290 San Francisco 2.23 0.40

9414750 Alameda 1.07 0.44

9419750 Crescent City -0.41 0.37

9439040 Astoria -0.19 0.43

9443090 Neah Bay -1.71 0.35

9447130 Seattle 2.26 0.30

9449880 Friday Harbor 1.14 0.32

9450460 Ketchikan 0.09 0.35

9451600 Sitka -1.87 0.30

9452210 Juneau -12.25 0.37

9452400 Skagway -16.37 0.51

9453220 Yakutat -5.63 0.37

The linear MSL trends from Section IV, determined with the entire data sets for each station, are

also marked in Figures 32 and 33 with filled circles. At only three out of the 60 stations is the longer

term trend clearly outside of the 95% confidence interval for the 1950-1999 trend. At Eastport,

Portland, and Boston, the MSL rise over the past 50 years has been significantly lower than the

longer term trends. At no station is the trend for the past 50 years significantly greater than the

longer term trend.
50

-5

0

5

10

15

BERMUDA EASTPORTBAR HARBORPORTLANDBOSTONWOODS HOLENEWPORTPROVIDENCENEW LONDONMONTAUKWILLETS POINTTHE BATTERYSANDY HOOKATLANTIC CITYPHILADELPHIALEWESBALTIMOREANNAPOLISSOLOMONS ISLANDWASHINGTONGLOUCESTER POINTSEWELLS POINTWILMINGTONCHARLESTONFORT PULASKIFERNANDINA BEACHMAYPORTKEY WESTST. PETERSBURGCEDAR KEYPENSACOLAGRAND ISLEGALVESTON PIER 21ROCKPORTPORT ISABEL

95% Confidence Interval of 1950-1999 Linear Trend (mm/yr)

Figure 32. 95% confidence intervals for 1950-1999 linear trends (mm/yr) from monthly

MSLs at Atlantic NWLON stations. Also shown as filled circles are the linear trends

determined from all available data.

-20

-15

-10

-5

0

5

HONOLULUHILOJOHNSTON ATOLLSAND ISLANDGUAMPAGO PAGOKWAJALEINWAKE ISLANDSAN DIEGOLA JOLLALOS ANGELESSANTA MONICAPORT SAN LUISSAN FRANCISCOALAMEDACRESCENT CITYASTORIANEAH BAYSEATTLEFRIDAY HARBORKETCHIKANSITKAJUNEAUSKAGWAYYAKUTAT

95% Confidence Interval of 1950-1999 Linear Trend (mm/yr)

Figure 33. 95% confidence intervals for 1950-1999 linear trends (mm/yr) from monthly

MSLs at Pacific NWLON stations. Also shown as filled circles are the linear trends

determined from all available data.
51

Linear MSL trends were also calculated for every possible 50-year period at 5-year intervals for

sixteen of the NWLON stations with the longest records. The results are listed in Table C in

Appendix V. The 95% confidence intervals for the trends are plotted versus the mid-year of the 50-

year periods in Figures 34 to 49. A horizontal line is also plotted showing the overall station trend

obtained with all available data (Table 3). An examination of Figures 34 to 49 can identify periods

when the 50-year linear trend was significantly different than the overall linear trend. These periods

are listed in Table 8.

Table 8. 50-year periods when linear MSL trend differs significantly from overall trend

Station Mid-year of periods with higher trends Mid-year of periods with lower trends

Portland 1945, 1950, 1955 1970, 1975

Boston 1945, 1950 1965, 1970, 1975

The Battery 1930, 1935, 1940, 1945, 1950 1970

Baltimore 1935, 1940, 1945, 1950 1965, 1970

Charleston 1965, 1970

Cedar Key 1965

San Francisco 1935, 1940, 1945, 1950, 1970 1890, 1895, 1900, 1905, 1910, 1915

San Francisco* 1885, 1890, 1895, 1900, 1905, 1910, 1915

Seattle 1945 1925

* Relative to the trend since the 1906 San Francisco earthquake.

For some Atlantic stations, periods centered on the years 1930 to 1955 tend to have MSL trends

higher than the overall trend, while periods centered on the years 1965 to 1975 tend to have MSL

trends lower than the overall trend. For Seattle, the period centered on 1925 had a trend lower than

the overall trend while the period centered on 1945 had a trend higher than the overall trend. 

For San Francisco, where the 50-year MSL trend actually changes sign and becomes negative for

a period around 1900, the situation is less clear and depends on whether the overall trend or the trend

since the 1906 earthquake is used. In either case, the periods centered on 1890 to 1915 had

significantly lower trends. In fact, the greatest deviation at any station from the overall trend was

at San Francisco for the period 1875-1924. The 50-year trend was -0.76 mm/yr, which is 2.17

mm/yr less than the overall trend and 2.88 mm/yr less than the trend since 1906. The San Francisco

earthquake, which resulted in substantial fault slippage less than 6 km from the water level gauge,

may have caused a small drop in water level and may also have changed the rate of vertical land

motion.
52

-2

0

2

4

6

8

10

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

M id -Y e a r o f E a c h 5 0 -Y e a r P e rio d

Linear Mean Sea Level Trend (mm/yr)

Figure 35. 95% confidence intervals for Portland MSL trends determined from 50-year

periods. Horizontal line indicates MSL trend from all data.

-2

0

2

4

6

8

10

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

M id -Y e a r o f E a ch 5 0-Y e a r P e rio d

Linear Mean Sea Level Trend (mm/yr)

Figure 36. 95% confidence intervals for Boston MSL trends determined from 50-year

periods. Horizontal line indicates MSL trend from all data.

-2

0

2

4

6

8

10

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

M id -Y e a r o f E a c h 5 0 -Y e a r P e rio d

Linear Mean Sea Level Trend (mm/yr)

Figure 34. 95% confidence intervals for Honolulu MSL trends determined from 50-year

periods. Horizontal line indicates MSL trend from all data.
53

-2

0

2

4

6

8

10

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

M id -Y e a r o f E a c h 5 0 -Y e a r P e rio d

Linear Mean Sea Level Trend (mm/yr)

Figure 37. 95% confidence intervals for The Battery MSL trends determined from 50-year

periods. Horizontal line indicates MSL trend from all data.

-2

0

2

4

6

8

10

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

M id -Y e a r o f E a ch 5 0-Y e a r P e rio d

Linear Mean Sea Level Trend (mm/yr)

Figure 39. 95% confidence intervals for Philadelphia MSL trends determined from 50-year

periods. Horizontal line indicates MSL trend from all data.

-2

0

2

4

6

8

10

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

M id -Y e a r o f E a c h 5 0 -Y e a r P e rio d

Linear Mean Sea Level Trend (mm/yr)

Figure 38. 95% confidence intervals for Atlantic City MSL trends determined from 50-year

periods. Horizontal line indicates MSL trend from all data.
54

-2

0

2

4

6

8

10

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

M id -Y e a r o f E a c h 5 0 -Y e a r P e rio d

Linear Mean Sea Level Trend (mm/yr)

Figure 41. 95% confidence intervals for Charleston, SC MSL trends determined from 50-year

periods. Horizontal line indicates MSL trend from all data.

-2

0

2

4

6

8

10

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

M id -Y e a r o f E a c h 5 0 -Y e a r P e rio d

Linear Mean Sea Level Trend (mm/yr)

Figure 42. 95% confidence intervals for Fernandina Beach MSL trends determined from 50-

year periods. Horizontal line indicates MSL trend from all data.

-2

0

2

4

6

8

10

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

M id -Y e a r o f E a c h 5 0 -Y e a r P e rio d

Linear Mean Sea Level Trend (mm/yr)

Figure 40. 95% confidence intervals for Baltimore MSL trends determined from 50-year

periods. Horizontal line indicates MSL trend from all data.
55

-2

0

2

4

6

8

10

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

M id -Y e a r o f E a c h 5 0 -Y e a r P e rio d

Linear Mean Sea Level Trend (mm/yr)

Figure 44. 95% confidence intervals for Cedar Key MSL trends determined from 50-year

periods. Horizontal line indicates MSL trend from all data.

-2

0

2

4

6

8

10

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

M id -Y e a r o f E a c h 5 0 -Y e a r P e rio d

Linear Mean Sea Level Trend (mm/yr)

Figure 43. 95% confidence intervals for Key West MSL trends determined from 50-year

periods. Horizontal line indicates MSL trend from all data.

-2

0

2

4

6

8

10

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

M id -Y e a r o f E a c h 5 0 -Y e a r P e rio d

Linear Mean Sea Level Trend (mm/yr)

Figure 45. 95% confidence intervals for Galveston Pier 21 MSL trends determined from 50-

year periods. Horizontal line indicates MSL trend from all data.
56

-2

0

2

4

6

8

10

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

M id -Y e ar o f E a ch 5 0-Y e a r P e rio d

Linear Mean Sea Level Trend (mm/yr)

Figure 48. 95% confidence intervals for Seattle MSL trends determined from 50-year

periods. Horizontal line indicates MSL trend from all data.

-2

0

2

4

6

8

10

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

M id -Y e ar o f E ach 50-Y e ar P e rio d

Linear Mean Sea Level Trend (mm/yr)

Figure 47. 95% confidence intervals for San Francisco MSL trends determined from 50-year

periods. Lower horizontal line is MSL trend from all data (1.41 mm/yr). Upper horizontal line

is the MSL trend from data since the 1906 San Francisco earthquake (2.13 mm/yr).

-2

0

2

4

6

8

10

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

M id-Ye a r of E a ch 5 0 -Ye a r P e riod

Linear Mean Sea Level Trend (mm/yr)

Figure 46. 95% confidence intervals for San Diego MSL trends determined from 50-year

periods. Horizontal line indicates MSL trend from all data.
57

-2

0

2

4

6

8

10

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

M id -Y e a r o f E a c h 5 0 -Y e a r P e rio d

Linear Mean Sea Level Trend (mm/yr)

Figure 49. 95% confidence intervals for Ketchikan MSL trends determined from 50-year

periods. Horizontal line indicates MSL trend from all data.

In the past, MSL and other tidal datums for NWLON stations have been updated periodically using

a NOS-defined 19-year period of data incorporating one 18.6-year cycle of the nodal revolution of

the moon. The present datums are established for the 1960-1978 epoch, and they are in the process

of being updated to the 1980-1998 epoch. However, in recent years, several NWLON stations in

Texas and Alaska with rapid vertical land motions were shifted to a more recent MSL datum in

response to concerns for safe navigation. A more frequent MSL datum update may become

necessary for these stations. Given the long term sea level trend at a station, it should be possible

to predict when a new, statistically different datum can be established.

For a station with a large mean sea level trend, the average of 19 years of data will likely fall close

to the annual MSL value of the middle year. For example, the 1960-1978 epoch MSL datum should

be close to the 1969 intercept of the long term MSL trendline. In Figure 50, the square symbols

indicate the expected sea level 10 years after the mid-point of a datum epoch (e.g., in 1979), as a

function of the long term trend at each NWLON station (from Table 3). The 95% confidence

interval estimates the uncertainty of the sea level change, given the possible deviation from the long

term trend over a 10-year period as described by equation 6. Although the stations with slow trends

may still be close to their established MSL datum, those with rapid trends (above 8 mm/yr) will have

already moved by a statistically significant amount.

NOS has traditionally updated the tidal datums to a new epoch about every 20 years. Therefore, a

period of about 30 years will elapse after the middle year of the established datum epoch to the time

that a new datum comes into effect. The present process of updating to a new epoch is expected to

be competed in 2002. In this case, 33 years will have elapsed from the middle year of the current

epoch to the time a new epoch is in place (1969 to 2002). Figure 51 shows the expected 30-year

change in sea level as a function of trend. All stations with trends greater than 1.5 mm/yr will have

shifted by a statistically significant amount from their established MSL datum. The stations with

rapid MSL trends, greater than 8 mm/yr, will have shifted by at least 0.2 m and as much as 0.4 m.

To prevent these station’s sea levels from drifting away from their datum by such a large amount,

their datums require updating more often than every 20 years.
58

-90

-60

-30

0

3 0

6 0

9 0

120

150

180

210

240

270

300

330

360

390

420

450

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

M S L T ren d (m m /yr)

Estimated Change in Mean Sea Level (mm)

Figure 51. Expected absolute value of sea level change in 30 years (mm) as a function of the

MSL trends of NWLON stations. 95% confidence intervals derived from equation 6.

-90

-60

-30

0

3 0

6 0

9 0

120

150

180

210

240

270

300

330

360

390

420

450

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

M S L T ren d (m m /yr)

Estimated Change in Mean Sea Level (mm)

Figure 50. Expected absolute value of sea level change in 10 years (mm) as a function of the

MSL trends of NWLON stations. 95% confidence intervals derived from equation 6. 
59

VIII. CONCLUSIONS

The primary purpose of this report is to update the calculated mean sea level trends for NWLON

stations up to the end of 1999. The data set analyzed consisted of a total of 117 NWLON stations

having monthly MSL records over a span of at least 25 years. NWLON stations are located on the

U.S. east and west coasts, the Gulf of Mexico, the Caribbean, Bermuda, Alaska, and Pacific Ocean

islands (Figures 1 to 7). The two oldest stations, San Francisco and The Battery (New York) have

monthly MSL records beginning in 1854 and 1856, respectively. The data were collected over the

years using a variety of different technologies and with some small shifts in instrument location.

However, there has been careful maintenance of station datums through regular leveling to stable

benchmarks, thus allowing diverse data sets to be combined into time series.

MSL trends are usually obtained by a simple linear regression of annual MSL data. To obtain an

annual MSL value requires a complete year of data to average out the seasonal cycle. The standard

error of trends from a linear regression assumes that the residual time series is random. However,

the residuals are dominated by a 3-5 year variability, most commoly caused by ENSO conditions

in the tropical Pacific Ocean affecting ocean temperatures, winds, and currents around the globe.

The residual is not a random time series but is, to first order, an autoregressive process of order 1.

Therefore, in this report, MSL trends are determined by a linear regression of monthly data with

autoregressive residuals of order 1. In addition to the linear trend, the average seasonal cycle, the

monthly residual time series, and the autoregressive coefficient of order 1 are determined along with

accurate estimates of their standard errors.

The calculated MSL trends (Figures 8 to 10) range between 9.85 mm/yr for Grand Isle, LA to -16.68

mm/yr for Skagway, AK. The MSL trends combine the global MSL rise (which is estimated by

numerous studies to be between 1.0 and 2.4 mm/yr) and the local vertical land motion. The

calculated MSL trends are consistent with previously published station trends. In Appendices II,

III, and IV, are time series plots for each station of the monthly MSL with the seasonal cycle

removed, the seasonal cycle, and the MSL residual after both the seasonal cycle and the trend are

removed. 

The location and timing of any major earthquakes near stations in tectonically-active areas are noted

since an associated vertical offset or a change in the MSL trend is possible. At three stations

(Unalaska, Adak Island, and Seward), there are clear offsets caused by earthquakes in 1957 and

1964. At Unalaska and Adak Island, there are significant changes in MSL trends. For San

Francisco, there is the possibility of a small offset and/or a change in MSL trend caused by the 1906

earthquake. Therefore, three trends are presented for San Francisco: an overall trend, and the trends

before and after the earthquake.

The MSL residual plots, after both the seasonal cycle and the trend are removed, show broad

regional correlations between stations. Years with extreme high or low residuals are defined by first

applying a 5-month running average to the residual time series and then summing the number of

months with values above 0.1 m and the number of months with values below -0.1 m. Tables 5 and

6 show the regional extent of periods of anomalous mean sea levels which are primarily caused by

ENSO events.
60

The standard errors of the calculated trends are found to be inversely related to the time span of data

available (Figures 29 and 30). An inverse power relationship was derived empirically by fitting a

least squares line to a log-log plot of the standard errors for each station versus the year range of the

data. This relationship gives an estimated requirement of 50 to 60 years of data for obtaining linear

MSL trends having a 1 mm/yr precision with a 95% statistical confidence interval (Figure 31). For

a given length of data, the standard errors for trends at Pacific Ocean and western Gulf of Mexico

stations tend to be greater than the standard errors for Atlantic coastal stations, due to the greater

amplitude of the ENSO events in the Pacific. Two outlier stations, Guam and Chuuk, in the western

Pacific Ocean have standard errors even greater than the other Pacific stations.

In order to investigate whether MSL trends have been changing recently, the trend for the period

1950-1999 is calculated for the sixty NWLON stations with data spanning these years. When the

trend from each station’s entire data set is compared to the 1950-1999 trend, at only three stations

(Eastport, Portland, and Boston) is there a statistically significant difference (Figures 32 and 33).

At these three northern U.S. Atlantic stations, the recent trends are significantly lower than each

station’s overall trends. At none of the stations is the 1950-1999 trend significantly higher than the

station’s overall trend.

The possibility that there are other 50-year periods with MSL trends statistically different than the

overall trend is examined for sixteen of the NWLON stations with the longest data sets (Figures 34

to 49). Six Atlantic stations (Portland, Boston, The Battery, Baltimore, Charleston, and Cedar Key)

had periods centered on years between 1930 and 1955 with significantly higher trends and/or periods

centered on years between 1965 and 1975 with significantly lower trends. Seattle had a period

centered on 1925 with a significantly lower trend and a period centered on 1945 with a significantly

higher trend. For San Francisco, trends for 50-year periods centered from 1890 to 1915 are

significantly lower than both the overall trend and the trend since the 1906 San Francisco

earthquake. For periods centered on 1895, 1900, and 1905, the trend is actually negative. This

suggests that there may have been a small vertical offset and/or a change in trend caused by the

earthquake.

At most NWLON stations, the monthly MSL data are relative to the MSL datum established from

the present National Tidal Datum Epoch (1960-1978). However, at a number of stations in Texas

and Alaska where there are rapid rising or falling MSL trends, the datum was recently updated using

data from 1990-1994 in Texas and 1994-1998 in Alaska. Projections of future changes in MSL

(Figures 50 and 51) should be useful in deciding which station datums will require more frequent

updates.
61

ACKNOWLEDGMENTS

Tom Huppmann assisted in the collection of monthly mean sea level data from the DPAS database

by creating several SQL queries. Raymond Smith cleared up questions about anomalous data points

and provided leveling information for linking several data sets at nearby locations. This report was

reviewed by William Stoney, Stephen Gill, Stephen Lyles, Leonard Hickman, James Hubbard,

Steacy Hicks, and Kurt Hess and their comments helped to improve the report. The document was

prepared for publication by Brenda Via and Gina Stoney. The high quality of the data collected over

many decades is attributable to the personnel of the Center for Operational Oceanographic Products

and Services and its predecessors. Their sustained efforts in the operation and maintenance of the

water level stations and the processing and archiving of data over the years has resulted in an

invaluable resource for scientific research.
62
63

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