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Fabien Durand and Thierry Delcroix

Abstract

The thermal structure variability of the tropical Pacific is investigated using an objective analysis of about 250 000 temperature profiles (mainly XBT) collected during the 1979–96 time period. Mean conditions and seasonal variability are briefly described to set the context, and temperature anomalies are constructed relative to a mean seasonal cycle to focus on the ENSO (El Niño–Southern Oscillation) timescale. Heat content anomalies (0–450 m) built up in the western equatorial basin prior to the 1982–83, 1986–87, and 1997–98 El Niño events but not clearly prior to the 1991–92, 1993, and 1994–95 events, which are thus found “atypical.” Low-frequency migration of temperature anomalies located at the mean thermocline depth is evidenced eastward in the equatorial band, as well as westward along a narrow zonal band located slightly north of the mean position of the intertropical convergence zone (say, 10°–20°N). This indicates that ENSO-related temperature anomalies in the subsurface ocean are present generally in the western equatorial Pacific about 1–2 years before the appearance of temperature anomalies in the eastern equatorial Pacific. Similarly, this indicates that subsurface temperature anomalies tend to be present in the eastern Pacific basin around 14°N about 1–2 years before the appearance of temperature anomalies in the western Pacific basin at the same latitude. The likely mechanisms responsible for these migrations and the possible link between the eastward and westward migration are discussed.

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Thierry Delcroix and Christian Henin

Abstract

Direct current measurements (0–600 m; re. 600 m) were carried out every six months from January 1984 to June 1986 in the western tropical Pacific Ocean (165°E) from 20°S to 10°N. The Equatorial Intermediate Current (EIC) occurred beneath the Equatorial Undercurrent (EUC) in the 300–500 m depth range between 20°S and 2°N.

At 165°E, the mean (i.e., the average of six cruiser) EIC flow has a characteristic reversed-U shape centered at the equator. Its associated hydrological features are (i) the EIC transport water of salinity 34.6–34.9%; (ii) its upper and lower limits correspond to the 26.4 and 27.0 sigma-t surfaces; and (ifi) its velocity core is located in the 1 1 “-140C water. The average transport of the EIC is −7.0 ± 4.8 (106 m3 s−1 i.e., 35% of the mean EUC transport computed for the same cruises.

Individual cruises exhibit little variability in the vertical (250–500 m) and meridional (2°S–2°N) EIC structure: The EIC velocity core during these cruises is thinnest at the equator and ranges in magnitude from −5 to −20 cm s−1. A notable exception is the 3–8 July 1985 EIC measurements, which show a disappearance of the EIC velocity core at the equator (U = +20 cm s−1).

Our EIC observations show good agreement with EIC simulated from models forced by easterly winds.

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Thierry Delcroix and Catherine Gautier

Abstract

A method is developed to estimate oceanic heat content (0–300 m) variations from sea level measurements in the tropical Pacific. To this end, statistical relationships between heat content and steric level, used as a surrogate variable for the sea level, are derived from climatological data. These relationships are then applied on independent datasets and the predictive ability of the method is determined regionally by comparing heat content estimated from XBT and sea level measurements recorded in three tropical Pacific islands (Christmas Fanning and Truk) during the 1979–85 period.

Good qualitative agreements are found between the two heat content estimates with correlations R = 0.78 to 0.94 and rms differences of average temperature of 0.25° to 0.50°C over an observed range of 6°C. Quantitative disagreements are observed in the central Pacific during the fall 1982 (El Niñc) period. These deficiencies in the method are found to be primarily due to intense and unusual salinity fluctuations at the surface which notably contribute to sea level variations. The difference between heat content variations deduced from sea level and calculated ones (from XBT) is significantly correlated (R = 0.54) with these sea surface salinity fluctuations.

For the investigated areas, the adopted method thus indicates that: 1) 2- and 3-month averaged sea level measurements can account for 61% to 88% of the 0–300 m heat content variations and, 2) special attention is required in its application when intense and unusual sea surface salinity anomalies occur.

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Jean Benoit Nicet and Thierry Delcroix

Abstract

An analysis of El Niño–Southern Oscillation (ENSO) related precipitation changes in New Caledonia, southwestern tropical Pacific, based on 21 selected stations covering the 1969–98 period is performed. The analysis at the ENSO timescale is complemented by an investigation of basin-scale precipitation changes in order to set the context, by an examination of changes in the flow rates of two major rivers of the island, and by a comparison between actual precipitation (P) records and the output of a simple linear regression model. The results indicate that a 20%–50% decrease in precipitation generally occurs during El Niño events, and vice versa during La Niña events. In accord with these P changes, the flow rates of the two rivers can almost double during La Niña years, and decrease by more that 50% during El Niño years. The magnitudes of the precipitation anomalies are however not strictly proportional to the strength of ENSO. Hence, it is found that the simple linear regression model based on the Southern Oscillation index can be used to infer the timing, but not the magnitude, of P changes.

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Thierry Delcroix, François Masia, and Gérard Eldin

Abstract

Profiling current meter (PCM) measurements under a drifting buoy are compared with concurrent shipboard acoustic Doppler current profiler (ADCP) measurements carried out in the western equatorial Pacific in March 1991, from 10°S to 7°N along the 165°E meridian. The mean (ADCP minus PCM)±rms differences between zonal and meridional velocity components are 5.7±11.2 cm s−1 and 0.0±8.8 cm s−1, respectively, when PCM measurements are relative to 600 m. The mean±rms differences decrease to 2.3±7.8 cm s−1 and 0.0±6.3 cm s−1 when the PCM and ADCP data are both referenced to the same layer (on a mean, 16–240 m). As compared with ADCP, it is found that PCM underestimates velocities of less than 20 cm s−1 by about 25%.

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Thierry Delcroix, Gerard Eldin, and Christian Hénin

Abstract

As part of the international TOGA program, the ORSTOM Center in Nouméa (New Caledonia) initiated in January 1984 a series of semi-annual cruises along the 165°E meridian from 20°S to 10°N, across the equatorial current system of the western Pacific. This paper presents an analysis of the first six hydrographic (0-1000 m) and current (0-600 m) sections.

A detailed description of “typical” January 1986 vertical structures of temperature, salinity and zonal measured velocity is offered. Differences are noted with structures previously obtained in the tropical Pacific. Compared to the central and eastern Pacific, the 165°E dataset evidences a much weaker equatorial upwelling and deeper surface isothermal layer and subsurface currents. Compared to the few western Pacific measurements, the two speed cores of the Equatorial Undercurrent (EUC) previously reported at 100 and 200 m are not observed here.

Special attention is given to the eastward equatorial jet (2°S-2°N; 0-75 m) measured in January 1985 when westerly winds were present from the north of New Guinea to 160°E.

For the purpose of volume transport calculations, eastward flows at 165°E are not sufficiently separated to be easily differentiated. A definition based on an isodensity surface (sigma-t=23.5 kg m−3) is thus adopted to discriminate the EUC and the North and South Subsurface Countercurrents (NSCC, SSCC) from the North and South Equatorial Countercurrents (NECC, SECC). The EUC is assumed to lie within 2 degrees of the equator below sigma-t = 23.5 kg m−3. Using these current boundaries, transports of the South Equatorial Current (SEC), EUC and NECC agree within 30% with estimates previously computed in the western, central and eastern Pacific; e.g., the mean NECC transport is 27 ± 13 106 m3 s−1. A noticeable exception is the SECC transport which is two to four times as much as that estimated for the central Pacific. The weaker (stronger) EUC and the farthest northern (southern) NECC were observed during the three January (June-July) cruises.

Large transport variability was observed and calls for a denser time-space sampling rate of observation. Hence, the credibility of dynamic height and geostrophic currents calculated from XBT (0-400 m) and mean temperature-salinity (T-S) curves are investigated. Major limitations, stressed by the semiannual transects, are caused by:

1) notable density variations in the 400–1000 m layer, and

2) the effects of variability of the T-S relation in the 0–400 m layer.

These two points can each result in signals of as much as 6 dyn cm in the surface dynamic height and therefore significant errors in geostrophic velocities calculated from individual cruises. These errors are generally not accounted for when the geostrophic method is applied to XBT data. However, poleward of 2° latitude, a fair agreement is observed between mean geostrophic and measured currents (5 cm s−1 rms difference), after eliminating the errors introduced by the 400 db reference level and mean T-S curves. In the 2°S-2°N band, the agreement is only qualitative (30 cm s−1 rms difference) and better in the EUC than in surface flows.

Deeper temperature sampling and a better knowledge of T-S variability than the present one are particularly recommended to monitor the equatorial current system from XBTs in the western tropical Pacific Ocean.

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Frédéric Marin, Elodie Kestenare, Thierry Delcroix, Fabien Durand, Sophie Cravatte, Gérard Eldin, and Romain Bourdallé-Badie

Abstract

A large reversal of zonal transport below the thermocline was observed over a period of 6 months in the western Pacific Ocean between 2°S and the equator [from 26.2 Sv (1 Sv ≡ 106 m3 s−1) eastward in October 1999 to 28.6 Sv westward in April 2000]. To document this reversal and assess its origin, an unprecedented collection of ADCP observations of zonal currents (2004–06), together with a realistic OGCM simulation of the tropical Pacific, was analyzed. The results of this study indicate that this reversal is the signature of intense annual variability in the subsurface zonal circulation at the equator, at the level of the Equatorial Intermediate Current (EIC) and the Lower Equatorial Intermediate Current (L-EIC). In this study, the EIC and the L-EIC are both shown to reverse seasonally to eastward currents in boreal spring (and winter for the L-EIC) over a large depth range extending from 300 m to at least 1200 m. The peak-to-peak amplitude of the annual cycle of subthermocline zonal currents at 165°E in the model is ∼30 cm s−1 at the depth of the EIC, and ∼20 cm s−1 at the depth of the L-EIC, corresponding to a mass transport change as large as ∼100 Sv for the annual cycle of near-equatorial zonal transport integrated between 2°S and 2°N and between 410- and 1340-m depths. Zonal circulations on both sides of the equator (roughly within 2° and 5.5° in latitude) partially compensate for the large transport variability. The main characteristics of the annual variability of middepth modeled currents and subsurface temperature (e.g., zonal and vertical phase velocities, meridional structure) are consistent, in the OGCM simulation, with the presence, beneath the thermocline, of a vertically propagating equatorial Rossby wave forced by the westward-propagating component of the annual equatorial zonal wind stress. Interannual modulation of the annual variability in subthermocline equatorial transport is discussed.

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Laurent Terray, Lola Corre, Sophie Cravatte, Thierry Delcroix, Gilles Reverdin, and Aurélien Ribes

Abstract

Changes in the global water cycle are expected as a result of anthropogenic climate change, but large uncertainties exist in how these changes will be manifest regionally. This is especially the case over the tropical oceans, where observed estimates of precipitation and evaporation disagree considerably. An alternative approach is to examine changes in near-surface salinity. Datasets of observed tropical Pacific and Atlantic near-surface salinity combined with climate model simulations are used to assess the possible causes and significance of salinity changes over the late twentieth century. Two different detection methodologies are then applied to evaluate the extent to which observed large-scale changes in near-surface salinity can be attributed to anthropogenic climate change.

Basin-averaged observed changes are shown to enhance salinity geographical contrasts between the two basins: the Pacific is getting fresher and the Atlantic saltier. While the observed Pacific and interbasin-averaged salinity changes exceed the range of internal variability provided from control climate simulations, Atlantic changes are within the model estimates. Spatial patterns of salinity change, including a fresher western Pacific warm pool and a saltier subtropical North Atlantic, are not consistent with internal climate variability. They are similar to anthropogenic response patterns obtained from transient twentieth- and twenty-first-century integrations, therefore suggesting a discernible human influence on the late twentieth-century evolution of the tropical marine water cycle. Changes in the tropical and midlatitudes Atlantic salinity levels are not found to be significant compared to internal variability. Implications of the results for understanding of the recent and future marine tropical water cycle changes are discussed.

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