NOAA Technical Report NOS CO-OPS 051 

 

ELEVATED EAST COAST SEA LEVEL ANOMALY: 

June – July 2009

Silver Spring, Maryland 

August 2009 

noaa National Oceanic and Atmospheric Administration

U.S. Department Of Commerce 

National Ocean Service 

Center for Operational Oceanographic Products and Services 
 2 

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) provides the National infrastructure, science, and technical expertise to collect and 

distribute 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 tidal 

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), a national network of Physical Oceanographic RealTime Systems (PORTS) in major U. S. harbors, and the National Current Observation Program 

consisting of current surveys in near shore and coastal areas utilizing bottom mounted platforms, 

subsurface buoys, horizontal sensors and quick response real time buoys. 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 051 

ELEVATED EAST COAST SEA LEVELS ANOMALY: 

July – June 2009

 

William Sweet

Chris Zervas 

Stephen Gill 

August 2009 

U.S. DEPARTMENT OF COMMERCE 

Gary Locke, Secretary

National Oceanic and Atmospheric Administration 

Dr. Jane Lubchenco,

Undersecretary of Commerce for Oceans and 

Atmosphere and NOAA Administrator 

National Ocean Service 

John H. Dunnigan, Assistant Administrator 

Center for Operational Oceanographic Products and Services 

Michael Szabados, Director
 

ii 

NOTICE 

Mention of a commercial company or product does not constitute an 

endorsement by NOAA. Use for publicity or advertising purposes of 

information from this publication concerning proprietary products or the 

results of the tests of such products is not authorized. 
iii 

 ELEVATED EAST COAST SEA LEVEL ANOMALY: 

JUNE – JULY 2009 

TABLE OF CONTENTS 

 

EXECUTIVE SUMMARY ............................................................................................................. vii 

PURPOSE ...........................................................................................................................................1 

INTRODUCTION ..............................................................................................................................3 

METHODS AND DATA ...................................................................................................................5 

FINDINGS ..........................................................................................................................................7 

 Observations, Predictions and Residuals ......................................................................................8 

 A Spatial and Temporal View of the De-trended Series ..............................................................10 

 Wind Forcing, Atmospheric Pressure and Sea Level Response ..................................................11 

 Florida Current Transport and Sea Level Response ...................................................................14 

 Satellite Perspective of the Event .................................................................................................17 

SUMMARY DISCUSSION .............................................................................................................19 

 Magnitude of Event .....................................................................................................................19 

 Inverse Barometer Effect .............................................................................................................19

 Effects of Wind Forcing ...............................................................................................................20

 Effects of Florida Current Transport ...........................................................................................21 

 Wind and FC Transport Interactions ...........................................................................................21

 Satellite Altimetry .........................................................................................................................22

 Future Work .................................................................................................................................22

ACKNOWLEDGMENTS ................................................................................................................25

REFERENCES .................................................................................................................................27
iv 

LIST OF FIGURES 

Figure 1. NOAA stations, NOAA buoys, and Florida Current transport measurement site ..............4 

Figure 2. Illustration of SL predictions based on 5 to 19 years of data and the seasonal SL cycle 

determined by averaging values from each month over the entire series (> 30 years) .....5 

Figure 3. a) Plot of 6-min data from Atlantic City, NJ and Charleston, SC between May 15 and 

July 14, 2009 and b) 39-hour lowpass filtered series of observations along East Coast ..7 

Figure 4. Monthly mean SL observations at East Coast stations from January 2000 to July 2009 ..8 

Figure 5. 30-day lowpass filter of SL observations, tidal predictions and residuals at Atlantic City, 

NJ and Charleston, SC from January 2006 to mid-July 2009 ..........................................9 

Figure 6. June 2009 mean SL observations adjusted for the long-term average seasonal SL cycle 

(red) and the long-term relative SL rise (blue). The vertical line shown on the plot 

separates the two regional groups ...................................................................................10 

Figure 7. Average of the mean monthly SL observations adjusted for the average seasonal cycle 

and long-term relative SL rise for 29 stations north of Cape Hatteras (Eastport to CBBT 

in blue) and 9 stations south of Cape Hatteras (Beaufort to Key West in red) ...............11 

Figure 8. 30-day lowpass filtered SL residual time series and southwesterly/northeasterly winds 

(SW/NE) and northwesterly/southeasterly winds (NW/SE) at NOAA buoys 44008 

Nantucket and 41004 Edisto. Correlation coefficients and phase lead (+) of the SW/NE 

winds to the SL residual (shown in parentheses) ............................................................12 

Figure 9. 7-day lowpass filtered SL residual, southwesterly/northeasterly (SW/NE) and 

northwesterly/southeasterly winds (NW/SE) and atmospheric pressure at sea level ..13 

Figure 10. a) The June 2000 – June 2009 average seasonal cycle of FC transport based on a 90-

day lowpass filtered series and SL predictions above MSL and b) diagram showing 

cross-shore sea slope with low, average, and high FC transport (adaption of Figure 2 in 

Noble and Gelfenbaum, 1992) ......................................................................................15 

Figure 11. a) 30-day lowpass filtered series of FC transport and SL residual time series at NOAA 

stations with correlation coefficients and phase lead (+) of the FC (shown in 

parentheses) and b) 7-day lowpass filtered series since April 2009 ...............................16 

Figure 12. Merged satellite altimetry of absolute dynamic topography (cm) and SL anomaly (cm) 

along the eastern U.S. coast and western Atlantic during periods of low (October 2008 

and June 2009) and high (December 2008) Florida Current transport 

(http://www.aviso.oceanobs.com/duacs) ........................................................................18 
v 

LIST OF TABLES

Table 1. Summary of correlations between 30-day lowpass filtered series. A negative value 

implies an inverse relationship and the phasing is shown in days (d) and positive values 

indicate that the wind and Florida Current transport lead/force a SL response at the 

NOAA stations ................................................................................................................20 
vi 
vii 

EXECUTIVE SUMMARY 

CO-OPS stations recorded higher than normal sea levels (SL) along the U.S. East Coast in June 

and July 2009. Near-peak levels in the latter half of June coincided with a perigean-spring tide, 

an extreme predicted tide when the moon is closest to the Earth during a spring tide. This tidal 

event added to the observed SL anomaly, produced minor coastal flooding, and caught the 

attention of many coastal communities because of the lack of coastal storms that normally cause 

such anomalies. 

In terms of SL heights, the event was not very abnormal as many locations have higher levels in 

the late-summer. June – July 2009 SL heights are anomalous because of their unexpected timing 

and geographic scope. The SL event is anomalous in terms of its ‘residual’ values unaccounted 

for within the predictions of the earth-moon-sun tides and normal seasonal cycles of the winds 

and atmospheric pressure, ocean currents, and heating/cooling of coastal waters. The mean 

residual SL values for June 2009 were significant (> 0.2 m) from North Carolina to New Jersey, 

substantial (> 0.1 m) from Florida to Maine, and the most extreme to occur simultaneously over 

the entire East Coast during a spring/summer period as far back as 1980. 

There are two probable mechanisms responsible for the June – July 2009 high SL residuals. The 

first is northeasterly (NE) wind forcing. In June 2009, winds over the entire geographic area 

from Cape Hatteras, NC to the Gulf of Maine had a moderate NE wind component, whose 

transport caused coastal SL to rise. South of Cape Hatteras, winds were primarily southwesterly 

(SW). The other mechanism is the changing transport of the Florida Current, which is measured 

in the Florida Straits before it supplies the Gulf Steam off of Cape Hatteras, NC. When the 

Florida Current / Gulf Stream transport is low, the eastward-rising cross-current slope relaxes 

and raises coastal SL. In June 2009, the SL residual rise was concurrent with a noted decrease in 

transport of the Florida Current. 

The June – July 2009 SL event decays in mid-July 2009 as the SL residuals diminish. During 

this period, transport of the Florida Current sharply increases and the winds oscillate between 

SW and NE along the East Coast. 

The June – July 2009 SL anomaly is unique in that the NE winds were not at a multi-year high or 

the Florida Current transport at its low. But the coupled effect of the two forces created SL 

residuals that were at highest levels all along the East Coast. Highest SL residuals between 

North Carolina and New Jersey highlight the region of greatest overlap of the two forces. 
viii 
1 

ELEVATED EAST COAST SEA LEVEL ANOMALY: 

JUNE – JULY 2009 

PURPOSE

The purpose of this report is to describe and document a significant anomaly in the elevations of 

sea level along the East Coast during June and July of 2009. Oceanographers and web-services 

personnel in the NOAA Center for Operational Oceanographic Products and Services (CO-OPS) 

started receiving dozens of inquiries from the general public in early June when they noticed the 

tides were running higher than predicted over several days. The peak number of inquiries was 

associated with the peak elevations of the event in late June 2009. During that time period (June 

20-25), the sea level anomaly coincided with extreme predicted tides caused by increased tideproducing forcing due to the near conjunction of the moon’s perigee (moon closest to the earth in 

the monthly orbit) and spring tide (a new moon). These perigean-spring tides, which cause 

water levels a few centimeters above normal spring tides, can cause minor coastal flooding even 

in the absence of a coastal storm or sea level anomaly. The amplitude and the extent of the 

anomaly caught everyone by surprise, because there was no coastal storm or strong weather front 

present that would normally cause these types of anomalies. 

The following text was used as an alert on the Center for Operational Oceanographic Product 

and Services web-site (http://tidesandcurrents.noaa.gov): 

“Observed tides have risen above predicted elevations along the entire U.S. East Coast from 

Maine to Florida since June. From June 19 thru June 24, water levels were between 0.6 to 2.0 

feet above normal depending upon location. As of July 1, anomalies continued running lower at 

0.3 to 1.0 ft. above normal. It is not unusual for smaller regions and estuaries along the East 

Coast to experience this type of event this time of year; however, it is significant that the 

geographic extent of this event covers the entire East Coast. The Center for Operational 

Oceanographic Products and Services (CO-OPS) will continue to monitor this event and will 

provide further information on the causes, amplitudes, geographic extent, and the duration of the 

event.” 

This sea level anomaly event should not be confused with a change in the relative trends in mean 

sea level determined from the NOAA tide stations. This event is of much shorter duration and 

would only affect relative sea level trends if the anomalies started occurring with increasing 

frequency and longer duration. 
2 
3 

INTRODUCTION 

Water levels vary over many time scales, from tidal forcing to longer-term sea level rise 

associated with climatic changes. Non-tidal changes vary in duration and magnitude and can be 

storm driven, or reflect seasonal, annual, inter-annual, and decadal sea level changes. Seasonal 

changes occur primarily from changes in wind forcing and atmospheric pressure, ocean-currents, 

and density (heating/cooling) of the shelf waters. Inter-annual and longer variability can cause 

significant changes in the timing and magnitude of the normal seasonal/annual patterns. Often, 

multiple forcing mechanisms work in tandem and create feed-back loops making it difficult to 

discern the specific response from a particular forcing parameter. In order to support the COOPS mission of providing reliable measurements and accurate predictions of water levels and 

establish longer-period (i.e., weekly and longer) sea level heights and trends, it is necessary to 

assess the sea level responses that occur from mechanisms forcing change. 

This document presents data from a sampling of NOAA stations (Figure 1) to highlight the 

extent and magnitude of an elevated sea level anomaly that occurred June – July 2009 along the 

U.S. East Coast. Important to this study are the concepts of observations and predictions of 

water level and their differences or ‘residuals’. Water level observations are a measurement 

made by NOAA stations and are analogous to ‘lines in the sand’, or the heights at which water is 

located at a given time. Tide predictions are a product provided by CO-OPS that quantify 

through harmonic analysis the site-specific time and height of the periodic motion driven by the 

interaction of the earth-moon-sun system. There are two long-period tidal constituents inherent 

to NOAA predictions that are important to this study. They are the 365-day period Sa (solar 

annual) and 183-day period Ssa (solar semiannual) harmonic constituents, which have a 

theoretical tide-producing force origin with small amplitudes. However, in practice, they are 

used in an attempt to capture the periodicity of the sea level response that results from seasonal 

meteorological forcing. These constituents are derived from an observed-mean annual sea level 

signal and are usually computed from 5 to 19 years of data, depending on the station. Thus, the 

tidal predictions are unable to account for either inter-annual variability or the intra-annual

variations that deviate from the mean annual sea level prediction. The residuals, or the portion of 

the observations unaccounted for by the predictions, will reflect all non-tidal variability induced 

by the forcing mechanisms described above. In this report, the June – July 2009 sea level 

anomaly is highlighted in terms of residual values. 

The document is laid out in the following manner. Examples of six-minute observations, tidal 

predictions and residual data provided at the CO-OPS website (http://tidesandcurrents.noaa.gov) 

are shown to highlight the event. Time series of daily observations from multiple stations along 

the East Coast are used to illustrate the magnitude of the event as would be witnessed by a local 

observer. Site-specific predicted and residual cycles since 2006 are shown at two locations to 

assess the timing and magnitude of the June – July 2009 event. To put the event in geographical 

perspective, the June 2009 monthly averaged sea level data from East Coast stations with >30 

years of data are de-trended by the site-specific seasonal signal and relative sea-level trend 

(Zervas, 2001). For historical perspective, the East Coast stations are then grouped into a 

northern or southern region and their monthly de-trended averages are shown since 1980. Time 

series of wind collected at two NOAA buoys and of the transport strength of the Florida Current 

are shown and correlated to sea levels at multiple stations to highlight likely forcing factors 

involved in the event. Time series of atmospheric pressure at multiple NOAA stations are also 
4 

included to determine the influence of the inverse barometer effect. Products derived from sea 

surface height (SSH) data from satellite altimetry measurements are shown to better discern the 

regional influence of the anomalous conditions. Lastly, reasons for inter-annual variability of the 

forcing mechanisms are described in context of the literature and on-going research efforts. 

Figure 1. NOAA stations, NOAA buoys, and Florida Current transport measurement site. 
5 

METHODS AND DATA

Water levels are obtained from the CO-OPS website located at http://tidesandcurrents.noaa.gov. 

Water level data are relative to mean sea level (MSL), which is a tidal datum currently in effect 

for the 1983-2001 National Tidal Datum Epoch (NTDE). Throughout this report, the term ‘sea 

level (SL)’ will be used to describe longer period (> 7 days) water levels and are shown relative 

to the MSL datum. Also reported are the tidal predictions reported relative to the same MSL 

datum. Predicted values are based upon 37 harmonic constituents derived by averaging > 5, 1-

year least-squares harmonic analyses of observed hourly water level heights. The residual values 

are the difference between the observed and predicted values. Time series of water levels from 

five NOAA stations are investigated to highlight the June – July 2009 SL event: 1) Nantucket, 

MA, 2) Atlantic City, NJ, 3) Duck, NC, 4) Charleston, SC, and 5) Trident Pier at Cape 

Canaveral, FL. Time series of atmospheric pressure during the event are also shown, but of the 

five stations, they are only available at the Nantucket, Duck and Trident Pier locations. 

In order to place the magnitude of the June – July 2009 SL anomaly into a long-term perspective, 

a ‘de-seasonalized’ and ‘de-trended’ monthly series are shown. The cycle removed from the deseasonalized series is composed of average heights relative to MSL for each month over the 

entire series. Generally, this seasonal cycle is very close to the predicted values based upon 

harmonic constituents. However, since the predictions are computed from harmonic constituents 

averaged over time periods of 5 to 19 years and the seasonal cycle is computed over the series 

length, slight differences occur (Figure 2). The de-trended series also includes the removal of 

the series-long (> 30 yr) linear fit that represents the site-specific relative SL rise (Zervas, 2001). 

Figure 2. Illustration of SL predictions based on 5 to 19 years of data and the seasonal SL cycle 

determined by averaging values from each month over the entire series (> 30 years). 

Time series of winds are shown for the NOAA buoy 41004 (Edisto), which is located 41 nm 

southeast of Charleston, SC (Figure 1). Wind is also shown for buoy 44008 (Nantucket), which 

is located 54 nm southeast of Nantucket, MA. Both sets of wind velocity vectors are rotated to 

direction of maximum (minimum) variance of the Edisto time series for consistency, namely 

45°/225° (135°/315°) from true north. These wind observations are thought to be representative 
6 

of large regions for application to the analyses presented here. Comparisons to the large-scale 

wind field of the SeaWinds satellite microwave scatterometer sensor (QSCAT) at 

www.remss.com attest to their general agreement. Throughout the document, wind directions 

will be referred to in the standard meteorological sense, i.e., from the direction of flow. Time 

series of the transport strength of the Florida Current (FC) is shown as measured between Florida 

and the Bahamas near 27°N (Figure 1). A detailed history of cable measurements of the Florida 

Current can be found in Larsen (1992). Also, merged daily composites of regional-scale 

dynamic ocean topography and SL anomaly are shown. These data are derived from satellite 

altimetry and gridded to 0.3 degrees and an accuracy of ~4 cm. The absolute dynamic height is 

relative to the 1993-1999 mean SSH adjusted for the geoid and calibrated by in-situ based 

climatology as well as the SL anomaly, which is a date-specific SSH anomaly from the 1993-

1999 SSH mean. Further documentation is available at www.aviso.oceanobs.com/duacs. 

Daily time series of water level observations, predictions, and residuals, atmospheric pressure, 

wind data, and FC transport have been lowpass filtered by a cosine Lanczos filter (Emery and 

Thompson, 2004). Lowpass filtered data series have cutoff frequencies (half amplitude) of 39-

hour, 7-day, 30-day, and 90-day and are appropriately labeled within the text. Filtered data have 

been truncated at the beginning and end by half the cutoff frequency (i.e., 15 days for the 30-day 

filter). The cross correlation function is computed between various 30-day lowpass series using 

the MATLAB (Mathworks, 2009) function XCOV, which is the cross covariance between the 

de-meaned time-series. Coefficients are normalized between -1 and 1 and imply strong inverse 

(negative) and direct (positive) correlations. Coefficients are shown at the corresponding 

lead/lag phases and are significant at the 99% confidence level. The cross correlation function 

was used instead of a frequency-based coherence function since it can handle the occasional 

large gaps in the data sets. Coherency analyses and spectral analyses of continuous SL, wind and 

FC data segments confirm significant energy and coherence in the 25 – 50 day band, validating 

the usage of the 30-day lowpass filter series used in the correlation analyses (Maul et al., 1990). 
7 

FINDINGS 

Sea levels during June 2009 were consistently > 0.2 m above predicted tidal elevations along 

most of the eastern U.S. seaboard. Examples from Charleston, SC and Atlantic City, NJ show a 

series of 6-minute water level measurements available on the CO-OPS website (Figure 3a). It 

can be seen that on ~6/22/09, water levels were at their highest values. In fact, near-peak 

residual values for the June – July 2009 event, which topped 0.4 m at both locations, occurred 

within a couple days of 6/22/09, when a perigean-spring tide was occurring. It is important to 

note that while the observations and predictions both gradually increased due to the tidal event, 

the residual values remain relatively constant. Water levels were indeed high, but the effects of 

the perigean-spring tide are included in the predictions, and thus the residual increase is in 

response to other underlying forcing mechanisms. 

Figure 3. a) Plot of 6-min data from Atlantic City, NJ and Charleston, SC between May 15 and 

July 14, 2009 and b) 39-hour lowpass filtered series of observations along East Coast. 
8 

The SL rise associated with the June – July 2009 event begins in approximately late May, as 

shown by multiple de-tided observations along the East Coast (Figure 3b). The de-tided series 

closely match the residual (observed – predicted) series over this short time period. It is 

important to comment on the large spike in water levels in mid-May shown at Springmaid Pier in 

Myrtle Beach, SC and Trident Pier located at Cape Canaveral, FL (Figure 3b) and in the residual 

at Charleston, SC in Figure 3a. This multi-day spike is not considered as part of the widespread 

SL anomaly. A slow-moving cold front produced a strong NE wind between May 19 and 23, 

2009 that temporarily raised water levels ~0.4 m from South Carolina to Florida. This event will 

be further discussed in later sections. 

Observations, Predictions and Residuals 

Since water levels are persistent across the tidal cycle, multi-day averages and lowpass filtered 

values of observations, predictions and residuals are investigated. Figure 4 is a plot of monthly 

mean SL observations from January 2000 to July 2009 from five stations along the East Coast. 

Common to most stations is a SL that is highest (> 0.2 m) in late summer, lowest in late winter 

(< -0.1) and has a secondary high in late spring that is more prevalent towards the south. The 

June 2009 SL values are not particularly abnormal in terms of their overall magnitude, though 

the middle Atlantic stations at Duck, NC and Atlantic City, NJ are close to their respective series 

high. The June 2009 SL values are anomalous in that many have already achieved heights 

similar to their respective historical late-summertime highs. That is, the high-water values are 

occurring earlier than expected. Comparison of the site-specific SL predictions will determine 

magnitudes of the anomalies in terms of their residuals. 

Figure 4. Monthly mean SL observations at East Coast stations from January 2000 to July 2009. 

Figure 5 shows the 30-day lowpass filter series of SL at Charleston, SC and Atlantic City, NJ 

from January 2006 to July 2009. The predictions, which reflect the site-specific Sa and Ssa 

constituents, represent the periodic intra-annual SL variability. The changes occur primarily in 
9 

response to seasonal meteorological and oceanic forcing. The predictions include effects from 

the steric cycle of seasonal heating and cooling of the coastal waters that cause SL to be higher in 

late summer and lower in the late winter. The predictions at Charleston have a slightly larger 

range and contain a more noticeable double peak than the predictions at Atlantic City. Both 

locations have predictions that are lowest in February, highest in October, a lessened peak in 

April/May with a subsequent low in June/July. The residual series show a positive anomaly 

associated with the June - July 2009 event peaking at ~0.25 m at both locations in the latter half 

of June. Actual SL observations with equal or greater magnitudes have occurred in the last three 

years at both locations. A few noted high SL events occurred in September 2006 at both 

locations and in September/October of 2007 and 2008 at Charleston, SC. The residuals, 

although, were less since the predictions are both locations are higher in September/October. 

Figure 5. 30-day lowpass filter of SL observations, tidal predictions and residuals at Atlantic 

City, NJ and Charleston, SC from January 2006 to mid-July 2009. 
10 

A Spatial and Temporal View of the De-trended Series

Monthly mean SL values are examined at long-term NOAA stations (> 30 years) to determine 

the geographical extent of the June – July 2009 SL anomaly along the East Coast and compare it 

with previous events. Data were obtained relative to the MSL datum for the 1983-2001 NTDE. 

The series have the average seasonal SL cycle, derived over the entire data series from each 

station, removed to create a ‘de-seasonalized’ SL series. The ‘de-seasonalized’ series are quite 

close to the longer-period SL predictions at a particular station (see Figure 2). The June 2009 

mean SL values with the seasonal cycle removed are displayed in red in Figure 6. The highest 

June SL values (> 0.2 m) are from Sandy Hook, NJ to Chesapeake Bay Bridge Tunnel (CBBT), 

VA although the SL values are substantial (> 0.1 m) from Eastport, ME to Vaca Key, FL. 

Figure 6. June 2009 mean SL observations adjusted for the long-term average seasonal SL cycle 

(red) and the long-term relative SL rise (blue). The vertical line shown on the plot 

separates the two regional groups. 

Absent in the predictions and thus inherent to the residual (observed – predicted) values is the 

long-term relative SL rise that has occurred since 1992, or the middle of the 1983-2001 NTDE. 

In the subsequent 17 years, SL rise has averaged ~3 mm/yr on the East Coast, resulting in an 

average rise of ~0.051 m. In Figure 6, each station in blue is further de-trended by its long-term 

relative SL rise. This de-trended value is a better representation of the true magnitude of the 

anomaly, independent of the time elapsed since the establishment of the NTDE. 

The de-trended SL series are highly correlated between neighboring stations along the coastline 

(not shown). A time series of the averages from 1980 for the two regional groups of stations – 

north of Cape Hatteras (Eastport to CBBT) and south of Cape Hatteras (Beaufort to Key West) is 
11 

shown in Figure 7. The average de-trended SL series north and south of Cape Hatteras are 

somewhat correlated with each other (0.50). Some months have large values north and south of 

Cape Hatteras, and in some months, large values exist in one region but not the other. 

Figure 7. Average of the mean monthly SL observations adjusted for the average seasonal cycle 

and long-term relative SL rise for 29 stations north of Cape Hatteras (Eastport to 

CBBT in blue) and 9 stations south of Cape Hatteras (Beaufort to Key West in red). 

The de-trended SL series allows an unbiased comparison of the June 2009 values to previous 

events. The greatest values in both regions occurred in January and February 1998 when a series 

of long-lasting, slow-moving winter storms significantly raised SL along the entire East Coast 

(Deitemyer, 1998). Similar events occurred in January-March 1983, March 1987, and February 

1999. On the other hand, winter storms in some years have significantly affected SL in one of 

the regions but not the other. There are only a few instances where large de-trended values occur 

in summer (Figure 7). There are large values only in the north for September 1996 and large 

values only in the south for August 1995 and September 1999. The June – July 2009 values are 

the highest shown to simultaneously occur within both regions during the spring/summer period. 

Wind Forcing, Atmospheric Pressure and Sea Level Response 

The 30-day lowpass filtered residuals for five stations along the East Coast show the magnitude 

and widespread extent of the June – July 2009 SL event (Figure 8). It can be seen that the 

southwesterly/northeasterly (SW/NE) component of the wind has a particularly strong effect on 

SL along the East Coast. Northeasterlies (negative) are largely oriented alongshore and cause an 

Ekman-driven convergence along the coast and SL to rise (positive). On the other hand, SW 
12 

(positive) winds cause a divergence away from the coast or set down of SL. The winds offshore 

at the Nantucket buoy establish a stronger negative correlation to SL than the winds offshore of 

Edisto (Figure 8 legend in parentheses) most likely due to the fact that Nantucket is further 

offshore and less influenced by boundary effects from land. 

Figure 8. 30-day lowpass filtered SL residual time series and southwesterly/northeasterly winds 

(SW/NE) and northwesterly/southeasterly winds (NW/SE) at NOAA buoys 44008 

Nantucket and 41004 Edisto. Correlation coefficients and phase lead (+) of the 

SW/NE winds to the SL residual (shown in parentheses). 

Near the beginning of May 2009, the winds at the Edisto buoy turn to become a NE wind; it 

should be noted, though, that the 30-day lowpass series primarily reflects the strong 5-day NE 

wind event in mid-May mentioned earlier. Residuals quickly respond at the nearby stations, 

increasing > 0.25 m at Trident Pier, FL and Charleston, SC. Interestingly, even while the winds 

reverse and become SW at the start of June, the residuals rise by ~0.15 m to a value of ~0.25 m 

by mid-June under continued SW wind forcing at Edisto. In contrast, the winds at the Nantucket 

buoy are SW in May, becoming NE winds by the start of June. The NE wind peaks in mid-June 

and become SW by July. The residuals at Duck, NC, Atlantic City, NJ and Nantucket, MA all 
13 

rapidly respond to the Nantucket winds. Residuals increase by > 0.25 m from mid-May to midJune at all locations under NE wind forcing, with highest levels of ~0.3 m observed at Duck. 

A few other NE wind events put the current event into perspective. The NE winds that occurred 

around October 2008 and March 2009 had magnitudes of > 3 m/s at both locations, which are 

similar to speeds of the June 2009 NE wind event (Figure 8). During the October 2008 event, 

Charleston, SC and Trident Pier, FL had SL observations slightly higher than during the June – 

July 2009 event (Figs. 4 and 8). During the March 2009 event, SL was lower at each location. 

However, in both occasions the residuals were lower due to the fact that the predictions normal 

for these time periods are also higher. There are two other NE wind events worth noting that 

exceeded the speeds during June 2009 and that happened at nearly the same time of year. During 

May of 2007, a NE wind > 4 m/s occurred at the Edisto buoy and during May/June of 2006, a 

NE wind of ~6 m/s occurred at the Nantucket buoy (Figure 8). Neither event registered at both 

buoys, similar to the June 2009 NE event. In both cases, except the response at Trident Pier, FL 

in May 2007 that reached ~0.3 m, the SL residuals were considerably less than the values 

reached during the June – July 2009 SL event. These results hint at the important contribution of 

another forcing mechanism upon the SL response during the June – July 2009 SL event. 

Figure 9. 7-day lowpass filtered SL residual, southwesterly/northeasterly (SW/NE) and 

northwesterly/southeasterly winds (NW/SE) and atmospheric pressure at sea level. 
14 

A higher resolution view of the winds and SL residuals from April to July 2009 are shown in 

Figure 9. In most cases the winds lead the SL signal by a couple days, though the 7-day filter 

makes this comparison approximate. A pattern similar to that described for Figure 8 emerges. 

The winds oscillate between SW and NE at the Nantucket buoy, and the residuals show a strong 

inverse response, though somewhat muted at Nantucket, which is probably related to its position 

seaward towards the shelf break. The > 0.3 m residual increase in the last quarter of May 2009 

at Duck and Atlantic City from a multi-day NE wind at the Nantucket buoy remain elevated as 

the wind reverses to a light SW wind at the start of June. An even stronger NE wind event at the 

Nantucket buoy in early July causes only a slight (~0.02) rise in water levels, indicating another 

factor causing a general drawdown of SL during this period. The strong NE winds at Edisto on 

and around ~5/19/09 mentioned earlier forces a rapid, slightly lagged rise of residuals at 

Charleston and Trident Pier. It is important to note that SL continues to rise at Charleston and 

Trident Pier in June under greatly diminished wind forcing in the opposite direction. 

Also shown in Figure 9 are time series of atmospheric pressure collected at the NOAA stations. 

Inverse barometer effects from atmospheric pressure changes are such that an increase (decrease) 

of one millibar in pressure results in a fall (rise) of one centimeter in water level. The pressure 

changes need to be of a sufficient period (> 4 days) to allow SL to adjust (Wunsch et al., 1969; 

Garrett and Toulany, 1982). Atmospheric pressures are very similar between Duck and 

Nantucket, and these two series have a larger range and variability than at Trident Pier. Of 

interest to the June – July 2009 SL event is the drop of ~15 mbar that occurs in the last week of 

May at Duck and Nantucket. During this period, the winds turn from SW to NE and SL 

residuals rise at Duck and Atlantic City by > 30 cm, or twice the amount attributed to the inverse 

barometer effect. At Nantucket, the residuals rise by ~20 cm. During the first half of June, 

atmospheric pressure remains relatively constant at all locations, though SL residuals at Atlantic 

City and Duck rise by > 10 cm and > 20 cm at Charleston. In mid-June, the most notable drop in 

atmospheric pressure occurs, ~25 mbar at Nantucket, ~15 mbar at Duck, and ~5 mbar at Trident 

Pier. The SL residuals rise by ~25 cm at Nantucket, > 10 cm at Atlantic City, a > 5 cm at Duck 

and a > 20 cm rise occurs at Trident Pier and Charleston, though initiating prior to the pressure 

drop. From late June into mid-July, pressure rises by ~10 mbar at Duck and Nantucket and ~5 

mbar at Trident Pier. During the same period, the overall drop in SL residuals is ~20 cm at 

Duck, Atlantic City and Nantucket and > 30 cm at Trident Pier and Charleston. 

Florida Current Transport and Sea Level Response

There are two distinct patterns of seasonal SL predictions along the East Coast (i.e., Figure 2). 

The shape of the predictive curves reflects the summation of the forcing specific to an individual 

station. Stations north of the Chesapeake Bay tend to have SL that steadily increases from spring 

until early fall and then decreases until late winter, driven primarily by the steric cycle of 

seasonal heating and cooling of the water column (see http://tidesandcurrents.noaa.gov/sltrends). 

As shown in Figure 2, the SL prediction at Charleston, SC has a more pronounced ‘lower’ high 

peak in the late spring than occurs at Atlantic City, NJ. The existence of a double peak reflects 

the annual variability in transport strength of the Florida Current (FC), which largely supplies 

and becomes the Gulf Stream off of the coast of the Carolinas. Figure 10a shows the seasonal 

cycle of the FC composed of its 90-day lowpass filtered series between June 2000 and June 
15 

2009, whose values have been grouped by month and subsequently averaged. Also shown are 

the SL predictions at East Coast stations that are largely inverse to the FC signal. 

Figure 10. a) The June 2000 – June 2009 average seasonal cycle of FC transport based on a 90-

day lowpass filtered series and SL predictions above MSL and b) diagram showing 

cross-shore sea slope with low, average, and high FC transport (adaption of Figure 2 

in Noble and Gelfenbaum, 1992). 

The 2000 – 2009 FC average transport cycle is maximum during the summer and greatly 

diminished during the fall (Figure 10a), similar to other findings (Blaha, 1984; Noble and 

Gelfenbaum, 1992; Baringer and Larsen, 2001). Typical FC transport values range on the order 

of 20 – 40 Sv in the Florida Straits, with annual, inter-annual and decadal variability normally < 

5 Sv (Baringer and Larsen, 2001). The inverse response of the predicted coastal SL to the FC 

transport is a dynamic adjustment to the changing geostrophy of the current system. Crosscurrent sea-slope changes of > 0.25 m seasonally occur within the Florida Current / Gulf Stream 

that extend across the continental shelf to the shore with a relatively small decay (Noble and 

Gelfenbaum, 1992), affecting SL from Florida to Delaware (Blaha, 1984; Ezer, 2001). For 

instance, when the FC is strong, the cross-current gradient steepens and effectively lowers SL 

along the coast (Figure 10b). The opposite occurs when the FC weakens. The influence of the 

FC is greatest at Trident Pier, FL (of the stations shown) such that it has lower predictions in 

June and higher values in October (Figure 10a). The influence of the FC upon predictions 

gradually lessens northward as the Florida Current / Gulf Stream heads offshore near Cape 

Hatteras, NC. 
16 

Figure 11a shows the 30-day lowpass filtered series of FC transport and SL residuals along the 

East Coast. The FC seasonal cycle (Figure 10a) has not been removed because it is a small part 

of the total variance and not well determined with only nine years of data (Meinen et al.,

submitted). High (low) residual SL along the eastern seaboard occurs during periods of low 

(high) FC transport. For example, high FC transport in December 2008 coincided with a large 

SL residual decrease along much of the East Coast. Conversely, the transport of the FC during 

the June – July 2009 SL event drops in mid-May from ~32 Sverdrup (Sv, 106

 m3/sec) to a low of 

~27 Sv in the latter half of June (Figure 11a). As the FC transport drops, the residual SL at all 

locations rise nearly (< 1 day) in phase. The FC regains strength in July, increasing to ~35 Sv in 

mid-July with the residuals all steadily falling ~0.2 m to < 0.1 m in the process. The correlation 

coefficients of the FC to the SL residuals are negative (low transport = high SL residual) and 

decrease to the north as would be expected as the Florida / Gulf Stream current system moves 

further offshore (Figure 11a legend in parentheses). 

Figure 11. a) 30-day lowpass filtered series of FC transport and SL residual time series at 

NOAA stations with correlation coefficients and phase lead (+) of the FC (shown in 

parentheses) and b) 7-day lowpass filtered series since April 2009. 

The 7-day lowpass filtered series in Figure 11b show a higher resolution view of the SL 

responses at Trident Pier, Charleston and Duck. The series show a strong inverse relationship, 
17 

though the Duck SL series (and the stations to the north) contain additional variability associated 

with wind forcing captured at the Nantucket buoy (Figure 9). The residual rise at Charleston and 

Trident Pier and FC slow down that occurs on ~5/21/09 in Figure 11b coincides with the strong 

NE wind of ~5/19/09 at the Edisto buoy (Figure 9), suggesting that short-period NE winds can 

act to slow the FC transport. The strong rebound of FC transport strength from ~29 to 40 Sv that 

occurs around the second week of July 2009 is coincident to a large drop in residual values, > 

0.25 m at Charleston and Trident Pier. 

A few historical periods with low FC transport offer comparison to the June – July 2009 SL 

event. The FC transport was lower during the months of April (26 Sv) and October (25 Sv) in 

2008 than the low during the June – July 2009 SL event (27 Sv). During these two earlier 

events, the station residuals were quite high (Figure 11a). During April 2008, the winds were 

NE at Edisto at ~2 m/s, but data was missing at Nantucket. Residuals were slightly higher at 

Atlantic City, NJ by ~0.02 m, but elsewhere slightly lower. During October 2008, there was 

slightly higher NE forcing (~4 m/s) at both locations compared to the June – July 2009 event. 

Residuals at Trident Pier were slightly higher (~0.02) than the June – July 2009 SL event, but the 

other residuals were much (~0.05 m) less. Again, since the predictions are slightly higher during 

April and much higher during October, the residuals during these periods generally happen to be 

less (Figure 10a). 

Satellite Perspective of Event 

Images of absolute dynamic ocean topography and SL anomaly derived from merged sea surface 

height (SSH) altimetry for times of recent high FC transport (see Figure 11a) on 12/10/08 and 

low transport on 10/1/08 are compared to the June – July 2009 SL event (6/21/09 image) in 

Figure 12. Measurements of the dynamic topography allow investigations of general circulation 

patterns of geostrophic flows. The dynamic topography shows generally a sharper gradient 

across the FC within the South Atlantic Bight (Florida to North Carolina) during the period of 

high FC transport than during the low-transport periods. A weaker (stronger) eastward-rising 

gradient is indicative of weaker (stronger) northward geostrophic flows. 

The SL anomaly in Figure 12 illustrates the SSH variability due to changing currents from 

seasonal and inter-annual forcing. During the high-transport period of 12/10/08, anomalously 

low (0 to -0.2 m) SL/SSH occurs over much of the East Coast. Conversely, during the low FC 

transport period of 10/1/08 and 6/21/09, anomalously high SL/SSH of < 0.4 m and < 0.2 m 

occur, respectively, over the East Coast. It is important to note that the satellite images of SL 

anomalies do not have a seasonal correction applied. Thus, an observer might have measured 

higher sea heights on 10/1/08. However, the SL residuals shown in Figure 11a, which have the 

mean seasonal series removed via predictions, correctly report a higher magnitude during the 

June – July 2009 event. 
18 

Figure 12. Merged satellite altimetry of absolute dynamic topography (cm) and SL anomaly 

(cm) along the eastern U.S. coast and western Atlantic during periods of low (October 

2008 and June 2009) and high (December 2008) Florida Current transport 

(http://www.aviso.oceanobs.com/duacs). 

Figure 12 also provides inference about the geostrophic currents present during each described 

FC transport stage. With low FC transport on 10/1/08 and 6/21/09, the dynamic topography has 

a negative gradient (eastward decline) between the coast and the landward-edge of the Gulf 

Stream in the area north of Cape Hatteras to Long Island, NY. This suggests a southwardflowing current along the shelf landward of the Gulf Stream (Csanady, 1978). A near-shore 

current flowing southward is required by geostrophy to have a higher SL closer to shore than 

seaward of the current. However, during high FC transport on 12/10/08, this southward current 

appears non-existent and not recognizable through the satellite images of dynamic topography. 
19 

SUMMARY DISCUSSION

Magnitude of Event 

Sea level (SL) along much of the eastern U.S. was higher than normal for much of June and July 

2009, enough to cause significant attention from coastal communities and gain noticeable media 

attention. At the height of the event in the latter half of June 2009, the event was exacerbated by 

the simultaneous occurrence of a perigean-spring tide (Figure 3a). In terms of observed SL, this 

event was not extremely abnormal. In fact, SL in many of these locations has exceeded levels 

observed during the June – July 2009 event numerous times over the last decade 

(http://tidesandcurrents.noaa.gov). The June – July 2009 SL event is anomalous in terms of its 

timing and magnitude compared to the site-specific predictions that CO-OPS produces at the 

NOAA tidal stations (Figure 5). The ‘residual’ values, which represent the height unaccounted 

for by the predictions, were anomalously high from early June into July 2009 from Florida to 

Maine (Figure 6). When the site-specific SL is further normalized through adjustments that ‘deseasonalize’ and remove the long-term relative SL rise (climate-related) from the series-long (> 

30 years) record, the high-water anomaly is unique in that it occurs over large regions both north 

and south of Cape Hatteras, NC. Close inspection of the regional averages, defined as north of 

Cape Hatteras (Eastport, MA to Chesepeake Bay Bridge Tunnerl [CBBT]) and south of Cape 

Hatteras (Beaufort, NC to Key West, FL), reveal that the June – July 2009 SL event is the most 

extreme high-water event to occur in both regions simultaneously during spring/summer period 

since 1980 (Figure 7). The other events that matched or exceded the de-trended values of June 

2009 in both regions occurred during the late winters of 1983, 1987, 1998, and 1999. The June – 

July 2009 SL event was highest (> 0.2 m) from Sandy Hook, NJ to Chesapeake Bay Bridge 

Tunnel (CBBT), VA although the values were substantial from Eastport, ME to Vaca Key, FL. 

Inverse Barometer Effect 

There are multiple mechanisms contributing to the high SL residuals during the June – July 2009 

SL event. Intrinsic to SL variability are inverse barometric effects from localized changes in 

atmospheric pressure. This force causes SL to rise (fall) as the overlying atmospheric pressure 

decreases (increases). The inverse barometer effect is in addition to any wind-forced transport 

that may result from large-scale atmospheric pressure differences and their wind fields. During 

the June – July 2009 SL event, the correlation coefficient between the 7-day lowpass filtered 

atmospheric pressure and SW/NE winds at Nantucket NOAA station and offshore buoy, 

respectively, is 0.45 with little phase lag (not shown). This relationship implies that often times, 

high (low) pressure on land is associated with SW (NE) winds at the buoy, which act to reinforce 

the SL response (i.e., SW winds cause SL to drop as does higher atmospheric pressure). 

However, the relationship often is reversed due to the distance between stations and the size and 

position of non-stationary pressure systems causing a particular wind field. The effects of 

changing atmospheric pressure were not directly removed from the SL residuals since the 

residuals are already adjusted for a mean seasonal signal of atmospheric pressure inherent to SL 

predictions. In order to make an adjustment would require deriving an anomalous series from a 

seasonal mean signal of atmospheric pressure. Over this June – July 2009 SL event, though, the 

absolute changes in atmospheric pressure are considered to cause an approximate change in SL. 
20 

Changes in SL residuals from the inverse barometer effect during the June – July 2009 SL event 

were most prominent at Nantucket, i.e., the 25 cm SL residual rise from a ~25 mbar drop in 

pressure in mid-June (Figure 9). However, at the other locations, SL residual responses 

contained much smaller influences from atmospheric pressure variability. For instance, the ~20 

cm SL residual increases in the first half of June at Charleston, Duck and Atlantic City occur 

under relatively stable atmospheric pressure. Also, SL residuals decrease in early July with an 

overall drop of ~20 cm at Duck, Atlantic City and Nantucket and > 30 cm at Trident Pier and 

Charleston during a period when atmospheric pressure rises by only ~10 mbar at Duck and 

Nantucket and ~5 mbar at Trident Pier. Thus, it can be seen that other forcing mechanisms are 

primarily driving the SL residual variability observed during the June – July 2009 SL event. 

Effects of Wind Forcing 

The first is wind forcing. Winds recorded at offshore NOAA buoys ~40 nm southeast of 

Charleston, SC and ~50 nm southeast of Nantucket, MA both had fairly persistent and moderate 

northeasterly (NE) component during the months of May and June, respectively (Figure 8). 

Though, it should be pointed out that the NE winds at the Edisto buoy in Figure 8b were 

primarily concentrated over a 5-day event in mid-May (Figure 9). The direction of this wind 

component is alongshore and acted to raise SL via Ekman transport, which is 90 degrees to the 

right in the northern hemisphere. The absolute magnitude of the correlations between the 30-day 

lowpass filtered southwesterly/northeasterly (SW/NE) winds and the stations’ residual SL values 

(observed – predicted) are highest (> 0.5) between the winds at the Nantucket buoy and residuals 

at the mid-Atlantic stations of Charleston, SC, Duck, NC, and Atlantic City, NJ (Table 1). In 

this area, the wind forcing leads the SL response by < 3 days, a timeframe typical of a regional 

wind-forced response (Emery and Thomson, 2004). The high-resolution view of the 7-day 

lowpass filtered winds and residuals clearly reveal that since April 2009, the two series are 

inversely related (Figure 9). However, the magnitude of the SL response varies to similar winds 

and indicates that another mechanism is also affecting the residual SL values. 

Table 1. Summary of correlations between 30-day lowpass filtered series. A negative value 

implies an inverse relationship and the phasing is shown in days (d) and positive values indicate 

that the wind and Florida Current transport lead/force a SL response at the NOAA stations. 
21 

Effects of Florida Current Transport 

The other mechanism is the changing strength of the Florida Current (FC) transport. The FC is 

measured in-situ between Florida and the Bahamas near 27°N. The FC flows northward along 

the shelf break of the Southeast US and largely supplies the Gulf Steam near Cape Hatteras, NC. 

Variability in the 30-day lowpass filtered FC transport strength has a high negative correlation (< 

-0.6) to SL ~south of Cape Hatteras, but is still considerable north of it (Table 1). The dynamics 

are such that when the FC transport is high, its cross-stream slope is steep, which causes a drop 

in SL landward to the coast (Figure 10b). The opposite occurs when the FC transport is low. 

Changes of SL to the longer-period (30-day lowpass series) fluctuations in the FC transport 

occur quickly (< 1 day) as shown in Table 1. The geographical extent of the influence of the FC 

transport is hinted at by the correlation coefficients whose absolute magnitudes progressively 

decrease to the north (Table 1) similar to findings of Blaha (1984) and Ezer (2001). Similarly, 

the changing influence of the FC transport is observable and inherent to the long-term seasonal 

SL cycles at NOAA stations (Figure 10a). The FC signal causes a double peak in the seasonal 

cycle, compared to a single peak which would be expected from normal seasonal heating/cooling 

of the water column. When the FC transport is highest in July, stations approximately south of 

Atlantic City, NJ have a low in their seasonal SL cycle (Figure 10a). When the FC transport is 

low in October, the stations have an enhanced high. These seasonal SL patterns are quite 

different than those approximately north of Atlantic City, NJ, where the signals are more 

controlled by the steric cycle, peaking in late August and at a minimum in early February. 

During the June – July 2009 SL event, the formation of the high SL residual coincided with a 

noted decrease in FC transport strength (Figure 11). At the peak of the event, the FC transport 

bottomed out at ~26 Sv and the residuals peaked at values > 0.25 m (7-day lowpass series in 

Figure 11b). Changes within the two signals are nearly concurrent to one another, especially at 

the more southern stations (Figure 11 and Table 1). The June – July 2009 SL event decays at all 

locations in early July, when the residuals became greatly reduced and even negative with the 

onset of a strong transport increase (~10 Sv) of the FC (Figure 11b), even while the winds 

oscillate between SW and NE (Figure 9). 

Wind and FC Transport Interactions 

Short-period winds appear to influence the transport strength of the FC measured within the 

Florida Straits during the June – July 2009 SL event. The 7-day lowpass filtered series 

highlights a strong NE wind event at the Edisto buoy commencing on ~5/19/09 (Figure 9b) and a 

subsequent drop ~2 days later in FC transport (Figure 11b). The 7-day filter makes exact 

phasing of these series difficult. However, both the filtered (and unfiltered, not shown) residuals 

at Trident Pier and Charleston and the NE wind peaked ~2 days prior to the low of the FC 

transport (Figure 11b). Historical investigations have noted similar interactions. Lee et al.

(1985) and Lee and Williams (1988) found that short-term variations in FC transport with 

periods of < 10 days were correlated to local northerly/southerly winds. For instance, a NE (SW) 

wind causes a landward (seaward) Ekman transport that diminishes (strengthens) the crossstream sea slope and slows (accelerates) the northward geostrophic transport of the current. In 

addition, multi-year correlations and phasing between the January 2006 – June 2009 30-day 

lowpass filtered SW/NE wind components at Edisto (0.40, 7 days) and Nantucket (0.54, 3 days) 
22 

and the FC transport suggest that longer-period NE (SW) winds may have caused a decrease 

(increase) in FC transport. This finding is weakly supported by our results, but in agreement 

with findings by Lee and Williams (1988) who concluded that the primary contribution to the 

annual FC transport cycle was from weather events longer than five days, though the winds used 

in the their study were regional over the Straits of Florida. Another study found a strong 

relationship between monthly variability in the easterly trade winds over the Caribbean and the 

seasonal FC transport patterns (Lee et al., 1996). 

The correlation coefficients between the SW/NE winds at Edisto (0.40) and Nantucket (0.54) and 

the FC transport suggest a quasi-independent contribution from both forces to the SL residual 

along the coast. It is realized, although, that the two forces are somewhat mutually correlated 

and a more complicated multivariate correlation analysis is required to determine the specific 

contribution from each forcing mechanism. However, a couple points are worth noting. During 

the June – July 2009 SL event, longer-period NE winds were not at their displayed series-long 

highest value (Figure 8) nor was the FC transport strength at its lowest value (Figure 11a). On 

the other hand, the SL residuals were at widespread highs, hinting at the importance of the 

coupled effect of the two forcing mechanisms. The fact that the highest de-trended (seasonally 

and long-term relative SL rise) SL values were located within the mid-Atlantic areas from 

Virginia to New Jersey is supportive of the overlapping effect of the two forcing mechanisms 

(Figure 6). High correlations of SW/NE winds at Nantucket (-0.67) and FC transport (-0.58) to 

the SL residuals at Duck, NC (Table 1), highlight this region as heavily influenced by both 

forces. 

Satellite Altimetry 

Satellite altimetry data offers an interesting view of the June – July 2009 SL event in terms of 

other recent high- and low-transport periods of the FC (Figure 12). Snapshots of sea surface 

height (SSH) derived products during the June – July 2009 event (June 21, 2009) and another 

low-transport event in October 2008 reveal a somewhat weakened cross-stream gradient in the 

dynamic topography, high SL anomalies along the East Coast, and the presence of a southwardflowing current from Long Island, NY to Cape Hatteras, NC. During a high-transport period in 

December 2008, the opposite conditions generally occurred. More analysis needs to be 

performed to see if this type of relationship is persistent and statistically significant for multiple, 

discrete periods of high- and low-FC transport. It is plausible that variability within the Florida / 

Gulf Stream system might substantially affect coastal SL within the mid-Atlantic Bight (North 

Carolina to Massachusetts) by regulating the strength of the southward flow normal along the 

shelf (Csanady, 1978) and the higher coastal SL it requires due to geostrophic constraints. 

Future Work 

The bigger question exists as to what causes the inter-annual variability in both winds and FC 

transport that caused the June – July 2009 SL event. On-going research is trying to decipher the 

complexities involved. Some recent findings have shown that multi-year FC variability is highly 

anti-correlated to variability in the North Atlantic Oscillation (NAO) though with ~18 month 
23 

lags between signals (Baringer and Larsen, 2001) that makes the relationship less obvious to the 

observer. The NAO is a climatic phenomenon defined by fluctuations in the differences of sealevel atmospheric pressure between the Icelandic Low and the Azores high that affects the 

westerly wind strength over the northern Atlantic. The NAO also modifies the wind-stress curl 

over regions of the subtropical Atlantic, which appears to be an important factor driving the FC 

transport rates (Dinezio et al., 2009). Continued work focusing on the inter-connections between 

the wind and currents and their effects upon coastal SL will be critical to addressing and 

ultimately better predicting intra-annual and inter-annual SL anomalies along the East Coast. 

The unexpected anomalous SL event during June – July 2009 will lead to further applied 

research in detecting and reporting out on similar events as they occur in the future. CO-OPS is 

investigating various types of new products to alert the public and to explain events to them. 
24 
25 

ACKNOWLEDGMENTS

Time series of transport of the Florida Current cable and section data are made freely available 

on the Atlantic Oceanographic and Meteorological Laboratory web page 

(www.aoml.noaa.gov/phod/floridacurrent/) and are funded by the NOAA Office of Climate 

Observations. We wish to acknowledge use of the Ferret program for altimetry-derived graphics 

in this paper. Ferret is a product of NOAA's Pacific Marine Environmental Laboratory. 

(Information is available at http://ferret.pmel.noaa.gov/Ferret/). The altimeter graphics were 

produced by Ssalto/Duacs and distributed by Aviso, with support from Cnes

(http://www.aviso.oceanobs.com/duacs/). The authors thank the technical and science reviews 

provided by John Boon of Virginia Institute of Marine Sciences (VIMS) and Mark Bushnell and 

Frank Aikman of NOAA. 

 
26 
27 

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