NOAA TECHNICAL MEMORANDUM ERL PMEL-108

TEMPERATURE ERRORS IN TAO DATA INDUCED BY MOORING MOTION

M.E. McCarty

L.J. Mangum

M.J. McPhaden

Pacific Marine Environmental Laboratory

7600 Sand Point Way N.E.

Seattle, WA 98115-0070

February 1997

Contribution No. 1839 from NOAA/Pacific Marine Environmental Laboratory
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NOTICE

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by 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. 1839 from NOAA/Pacific Marine Environmental Laboratory

__________________________________________________

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

Springfield, VA 22161
iii

CONTENTS

PAGE

ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

2. INSTRUMENTATION AND DATA PROCESSING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

3. DEPLOYMENT SELECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

4. TEMPERATURE REMAPPING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

4.1 General method of remapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

4.2 Adjusting pressure records for depth offsets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

4.3 Other pressure corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

4.4 Pressure record variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

5. RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

5.1 Temperature differences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

5.2 Depth-averaged temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

5.3 Displacement of 20(C isotherm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

5.4 Dynamic heights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

5.5 Changing the assumption about pressure sensor errors . . . . . . . . . . . . . . . . . . . . . . 11 

6. CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

7. ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

8. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

APPENDIX A: 0(, 80.5(E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

APPENDIX B: 5(N, 156(E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 

APPENDIX C: 0(, 156(E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 

APPENDIX D: 5(S, 156(E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 

APPENDIX E: 5(N, 155(W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 

APPENDIX F: 0(, 155(W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 

APPENDIX G: 0(, 125(W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 

APPENDIX H: 8(N, 110(W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 

APPENDIX I: 2(N, 110(W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 
Temperature Errors in TAO Data Induced by Mooring Motion

Marguerite E. McCarty, Linda J. Mangum, and Michael J. McPhaden

ABSTRACT. Moored subsurface temperature measurements from the Tropical Atmosphere Ocean

(TAO) Array are subject to errors resulting from vertical motion of the instrumented mooring line. The

magnitude of the errors depends on both environmental conditions and mooring design. This report

describes a method for estimating these errors using subsurface pressure data to determine vertical

excursions of the mooring line. For taut-line moorings examined, typical mooring motion-induced

temperature errors are largest in the upper thermocline, with root mean square (RMS) errors of

0.15(–0.45(C. Slack-line moorings typically have larger RMS errors (0.3(–1.1(C), in some cases with

maxima at the lowest instrumented depth. These errors are correctable using processing procedures

developed in this report.

1. INTRODUCTION

The Tropical Atmosphere Ocean (TAO) Array comprises about 70 moored buoys which

measure meteorological and oceanographic variables in the Equatorial Pacific. The moorings extend

from 137(E to 95(W, roughly between 8(N and 8(S. The TAO Array was established by the

Tropical Ocean Global Atmosphere (TOGA) Program as part of a 10-year (1985–1994) effort to

study climate variability on seasonal to interannual time scales, with the particular aim of increasing

understanding of the El Niño-Southern Oscillation phenomenon (McPhaden, 1993, 1995).

Development of the buoys and the array began under the Equatorial Pacific Ocean Climate Studies

(EPOCS) program and other predecessors of TOGA. The maintenance of the array since 1994 has

been assumed by the Global Ocean-Atmosphere-Land System (GOALS) component of the Climate

Variability and Prediction (CLIVAR) program; TAO is also viewed as a contribution to the Global

Climate Observing System (GCOS) and the Global Ocean Observing System (GOOS).

Most of the moorings in the TAO array are Autonomous Temperature Line Acquisition

System (ATLAS) moorings which measure winds, air temperature, relative humidity, sea-surface

temperature, subsurface temperature at 10 depths down to 500 m, and subsurface pressure at two

depths. These moorings have been deployed primarily in a taut-line configuration. However, at two

sites slack-line moorings have been maintained, one in relatively shallow water (5(S, 156(E) and

one in the strong and highly sheared Equatorial Undercurrent (0(, 125(W). 

Mooring line motion causes vertical displacement of the temperature sensors and introduces

error into temperature records assumed to be from fixed depths. Although most of the moorings are

taut-line moorings designed to minimize vertical excursions, the mooring will respond to surface

wind forcing and changes in the surface and subsurface currents. Typical watch circles for the

surface toroid on taut-line moorings are 2–4 km in waters near 4000 m deep. Further details on

mooring dynamics and design can be found in Berteaux (1976).
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Presented in this report is a procedure for using pressure records to estimate the error induced

in the temperature records by depth excursions of the mooring line. The method was applied to data

from seven taut-line ATLAS moorings and two slack-line moorings. The results are described and

discussed below.

2. INSTRUMENTATION AND DATA PROCESSING

The upper part of an ATLAS mooring consists of a toroidal float on which is mounted an

instrumentation tower. Seven hundred meters of wire rope connect to a bridle bolted to the underside

of the float. On a taut-line mooring, 3⁄4" nylon line makes up the majority of the remaining length

to the anchor. The length of the nylon line, which stretches under tension, is adjusted to yield a

scope for the entire mooring of about 0.985. Slack-line moorings are similar, but contain a section

of floatable polyolifin line just above the anchor and have a scope of around 1.35.

Temperature and pressure sensors are enclosed in pods incorporated into the temperature

cable, a polyurethane-jacketed, double-armored, three-conductor cable (Hayes et al., 1991). The

temperature cable runs parallel to the wire rope and is clamped to it; they move in tandem. ATLAS

temperature sensors are Yellow Springs Instrument Co., Inc. (YSI) model 46006 thermistors. The

temperature I/O boards convert thermistor resistence to voltage, voltage to frequency, and frequency

to counts, and output the counts. The combined instrumental error for SST measurements is

0.030(C. The combined instrumental error for subsurface temperature measurements is about 0.1(C

(Freitag et al., 1994). SST sensors are fixed on the mooring bridle at 1 m depth. Nominal depths for

subsurface temperature sensors on eastern Pacific (95(–140(W) buoys are 20, 40, 60, 80, 100, 120,

140, 180, 300, and 500 m. For western and central Pacific (137(E–155(W) moorings the nominal

temperature sensor depths are 25, 50, 75, 100, 125, 150, 200, 250, 300, and 500 m. 

Pressure measurements are made by Senso-Metrics ported semiconductor strain gauge

pressure transducers (model SP91CFD). The sensor range is 0 to 1000 psi (0 to 689 db; 1 psi equals

0.6895 db); no manufacturer’s specifications are available for long term stability estimates. Thermal

sensitivity for these sensors is approximately 0.02% of full scale per 1( Fahrenheit (or 0.25 db per

(C). To minimize thermal sensitivity during calibrations, all 500-m (300-m) pressure modules are

calibrated at 8(C (11(C). 

Pressure calibrations were performed at PMEL using a Ruska dead-weight tester and a

Calumet Industries Environator van. Sensors were placed in the environmental chamber and allowed

to equilibrate at the calibration temperature. Pressures were applied by adding plates to the

dead-weight tester for different pressures levels for first increasing pressure and then decreasing

pressure at five levels from 138 to 586 db. At least 4 samples were recorded and averaged at each

pressure level. A simple linear fit was then used to compute the regression coefficients for each

sensor. Sensors with maximum residuals for the linear fit less than 0.2 db were deployed in the field.

After recovery of the mooring, the pressure sensors were recalibrated using the above procedure.

Comparisons of pre- and post-deployment calibrations for individual sensors showed relatively large
3

differences, typically averaging 2 to 4 db for a 1-year deployment. In some cases, changes up to 15

db were observed.

The ATLAS data logger/transmitter in the buoy instrumentation tower averages temperature

and pressure values over 24 hours and telemeters them to shore via the Argos system (Hayes et al.,

1991). In most of the deployments selected for analysis in this report, pre-deployment calibration

values have been applied to the data. However, in two cases, the pressure records were recalculated

using the post-deployment calibrations. These will be noted below.

3. DEPLOYMENT SELECTION

The deployments were selected to cover a variety of geographic positions, oceanographic

conditions, water depths, and mooring scopes. These included moorings on and off the equator from

the eastern, western, and central Pacific; deployments from El Niño and non-El Niño periods;

relatively shallow and deep-water moorings; and taut-line and slack-line moorings. A single

mooring deployed in the central equatorial Indian Ocean in 1993 is also included in this report.

Deployments were selected for inclusion in this analysis only if they had complete or nearcomplete data records for a minimum of 6 months. The deployments necessarily had complete

pressure records for the time period. All selected moorings had complete subsurface temperature

records. The SST records, which are not affected by vertical excursions of the mooring line, were

incomplete in two cases; ET342 (0(, 156(E) had 6 days of missing values scattered through the first

half of the record and ET177 (8(N, 110(W) had a 3-month gap. A complete wind record for the

time period was considered desirable, but not necessary.

In addition, an attempt was made to choose only deployments for which the pressure sensors

displayed little or no drift and for which the buoy itself didn’t move during the deployment period.

It was possible to meet these two criteria in all but one case. One mooring, ET214, violated both

criteria, but it was used anyway because it was the only mooring in the Indian Ocean. ET214

showed an obvious drift in the 300-m pressure record (Fig. 1). The drift was corrected by a method

described below. Also, from late July 1993 to early January 1994 ET214 moved from its initial

position at 0.103(N, 80.54(E to 0.076(S, 81.142(E. Most of the movement occurred in one stage

in early November 1993 during a period of strong westerly winds (Fig. 1). The move in November

appeared to have ended in water shallower than the initial deployment depth. Afterwards, the

pressure records showed shallower values, which is consistent with a slacker line (Fig. 1).

Table 1 lists the 9 moorings selected, along with their positions, the beginning and ending

dates for the portion of the records used in this analysis, the scopes, and the water depths.

4. TEMPERATURE REMAPPING

4.1 General method of remapping

The first step in remapping the temperature data was to estimate a depth for each daily value

from each sensor. The top of the temperature cable was assumed to be at 1 m depth (actual depth
4

Table 1. Position, mooring designator, beginning and end dates of data used, mooring scope, and

water depth for each mooring selected for this analysis.

Position Mooring Begin End Scope Depth

(lat, long) designator (m)

0(, 80.5(E ET214 25 Jul 93 07 Jan 94 0.986* 4662*

5(N, 156(E ET344 22 Jul 95 23 Apr 96 0.985 3607

0(, 156(E ET342 20 Jul 95 23 Apr 96 0.984 1973

5(S, 156(E ET310 28 Dec 94 24 Apr 96 1.349 1500

5(N, 155(W ET297 24 Oct 94 28 Dec 95 0.985 4600

0(, 155(W ET296 22 Oct 94 26 Dec 95 0.986 4676

0(, 125(W ET288 09 Sep 94 30 May 95 1.357 4651

8(N, 110(W ET177 20 Oct 92 11 Sep 93 0.988 4250

2(N, 110(W ET178 05 Nov 92 08 Jul 93 0.985 3819

* Mooring moved. Scope and depth listed are the values at the deployment site. Final scope and depth

are unknown.

Fig. 1. Time series plots of 300-m pressures, 500-m pressures, latitude, and longitude from the Indian Ocean mooring,

ET214, nominally located at 0(, 80.5(E.
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is 1.2 m); the cable was assumed to form a straight line 299 m long between the 1-m fixed point and

the depth indicated by the 300-m pressure sensor, and a straight line 200 m long between the 300-m

and 500-m pressure sensor depth values. The pressure data were converted to depths using the

method of Saunders and Fofonoff (1976). The temperature sensor depths were estimated by linear

interpolation between the 1-m and the two subsurface pressure sensors. In some cases, before being

used in the temperature remapping scheme, the pressure records were subjected to adjustments

described below in sections 4.2 and 4.3.

After estimating a set of sensor depths for a single day, the temperature values for that day

were remapped by linear interpolation to the standard ATLAS depths for that mooring. Instead of

extrapolating adjusted sensor depths to 500 m, a 450-m value was chosen as the deepest standard

level in the remapped data. An uncorrected 450-m temperature record was also calculated by linear

interpolation between uncorrected records at the nominal depths of 300 and 500 m.

4.2 Adjusting pressure records for depth offsets

The maximum possible depth of an instrument is determined by the length of the mooring line

between the instrument and the point where the line is fixed to the bottom of the bridle, plus the

distance from the bottom of the bridal to the surface. When the moorings are deployed the

instruments are attached to the wire rope at or near marked, premeasured points. If the instrument

is placed exactly at the premeasured point and there was no error in measuring and marking the line,

the set of maximum possible depths for a mooring will be identical to the nominal depths as listed

in section 2. The wire rope was measured by a Reelomatic mechanical counter manufactured by

Olympic Instruments (Vashon Island, Washington). The in-house estimate of the error for this

measurement is ±1% of the sensor depth (Hugh Milburn, personal communication). In addition,

when the instruments are deployed, it can be difficult to mount them exactly at the marked points,

introducing additional error, though typically less than ±1 m.

For the purposes of this analysis, the possible errors in measurement and instrument placement

were ignored because of concerns about the potential for even larger pressure sensor errors. It was

assumed that the maximum depth that an instrument could possibly reach was equal to the nominal

depth of the instrument. Thus, in the interpolation scheme described above, the line lengths used are

determined by the nominal depths of the sensors. The alternative approach of assuming that the

pressure records were correct, but the line measurement was in error, is discussed below in section

5.5.

When depth values were calculated from the pressure sensor readings, some deployments had

depth values deeper than the nominal depths of the sensors. For these deployments, the calculated

depth record was examined and the deepest value was noted. This value is called the maximum

measured depth for the sensor. The expected effect of remapping temperature records is that the

remapped temperatures will be colder than the unremapped temperatures, due to the instruments

being displaced upward by line motion into warmer water. In cases where the maximum measured
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Table 2. Sites that required pressure records to be adjusted and the adjustment

required for each record. Pressure records were adjusted such that when depths

were calculated from the pressure values, the maximum depth was equal to the

nominal depth of the sensor. Adjustments needed are shown in db for the 300-m

pressure record (p300) and the 500-m pressure record (p500).

Position Mooring p300 adjustment p500 adjustment

(lat, long) designator (db) (db)

5(N, 156(E ET344 –3.61 N/A

5(S, 156(E ET310 –0.21 –3.21

5(N, 155(W ET297 –3.21 –9.21

0(, 155(W ET296 –0.32 –0.83

0(, 125(W ET288 –2.72 N/A _____ _____

Mean –2.01 –4.42

Std. Dev. 1.63 4.32

depth indicated by a pressure sensor was deeper than the nominal depth, the remapped temperatures

could be warmer than the original temperatures for much of the water column, masking the effect

of the upward excursions.

In order to see the effects of the upward displacements for those moorings where the

maximum measured depth was deeper than the nominal depth, pressure records were adjusted by

adding a constant offset that yielded a corrected maximum measured depth equal to the nominal

depth. Five of the chosen deployments required correction of either the 300-m pressure record or

both the 300- and 500-m pressure records. Corrections ranged from –0.21 to –9.21 db (Table 2).

The fact that pressure records are obviously offset downward for some deployments implies

that for other deployments the pressure records may be offset upward by similar amounts. There is

no way to distinguish between a record that is offset upward and a record from an instrument that

never reached its maximum possible depth because it was continually displaced upward by stresses

on the mooring line.

4.3 Other pressure corrections

The 300-m pressure record from the Indian Ocean mooring, ET214, showed an obvious drift

(Fig. 1). An attempt was made to correct the record for the apparent change in sensor calibration.

An estimate of the trend was made for the first 58 days of the record, where the pressure record was

a fairly straight line. The trend was estimated by linear least squares regression. That trend was then

removed from the entire record length, and the mean of the record was adjusted so that the mean of

the first week of corrected data equaled the mean of the first week of uncorrected data. The size of

the correction varies from negligible in July 1993 to about 5 db (deeper) in January 1994 (Fig. 2).
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Fig. 2. Time series plot of the 300-m pressure records from 0(, 80.5(E. Dotted line is the original pressure record. Solid

line is the pressure record after correcting for the estimated drift of the pressure sensor.

For the deployments at 0(, 125(W (ET288) and at 0(, 155(W (ET296), the post-deployment

calibrations of the pressure sensors were judged to be of higher quality than the pre-deployment

calibrations. The pressure records were recalculated using the post-deployment calibration

coefficients.

4.4 Pressure record variability

After adjustment, the 300-m pressure records for the chosen deployments typically exhibit

standard deviations of about 1–3 db (Table 3). The standard deviations of the adjusted 500-m

pressure records are about twice that. The deployment at 8(N, 110(W (ET177) shows the smallest

variability in the pressure records. The standard deviation of the ET177 300-m pressure record is

0.8 db; for the 500-m record it is 1.5 db. At 0(, 125(W (ET288) the combination of a slack-line

mooring and a fast, highly-sheared current regime led to large excursions of the 300-m and 500-m

sensors. The standard deviation for the 0(, 125(W 300-m pressure record is 8.1 db; for the 500-m

record it is 20.0 db. 

Statistics for the 0(, 80.5(E pressure records were calculated in two parts: 25 July 1993 to 7

November 1993 (before the buoy moved) and 12 November 1993 to 7 January 1994 (after it moved).

Prior to the move, the 300-m pressure sensor data have a mean of 297 db; the mean after the

move is shallower, 292 db. Similarly, before the move, the 500-m pressure sensor data have a mean

value of 493 db; the mean for the data after the move is 481 db. 

The remapped temperature, adjusted pressure, and wind data for each deployment are

presented in the appendices.

5. RESULTS

5.1 Temperature differences

In order to examine the effect of remapping the temperature records, a temperature difference

field was calculated for each deployment by subtracting the remapped temperature records from the
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Table 3. 300-m pressure mean and standard deviation; 500-m pressure

mean and standard deviation. Pressure records were subjected to

adjustments described in the text. Statistics for the mooring at 0(, 80.5(E

are divided into two parts: (a) before the move into shallower water, 25

July 1993 to 7 November 1993; (b) after the move, 12 November 1993 to

7 January 1994.

300-m pressure 500-m pressure

Position Mean Std. dev. Mean Std. dev.

(lat, long) (db) (db) (db) (db)

0(, 80.5(E (a) 297 2.5 493 5.1

0(, 80.5(E (b) 292 2.7 481 4.5

5(N, 156(E 300 1.3 499 2.7

0(, 156(E 298 2.0 497 3.6

5(S, 156(E 299 3.6 489 11.2

5(N, 155(W 300 1.3 500 2.7

0(, 155(W 300 1.1 499 2.8

0(, 125(W 289 8.1 467 20.0

8(N, 110(W 300 0.8 498 1.5

2(N, 110(W 298 2.7 494 5.6

original records. Contour plots of the temperature difference fields, profiles of the mean and

standard deviations of the difference records, and profiles of the mean and standard deviations of

the vertical displacements are presented in the appendices. The plots show that for all the moorings

except the one at 0(, 125(W, the maximum temperature difference occurs in the upper thermocline.

A typical RMS value for the temperature differences from the record showing the maximum effect

of remapping is in the 0.15(–0.3(C range; typical mean and standard deviation values for the same

record are in the 0.1(–0.25(C range (Table 4). For points above and below the thermocline the

temperature difference records generally have RMS differences of around 0.1(C or less, though a

few show secondary maxima of around 0.2(C at the interpolated 450-m depth, where the line

experiences a greater range of motion.

Two notable outliers are the slack-line mooring at 0(, 125(W and the Indian Ocean mooring

at 0(, 80.5(E, which apparently converted itself to a slack-line mooring by moving to shallower

water. The Indian Ocean mooring has the record with maximum temperature differences in the

thermocline, but with an RMS value of 0.45(C prior to the move and 1.1(C after the move. The

mooring at 0(, 125(W showed the maximum temperature difference at the (interpolated) depth of

450 m. At this site the 500-m sensors experienced vertical displacements with a mean of about 35 m.

The large vertical displacement through a gentle temperature gradient yielded greater temperature

differences for the interpolated 450-m record than the smaller displacements through the

thermocline. The RMS temperature difference at 450 m was 0.66(C; a secondary maximum of

0.51(C occurred in the thermocline at 120 m. It is interesting to note that the other slack-line
9

Table 4. For each mooring, the nominal depth at which mean T (original

temperature minus remapped temperature) is greatest, and the mean, standard

deviation, and RMS T for that depth. Statistics for the 0(, 80.5(E mooring are

divided into 2 parts: (a) before the move into shallower water, 25 July 1993 to 7

November 1993, and (b) after the move, 12 November 1993 to 7 January 1994.

Position Depth of max Max mean Std. dev. RMS

(lat, long) Mean T (m) T ((C) T((C) T((C)

0(, 80.5(E(a) 100 0.353 0.284 0.453

0(, 80.5(E(b) 120 0.946 0.552 1.110

5(N, 156(E 125 0.119 0.096 0.153

0(, 156(E 125 0.219 0.167 0.275

5(S, 156(E 200 0.247 0.241 0.345

5(N, 155(W 120 0.206 0.214 0.297

0(, 155(W 120 0.144 0.093 0.172

0(, 125(W 450 0.571 0.321 0.655

8(N, 110(W 60 0.141 0.072 0.158

2(N, 110(W 60 0.168 0.186 0.250

mooring, ET310, at 5(S, 156(E, sited in a less energetic current regime than the 0(, 125(W

mooring, had a maximum RMS temperature difference of 0.34(C in the thermocline and displayed

a secondary maximum of 0.27(C at 450 m. 

5.2 Depth-averaged temperatures

Depth averages of both the original and remapped temperature records were calculated

between 0 and 450 m to assess the effect of remapping over the whole of the instrumented depth.

The depth averages for each mooring are presented in the appendices as overplotted time series and

as scatter plots. For the taut-line moorings, typical RMS differences between depth averages from

remapped and unremapped data are 0.05 to 0.1(C. The slack-line moorings, including that at 0(,

80.5(E after the move, show larger RMS differences, with the one at 0(, 125(W showing an RMS

difference of 0.31(C. (Table 5).

5.3 Displacement of 20(C isotherm

The effect of remapping the temperature records can also be examined in terms of isotherm

displacement. For each mooring, daily values for the depth of the 20(C isotherm were calculated

by linear interpolation from both the unremapped data and the remapped data. The RMS

displacements for taut-line moorings range from 0.4 to 2.0 m. All but one of the taut-line moorings

have RMS displacements of 1.0 m or less. The RMS displacements for the slack-line moorings

range from 2.9 to 3.8 m (Table 6). Time series plots and scatter plots of the isotherm depths can be

found in the appendices.
10

Table 5. T mean, standard deviation and RMS where T represents

the depth-averaged original temperatures minus the depth-averaged

remapped temperatures. Depth averages were calculated between 0 and

450 m. Statistics for the 0(, 80.5(E mooring are divided into 2 parts: (a)

before the move into shallower water, 25 July 1993 to 7 November

1993, and (b) after the move, 12 November 1993 to 7 January 1994.

Position T T T

(lat, long) Mean ((C) Std. dev. ((C) RMS ((C)

0(, 80.5(E(a) 0.10 0.05 0.11

0(, 80.5(E(b) 0.18 0.05 0.18

5(N, 156(E 0.04 0.03 0.05 

0(, 156(E 0.10 0.06 0.12

5(S, 156(E 0.12 0.12 0.17

5(N, 155(W 0.03 0.02 0.04

0(, 155(W 0.04 0.02 0.05

0(, 125(W 0.27 0.15 0.31

8(N, 110(W 0.04 0.01 0.04

2(N, 110(W 0.08 0.06 0.09

Table 6. Mean remapped 20(C isotherm depth and the mean, standard deviation, and RMS

displacement (z) of the 20(C isotherm depth. z is defined as the difference between the

original minus remapped 20(C isotherm depths. Statistics for the 0(, 80.5(E mooring are

divided into two parts: (a) before the move into shallower water, 25 July 1993 to 7

November 1993, and (b) after the move, 12 November 1993 to 7 January 1994.

Position z(20.0() z(20(C) z(20(C) z(20(C)

(lat, long) Mean (m) Mean (m) Std. dev.(m) RMS (m)

0(, 80.5(E(a) 110.3 1.5 1.0 1.8

0(, 80.5(E(b) 121.5 2.7 1.0 2.9

5(N, 156(E 153.6 0.7 0.6 0.9

0(, 156(E 173.4 1.7 1.1 2.0

5(S, 156(E 198.1 2.0 2.1 2.9

5(N, 155(W 111.2 0.6 0.5 0.7

0(, 155(W 103.8 0.6 0.5 0.8

0(, 125(W 85.1 3.1 2.2 3.8

8(N, 110(W 66.9 0.4 0.2 0.4

2(N, 110(W 68.5 0.7 0.6 1.0
11

Table 7. DH mean, standard deviation and RMS, where DH represents the

dynamic height from the original temperatures minus the dynamic height from the

remapped temperatures. Dynamic heights were calculated between 0 and 450 m.

Statistics for the 0(, 80.5(E mooring are divided into two parts: (a) before the

move into shallower water, 25 July 1993 to 7 November 1993, and (b) after the

move, 12 November 1993 to 7 January 1994.

Position DH DH DH

(lat, long) Mean (dyn. m) Std. dev. (dyn. m) RMS (dyn. m)

0(, 80.5(E(a) 0.014 0.008 0.016

0(, 80.5(E(b) 0.014 0.008 0.016

5(N, 156(E 0.005 0.004 0.006 

0(, 156(E 0.010 0.006 0.012

5(S, 156(E 0.010 0.010 0.015

5(N, 155(W 0.003 0.002 0.004

0(, 155(W 0.003 0.002 0.004

0(, 125(W 0.021 0.012 0.024

8(N, 110(W 0.003 0.001 0.004

2(N, 110(W 0.006 0.005 0.008

5.4 Dynamic heights

Dynamic heights between 0 and 450 db were calculated for each deployment from both the

original temperatures and the remapped temperatures. Salinities for the dynamic height calculation

were interpolated from a set of T-S values constructed for each site from the World Ocean Atlas

annual temperature and salinity climatologies (Levitus et al., 1994; Levitus and Boyer, 1994). Time

series plots and scatter plots of both sets of dynamic heights are shown for each deployment in the

appendices. Except for the slack-line moorings, the RMS differences between the two sets of

dynamic heights for each site range from 0.003 to 0.012 dyn m. The RMS differences for the slackline moorings are greater. The largest RMS difference, 0.024 dyn m, is from the 0(, 125(W site

(Table 7).

5.5 Changing the assumption about pressure sensor errors

To examine the significance of adjusting the pressure records rather than adjusting the length

of the line in the temperature remapping procedure, two of the five moorings that required pressure

adjustments were remapped a second time using the assumption that the pressure sensors were more

accurate than the line measurement. In these two cases, the unadjusted pressure records were used

in the remapping procedure, and the maximum measured depths were used to determine line lengths.

The cases that were remapped a second time are the 0(, 125(W mooring, which in the first

remapping showed the greatest error due to line motion, and the 5(N, 155(W mooring, which

required the largest adjustment to the pressure records for the first remapping. For the second

remapping of the 5(N, 155(W data, the line lengths were set to yield a maximum possible depth for
12

Table 8. Mean, standard deviation and RMS of T(120 m), T(450 m), T, z(20(C), and DH for time series based

on temperatures remapped by two different methods. T(120 m) represents the original 120-m temperature minus the

remapped 120-m temperature. T(450 m) represents the original 450-m temperature minus the remapped 450-m

temperature. T represents the depth averaged original temperatures minus the depth averaged remapped temperatures.

z(20(C) represents the depth of the 20(C isotherm calculated from the original temperatures minus the depth of the

20(C isotherm calculated from the remapped temperatures. DH represents the dynamic height from the original

temperatures minus the dynamic height from the remapped temperatures. Temperature remapping done by Method 1

was based on pressure records that had been adjusted so the maximum measured depth equaled the nominal depth.

Temperature remapping done by Method 2 was based on the assumption that actual pressure sensor depth equaled the

maximum measured depth. 

(a) 5(N, 155(W

Method 1 Method 2

Variable Units Mean Std. dev. RMS Mean Std. dev. RMS

T(120m) (C 0.21 0.21 0.30 0.21 0.21 0.29

T(450 m) (C 0.03 0.02 0.03 0.02 0.02 0.03

T (C 0.03 0.02 0.04 0.01 0.02 0.03

z(20(C) m 0.6 0.5 0.7 0.6 0.5 0.7

DH Dynamic m 0.003 0.002 0.004 0.002 0.002 0.003

(b) 0(, 125(W

Method 1 Method 2

Variable Units Mean Std. dev. RMS Mean Std dev. RMS

T(120 m) (C 0.41 0.30 0.51 0.41 0.30 0.51

T(450 m) (C 0.57 0.32 0.66 0.58 0.32 0.66

T (C 0.27 0.15 0.31 0.27 0.15 0.31

z(20(C) m 3.1 2.2 3.8 3.1 2.2 3.8

DH Dynamic m 0.021 0.012 0.024 0.021 0.012 0.024

the 300-m sensors of 303.2 m and a maximum possible depth for the 500-m sensors of 509.1 m. For

0(, 125(W, line lengths were set so that the maximum possible depth of the 300-m sensors was

302.7 m. The maximum measured depth from the 0(, 125(W 500-m pressure sensor was shallower

than 500 m, so the maximum possible depth of the 500-m sensors was set to the nominal depth. 

The errors calculated assuming that the deepest measured depth indicates the position of the

sensor on the mooring line are, for all variables (temperatures from specific depths, depth averaged

temperatures, 20(C isotherm depth, and dynamic heights), virtually indistinguishable from those

calculated by the previous method (Table 8).
13

6. CONCLUSION

Remapping temperature records by the above method indicates that the effect of depth

excursions is typically 11⁄2 to 21⁄2 times greater than instrumental temperature error for taut-line

moorings. The effect on slack-line moorings is greater. The size of these corrections underscores

the need for accurate and reliable pressure sensors on the ATLAS moorings so that corrections can

be made in both real-time and post-deployment processing. This analysis also underscores the need

for more accurate line measurement to remove ambiguities about how corrections are applied. To

address this need, a new line measuring system with linear drive belts (model MVP FC-25)

manufactured by P.T.T. Inc. (Hayward, California), with a manufacturer’s stated accuracy of 0.07%,

has recently been acquired.

A move has begun to replace the present style of ATLAS mooring with a reengineered nextgeneration ATLAS system. The reengineered ATLAS system will be deployed in either taut-line

or slack-line configurations, but with the temperature cable replaced by inductively coupled sensors

mounted on the wire rope. The new design allows pressure sensors at three depths rather than two.

The sensor of choice at present is the Paine model 2111-30-660-02 strain-gauge-type pressure sensor

with a nominal accuracy of ±2 db (Milburn et al., 1996).

It is anticipated that eventually most, if not all, of the TAO array will be instrumented with

next generation ATLAS moorings of the slack-line variety. Hence, temperature remapping for depth

excursions will become a mandatory part of data processing to correct for mooring motion-induced

temperature errors. Ongoing efforts at PMEL are directed at refining and further testing of error

estimation algorithms discussed in this report. We are also developing procedures whereby

necessary temperature corrections can be applied on a routine basis to real-time and delayed-mode

ATLAS mooring data streams.

7. ACKNOWLEDGMENTS

Thanks are extended to Ann Nimersheim for comparisons of pre- and post-deployment

calibrations and to Linda Stratton for reprocessing pressure records with post-deployment

calibrations. The authors also thank Hugh Milburn for his discussion of mooring line measurement.

Production of this paper was supported by NOAA’s Environmental Research Laboratories and

Office of Global Programs. Contribution number 428 from the Joint Institute for the Study of the

Atmosphere and the Ocean.

8. REFERENCES

Berteaux, H.O. (1976): Buoy Engineering. John Wiley & Sons, New York, 314 pp.

Freitag, H.P, Y. Feng, L.J. Mangum, M.J. McPhaden, J. Neander, and L.D. Stratton (1994):

Calibration procedures and instrumental accuracy estimates of TAO temperature, relative

humidity and radiation measurements. NOAA Technical Memorandum ERL PMEL-104

(NTIS PB95-174827), 32 pp.
14

Hayes, S.P., L.J. Mangum, J. Picaut, A. Sumi, and K. Takeuchi (1991): TOGA-TAO: A moored

array for real-time measurements in the tropical Pacific Ocean. Bull. Am. Meteorol. Soc., 72,

339–347. 

Levitus, S., R. Burgett, and T.P. Boyer (1994): World ocean atlas 1994, Volume 3: Salinity. NOAA

Atlas NESDIS 3, 99 pp. 

Levitus, S., and T.P. Boyer (1994): World ocean atlas 1994, Volume 4: Temperature. NOAA Atlas

NESDIS 4, 117 pp. 

McPhaden, M.J. (1993): TOGA-TAO and the 1991–93 El Niño-Southern Oscillation Event.

Oceanography, 6, 36–44.

McPhaden, M.J. (1995): The Tropical Atmosphere-Ocean Array is completed. Bull. Am. Meteorol.

Soc., 76, 739–741.

Milburn, H.B., P.D. McLain, and C. Meinig (1996): ATLAS buoy—reengineered for the next

decade. Proceedings of the MTS/IEEE Oceans 96 Conference, Ft. Lauderdale, Florida, 24–26

September 1996, IEEE, New York, vol. II, 698–702.

Saunders, P.M., and N.P. Fofonoff (1976): Conversion of pressure to depth in the ocean. Deep-Sea

Res., 23, 109–111.
APPENDICES

(Click on the appendix below you wish to view)

APPENDIX A: 0(, 80.5(E

APPENDIX B: 5(N, 156(E

APPENDIX C: 0(, 156(E

APPENDIX D: 5(S, 156(E

APPENDIX E: 5(N, 155(W

APPENDIX F: 0(, 155(W

APPENDIX G: 0(, 125(W

APPENDIX H: 8(N, 110(W

APPENDIX I: 2(N, 110(W