NOAA Technical Memorandum ERL PMEL-115

COARE Seacat data: Calibrations and quality control

procedures

H.P. Freitag,1 M.E. McCarty,1 C. Nosse,2 R. Lukas,2 M.J. McPhaden,1 and M.F. Cronin1

1Pacific Marine Environmental Laboratory

7600 Sand Point Way NE

Seattle, WA 98115

2School of Ocean and Earth Science and Technology (SOEST)

University of Hawaii

1000 Pope Road

Honolulu, HI 96822

March 1999

Contribution 2034 from NOAA/Pacific Marine Environmental Laboratory

Contribution 595 from the Joint Institute for the Study of the Atmosphere and Ocean

Contribution 98-02 from the School of Ocean and Earth Science and Technology (SOEST)
NOTICE

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NOAA/ERL. Use of information from this publication concerning proprietary products or the

tests of such products for publicity or advertising purposes is not authorized.

Contribution No. 2034 from NOAA/Pacific Marine Environmental Laboratory

Contribution 595 from the Joint Institute for the Study of the Atmosphere and Ocean

Contribution 98-02 from the School of Ocean and Earth Science and Technology (SOEST)

For sale by the National Technical Information Service, 5285 Port Royal Road

Springfield, VA 22161

ii
Contents iii

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2. Instrumentation and Mooring Design . . . . . . . . . . . . . 2

3. Methods of Calibration and Quality Analysis . . . . . . . . 8

3.1 Pre-Deployment and Post-Recovery Calibrations . . . . . . . 10

3.2 In Situ Stability Analysis . . . . . . . . . . . . . . . . . . . . 13

3.3 In Situ CTD Comparisons . . . . . . . . . . . . . . . . . . . . 16

3.4 Salinity Spikes . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.5 Remaining Inversions . . . . . . . . . . . . . . . . . . . . . . 21

3.6 Errors Induced by Depth Excursions . . . . . . . . . . . . . . 21

4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 25

6. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Appendix A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Appendix B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

List of Figures

1 TAO moorings in the western equatorial Pacific Ocean during COARE 2

2a Distribution of salinity data from moorings along 156◦E longitude . 3

2b Distribution of salinity data from moorings along 165◦E longitude . 4

2c Distribution of salinity data from moorings near 0◦, 154◦E, 0◦, 160◦30

E,

and 0◦, 170◦E............................... 5

3 Schematic drawing of a taut-line ATLAS mooring . . . . . . . . . . . 7

4 Salinity differences between 5-m and 1-m Seacats on FU6 deployment

at 0◦, 156◦E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5 Differences between post-recovery conductivity calibrations and predeployment conductivity calibrations as a function of Seacat depth . 12

6 Salinity and density differences between 11-m and 1-m Seacats deployed at 0◦ 156◦E . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

7 Comparison of CTD and moored Seacat measurements . . . . . . . . 17

8 Intercomparison cast of co-located CTD and Seacat . . . . . . . . . 18

9 Seacat salinity and temperature at 1 m and 5 m at 0◦, 156◦E . . . . 20

10Distribution of 11-m minus 3-m density (sigma-theta) differences at

0◦, 156◦E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

11 Salinity corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
iv Contents
COARE Seacat data: Calibrations and quality control procedures

H.P. Freitag,1 M.E. McCarty,1 C. Nosse,2 R. Lukas,2 M.J. McPhaden,1 and M.F. Cronin1

Abstract. An intensive set of quality analysis routines was applied to salinity data from twelve

mooring sites of the Tropical Ocean-Atmosphere (TAO) Array within the western Pacific warm

pool. Primary sources of error identified were 1) biological fouling and abrasive scouring of the

conductivity cell, and 2) temperature/conductivity sensor response time mismatch in the presence

of high-frequency temperature variability and low flush rate. Quality control methods included

application of both pre-deployment and post-recovery calibrations, stability analysis among sensors,

and comparison with CTD cast data. Corrections were applied by modeling error bias as linear or

higher order polynomials. After correction, salinity data are believed to be accurate to within 0.02

psu in most cases.

1. Introduction

The western Pacific warm pool is a region with strong influence upon the

El Ni ̃no/Southern Oscillation phenomena and thus world climate variations.

The region is characterized by a warm surface mixed layer, high precipitation, intense westerly wind events, and strong ocean-atmosphere coupling.

During the latter half of the 10-year Tropical Ocean Global Atmosphere

(TOGA) program, the TOGA Coupled Ocean-Atmosphere Response Experiment (COARE) sought to describe and understand the processes at work

in the coupled system of the western Pacific warm pool, and their influence

on global climate (Webster and Lukas, 1992).

The large flux of fresh water into the warm pool from precipitation has

a strong influence on the dynamics of the upper ocean. At times the depth

of the mixed layer is determined as much by salinity as by temperature.

During such times the mixed layer may be separated from the thermocline

by a salt-stratified barrier layer that inhibits cooling of the mixed layer from

below (Lukas, 1988; Godfrey and Lindstrom, 1989; Lukas and Lindstrom,

1991; Sprintall and McPhaden, 1994). Because the measurement of salinity

is an important factor in understanding the processes active in ocean dynamics and the flux of energy between ocean and atmosphere in the warm

pool, the objectives of the oceanographic component of COARE included

the determination of salinity space-time structure as well as the net flux of

salt and freshwater into the region.

Salinity measurements from moorings of the Tropical Ocean-Atmosphere

(TAO) Array (Hayes et al., 1991; McPhaden, 1993) in the warm pool were

begun in November 1988 (Sprintall and McPhaden, 1994) and continue to

the present. In preparation for COARE, the number of moorings from which

salinity was measured was substantially increased through the cooperative

efforts of the Pacific Marine Environmental Laboratory (PMEL), the University of Hawaii (UH), and L’Institut Fran ̧cais de Recherche Scientifique pour

1NOAA/Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, Seattle,

WA 98115 2School of Ocean and Earth Science and Technology (SOEST), University of Hawaii,

1000 Pope Road, Honolulu, HI 96822
2 Freitag et al.

Figure 1: Location of TAO moorings in the western equatorial Pacific Ocean during COARE. ATLAS

moorings from which Seacat data were obtained denoted by solid triangles. PROTEUS moorings with

Seacat data denoted by solid squares. Atlas moorings without Seacat data denoted by open triangles.

le D ́eveloppement en Coop ́eration (ORSTOM), each of which contributed instruments, research-vessel time, and personnel. This memorandum will cover

moorings deployed between fall 1991 and winter 1994, approximately 1 year

before and after the COARE Intensive Observing Period (IOP), which was

from November 1992 to February 1993. Mooring locations at which salinity measurements were made during this period are shown in Fig. 1. Data

quality issues for TAO moored salinity measurements previous to the period

covered by this memorandum were discussed by Sprintall and McPhaden

(1994). Salinity measurements from TAO moorings continued after COARE

will be quality controlled in a manner similar to those data described here.

The majority of measurements were made between 1 m and 100 m below

the sea surface (Fig. 2). At two sites (0◦, 156◦E and 0◦, 165◦E) measurements

were routinely made down to 200 m. Measurements were extended to 750 m

at two moorings (2◦S, 156◦E and 2◦S, 165◦E) near TOPEX/POSEIDON

crossover sites for verification of satellite altimetry measurements for 5–7

months concurrent with the IOP (Picaut et al., 1995; Gourdeau et al., 1995).

2. Instrumentation and Mooring Design

In the warm pool, vertical salinity differences in the upper 100 m, horizontal

differences over the 200 km to 1000 km distance between moorings, and temporal differences over periods of hours to years are all order 1 psu. Salinity

differences that define the vertical extent of the mixed layer may be of order

0.2 psu. Accurate determination of these signals requires moored salinity

sensors with relatively high accuracy and small drift characteristics.
COARE Seacat Data 3

Figure 2a: Distribution of salinity data from moorings deployed along 156◦E longitude. Mooring names

are indicated along the time axis.
4 Freitag et al.

Figure 2b: Distribution of salinity data from moorings deployed along 165◦E longitude. Mooring names

are indicated along the time axis.
COARE Seacat Data 5

Figure 2c: Distribution of salinity data from moorings deployed near 0◦, 154◦E, 0◦, 160◦30E, and 0◦,

170◦E. Mooring names are indicated along the time axis.
6 Freitag et al.

Conductivity and temperature data considered in this report were measured with Sea-Bird Electronics, Inc., Model SBE-16 Seacat ConductivityTemperature recorders. The Seacat SBE-16 employs an internal-field conductivity cell with a range of 0to 7 S m−1, and a pressure-protected thermistor with a range of –5 to 35◦C. The internal-field conductivity cell allows

anti-foulant devices to be placed at both ends of the cell to reduce the effect

of biological fouling on the cell. The conductivity cell has a nominal accuracy

of 0.001 S m−1 month−1, and a resolution of 0.0001 S m−1; the thermistor

has an accuracy of 0.01◦C per 6 months and a resolution of 0.001◦C. These

accuracies and all other specifications cited here were provided by the manufacturer.

The sensors are interfaced to a Wein-bridge oscillator which generates

temperature or conductivity dependent frequencies. The frequencies are

digitized by counting integer and fractional cycles over a time interval of

0.125 seconds. The time base is determined by a precision temperaturecompensated crystal oscillator. The Seacat interface circuit is corrected for

electronic drift before each measurement by referencing stable Vishay resistors. The crystal oscillator has an accuracy of ±3 ppm over a temperature

range of –5◦C and 35◦C and it ages less than ±2 ppm yr−1.

The real-time clock in the Seacat is a 32,768 Hz watch crystal oscillator

which is corrected for initial error, temperature-induced drift, and aging by

the temperature-compensated crystal oscillator, yielding an accuracy of ±2.6

min yr−1.

During COARE, Seacats were installed on several ATLAS (Autonomous

Temperature Line Acquisition System) moorings. The upper section of an

ATLAS mooring consists of a toroidal float with an instrumentation tower

mounted on the upper side and a bridle mounted to the underside. Beneath

the bridle, jacketed wire rope extends to a depth of (typically) 700 m. A

500-m-long, polyurethane-jacketed, double-armored, three-conductor cable

with attached temperature and pressure sensors runs parallel to the wire

rope. When Seacats are mounted on ATLAS moorings, they are mounted

on either the bridle or on the wire rope section of the mooring line.

Usually the moorings were taut-line moorings, but at one site (5◦S,

156◦E) slack-line moorings were also deployed. For a taut-line mooring

(Fig. 3), 3⁄4" nylon line connects the wire rope to the acoustic release and

the anchor. The nylon line, which stretches under tension, is adjusted to

yield a scope of 0.985 (ratio of mooring length without tension to the water

depth) which limits mooring excursions in both the horizontal and vertical. For a slack-line ATLAS mooring, a section of polyolefin line is included

just above the anchor, and the mooring length is adjusted within the nylon

section to a scope of 1.35.

At two sites (0◦, 156◦E and 0◦, 165◦E), Seacats were installed on tautline PROTEUS (PRofile TElemetry of Upper ocean currentS) moorings

(McPhaden et al., 1991). The upper portion of a PROTEUS mooring consists of a toroidal float with an instrumentation tower and bridle attached.

A downward-looking RDI Acoustic Doppler Current Profiler (ADCP) is

mounted in the toroid. When Vector Averaging Current Meters (VACMs)

are included in the mooring line, the wire rope is broken into sections with
COARE Seacat Data 7

	

















	












	


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Figure 3: Schematic drawing of a taut-line ATLAS mooring. Number and placement of Seacats varied from

site to site and, in some cases, from deployment to deployment at a given site.
8 Freitag et al.

the VACMs shackled between pieces. The deepest VACM is typically at

200 m. Seacats and MTRs (mini-Temperature Recorders, designed and built

by PMEL) are mounted directly onto the wire rope. Fairings are typically

placed on the upper portion of the wire rope to a depth of 250m. The majority of the mooring length is made up of 3⁄4" nylon line, which is adjusted

to give an overall mooring scope of 0.985. The deepest part of the mooring

consists of 32 glass balls, an acoustic release, and the anchor.

3. Methods of Calibration and Quality Analysis

Each institution (PMEL, UH, ORSTOM) was responsible for the processing

and quality analysis of data from the instruments which they contributed to

the moored Seacat array. The principles used were generally the same (e.g.,

application of pre-deployment and post-recovery calibrations, stability analysis, and comparison to CTDs), although the exact methods used differed in

some respects. For TOPEX/POSEIDON validation, ORSTOM interpolated

some of their time series from their original sample intervals (10, 20, or 30

min) to 5-min intervals in a manner that took into account high-frequency

variability measured by Seacats sampling at 5 min (Gourdeau et al., 1995).

In the present case, ORSTOM data were analyzed (by PMEL researchers)

at their original sample interval to be consistent with the methods used on

the PMEL and UH data.

The manufacturer has found that most Seacats drift toward lower conductivity values with time (Nordeen Larson, Sea-Bird Electronics, Inc., personal communication). This negative drift is primarily attributed to biological or geochemical fouling which decreases the effective cross-sectional area

of the cell, and secondarily to fouling or loss of material on the electrode.

Conversely, most Seacats deployed on TAO moorings drifted towards higher

conductivity values. While the cause of this positive drift is not totally

understood, it is suspected that abrasive scouring of the cell from strong,

equatorial currents carrying high concentrations of plankton may be a factor. The manufacturer has reported that electron microscope photographs of

cells deployed on TAO moorings have shown grooves running along the axis

of the cell, which are not apparent on new cells. The first Seacats to be used

on TAO moorings were configured with the conductivity cells in a horizontal

orientation. In an attempt to reduce the flow rate through the cell (and

presumably scouring of the cell) newer Seacats purchased by PMEL, as well

as those purchased by UH and ORSTOM, were configured with vertically

oriented conductivity cells. Unfortunately, conductivity drifts remained positive with either cell orientation, indicating that either flow rates were not

significantly reduced by this change, or that the positive drifts were the

result of some other process.

Biological growth and loss of material from the electrode are also factors

in the drift of TAO moored sensors. When possible, instruments were calibrated after recovery in both an “as is” state and again after cleaning of the

cell and replatinizing of the electrode. In most cases the second calibration

indicated an even larger positive conductivity drift. It is assumed that nega-
COARE Seacat Data 9

tive drift, such as bio-fouling which was removed by the cleaning process, had

partially compensated for the drift due to scouring. An attempt was made

to minimize the effect of bio-fouling by installing anti-fouling plugs supplied

by the manufacturer on the conductivity cells. It is thought that the antifoulant is effective for a period of at least 3 months, typically 6 months, and

perhaps longer in some cases (Nordeen Larson, Sea-Bird Electronics, Inc.,

personal communication). Our experience is generally consistent with this

view. Differences between some sensor pairs placed within the mixed layer

tended to be linear during the first months of a deployment, but increased

nonlinearly thereafter (see discussion below on in situ stability analysis).

This change in drift rates could be due to the depletion of the anti-foulant

resulting in an increase in biologic growth rate on the cell.

A preliminary step in the processing of the data was to perform gross

error checks. This process identified missing or obviously bad data. Once

identified, these data points were either corrected or flagged as bad, depending on the length of time involved. Periods of bad or missing data of up

to 6 hours in duration were filled by linearly interpolating between good

data points before and after the bad or missing data. Longer periods of bad

or missing data were flagged. Identification could often be automated by

passing the data though a window (e.g., 10◦C to 33◦C for temperature, 29

psu to 36 psu for salinity) beyond which data values were not expected. In

some cases the actual window limits were determined by the location of the

instrument (i.e., at the surface, in the mixed layer, in the thermocline, etc).

In addition to this automatically applied windowing technique, a subjective

analysis was performed by plotting each time series and looking for spikes

or other suspicious data. Once identified, comparisons to other time series

from the same mooring, when available, were used to determine whether to

manually correct the questionable values. In general, the amount of data

affected by gross error checking was minimal, ranging from zero to order 10

samples per time series (order 0.1%). Time series with more significant rates

of gross errors are noted in Appendix A.

The next step in quality analysis of salinity data was to apply both

pre-deployment and post-recovery calibration coefficients in a linear, timedependent combination so that pre-deployment calibrations predominate in

the early portion of the time series and post-recovery calibrations predominate in the latter portion. This step removed a linear drift, the sources of

which could be residual instability of the electronics, loss of material from

the electrode, and/or abrasive scouring of the conductivity cell. In some

cases this step was retracted if it appeared that the linear drift correction

was inaccurate. Subsequent analysis included stability analysis (comparing

instruments co-located in or near the mixed layer), comparison of moored

Seacat and CTD data, and attachment of recovered Seacats onto the CTD

frame for comparison. Details on each step of the analysis are given below.

All methods of quality analysis could not be applied to all time series. Some

instruments were damaged while on the mooring and thus could not be calibrated after recovery. Instruments located below the mixed layer and those

which were singly located within the mixed layer could not have a vertical

stability check performed (in other than a large scale sense, which would
10 Freitag et al.

not identify the typically small rates of drift). Some deployments did not

have sufficient CTD casts to make a meaningful comparison, and not all

Seacats were attached to the CTD frame for comparison. Appendix A lists,

by deployment, the particular methods used for each time series.

3.1 Pre-Deployment and Post-Recovery Calibrations

Prior to deployment and after recovery, the Seacats were sent by Sea-Bird

Electronics, Inc. to the Northwest Regional Calibration Center for calibration. The instruments were calibrated in two baths with different salinities

at several temperatures. Typically the salinity of one bath was 35 psu, and

frequency outputs from the instruments in that bath were recorded at nominal temperatures of 31, 23, 15, 7, and –1◦C. The typical salinity of the

second bath was 15 psu, and measurements were made near 27, 19, 11, and

3◦C. The temperatures of the baths were known to within 0.003◦C and the

salinities to within 0.003 psu (Andy Heard, Sea-Bird Electronics, Inc., personal communication). Frequency output for the dry conductivity cell in air

was also measured.

The conductivity outputs measured during the post-recovery calibration

of a Seacat are influenced by the handling of the instrument after it is recovered. Unfortunately, a method of handling doesn’t exist that can be

expected to preserve the conductivity cell in a state identical to that which

it had upon recovery. Allowing the cells to dry after recovery, as PMEL researchers often did, allowed evaporative deposits (which may take some time

to dissolve when re-immersed) and desiccation of the biota, both of which

change cell geometry. The method followed by UH researchers, which was to

keep the conductivity cell filled with distilled water, prevented the changes

to cell geometry that drying would allow. However, keeping the cells in fresh

water for weeks to months (often necessary for western Pacific cruises) may

tend to rinse off or dilute the concentration of biota. Keeping cells in salt

water would have allowed further biological fouling and chemical deposition

to occur.

In spite of the uncertainties introduced by the inability to keep the Seacats in the same state they were upon recovery, applying the post-recovery

conductivity calibrations as well as the pre-deployment calibrations generally improved data quality, in that drifts in the data were decreased. The

1-m and 5-m hourly salinities from mooring FU6 present an example of

the improvement that can be made in data quality by applying both predeployment and post-recovery calibrations to salinity records (Fig. 4). A

6-week gap in the time series was caused by the mooring line having parted

in December of 1992. The 1-m and 5-m Seacats were recovered within days

of the mooring failure and the mooring was redeployed in February 1993 using the same Seacats. The majority of salinity differences between the 1-m

and 5-m instruments, calculated using only the pre-deployment calibration

values (Fig. 4a), were near zero when first deployed, but drifted to an offset

of about 0.02 psu by the time the mooring line broke. When redeployed in

February, the offset had not significantly changed from the December value.

Even though these instruments were out of the water for weeks after the
COARE Seacat Data 11

Figure 4: Salinity differences between 5-m and 1-m Seacats on FU6 deployment at 0◦,

156◦E. Salinity computed using (a) pre-deployment calibrations only and (b) a linear combination of pre-deployment and post-recovery calibrations.

mooring parted, and dry for months after the final recovery and before postrecovery calibrations were performed, application of linear drift corrections

based on the pre-deployment and post-recovery calibration values removed

the drift of the 5-m salinities relative to the 1-m values (Fig. 4b).

Salinity differences between pre-deployment and post-recovery calibrations were typically 0.06 psu or less for Seacats in the upper 100 m (Fig. 5)

and were typically in the direction consistent with cell abrasion. Below

100 m, differences were smaller and of opposite sign.

While the above procedure often successfully removed apparent sensor

drifts, in some cases its application resulted in increased instability. In these

cases, only pre-deployment calibrations were applied to the data, after which

other calibration methods (as described below) may have been performed.

Differences in temperature calibrations were typically 0.003◦C or less,
12 Freitag et al.

Figure 5: Differences between salinity values calculated using post-recovery conductivity calibrations minus

values calculated using pre-deployment conductivity calibrations as a function of Seacat depth. Black line

is the mean. Grey lines are the mean ± one standard deviation. Negative values indicate an instrumental

drift towards higher salinities between pre-deployment and post-recovery calibrations.
COARE Seacat Data 13

which was comparable to the accuracy of the calibrations themselves. Linear

temperature drift corrections based on both pre-deployment temperature

calibrations and post-recovery calibrations were applied by UH researchers

when available. Only pre-deployment calibrations were used for ORSTOM

and PMEL temperature records.

3.2 In Situ Stability Analysis

Applying drift corrections based on pre-deployment and post-recovery calibrations did not correct all the questionable data. In the mixed layer,

sustained periods of instability and trends in salinity or density differences

between adjacent instruments could still be found in the data after correcting

for linear calibration drift.

Problematic data were identified by PMEL researchers by examining

time-series plots of the differences between adjacent instruments for temperature, salinity, and density. When large, sustained inversions were found in

the plots of differences, further examination was needed to determine which

instrument of the pair was in error. Often the biased instrument could be

identified by comparison with data from a third instrument. When it was

not possible to determine which record was in error, no correction was made.

Once the biased data were identified, they were corrected by modeling

the bias for time-periods when the surface layer was well mixed or, at most,

weakly stratified. These periods were determined using a stability criteria

based on density differences among records with no apparent drift. When

differences among these less questionable data were ≤0.02 kg m−3, the surface layer was considered to be well mixed (to within the accuracy of the

instruments) and the correction based on these data was considered a highquality correction. When density differences for the less-questionable data

were between 0.02 kg m−3 and 0.1 kg m−3, the surface layer was considered

to be weakly stratified and the resulting correction was considered to be of

lower quality. When applied, criteria used for such corrections are noted in

Appendix A.

For each time step where the well-mixed/weakly-stratified criterion was

met, PMEL researchers modeled the error by calculating salinity as a linear

function of depth from the less-questionable records. The error in the biased

instrument was defined as the difference between its value and the linear

fit at the depth in question. To generate an error time series for the entire

record, a first-, second-, or higher-order polynomial function in time was

fit to the estimated errors using least-squares regression techniques. The

resulting relationship was subtracted from the measured data to obtain a

corrected record at all times. In some cases, different sections of the record

were modeled using different order polynomials.

An example of this method is shown in Fig. 6. Salinity differences between the 11-m and 1-m salinities from mooring FU4, after correction for

calibration drift, were characterized by a small linear drift, with the 11-m

record becoming relatively saltier, during the first 5 months of the record

(Fig. 6a). In the last month of the record, the drift was non-linear and in

the opposite direction, probably indicating a failure of an anti-foulant. The
14 Freitag et al.

Figure 6: (a) Salinity and (b) density (sigma-theta) differences between 11-m and 1-m Seacats deployed

at 0◦ 156◦E. A linear combination of post-recovery and pre-deployment conductivity calibrations have been

applied. (c) Salinity bias of 11-m data relative to 1-m and 30-m data (dots) has been modeled (solid line)

as a linear function in time for the first 5 months and as a 2nd order polynomial for the last month. (d)

Density differences after modeled bias has been removed.
COARE Seacat Data 15

relative salinity drift led to apparently unstable density differences between

11 m and 1 m in the last month of the mooring (Fig. 6b). Comparison with

data from other depths identified the 11-m record as being in error. Using

the stability correction scheme described above, a linear correction was modeled for the first 5 months, and a quadratic correction for the last month

(Fig. 6c). After application of the modeled correction to the 11-m salinity,

the trend towards increased stability in the first 5 months and the apparent

instability in the last month was removed (Fig. 6d).

UH researchers considered the upper water column to be well mixed during windy periods, defined as periods of sustained wind speeds over 8 m s−1.

Salinity records that looked suspicious because of lack of agreement with

CTD casts taken near individual moorings were confirmed as erroneous by

examining stability during windy periods. It was assumed that during wellmixed periods, density instabilities should not be present for more than 2–3%

of the well-mixed times. A single instrument in the mixed layer, usually below 15 m to avoid biological fouling which seemed to most affect instruments

near the surface, was selected and the mean differences between salinity data

during well-mixed periods from that instrument and data from the other

instruments in the mixed layer were calculated. Records with mean differences that suggested salinity and/or density instability in the mixed layer,

and that also agreed poorly with CTD data, were regarded as questionable.

The mean difference, for the well-mixed, windy periods, between data from

an instrument regarded as good and data from the questionable instrument,

was used as a correction. This correction was added to the questionable Seacat data beginning with the time of the first CTD/Seacat bias. For times

when CTD data were not available for moorings, windy periods were used

to both identify and correct suspicious data due to instability.

In contrast to well-mixed, windy periods, when instabilities are assumed

to be rare, some short periods of increased instability (e.g., nighttime cooling) are to be expected, but should not dominate the time series. Identification of questionable salinity data was performed by UH researchers by

looking for vertically adjacent records which displayed instability relative to

each other for a high percentage of the deployment time, in a manner similar

to the methods employed by PMEL. UH researchers considered data to be

questionable if density instabilities were present for more that 10% of the

mooring deployment time. If a time series plot or statistics of the density

differences between two such records revealed a trend in salinity, steps were

taken to identify and correct the record containing the salinity drift. Density differences with other nearby records were calculated and examined for

corresponding trends. When the record containing the drift was identified,

a correction was calculated based on an adjacent good record. The salinity

differences were calculated between the good and the questionable records.

The mean was removed; then all differences greater than ±0.02 psu were

removed. A linear regression was fit to the remaining values in order to

compute a drift correction.
16 Freitag et al.

3.3 In Situ CTD Comparisons

An attempt was made to gather as many CTD casts as possible for in situ

quality checks of Seacat salinities. From one to three CTD casts were made

at the times of many of the mooring deployments and recoveries (e.g., Shinoda et al., 1995). During the IOP the presence of several ships within the

COARE Intensive Flux Array (centered at 2◦S, 156◦E) permitted additional

casts to be made near a few moorings (e.g., Delcroix et al., 1993). In some

cases, comparisons provided a consistent picture, showing either that differences were small (0.01 psu or less) or that there was a bias in the Seacat

measurement. In other cases, the comparisons were ambiguous, probably

because of temporal and spatial variability in the salinity field. In regions

of high vertical salinity gradient, slight depth offsets would lead to large

salinity differences. Horizontal variations could also add uncertainty, since

casts were typically made roughly a mile from the mooring. Rather than

the actual time at each depth, processed CTD files recorded only the time

when the instrument entered the water or when it reached the greatest depth

(which could be an hour or more after the cast started). For this reason Seacat samples from a few sample intervals (ranging from 5 min to 60min) near

the cast time were often used in the analysis. In addition, salinity comparisons were made at fixed temperatures, rather than fixed depths, assuming

small bias between Seacat and CTD temperature.

An example of a Seacat/CTD comparison follows for mooring FU2 which

was placed near 0◦, 156◦E on 28 August 1991. Three CTDs were made within

hours of deployment. Seacat/CTD salinity differences were large (up to 0.097

psu) in the upper 50m, but inconsistent between casts and Seacat samples

(e.g., differences ranged from –0.096 psu to 0.097 psu at 30 m; see Fig. 7a for

an example of one of the three casts). The inconsistency of differences was

due to a lack of a well-defined mixed layer. Two more casts were made 11

days later, at which time the upper ocean was well mixed to 60m. (Figure 7b

is an example of one of the two casts.) In this case Seacat/CTD differences

were more stable between the two CTD casts and different Seacat samples.

Mean differences at 3 m, 30 m, and 51 m were 0.010 psu or less for each

cast. It was felt that given the uncertainties described above, differences of

this magnitude were less than the accuracy of the method, and thus it was

decided that the CTD and Seacats were not significantly different at these

depths. Conversely, a fairly consistent Seacat/CTD difference with mean of

0.020 psu at 11 m was sufficiently large to determine that the Seacat at this

depth was in error. The bias found from this CTD comparison was consistent

with the bias found using the stability analysis method described above. The

stability analysis method also showed that the 11-m Seacat bias was variable

with time, and thus corrections were made using that method. Nevertheless,

it was encouraging to find that the two methods gave a consistent estimate

of bias in September.

In addition to using CTD comparison methods similar to those described

above, UH researchers conducted CTD-Seacat intercomparison casts to aid

in Seacat calibration. On several occasions, after mooring recoveries and

prior to mooring deployments, Seacats were strapped to a CTD rosette, with
COARE Seacat Data 17 Figure 7: Temperature-salinity diagram of CTD cast (solid line) made near 0◦, 156◦E (a) on August 28, 1991 and (b) on 9 September 1991. Symbols represent Seacat measurements at approximately the same time as the CTD cast.
18 Freitag et al.

     









	

		

	




     















	

		

	


	


Figure 8: Conductivity readings recorded during a portion of an intercomparison cast involving a CTD

(points marked with o’s in top panel) and Seacat S/N 781 (points marked with x’s in top panel). During

this portion of the intercomparison cast, the CTD and Seacat were held at 30m. The mean CTD–Seacat

conductivity difference (bottom panel) is –0.00423 S m−1.

the sensors placed roughly at the same vertical position. A CTD cast was

then conducted with this instrument arrangement. During the cast, the CTD

was held stationary at several depths for minutes at a time to obtain CTDSeacat intercomparisons. After the CTD data were post-cruise calibrated

and processed, mean and standard deviation statistics were determined for

the CTD-Seacat temperature and conductivity differences during the times

when the CTD was held vertically stationary.

When available, these intercomparisons were used to identify and correct

questionable Seacat data in conjunction with the regular CTD comparisons

as well as the windy period analyses and the stability analyses. If supported

by CTD comparisons and/or well-mixed (windy) period and/or stability

analyses, offset or drift corrections were made using the mean differences

produced by the CTD-Seacat intercomparisons as a correction. For exam-
COARE Seacat Data 19

ple, the ET 193 deployment (located at 0◦, 160.5◦E from March 1993 to April

1994) showed unstable density analyses during well-mixed (windy) periods.

Intercomparison casts were taken before this deployment and upon using the

corrections suggested by the intercomparison casts, the well-mixed (windy)

periods showed stable density profiles. To illustrate, during an intercomparison cast, the Seacat placed at 30m for ET 193 (Seacat S/N 781) showed a

mean conductivity difference of –0.00423 S m−1 when held at 25 m during

the intercomparison cast (Fig. 8). This mean difference was used to correct

the 30-m Seacat (S/N 781) and similar differences were used to correct the

other Seacats on this mooring to help produce stable density profiles during

well-mixed (windy) periods observed by this mooring.

3.4 Salinity Spikes

Some salinity time series had a few positive spikes or periods of unusual high

frequency variability. These spikes were most common at 1-m or 3-m time

series and were distinct from negative salinity spikes due to rainfall. These

spiky salinity periods were often correlated with strong daytime heating and

high frequency variability in the temperature time series such as on November 3, 14, and 18, 1992, at 0◦, 156◦E (Fig. 9). On 19 November, when similar

heating occurred, but without the high frequency variability, salinity variability was relatively small. Note that in the example shown the spikes are

limited to less than 5 m depth. The strong diurnal heating was associated

with days with low wind speed, during which a thin layer of surface water

was heated in the presence of weak mixing. The high frequency variability

in the temperature time series was perhaps due to small gusts of wind overturning the heated layer for short periods. A physical process generating

positive salinity spikes of 0.4 psu or more exhibited here is unlikely. Using

observed temperature and wind values in the bulk formula for latent heat

flux, evaporative increases in salinity due to enhanced SST should have been

much less than 0.1 psu. In addition, no source of such high salinity water

was available immediately below the surface.

It is presumed that the source of much of this salinity noise was a mismatch in the response times of the temperature and conductivity sensors.

The Seacat temperature time constant is 0.5 s, while the conductivity time

constant is a function of the cell flush rate. Anti-fouling plugs and low wind

speed (perhaps associated with calm seas and a low surface-current speed)

would both tend to decrease the flush rate. Under these circumstances,

the conductivity time constant would exceed 1 s (Nordeen Larson, Sea-Bird

Electronics, Inc., personal communication).

These salinity spikes were corrected by PMEL researchers on a caseby-case basis. Identification and characterization of periods with erroneous

spikes was aided by graphical computer software which displayed the temperature and salinity of the noisy instrument and, when available, rainfall

and/or the salinity of an adjacent instrument located a few meters below the

first. Methods of salinity correction included substitution of the adjacent instrument’s data, linear interpolation (in time), or smoothing. Substitution

was used when salinity at both levels was nearly identical during the nights
20 Freitag et al.

Figure 9: Seacat salinity and (b) temperature at 1 m (black) and 5 m (grey) at 0◦, 156◦E.
COARE Seacat Data 21

before and after the day with noisy salinity. When salinity at both levels

differed at night, or when no adjacent time series was available, then a subjective choice of when and how to interpolate or smooth was made. When

spiky data were identified, but no satisfactory correction could be made, the

data were flagged as bad. Salinity data that were modified in these ways are

identified by their quality index in the data archives. Time series modified

in this manner are indicated in Appendix A. In some cases, surface salinity

data were considered suspicious due to spikiness, but because no corroborating data (rainfall or nearby salinity) were available, no correction was

attempted. These time series are also noted in Appendix A.

3.5 Remaining Inversions

Most density instability remaining in the corrected records involved nearsurface (1-m or 3-m) time series and can be explained by nighttime cooling.

Histograms of density differences, the time duration of inversions, and diurnal variation of inversions were examined to come to this conclusion. For

example, remaining near-surface density inversions represented 16% of the

3–11 m data pairs from mooring FU2 (Fig. 10a), but the magnitudes of

the great majority of these inversions were 0.01 kg m−3 or less (within the

estimated accuracy of the instrument). Inversions between the 11-m and

30-m instruments represented less than 7% of data pairs and virtually none

exceeded 0.01 kg m−3. Moreover, the inversions between the 3-m and 11-

m instruments were short-lived, and larger magnitude inversions were more

likely to be short-lived. Sixty-one percent of all near-surface inversions were

single (30-min) samples (Fig. 10b), and 91% of inversions with magnitude

greater than 0.01 kg m−3 were single samples (Fig. 10c). A diurnal pattern

to the remaining inversions also existed, with the greatest number of inversions occurring during the local night and early morning, and the smallest

number occurring in the early afternoon (Fig. 10d). The inversions being

near-surface, small, short-lived, and at night or early morning was consistent

with nighttime, cooling-induced, small instabilities that were quickly mixed

down.

Seacat records which, after correcting, still contained questionable instabilities are noted in Appendix A.

3.6 Errors Induced by Depth Excursions

Temperature and salinity data from Seacats on both taut-line and slackline moorings are subject to errors resulting from vertical displacement of

sensors from their nominal depths. Most Seacats were mounted on taut-line

moorings, which minimize vertical excursions of the mooring line. However,

even taut-line moorings respond in some degree to surface wind forcing and

to surface and sub-surface currents. Relative displacements, which are zero

at the surface, increase monotonically with depth. McCarty et al. (1997)

show that mooring-motion-induced errors in taut-line ATLAS temperature

records were largest in the upper thermocline, with root mean square (RMS)

values of 0.15◦C to 0 .45◦C. Errors below 200 m in the western Pacific ranged
22 Freitag et al. Figure 10: (a) Distribution of 11-m minus 3-m density (sigma-theta) differences at 0◦, 156◦E. (b) Cumulative distri- bution of duration (hours) of all instabilities. (c) Cumulative distribution of duration for instabilities ≥0.01 kg m−3. (d) Diurnal distribution of all instabilities. Nighttime hours (1800 to 0600 local) are 0800 to 2000 GMT.
COARE Seacat Data 23

from 0.02◦C to 0 .15◦C. Errors in the mixed layer were negligible on both

taut-line and slack-line moorings.

The induced errors in the Seacat temperature records are assumed to

be similar in magnitude to the errors induced in the records from ATLAS

temperature sensors. Salinity errors due to vertical excursions of the Seacats

are expected to be small, because the salinity gradients are for the most part

weak. Also, most Seacats were located in the upper 100 m to 200 m of the

mooring line, where vertical excursions were small. No Seacats were deployed

below 3 m on slack-line moorings. In the surface mixed layer, salinity errors

should be negligible for both taut-line and slack-line moorings. In the upper

thermocline, a typical mean value for vertical displacement of the 200 m

depth of a taut-line ATLAS buoy was 2.5 m. Assuming a salinity gradient

of 1 psu per 100 m, an error due to mooring-line motion of 0.025 psu could

be expected in a Seacat salinity record from 200 m.

Correction of either the salinity or temperature records for depth excursions was not attempted.

4. Summary

After application of the quality analysis routines described above, the

COARE array of Seacats deployed on TAO moorings has produced a highquality salinity data set that may be used to describe the structure of the

salinity field in both time and space. In addition, these data may be used to

investigate the salinity balance of the region (Cronin and McPhaden, 1997).

Errors in the data, due primarily to cell abrasion and biological fouling, have

been corrected using an intensive set of analysis methods that required sensor calibration before and after use, coincident CTDs, and the placement

of multiple sensors within the mixed layer. The sign of the corrections was

generally such that salinity values were lower after correction (Fig. 11). Most

corrections were near zero at the beginning of a deployment and increased

in magnitude with time. Consequently, longer deployments typically had

larger corrections than shorter deployments. In a few cases, corrections near

the beginning of the record are not near zero because: 1) pre-deployment

calibration was many months before deployment (significant only for UH

processed data), 2) CTDs at the time of deployment indicated an offset, or

3) the instrument was redeployed shortly after recovery, with no laboratory

calibration in between. A few time series have sharp jumps in the corrections which were the result of either: 1) stability analysis indicating a sharp

offset in salinity, 2) or stability analysis indicating a specific correction for

only a portion of the record (this might occur if the thickness of the mixed

layer changed). In some cases, salinity data after these sharp offsets in the

corrections occurred are considered to be questionable and of lower quality

than other data in general. These cases are identified in Appendix A. The

largest correction was 0.435 psu, but corrections of this magnitude occurred

in only a few cases and generally only during the last month or two of a

year-long deployment. The vast majority of the 173 time series processed

had much smaller corrections. The RMS of all maximum corrections was
24 Freitag et al.

Figure 11: Daily mean salinity differences between quality controlled salinity time series minus time series

based on pre-deployment calibrations. Negative values imply that the data were fresher after correction.
COARE Seacat Data 25

0.062 psu. Corrections for instruments placed at 150 m and below were significantly smaller than those above, with a tendency towards higher salinity

after correction.

In addition to these long-term drift corrections, the effect of despiking

several near-surface salinity time series is also shown in Fig. 11. The differences shown are those of daily averaged data. The magnitude of changes to

individual data points was at times much larger. For most time series the

largest individiual spike removed ranged between 0.2 and 0.8 psu. For one

particularly noisy 1-m time series (FU8) the largest individual spike removed

was 1.8 psu.

Given the uncertainties inherent in the calibration of moored conductivity sensors, as discussed above, a precise, absolute accuracy of these salinity

measurements cannot be given, but intercomparison of Seacat salinities and

comparisons to CTD salinities have led to the conclusion that the corrected

data are accurate to within 0.02 psu or less in the majority of cases. Instances when this level of accuracy was not met would include times when

there were insufficient co-located instruments or CTDs to perform intercomparisons, times when well-mixed conditions were not sufficiently present,

and times when salinity spiking near the surface was large and adequate

correction methods were unavailable.

Daily mean values of salinity and density (sigma-theta) are displayed in

Appendix B. Hourly mean temperature, salinity and density are available on

the World Wide Web at http://www.pmel.noaa.gov/coare/coare-seacatdeliv.html.

5. Acknowledgments

We would like to thank ORSTOM for furnishing instruments and processed data

from the TOPEX/POSEIDON validation. Discussions with Nordeen Larson, Rick

Baumann, and Andy Heard of SeaBird Electronics, Inc., provided valuable information regarding the operation, calibration, and performance of the instruments. This

work was supported by NOAA’s Office of Global Programs, the National Science

Foundation (Grant no. OCE-9024452) and the University of Hawaii.

6. References

Cronin, M.F., and M.J. McPhaden (1997): The upper ocean salinity balance in the

western Pacific. J. Geophys. Res., 103(C12), 27,567–27,589.

Delcroix, T., G. Eldin, C. Henin, F. Gallois, J. Grelet, M. Inall, S. Keene, M.-J.

Langlade, C. Menkes, F. Masia, and K. Richards (1993): Campagne COAREPOI `a bord du N.O. Le Noroit, 1er d ́ecembre 1992–2 mars 1993. Rapports

de Missions, Sciences de la Mer, Oc ́eanographie Physique, No. 10, ORSTOM,

Centre de Noum ́ea, New Caledonia, 338 pp.

Godfrey, J.S., and E.J. Lindstrom (1989): The heat budget of the equatorial western

Pacific surface mixed layer. J. Geophys. Res., 94, 8007–8017.

Gourdeau, L., M.-J. Langlade, A. Busalacchi, M.J. McPhaden, P. Freitag, F. Gonzalez, M. Eble, and R. Weller (1995): Data preparation for the open-ocean

validation of TOPEX/POSEIDON sea level in the western equatorial Pacific.

Notes Techniques, Sciences de la Mer, Oc ́eanographie Physique, ORSTOM,

Centre de Noum ́ea, New Caledonia, 12, 43 pp.
26 Freitag et al.

Hayes, S.P., L.J. Mangum, J. Picaut, A. Sumi, and K. Takeuchi (1991): TOGATAO: A moored array for real-time measurements. Bull. Amer. Meteorol. Soc.,

72, 339–347.

Lukas, R. (1988): On the role of western Pacific air-sea interaction in the El

Ni ̃no/Southern Oscillation phenomenon. Proc. of the U.S. TOGA Western Pacific Air-Sea Interaction Workshop, R. Lukas and P. Webster (eds.), Honolulu,

16–18 September 1987, U.S. TOGA Rept. USTOGA-8, Univ. Corp. Atmos.

Res., 43–69.

Lukas, R., and E.J. Lindstrom (1991): The mixed layer of the western equatorial

Pacific Ocean. J. Geophys. Res., 96, suppl., 3343–3357.

McCarty, M.E., L.M. Mangum, and M.J. McPhaden (1997): Temperature errors in

TAO data induced by mooring motion. NOAA Tech. Memo. ERL PMEL-108

(NTIS PB97-146492), Pacific Marine Environmental Laboratory, Seattle, WA,

68 pp.

McPhaden, M.J., H.B. Milburn, A.I. Nakamura, and A.J. Shepherd (1991):

PROTEUS—Profile Telemetry of Upper Ocean Currents. Sea Technol., 32, 10 –

19.

McPhaden, M.J. (1993): TOGA-TAO and the 1991–93 El Ni ̃no/Southern Oscillation Event. Oceanography, 6, 36–44.

Picaut, J., A.J. Busalacchi, M.J. McPhaden, L. Gourdeau, F.I. Gonzalez, and E.C.

Hackert (1995): Open-ocean validation of TOPEX/POSEIDON sea level in the

western equatorial Pacific. J. Geophys. Res., 100, 25,109–25,127.

Shinoda, T., S. DeCarlo, S. Kennan, R. Lukas, F. Santiago-Mandujano, and J.

Snyder (1995): Shipboard measurements from R/V Moana Wave during TOGA

COARE enhanced monitoring cruises. SOEST Tech. Rept. 95-7, School of Ocean

and Earth Science and Technology, University of Hawaii, 201 pp.

Sprintall, J., and M.J. McPhaden (1994): Surface layer variations observed in multiyear time series measurements from the western equatorial Pacific. J. Geophys.

Res., 99, 963–979.

Webster, P.J., and R. Lukas (1992): TOGA COARE: The Coupled OceanAtmosphere Response Experiment. Bull. Am. Meteorol. Soc., 73, 1377–1417.
COARE Seacat Data 27

Appendix A

This appendix contains details of salinity quality analysis for each Seacat

time series. Pages are ordered from west (0◦ 154◦E) to east (0◦ 170◦E) and

from north to south along longitudes with multiple mooring sites. Each page

contains a table which lists mooring name, latitude, longitude, deployment

and recovery dates, Seacat serial numbers (S/N), sample rates (∆T) and

depths of the instruments. In addition, the tables indicate what type of processing was used. For temperature, they indicate whether pre-deployment

calibrations were used exclusively, or a linear combination of pre-deployment

and post-recovery calibrations. These entries are found for salinity as well,

plus entries indicating adjustments made based upon stability analysis, CTD

comparisons (with either moored Seacats or Seacats attached to the CTD),

and a column showing the maximum long-term salinity drift (∆S) between

the time series generated using only pre-deployment calibrations and the

final version of the time series.

In addition to the tables, any other pertinent facts are listed below the

tables. These may include: moorings parting and drifting, Seacats going to

the bottom, instrument failures for incomplete records, order of polynomials

used and other necessary details for stability corrections, and the details of

CTD-based corrections.
28 Freitag et al.

Mooring ET146

Latitude: 0◦ 0.2S Longitude: 153◦ 59.2E

Deployment Period: 5 May 1992–7 March 1993

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

3 804 10 • • 0.0

5 803 10 •• • –0.058

10 799 10 • • •• 0.006

15 801 10 • • 0.0

20 802 10 • • 0.0

30 784 60 •• • 0.096

50 78160 •• • 0.051

75 736 60 • • 0.0

The 3 m Seacat (S/N 804) drifted towards fresher values starting on 19

October 1992. The drift was of order –0.25 psu. As it is unclear how to best

correct for this drift, the 3 m salinity data recorded after 19 October 1992

remain uncorrected and should be considered highly suspect.

A jump of 0.058 psu was detected through the use of stability analyses

in the 5 m Seacat (S/N 803) salinity data which seemed to affect all salinity

points after 31 December 1992. These points were corrected with an offset

correction of –0.058 psu but should be considered suspect. Furthermore, a

salinity drift of order –0.15 psu/month started on 30 August 1992. Thus, all

salinity points for the 5 m Seacat recorded after 30August 1992 should be

considered highly suspect as it is not clear how to correct for this drift.

The 10m Seacat (S/N 799) became very unstable and began to record

inaccurate readings (a severe drift caused salinities less than 33 psu to be

recorded) starting on 10January 1993 so data after this day were eliminated.

The conductivity cell for the 10m Seacat was found to have corrosion damage

when it returned to the factory for recalibration so this could have been a

source for the inaccurate and unstable readings. A CTD cast taken just after

the deployment of this mooring as well as stability analyses and analyses

performed during well-mixed times suggested that all salinity data from this

instrument required an adjustment of 0.006 psu.

Stability analyses showed a drift in the 15 m Seacat (S/N 801) salinities

beginning on 28 November 1992. Salinities from this instrument began to

drift to saltier (larger) values at this time. As it is difficult to determine

how to correct for this drift, all 15 m salinity points after 28 November 1992

should be considered highly suspect.

Two drops in salinity were detected using stability analyses for the 30m

Seacat (S/N 784). Sustained drops in salinity were seen on both 26 October

1992 and 21 January 1993. Offset corrections were applied to the onset

of each drop and the corrections were continued from that point through

the rest of the deployment. Salinity points after the 26 October 1992 drop

were corrected by 0.037 psu and points after the 21 January 1993 drop

were corrected by 0.059 psu. All these corrected salinity points should be

considered suspect.
COARE Seacat Data 29

Stability analyses show a salinity drop in the 50m Seacat (S/N 781)

salinity data on 27 October 1992. An offset correction of 0.051 psu was

applied to this drop and all salinity points recorded after its onset. Hence,

all salinity points recorded after 27 October 1992 for the 50m Seacat should

be considered suspect.

Quality analysis was performed at the University of Hawaii.
30 Freitag et al.

Mooring ET188

Latitude: 0◦ 1.1N Longitude: 153◦ 59.5E

Deployment Period: 7 March 1993–25 November 1993

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

3 800 10 • • 0.0

5 797 10 • • 0.0

This mooring was heavily vandalized. The mooring line parted somewhere below 500 m and the mooring was set adrift starting on 20 April

1993. Data recorded after 20April 1993 had a constantly changing location

associated with them. On 16 November 1993, the mooring line was again

severed and all equipment below 9 meters was lost.

The 3 m Seacat (S/N 800) began to record wild readings (negative temperature, negative salinity) on 31 October 1993 until the instrument was

recovered. These data were eliminated from the record.

Salinity data from the 3 and 5 m Seacats (S/N 800 and S/N 797) should

be considered highly suspect from 22 June 1993 until the end of the deployment. One of the Seacats experienced a large change in salinity (on order of

> 0.3 psu) on 22 June and salinities remained offset between the two sensors

until the end of the deployment. As it is difficult to determine which sensor

was responsible for the salinity offset, salinity data from both Seacats (S/N

800 and S/N 797) should be considered highly suspect from 22 June 1993

until the end of the deployment.

Quality analysis was performed at the University of Hawaii.
COARE Seacat Data 31

Mooring ET116

Latitude: 5◦ 0.0N Longitude: 156◦ 1.0E

Deployment Period: 31 August 1991–18 September 1992

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

3 800 10 •• • 0.019

Seacat S/N 800 failed on 15 February 1992 due to a bad battery pack.

All salinity data from Seacat S/N 800 was offset corrected by 0.019 psu

after two near-mooring CTD casts, taken one week after deployment, showed

the instrument to be recording salinities which were 0.019 psu too low.

Quality analysis was performed at the University of Hawaii.
32 Freitag et al.

Mooring ET171

Latitude: 5◦ 0.5N Longitude: 156◦ 0.2E

Deployment Period: 23 September 1992–21 December 1993

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

3 1096 10 •• • 0.0

Seacat S/N 1096 filled its memory before recovery.

Five near-mooring CTD casts conducted during the course of this mooring deployment were used to verify the salinity of Seacat S/N 1096. The five

near-mooring CTD casts were always at least within 0.02 psu of the Seacat

and at times, the CTD and Seacat S/N 1096 compared even more favorably.

Hence, no adjustments were made to the Seacat salinities.

Quality analysis was performed at the University of Hawaii.
COARE Seacat Data 33

Mooring ET242

Latitude: 4◦ 59.8N Longitude: 156◦ 3.7E

Deployment Period: 22 December 1993–21 July 1995

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

1622 60 • • –0.060

This was a longer than normal (19 month) deployment. Based upon the

performance of other Seacats deployed for long periods, it could be expected

that salinity drift could be very non-linear toward the latter portion of the

time series. Since this mooring had only one Seacat, no stability analysis

could be performed. Comparisons were made to a CTD cast at deployment

and one in April 1994 and no clear evidence of Seacat bias was found.

The 1 m record had 9 days with periods of noisy salinity. These periods

were despiked using linear interpolation. A total of 14 hours of salinity

data were modified (0.1% of the total). The largest salinity spike removed

was an upward spike of 0.224 psu. In addition, the 1 m temperature data

contained several spikes, affecting 8 days of the record. Temperature data

were despiked using linear interpolation. Twelve hours of temperature data

were modified.

There were a large number of downward spikes in the last 3 months of

the time series. By comparison, the number of downward salinity spikes in

May to July 1995 was noticeably larger than the number occurring in the

corresponding period of 1994. Without a rain gauge or adjacent Seacats on

the mooring, it was impossible to distinguish instrumental error from rain

events. The data for May to July 1995 should be used with caution.

Quality analysis was performed at PMEL.
34 Freitag et al.

Mooring ET168

Latitude: 2◦ 0.2N Longitude: 156◦ 1.3E

Deployment Period: 17 September 1992–20 February 1993

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

3 1091 10 • •• –0.006

5 1102 10 • • 0.0

10 1086 10 •• • –0.005

15 1095 10 • • 0.0

20 1100 10 • •• –0.007

30 1089 10 • • 0.0

45 1098 10 • • 0.0

70 1101 10 • • 0.0

A boat tied off and then moved this mooring 2 miles to the northwest of

its original position on 3 February 1993. From this day forward, wild and

unstable readings were seen for the 3, 5, 10, and 45 m Seacats. These data

from 3 February 1993 until the end of the deployment should be considered

suspect. Upon recovery, the Seacats were found with connector plugs and

anti-foulant caps missing as well as having electronics connectors, mounting

brackets, and handles broken. These events most likely occurred when the

mooring was towed and thus caused the wild and unstable salinity readings.

The mooring was also found with a fishing net tangled within it down to

65 m. The entanglement of this net could have been related to the mooring

being dragged.

A –0.0063 psu offset was applied to the 3 m Seacat (S/N 1091) salinity

data as determined by the 3–5 m salinity differences.

The 10m Seacat (S/N 1086) was corrected by a 2.9 × 10−5 psu/day

linear drift correction as determined by the salinity differences between the

10and 15 m Seacats.

The 15 m Seacat (S/N 1095) has a 10-day period of missing data near

the end of the deployment which was due to an instrument malfunction.

Beginning on 15 October 1992 a –6.17 × 10−5 psu/day linear drift correction was applied to the 20 m Seacat (S/N 1100) salinity data as determined by the 15–20m salinity differences. After the linear drift correction

was made, an overall offset correction of 0.0012 psu was applied to the 20

m Seacat (S/N 1100) salinity data as determined by the 15–20 m salinity

differences.

Quality analysis was performed at the University of Hawaii.
COARE Seacat Data 35

Mooring ET183

Latitude: 2◦ 0.1N Longitude: 156◦ 0.3E

Deployment Period: 21 February 1993–27 April 1994

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

10 1098 10 • • 0.0

15 1101 10 • • 0.0

The mooring line parted and all instruments fell to the bottom on 16

February 1994. As a result, the instruments at 3 and 5 m (Seacats S/N 1091

and S/N 1089) were lost at sea. The 20 m Seacat (S/N 1100) failed during

deployment and only returned 17 hours of data.

Density stability analysis showed that density became unstable between

the 10 and 15 m Seacats (S/N 1098 and S/N 1101) beginning on 25 November

1993. The unstable trend was long and sustained and originated due to a

drift in the salinity data between the 10and 15 m Seacats starting on 12

September 1993. As there is not enough information available to determine

which Seacat was drifting, all salinity points from both the 10and 15 m

Seacats (S/N 1098 and S/N 1101) should be considered suspect from 12

September 1993 until the end of the deployment.

Quality analysis was performed at the University of Hawaii.
36 Freitag et al.

Mooring FU2

Latitude: 0◦ 0.9S Longitude: 155◦ 24.2E

Deployment Period: 28 August 1991–28 February 1992

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

3 74130 • • –0.003

11 737 30 • • •• –0.015

30 744 30 • • –0.008

51740 30 • • –0.023

75 742 30 • • –0.022

101 739 30 • • –0.016

150 745 30 • • 0.004

201743 30 • • 0.003

Temperature and current meter records from this mooring indicate that

it was pulled upon (presumably by fishermen) for about 12 hours on 18

February 1992. All time series have been linearly interpolated (in time)

during this period.

After application of post-deployment calibrations, the 11 m salinity had

a non-linear bias when compared to the 3 m and 30m time series. A 4th

order polynomial was fit to the bias from the start of the time series through

2 February 1992. A 3rd order polynomial was fit to the bias from 3 February

1992 to the end of the time series.

At the time of deployment the upper water column was not well mixed,

making comparison to 3 CTD casts taken at that time of little value for

comparison to Seacats. Two CTD casts in September indicated small (<0.01

psu) differences with the exception of the 11 m Seacat. Differences at 11 m

were comparable to those indicated by the Seacat stability analysis given

above.

Quality analysis was performed at PMEL.
COARE Seacat Data 37

Mooring FU4

Latitude: 0◦ 0.3N Longitude: 155◦ 58.8E

Deployment Period: 1 March 1992–13 September 1992

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

1294 30 • • –0.033

11 173 30 • •• 0.053

30 170 30 • • –0.015

51292 30 • • –0.031

75 169 30 • • 0.010

101 291 30 • • –0.024

150 167 30 • • 0.020

201293 30 • • 0.008

This mooring broke in the nylon and was adrift from 20April 1992 to

6 May 1992. It was redeployed on 7 May 1992. Data were linearly interpolated (in time) for the 21 hours between recovery and redeployment. On 5

September 1992 the buoy was unshackled from the mooring line (presumably

by fishermen), and the majority of the mooring fell to the ocean floor. The

buoy, with the 1 m Seacat attached, drifted until recovered on 13 September.

The 1 m record had 15 days with periods of noisy salinity. These periods

were despiked by either linear interpolation or smoothing. A total of 106

hours of salinity data were modified (2.2% of total). The largest salinity

spike removed was an upward spike of 0.273 psu.

After application of post-deployment calibrations, the 11 m salinity had

a bias when compared to the 1 m and 30m time series. A linear fit to the

bias was applied from the start of the time series to 1 August 1992 and a

2nd order polynomial was fit thereafter.

Salinity differences with CTD casts taken near the mooring shortly after

deployment were small (<0.01 psu). Two CTDs taken in May when the

mooring was redeployed did not have data above 20m and the data below

did not provide a good Seacat/CTD comparison. No casts were taken at

recovery since the mooring was on the bottom.

Quality analysis was performed at PMEL.
38 Freitag et al.

Mooring FU6

Latitude: 0◦ 0.1S Longitude: 156◦ 2.3E

Deployment Period: 15 September 1992–24 December 1992

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

1386 30 • • –0.008

5 172 30 • • –0.021

11 745 30 • • –0.016

33 171 30 • • –0.016

57 74130 • • •• 0.017

83 737 30 • • 0.008

132 740 30 • • 0.007

183 744 30 • • 0.010

A snag in the mooring line between 30m and 50m resulted in Seacats

intended for 51 m and below being 18 m higher. The mooring parted at 30m

on 20December. The drifting surface portion of the mooring was recovered

on 24 December.

The 1 m record had 17 days with periods of noisy salinity. These periods

were despiked by either linear interpolation, substitution of the 5 m value

or smoothing. A total of 84.5 hours of salinity data were modified (3.5% of

total). The largest salinity spike removed was an upward spike of 0.474 psu.

There were four long-lived episodes of instability between the 33 m and

57 m instruments in October and November. Comparison with the 83 m

instrument indicated that the 57 m instrument had a bias of about 0.015

psu. The water column was well stratified during the first and last months

of the record, so no indication of bias was found at those times. Sixteen

CTD’s from Hakuho Maru during one of the November episodes confirmed

the estimate of bias. A constant adjustment of 0.015 psu was made to the

entire 57 m record. This adjustment resulted in a few (<1%) small (≤0.005

kg m−3) instabilities between 57 m and 83 m, but these were short lived and

mostly during the night.

Quality analysis was performed at PMEL.
COARE Seacat Data 39

Mooring FU6b

Latitude: 0◦ 0.5N Longitude: 156◦ 0.2E

Deployment Period: 8 February 1993–9 March 1993

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

1386 5 • • –0.011

5 1 72 5 • • –0.028

11 745 5 • • –0.020

50 171 5 • • –0.023

75 7415 • • 0.003

100 737 5 • • 0.011

150 740 5 • • 0.010

200 744 5 • • 0.014

These Seacats were redeployed from the recovered FU6 mooring. Instruments at 50m and below had spent the previous month and a half on the

sea floor.

Because of the short duration of this deployment, the sample rate was

increased to 5 minutes between samples.

The 1 m record had 7 days with periods of noisy salinity. These periods

were despiked by either linear interpolation or substitution of the 5 m value.

A total of 32.3 hours of salinity data were modified (4.5% of total). The

largest salinity spike removed was an upward spike of 0.357 psu.

The Seacat at 75 m had been adjusted by 0.015 psu during the previous

deployment, but the density gradient during the FU6b deployment was such

that no instability at 75 m was indicated. The mean differences between 24

CTDs from RV Natsushima and all Seacats between 5 and 75 m were ≤0.006

psu or less.

Quality analysis was performed at PMEL.
40 Freitag et al.

Mooring FU8

Latitude: 0◦ 0.3N Longitude: 156◦ 1.1E

Deployment Period: 10 March 1993–18 December 1993

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

1874 30 • • –0.056

5 1 21 0 30 • •• 0.028

10 292 30 • • –0.036

30 29130 • • –0.130

50 293 30 • • –0.045

75 170 30 • • –0.010

100 167 30 • • 0.003

150 169 30 • • –0.001

200 294 30 • • –0.002

The 1 m Seacat (S/N 874) salinity time series was quite noisy, especially

in the first half of the record. The noise was not related to unusually warm

days and high frequency temperature variability, as were some other surface

records. A 7-pt Hanning filter was applied to the complete salinity time

series, because of their relative ubiquity. While removing much of the noise,

to some degree this smoothing also decreased the magnitude of downward

salinity spikes related to rain events. The largest salinity spike removed was

a downward spike of 1.769 psu.

After application of post-deployment calibrations, the 5 m salinity had

a bias when compared to the 1 m, 10m and 30m time series. A 3rd order

polynomial fit to the bias was applied to the entire record.

Four CTD casts were taken at the time of deployment, but large castto-cast differences and instability in the upper portions of the casts made

Seacat/CTD comparisons difficult. There was a general tendency for the

Seacats in the mixed layer (after adjustment at 5 m described above) to be

lower than the CTD by 0.015 to 0.020 psu, but no adjustment was made

to the Seacats since the CTDs were so inconsistent among themselves. One

cast was made at the time of recovery, but did not provide any data for a

useful comparison to the Seacats.

Quality analysis was performed at PMEL.
COARE Seacat Data 41

Mooring TC2

Latitude: 0◦ 0.4N Longitude: 156◦ 1.8E

Deployment Period: 19 December 1993–29 April 1994

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

1744 30 • • –0.009

5 745 30 • • –0.014

10 970 30 • • –0.034

20 1107 30 • • –0.010

30 1110 30 • • –0.023

50 1111 30 • • –0.015

75 1242 30 • • –0.008

100 1244 30 • • –0.002

150 1245 30 • • 0.003

200 1116 30 • • 0.002

This mooring was dragged 6 nm on 24 March 1994. Data at 75 m and

below were interpolated for 10.5 hours while the mooring was moving because

it was apparent that they were above their nominal depths during this time.

Because instruments above 75 m were in the mixed layer during this time,

they did not appear to be affected by the movement of the mooring.

During the first 2 months after deployment, the 1 m Seacat (S/N 744)

salinity, and to a lesser degree temperature, had a lot of high-frequency

noise. This noise resulted in some relatively large (some greater than 0.1 kg

m−3) instabilities between the 1 m and 5 m density time series. Later in the

record there were some salinity spikes associated with unusually warm days

and high frequency temperature variability, and also some salinity spikes

not associated with temperature variability. To minimize all types of salinity errors, a 7-pt Hanning filter was applied to the complete salinity time

series. While removing much of the error, some residual error remained, and

to some degree this smoothing also decreased the magnitude of downward

salinity spikes related to rain events. The largest salinity spike removed was

a downward spike of 0.843 psu.

No significant biases were apparent after application of the post-deployment calibrations, but the number of density instabilities between the 1 m

and 5 m was higher than for most near-surface time series, even after the 1

m time series was smoothed. Instabilities less than 0.01 kg m−3 were 25% of

the total record. Furthermore, unlike most cases, the number of instabilities

did not decrease monotonically with depth. The percentage of instabilities

less than 0.01 kg m−3 were 6% between 5 m and 10m, and 17% between

10m and 20m. This implies that there was a small (<0.01 psu) bias in

the 5 m salinity, but differences of this magnitude are considered within the

accuracy of the instruments and therefore no adjustment was made.

Quality analysis was performed at PMEL.
42 Freitag et al.

Mooring ET165

Latitude: 1◦ 59.2S Longitude: 155◦ 53.8E

Deployment Period: 12 September 1992–22 February 1993

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

1623 30 • •• –0.009

3 1099 5 • • –0.035

5 1097 5 • •• –0.009

10 1090 5 • • –0.026

15 1088 5 • • •• –0.014

20 1093 5 • • •• –0.028

30 1085 5 • • 0.000

70 1092 5 • • 0.000

112 969 5 • • –0.008

132 970 5 • • –0.002

175 971 5 • • 0.007

225 1112 5 • •• 0.058

275 1113 5 • •• 0.027

400 1116 10 • • 0.007

750 1117 10 • • 0.009

Stability analysis showed that the 1 m Seacat values are too salty relative

to the 3 m and 10m records. (The 5 m record was excluded from the stability

analysis because it was short.) The 1 m salinity data were corrected by a

constant offset of –0.005 psu. The 1 m record had 15 days with periods of

noisy salinity. These periods were despiked by linear interpolation in time

or substitution of the 3 m time series. A total of 58 hours of salinity data

were modified (1.4% of total).

Seacats at 5 and 30 m (S/N 1097 and S/N 1088) have short data records

(data ends in mid-November 1992) due to bad battery packs.

A stability analysis showed a small drift (on order 0.008 psu) for the 5 m

Seacat (S/N 1097) for the entire deployment. Even after correcting this drift,

a significant number of inversions remained between 3 m and 5 m Seacats.

As no further determinations could be made regarding the calibration of the

5 m salinity data, data for this instrument should be treated with caution

if the data are used in cases where an accuracy of ±0.01 psu or better is

required.

Three analyses conducted during well-mixed periods suggested that after

19 December 1992, the 15 and 20 m Seacats (S/N 1088 and S/N 1099) needed

corrections of –0.024 and –0.028 psu, respectively, as their readings appeared

anomalously high compared to the 10 m Seacat (S/N 1090). Eleven nearmooring CTD casts helped to support this conclusion as they also showed

offsets of approximately these same amounts for the 15 and 20m Seacats.

Thus, the salinity data for the 15 and 20m Seacats (S/N 1088 and S/N

1099) were corrected after 19 December 1992 but should be treated with

caution.

The 30m Seacat (S/N 1085) showed an anomalous drop in salinity (on

order 0.02 psu) on 31 October 1992. After this date, salinity data from this
COARE Seacat Data 43

instrument were widely varying and, at times, less than the 20m Seacat

data. As neither a cause nor correction can be determined, caution must be

used when analyzing the 30m salinity data after 31 October 1992.

After application of linear corrections for post-deployment calibration,

Seacat data at 225 m (S/N 1112) and 275 m (S/N 1113) had mean biases

of –0.042 and –0.012 psu, respectively, relative to 17 CTDs taken between 8

December 1992 and 12 February 1993. No Seacat bias was indicated when

compared to 3 CTDs taken between 12 September and 26 September 1992.

A linear adjustment was applied between the time of deployment and 8

December 1992, increasing from zero to the means listed above. Thereafter

a constant adjustment equal to the mean bias was applied.

Quality analysis was performed on the 3 m through 70m Seacats at

the University of Hawaii. ORSTOM performed the initial quality analysis

on the remainder of instruments during TOPEX/POSEIDON validation.

PMEL performed a consistency check on the combined data set and made

adjustments to the 225 m and 275 m salinity based on CTD comparisons.
44 Freitag et al.

Mooring ET184

Latitude: 1◦ 58.7S Longitude: 156◦ 20.2E

Deployment Period: 23 February 1993–30 April 1994

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

3 1099 10 • • •• –0.126

5 1090 10 • • •• –0.260

10 965 10 • •• –0.056

15 1088 10 • • •• –0.301

20 1093 10 •• • –0.028

30 966 10 •• • 0.006

45 1092 10 •• • –0.012

70 967 10 ••• –0.017

Mooring parted between 3 m and 5 m on 10February 1994.

A near-mooring CTD cast conducted immediately after the deployment

of this mooring showed the Seacat salinities to be offset from the CTD.

Hence, the Seacat salinities were corrected through the information provided

by the near-mooring CTD cast. Corrections of 0.013, 0.014, –0.001, –0.024,

–0.028, 0.006, –0.012, and –0.020 psu were made to the 3, 5, 10, 15, 20,

30, 45, and 70 m Seacats respectively (S/N’s 1099, 1090, 965, 1088, 1093,

966, 1092, and 967) according to the offsets between the Seacats and the

near-mooring CTD cast.

Stability analysis suggests that the 3 m Seacat (S/N 1099) salinity record

had a fast and anomalous increase from 8 July 1993 through 17 July 1993.

After 17 July, the 3 m salinity appears to remain offset by 0.070 psu. Although a correction was applied to the anomalous salinity increase, caution

should be used when examining salinity data from the 3 m Seacat (S/N

1099) after 8 July 1993.

A sharp increase in salinity was seen for the 5 m Seacat (S/N 1090)

during a stability analysis. It appears that the sharp increase in salinity of

0.042 psu occurred on 14 August 1993 and left the salinity data offset by that

amount until the salinity data started to drift even more on 15 December

1993. This drift increased the salinity data until 13 January 1994 when it

appeared to reach a constant offset of 0.173 psu. Corrections have been

applied to the 5 m Seacat (S/N 1090) salinity data from 14 August 1993

until the mooring sank, but these data should be treated with caution.

The 15 m Seacat (S/N 1088) showed two salinity drifts during stability

analysis. The first drift showed a steady increase in salinity from the beginning of the deployment until 18 August 1993 when the pattern of the drift

could no longer be determined (although it did reach a maximum increase

of 0.043 psu). The second drift seen for the 15 m Seacat began 11 December

1993 and continued until the mooring sank, at which point the drift had

reached an increase of 0.301 psu. Corrections were made for these drifts but

caution should be used when examining the 15 m Seacat (S/N 1088) salinity

data from the beginning of the deployment until August 18, 1993 and from

11 December 1993 until the end of the record.

Quality analysis was performed at the University of Hawaii.
COARE Seacat Data 45

Mooring ET113

Latitude: 4◦ 58.8S Longitude: 156◦ 0.0E

Deployment Period: 25 August 1991–11 September 1992

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

3 797 10 •• • 0.006

Seacat S/N 797 failed on 16 February 1992 due to a bad battery pack.

All salinity data from Seacat S/N 797 was offset corrected by 0.006 psu

after a near-mooring CTD cast conducted 1.5 hours after deployment showed

the instrument to be recording salinities which were 0.006 psu low.

Quality analysis was performed at the University of Hawaii.
46 Freitag et al.

Mooring ET163

Latitude: 4◦ 58.9S Longitude: 156◦ 1.4E

Deployment Period: 10 September 1992–16 December 1993

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

3 1094 10 • • 0.0

ET163 was a slack-line mooring.

Seacat S/N 1094 failed on 14 December 1992. Data recorded after 24

November 1992 were eliminated as they were completely unrealistic.

Two near-mooring CTD casts were made for this mooring while Seacat

S/N 1094 was functioning. However, neither CTD cast compared well with

the CTD. As it was difficult to determine a clear way to correct the Seacat

data according to the CTD, they were left uncorrected. It should be noted

though, that since there were no corrections made, the CTD casts do suggest

that the salinity data for Seacat S/N1094 could be inaccurate by more than

0.05 psu.

Quality analysis was performed at the University of Hawaii.
COARE Seacat Data 47

Mooring ET241

Latitude: 5◦ 0.1S Longitude: 156◦ 1.7E

Deployment Period: 16 December 1993–26 December 1994

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

1623 60 • • –0.073

ET241 was a slack-line mooring.

Since this mooring had only one Seacat, no stability analysis could be

performed. There were no CTD data available at the Seacat depth.

The 1 m record had 19 days with periods of noisy salinity. These periods

were despiked using linear interpolation. A total of 33 hours of salinity data

were modified (0.4% of the total). Without a rain gauge or adjacent Seacats

on the mooring, it was impossible to distinguish downward (fresher) salinity

spikes due to instrumental error, from real data; no attempt was made to

adjust downward spikes. The largest salinity spike removed was an upward

spike of 0.381 psu.

Quality analysis was performed at PMEL.
48 Freitag et al.

Mooring ET148

Latitude: 0◦ 1.6S Longitude: 160◦ 32.8E

Deployment Period: 9 May 1992–12 March 1993

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

3 787 60 • •• –0.052

5 79160 • • –0.060

10 792 60 • • –0.032

15 793 60 • • –0.003

20 794 60 • • 0.0

30 795 60 • • 0.009

The 3 m Seacat (S/N 787) was lost at sea after this deployment so it does

not have a post-deployment temperature or conductivity calibration. Stability analyses performed after the other Seacats were calibrated suggested

that the 3 m Seacat (S/N 787) was recording salinities which were 0.052 psu

too salty from the beginning of the deployment. Thus, an offset correction

was made. Furthermore, stability analyses showed that the 3 m Seacat (S/N

787) started to drift to extremely fresh values starting on 20August 1992.

The drift appeared to become as large as 0.25 psu by December 1992. This

drift could be due to biological fouling of the conductivity cell, but it is

difficult to model as it is unknown if the drift continued beyond December

1992. Thus, all salinity data recorded after the drift began on 20August

1992 should be considered suspect

The time-series for the Seacat at 5 m (S/N 791) only lasted 73 days (until

19 July 1992) due to a battery connection problem.

The time-series for the Seacat at 10m (S/N 792) lasted for only 120

days (until 6 September 1992) for an unknown reason. The batteries were

still in good condition upon retrieval so it did not appear to be a battery

connection problem. However, a “rollover” problem did induce errors in the

time-series for the 10m Seacat (S/N 792) as 134 unrealistic temperature

and conductivity values were introduced into the record.

Quality analysis was performed at the University of Hawaii.
COARE Seacat Data 49

Mooring ET193

Latitude: 0◦ 1.1S Longitude: 160◦ 33.9E

Deployment Period: 13 March 1993–22 April 1994

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

3 804 10 •• • –0.007

5 803 10 •• • –0.015

10 802 10 • • •• 0.026

15 801 10 • • •• 0.241

20 784 60 • • •• 0.435

30 78160 • • •• 0.217

Intercomparison casts were made for these Seacats prior to this deployment and conductivity offset corrections were applied to the Seacat data in

accordance with the offsets suggested by the intercomparison casts.

The 10m Seacat (S/N 802) was found to be recording a mean density

inversion compared to other Seacats through stability analyses. The mean

inversion seemed to be caused by a drift in the 10m Seacat salinities so a

linear salinity drift correction of 9.1 × 10−5 psu per day was applied from the

beginning of the deployment until 17 December 1993. After 17 December

1993, the 10m Seacat (S/N 802) salinity data became widely varying until

the end of the deployment. The 10m salinity data during this period have

times when they were fresher than the 5 m salinity data and then saltier than

the 20and 30m salinity data. As there are no clues as to how to correct

these data (no near-mooring CTD casts and no well-mixed period analyses

available) the 10m salinity data (Seacat S/N 802) should be considered

highly suspect from 17 December 1993 until the end of the deployment.

A linear salinity drift correction of 0.0045 psu per day was applied to

the 15 m Seacat (S/N 801) from 21 February 1994 until the end of the

deployment as stability analyses showed this sensor to be drifting near the

end of the deployment. After the linear salinity drift correction was applied

to these points, a salinity offset correction was applied to the 15 m Seacat

(S/N 801) as stability analyses and analyses conducted under well-mixed

conditions showed the 15 m data to be too fresh from the beginning of the

deployment until 17 June 1993. Thus, a salinity offset correction of 0.0177

psu (based on 10and 15 m salinity differences) for these points was applied.

After 17 June 1993, the 10and 15 m salinity differences showed another drift

in the 15 m Seacat (S/N 801) salinity data so another linear drift correction

was applied. The linear drift correction was applied directly to the 15 m

salinity data from 17 June 1993 until 5 December 1993 at a rate of –0.0003

psu per day.

Through stability analyses, the 20m Seacat (S/N 784) was found to be

recording salinities which were too fresh due to probable instrument drift.

Thus, a linear salinity drift correction of 0.0062 psu per day was applied to

the 20m Seacat data from 16 February 1994 until the end of the deployment.

Stability analyses showed a drift in the 30m Seacat (S/N 781) salinity
50 Freitag et al.

data so a linear salinity drift correction of 0.0005 psu per day was applied to

the 30m salinity data for the entire deployment. After the linear drift correction for the 30m Seacat (S/N 781) salinity data, the 30m data appeared

to be offset from the 20m Seacat (S/N 784) salinity so an offset correction

of 0.0238 psu from the beginning of the deployment until 22 September 1993

was applied. This correction was based on the median value of the 20and

30 m salinity differences. An additional salinity offset correction of 0.0263

psu was applied to the 30m Seacat (S/N 781) salinity data from the beginning of the deployment until 22 September 1993 as two analyses during

well-mixed periods suggested that the 30m Seacat was recording salinities

which were 0.0263 psu (the mean difference from the two well-mixed periods)

too fresh during this time period.

Quality analysis was performed at the University of Hawaii.
COARE Seacat Data 51

Mooring ET244

Latitude: 5◦ 1.8N Longitude: 165◦ 0.2E

Deployment Period: 28 December 1993–26 April 1995

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

1624 60 • • –0.079

Since this mooring had only one Seacat, no stability analysis could be

performed. A comparison was made to 1 CTD cast and no clear evidence of

Seacat bias was found.

There were several periods in the ET244 1 m record where high frequency

variability in the salinity record appeared to be related to high frequency

variability in the temperature record. However, without rain data or additional salinity records, it was impossible to dismiss downward salinity spikes

as noise, and consequently, very little despiking was performed. Five days

of data were despiked by linear interpolation or by applying a 5-hour long

(five-point) Hanning filter. Only 0.3% of the salinity data were modified

by despiking procedures. The largest salinity spike removed was an upward

spike of 0.310 psu.

Quality analysis was performed at PMEL.
52 Freitag et al.

Mooring ET243

Latitude: 2◦ 0.2N Longitude: 164◦ 59.2E

Deployment Period: 26 December 1993–22 December 1994

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

1968 30 • •• –0.058

10 969 30 • •• –0.144

30 1112 30 • • –0.081

37 1113 30 • • –0.092

511243 30 • • –0.102

Both temperature and salinity data from the 1 m instrument were bad

after 11 September 1994.

The 1 m record had 54 days with periods of noisy salinity. These periods

were despiked by linear interpolation, substitution of the 10m salinity data,

or application of a seven-point (3.5 hour) Hanning filter. A total of 258

hours of data were modified (4% of the record). The largest salinity spike

removed was an upward spike of 0.557 psu.

The 10m record had a period of noisy temperature that extended from

15 March 1994 to 22 March 1994. Temperature and salinity data for that

time period are flagged.

There was a prolonged period of pronounced instability between the 1 m

and 10m instruments which began in mid March and persisted to mid

July. Comparison with the 30m and 37 m instruments indicated the 10m

instrument had a bias during the mid-March to mid-July time period. The

bias was modeled as a second-order polynomial which crossed 0.0 psu on 6

March 1994 and again on 22 July 1994 and which attained a maximum value

of 0.025 psu from 8 to 20 May 1994. The 10 m salinity data between 18:00

6 March 1994 and 17:00 22 July 1994 were adjusted by the modeled bias.

After mid-July, bias was apparent in the 1 m record (see below). Comparing

the 10m data to the deeper data, there is some suggestion that the 10m

values after late August are too fresh, but without 1 m data the possibility

of real stratification can not be eliminated.

After mid July, an extremely pronounced and persistent stratification

developed between 1 m and 10m and lasted until a few days before the

1 m salinity measurements failed. Comparison with the 30m and 37 m data

indicated that the 1 m record had a large bias during this time period. The

bias of the 1 m data was calculated for the periods when the water column

between 10m and 37 m was weakly stratified and then modeled as a second

order polynomial. The modeled bias is 0.0on 20July 94 and reached a

maximum value of 0.066 on 16 August 1994. The 1 m salinity between 3:00

20 July 1994 through 05:00 11 September 1994 was adjusted by the modeled

bias. This is not considered a high quality correction.

The 51 m time series ended about 1 month before the mooring was

recovered, but no failure mode was determined for the instrument. Between

37 m (30m) and 51 m there were several long-lasting, instability episodes,
COARE Seacat Data 53

at times reaching magnitudes >0 .0 5 kg m−3. Seven (five) of the instability

events lasted longer than 12 hours. In two cases the events lasted longer than

a day. This may indicate a bias in the 51 m record. No attempt was made

to correct the 51 m record, because the 51 m Seacat was infrequently in the

mixed layer, and because there were no instruments below for comparison.

The problematic instability events begin on 21 July 94. All 51 m data after

this point should be considered questionable.

Quality analysis was performed at PMEL.
54 Freitag et al.

Mooring FU3

Latitude: 0◦ 0.6N Longitude: 165◦ 0.8E

Deployment Period: 7 August 1991–7 February 1992

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

3 738 30 • • –0.020

11 174 30 • • •• 0.054

30 168 30 • • •• –0.025

51171 30 • • –0.032

75 386 30 • • –0.037

101 26 15 • • 0.013

151 27 15 • • –0.008

201172 30 • • 0.010

The mooring parted between 75 m and 100 m on 29 January 1992. The

lower portion of the mooring dropped to the ocean bottom, but was later

recovered. The upper portion drifted for 9 days, after which it was also

recovered.

Both the 11 m and 30m Seacat salinity time series drifted, relative to

the 3 m and 51 m instruments, beginning on 5 November 1991. Because the

drift rates were complicated, 4th order polynomials were fit for November–

December, and for January to recovery. Three CTDs taken on 26 January

1992 confirmed that the adjustments made to the 11 m and 30m Seacats

were necessary.

Quality analysis was performed at PMEL.
COARE Seacat Data 55

Mooring FU5

Latitude: 0◦ 1.7N Longitude: 164◦ 51.5E

Deployment Period: 8 February 1992–12 August 1992

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

1874 15 • • –0.021

Mooring FU5 was vandalized. Seacats from 11 m to 151 m were lost and

the Seacat at 201 m was damaged and returned no data.

The 1 m record had 76 days with periods of noisy salinity. These periods

were despiked by either linear interpolation or smoothing. A total of 294

hours of salinity data were modified (8% of total). The largest salinity spike

removed was an upward spike of 0.547 psu.

Quality analysis was performed at PMEL.
56 Freitag et al.

Mooring FU7

Latitude: 0◦ 0.6N Longitude: 164◦ 59.2E

Deployment Period: 24 August 1992–20 November 1992

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

1876 30 • • –0.026

11 743 30 • •• –0.010

30 27 30 • • N/A

51738 30 • • 0.033

75 742 30 • • –0.009

101 168 30 • • 0.002

151 26 30 • • 0.002

201739 30 • • 0.008

Mooring FU7 parted above the 30m Seacat on 5 November 1992. The

drifting surface portion of the mooring was recovered on 20November.

The 1 m record had 7 days with periods of noisy salinity. These periods

were despiked by either linear interpolation, or substitution of the 11 m

value. A total of 46 hours of salinity data were modified (2.2% of total).

The largest salinity spike removed was a downward spike of 0.358 psu.

The 11 m Seacat appeared to have a salinity bias when compared to 1 m

and 30m instruments, but corrections based on the 1 m and 30m time

series appeared to overcorrect the bias relative to 1 m. This may have been

because of problems with the 30m instrument (see below). A small, linear

(–.012 psu to 0.000 psu) correction was applied, based on the 1 m and 11

m differences. Three CTDs taken at deployment confirmed the need for the

correction at that time.

The 30m Seacat conductivity cell was found to be cracked when postcalibrated. Only pre-deployment calibrations were used since the time when

the cell cracked is unknown. This time series is probably less accurate than

most time series in this data base.

Quality analysis was performed at PMEL.
COARE Seacat Data 57

Mooring FU7b

Latitude: 0◦ 0.0N Longitude: 165◦ 0.4E

Deployment Period: 13 December 1992–15 March 1993

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

1876 30 • • –0.053

11 743 30 • •• –0.014

30 1212 30 • • –0.005

511243 30 • • –0.021

75 1211 30 • • –0.017

101 1245 30 • • 0.007

151 1242 30 • • 0.009

2011244 30 • • 0.016

Mooring FU7b parted deep in the mooring line and drifted for its entire

deployment period.

Seacats at 1 m and 11 m were recovered from mooring FU7 and deployed

without calibration between use.

The 1 m record had 5 days with periods of noisy salinity. These periods

were despiked by linear interpolation in time. A total of 34 hours of salinity

data were modified (1.5% of total). The largest salinity spike removed was

an upward spike of 0.385 psu.

A small (–0.007 psu) salinity correction was computed and applied to

the 11 m Seacat based on the 1 m and 30m time series.

No CTD’s were available for this mooring.

Quality analysis was performed at PMEL.
58 Freitag et al.

Mooring FU9

Latitude: 0◦ 0.1N Longitude: 165◦ 0.1E

Deployment Period: 23 March 1993–24 December 1993

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

1742 30 • • –0.040

11 26 30 • •• –0.052

30 173 30 • • –0.017

51 1211 30 • • –0.069

75 1212 30 • • –0.021

101 738 30 • • –0.029

151 739 30 • • 0.006

201168 30 • • –0.012

The 1 m record had 35 days with periods of noisy salinity. These periods

were despiked by linear interpolation in time, or substitution of the 11 m

salinity time series. A total of 271 hours of salinity data were modified (4.1%

of total). The largest salinity spike removed was an upward spike of 0.481

psu.

The 1 m and 11 m salinity and sigma-theta records exhibited several

problems relative to one another. From the beginning of the deployment

until mid-September there was a trend from slight instability to pronounced

stratification. From late September to the end of the deployment there

were two prolonged instability episodes separated by a period of pronounced

stratification from mid-October to mid-March. Comparison of the 1 m and

11 m records to deeper records indicated that the 11 m instrument was the

instrument in error. The bias was calculated for times between 23 March

and 23 August 1993 when the well-mixed criterion was met. The bias was

modeled by a second-order polynomial which crossed 0.0 on 12 April 1993,

and reached a maximum of 0.0139 psu on 10 August 1993.

For the latter half of the 11 m record, a second bias model was required.

Bias was calculated for weakly stratified times between 15 July and 24 December 1993. The bias was modeled by a second-order polynomial, which

crossed the first bias model on 10August 1993, reached a maximum value of

0.032 psu on 24 September 1993, crossed 0.0 on 22 November 1993, and was

–0.0440 psu at the end of the deployment. The 11 m record was adjusted by

the first polynomial between 23 March and 10August 1993, and adjusted

by the second polynomial between 10August and 24 December 1993. The

11 m data after 10August 1993 should be used with caution; the adjustment for that time period is not considered a high-quality correction, and

it introduces long-lasting (up to 17 hours), large (as much as 0.04 kg m−3)

instabilities between 1 m and 11 m during the period of late September to

mid-October.

The 30m salinity exhibited a large, sharp jump (∼0.4 psu) on 29 June

1993. Post-recovery calibration of the instrument did not reveal any problem with the instrument and the drift of the conductivity cell was typical.

The salinity data were judged to be uncorrectable after the jump, and were

omitted from the 30m record.

Quality analysis was performed at PMEL.
COARE Seacat Data 59

Mooring TC1

Latitude: 0◦ 0.1N Longitude: 165◦ 1.7E

Deployment Period: 25 December 1993–17 December 1994

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

1876 60 • • 0.000

Mooring TC1 was not recovered, but during a visit to the site on 15

April 1994, data up to that point were recovered from the 1 m Seacat. Since

no instruments were recovered, no post-deployment calibrations exist. No

data were recovered from subsurface Seacats, so no stability analysis could

be performed. There were no CTD data available at the 1 m Seacat depth.

The TC1 1 m record had 9 days with periods of noisy salinity. These

periods were despiked by linear interpolation. A total of 32 hours of data

were modified (1.2% of the record). The largest salinity spike removed was

an upward spike of 0.327 psu. One point (1 hour) in the temperature record

was also despiked by linear interpolation.

Quality analysis was performed at PMEL.
60 Freitag et al.

Mooring ET96

Latitude: 1◦ 58.8S Longitude: 164◦ 52.8E

Deployment Period: 31 March 1991–9 February 1992

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

10 622 30 • • –0.039

30 623 30 • • –0.061

102 624 30 • • –0.041

There are no obvious instability problems between Seacats. There were

five CTD casts taken at this site during the deployment. No significant

differences between CTD values and Seacat data could be identified.

The 10m salinity and temperature records end on 7 October 1991 and

had a 9-hour period of noisy values on 21 June 1991. All 10m data for that

time period are flagged. There were also a few short gaps (32 points in all,

no gap longer than 6 hours) which were filled by linear interpolation.

ORSTOM performed the initial quality analysis on these instruments.

PMEL later applied their own quality analysis as a consistency check, flagged

the noisy period on 21 June 1991, and filled the short gaps.
COARE Seacat Data 61

Mooring ET129

Latitude: 1◦ 59.8S Longitude: 164◦ 41.9E

Deployment Period: 9 February 1992–25 August 1992

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

10 965 10 • • –0.060

30 966 10 • • –0.028

101 967 10 • • –0.018

The 10m salinity and temperature records do not begin until 12 February

1992. There were no obvious instability problems between Seacat depths.

No CTD casts were taken at this site during this deployment.

ORSTOM performed the initial quality analysis on these instruments.

PMEL later applied their own quality analysis as a consistency check.
62 Freitag et al.

Mooring ET161

Latitude: 1◦ 59.4S Longitude: 164◦ 24.9E

Deployment Period: 26 August 1992–22 March 1993

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

10 1106 5 • • –0.038

37 1107 5 • • –0.054

62 1108 5 • • –0.057

87 1109 5 • • –0.038

112 1111 5 • • –0.004

137 1110 5 • • –0.004

175 968 5 • • 0.013

225 622 20 • • 0.010

275 624 20 • • 0.011

400 1114 10 • • 0.000

750 1115 10 • • 0.006

The 400 m salinity and temperature records end on 23 September 1992;

the 137 m records end on 6 December 1992.

There were no obvious instability problems between Seacat depths. Only

one CTD cast was taken at this site during this deployment. No significant

Seacat bias could be identified by comparison with the CTD data.

ORSTOM performed the initial quality analysis on these instruments.

PMEL later applied their own quality analysis as a consistency check.
COARE Seacat Data 63

Mooring ET197

Latitude: 1◦ 57.5S Longitude: 164◦ 22.7E

Deployment Period: 24 March 1993–16 April 1994

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

10 799 10 • • –0.011

37 793 60 ••• –0.032

62 794 60 ••• –0.079

Linear conductivity drift corrections of –4 × 10−6, –1 × 10−6 and –1.8

× 10−5 S/m per day were applied to the Seacats at 10, 37, and 62 m respectively (S/N 799, S/N 793, and S/N 794). These corrections were established

by comparisons of pre-deployment and post-deployment conductivity calibrations.

Conductivity offset corrections of 0.004219 and 0.004379 S/m were applied to the 37 and 62 m Seacats (S/N 793 and S/N 794). These offsets were

determined through both CTD intercomparison and near-mooring casts.

Quality analysis was performed at the University of Hawaii.
64 Freitag et al.

Mooring ET149

Latitude: 0◦ 2.5N Longitude: 169◦ 59.5E

Deployment Period: 12 May 1992–17 March 1993

Temperature Salinity

Depth ∆T ∆S

(m) S/N (min) Pre Pre/Post Pre Pre/Post Stability CTD (psu)

3 783 60 ••• –0.083

A near-mooring CTD cast conducted at the end of this deployment suggested that a –1.87 × 10−5 S/m per day linear conductivity drift correction

(established by comparison of pre-deployment and post-deployment conductivity calibrations) be used to allow salinity data from Seacat S/N 783 to

better match the CTD.

Quality analysis was performed at the University of Hawaii.
COARE Seacat Data 65

Appendix B

Time series plots of daily averaged salinity and density (sigma-theta). Plots

are organized by site from west (0◦ 154◦E) to east (0◦ 170◦E) and from north

to south along longitudes with multiple mooring sites. Most sites have one

page with all salinity time series and one page with all density time series.

Sites which had instruments at only one or two depths have both salinity and

density plotted on the same page. Sites which had many instruments have

two pages for salinity and two pages for density. Plotting scales have been

chosen at each site which clearly display the data for that site, but scales

may change from site to site. Most plots have one panel per instrument

depth. At some sites, instrument depths changed from one mooring to the

next. If time series from more than one depth are plotted in one panel, they

are identified by greytone differences.
66 Freitag et al.
COARE Seacat Data 67
68 Freitag et al.
COARE Seacat Data 69
70 Freitag et al.
COARE Seacat Data 71
72 Freitag et al.
COARE Seacat Data 73
74 Freitag et al.
COARE Seacat Data 75
76 Freitag et al.
COARE Seacat Data 77
78 Freitag et al.
COARE Seacat Data 79
80 Freitag et al.
COARE Seacat Data 81
82 Freitag et al.
COARE Seacat Data 83
84 Freitag et al.
COARE Seacat Data 85
86 Freitag et al.
COARE Seacat Data 87
88 Freitag et al.
COARE Seacat Data 89