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Gilles Reverdin

Abstract

The climatological seasonal cycle of the upper equatorial Indian Ocean is discussed. A summary of the observations is given. Near the surface and below the equatorial thermocline, the observations indicate an intense variability of the equatorial currants, primarily at the semiannual frequency. In the thermocline, an eastward flowing equatorial undercurrent present in March and April is the dominant feature. An analysis of the temperature profiles in the upper 500 meters also indicates at all depths, seasonal vertical displacements well above the estimated uncertainty. At the semiannual period, the different isotherms are oscillating in phase in the upper western Indian Ocean and an upward phase propagation is present in the east. Displacements at the annual period have shorter spatial scales and are usually smaller, the near surface and the deeper oceans am separated at this period by a level of no displacements in the upper thermocline. The equatorial thickening of the thermocline in March–April in the central and western Indian Ocean also suggests the presence of an eastward flowing undercurrent at that time.

To investigate the dynamics, a simplified wind forced linear model in the equatorial beta-plane is presented for a stratified Indian Ocean. The wind stress is applied as a body force over a mixed layer of specified thickness and below a dynamically induced mixing is included. A decomposition in vertical modes is done and the lower modes are solved with a numerical scheme from Cane and Patton. The dynamics of the higher modes are characterized by Yoshida jets (negligible zonal gradients) and an approximate solution is sought for them. The model response to wind forcing is sensitive to thew hypotheses with the surface equatorial currants being strongly dependent on the mixed layer depth and with the currents in the thermocline being controlled by mixing.

The model reproduces many of the most salient characteristics of the observed seasonal cycle, both for the currents and the isotherm vertical displacement. As in earlier studies, this suggests that the wind stress near the equator is an important forcing of the equatorial Indian Ocean, at least to depths of 500 m. However, typical differences between the observed seasonal cycle and the modeled one are of the order of 10 days in phase and a factor 1.5 in amplitude. Sensitivity studies suggest that these differences could not be overcome by tuning the model adjustable parameters, but that they are likely to arise from the crude representation of the forcing as a body force and the neglect of the nonlinearities.

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Nathalie Verbrugge and Gilles Reverdin

Abstract

The interannual variability of sea surface temperature (SST) in the North Atlantic is investigated from October 1992 to October 1999 with special emphasis on analyzing the contribution of horizontal advection to this variability. Horizontal advection is estimated using anomalous geostrophic currents derived from the TOPEX/Poseidon sea level data, average currents estimated from drifter data, scatterometer-derived Ekman drifts, and monthly SST data. These estimates have large uncertainties, in particular related to the sea level product, the average currents, and the mixed-layer depth, that contribute significantly to the nonclosure of the surface temperature budget. The large scales in winter temperature change over a year present similarities with the heat fluxes integrated over the same periods. However, the amplitudes do not match well. Furthermore, in the western subtropical gyre (south of the Gulf Stream) and in the subpolar regions, the time evolutions of both fields are different. In both regions, advection is found to contribute significantly to the interannual winter temperature variability. In the subpolar gyre, advection often contributes more to the SST variability than the heat fluxes. It seems in particular responsible for a low-frequency trend from 1994 to 1998 (increase in the subpolar gyre and decrease in the western subtropical gyre), which is not found in the heat fluxes and in the North Atlantic Oscillation index after 1996.

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Boris Dewitte and Gilles Reverdin

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The annual and interannual variability in the subthermocline equatorial Pacific is studied in a simulation of the tropical Pacific for the 1985–94 decade using a primitive equation high-resolution model. The study focuses on temperature variability and vertical energy fluxes. Similar to the observations made by W. S. Kessler and J. P. McCreary, the annual harmonic of vertical isotherm displacements presents phase lines sloping downward from east to west with off-equatorial maxima. Estimates of zonal and vertical phase speeds, the location of the maxima of isotherm displacements, and an analysis of ernergy fluxes suggest the presence of l = 1 Rossby waves. The simulated field is somewhat more trapped toward the equator than the observations. A linear simulation is carried out for the vertical standing modes with characteristics derived from the OGCM simulation. In the linear simulation, high-order meridional mode Rossby waves are more prominent than in the OGCM simulation. However, the (l = 1) Rossby wave in the linear model shares many characteristics with the one in the OGCM simulation. It is the result of both the reflection of surface-forced Kelvin waves on the eastern boundary and of direct forcing at the surface.

On interannual timescales the simulation also presents vertical propagation of Rossby waves from the eastern boundary, but not as deep as for the annual cycle and in a much thinner beam. The variability of vertical displacements exhibits an asymmetry with a larger amplitude north than south of the equator. The zone of large interannual variability originating at the eastern boundary extends westward following (l = 1) or (l = 2) WKB ray paths and reaches the western Pacific near 400 m at 3°–4° north but not south of the equator. The 3.3-yr harmonic is prominent in isotherm vertical displacements for this particular simulation. The analysis also suggests the presence of (l = 1) Rossby wave, but the phase line characteristics are also indicative of the contribution of higher-order meridional modes. At this frequency, the solution of the linear model for the (l = 1) Rossby mode agrees well with the OGCM (l = 1) Rossby wave contribution.

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Gilles Reverdin and Mark Cane

Abstract

Different wind field analyses are used to force a one layer adiabatic model of the near equatorial surface circulation in the Indian Ocean in 1979. The model simulates the major features of the observations: eastward jets were present in April-May and in October-November in the central Indian Ocean in phase with the local winds; the seasonal changes of thermocline depth in the western part of the basin are related to the near equatorial currents.

Significant discrepancies are also found. Some are due to the uncertainty in the wind fields. Correlation between different wind fields are only of the order of 0.75 for the low frequencies and magnitude can vary by a factor of 1.5. Others are attributed to model inadequacies especially the neglect of nonlinearity and the oversimplification of the vertical structure. There is an unrealistic energy focus in the central Indian Ocean though, in general, seasonal changes are underestimated by at least 30%.

Simpler dynamics failed to produce a reasonable agreement over the whole basin. A Yoshida jet did well for the currents in the central part of the basin, but did not reproduce the mass changes in the west. Sverdrup equilibrium reproduces the model zonal slope of the thermocline, but not the currents.

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Fabien Durand and Gilles Reverdin

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The Profiling Autonomous Lagrangian Circulation Explorer (PALACE) float is used to implement the Array for Real-Time Geostrophic Oceanography (ARGO). This study presents a statistical approach to correct salinity measurement errors of an ARGO-type fleet of PALACE floats. The focus is on slowly evolving drifts (typically with time scales longer than a few weeks). Considered for this case study is an ensemble of about 80 floats in the Irminger and Labrador Seas, during the 1996–97 period. Two different algorithms were implemented and validated based on float-to-float data comparison at depth, where the water masses are relatively stable over the time scales of interest. The first algorithm is based upon objective analysis of the float data, while the second consists of a least squares adjustment of the data of the various floats. The authors’ method exhibits good skills to retrieve the proper hydrological structure of the case study area. It significantly improves the consistency of the PALACE dataset with in situ data as well as with satellite altimetric data. As such, the method is readily usable on a near-real-time basis, as required by the ARGO project.

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Daniel Cadet and Gilles Reverdin

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An assimillaiion of surface data collected from ship reports, land and islands stations during summer 1975 is presented. The domain of this study is the summer MONEX area (35–100°E, 25°N-25°S) and the period covers May, June, July, August and September 1975. Objective analysis of the data for 304 individual half-days (an analysis every 12 h) was carried out over the domain for the wind, pressure, air temperature, sea surface temperature and water vapor mixing ratio fields. In this first part, the mean monthly fields are presented and climatological features are discussed. Charts of latent heat transfer are presented. The water vapor cross-equatorial flux (40–100°E) for the complete monsoon season (May-September) is 4.7=1010 tons day−1. It is shown that maximum input takes place in the Arabian Sea (75% of the total flux), especially between 45 and 60°E (more than 50% of the total flux). The cross-equatorial flux into the Bay of Bengal is not negligible, being about ⅓ of that over the western Indian Ocean (25% of the total flux).

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Gilles Reverdin and James Luyten

Abstract

Drifting buoys were released in the western Indian Ocean from 1979 to 1982. They reveal meanders of the eastward flowing monsoon drift in August–September 1979, 1980 and 1981. Oscillating meridional buoy drifts reach 80 cm s−1 and meridional displacements can exceed 3° for motions at a period close to 25 days. In 1979, this is related to oscillation below the thermocline, sensed by an array of current meters in the western equatorial Indian Ocean. It is likely that the origin of the oscillations is in the surface currents north of the equator.

The currents change in October with the formation of an intense eastward equatorial current in which most of the buoys are entrained. As the buoys drift rapidly towards the eastern Indian Ocean, meridional motions are still present but at shorter periods (12 days) than is observed below the thermocline (25 days). It is possible that is still a manifestation of the same oscillations, but with Doppler shifting and a strong influence of nonlinearities. Waves at 25 days are found in the three oceans. Significant differences in the circulation of relevance for the waves are a more intense seasonal cycle of the currents in the Indian Ocean with predominantly eastward currents near the equator and the presence of the waves in the western portion of the Ocean where a very intense varying circulation is found in the proximity of the Somali Coast during the summer monsoon.

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Alain Morliére, Gills Reverdin, and Jacques Merle

Abstract

Thirty-six hundred temperature profiles collected during 1984 were assimilated into a multilayer primitive equation model of the tropical Atlantic Ocean. The method consists in a monthly correction of the simulated temperature field. Each month, an observed field is computed from the temperature profiles with a successive correction analysis starting from the simulated field. The difference between the observed field and simulation is computed, the model is restarted from the previous month, progressively introducing the difference as a Newton forcing in the heat equation. The sensitivity to the initial mate is greatly reduced near the equator after six months, but persists for a longer time at higher latitudes. The assimilated temperature structure is closer to the observations than was the unassimilated simulation. The thermocline has strengthened, and low-frequency variability the equator is close to the observe one, resulting in a more realistic zonal slope of the thermocline. The current structure, although it still differs noticeably from the observations, is more realistic, with stronger near-surface countercurrents and a faster equatorial undercurrent.

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Verena Hormann, Luca R. Centurioni, and Gilles Reverdin

Abstract

Salinity measurements from drifters constitute an important in situ dataset for the calibration and validation of the sea surface salinity satellite missions. A total of 114 satellite-tracked salinity drifters were deployed within the framework of the first Salinity Processes in the Upper Ocean Regional Study (SPURS) experiment in the subtropical North Atlantic focusing on the period August 2012–April 2014. In this study, a subset of 83 drifters, which provided useful salinity measurements in the central SPURS region from a few weeks to more than one year, is evaluated and an ad hoc quality-control procedure based on previously published work and the new observations is described. It was found that the sampling algorithm of the drifters introduces a predominantly fresh bias in the noise level of the salinity data, probably caused by the presence of air bubbles within the measuring cell. Since such noise is difficult to eliminate using statistical methods, extensive editing was done manually instead. Such quality-control procedures cannot be routinely applied to the real-time data stream from the drifters. Therefore, a revision of the sampling algorithm of the drifter’s salinity sensor is needed. Comparisons of the drifter’s salinity measurements with independent datasets further indicate that the sensor can provide reliable observations for up to one year. Finally, little evidence was found that the quality of the drifter’s salinity measurements depends on the presence of the drogue.

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Gerd Krahmann, Martin Visbeck, and Gilles Reverdin

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A general circulation ocean model has been used to study the formation and propagation mechanisms of North Atlantic Oscillation (NAO)-generated temperature anomalies along the pathway of the North Atlantic Current (NAC). The NAO-like wind forcing generates temperature anomalies in the upper 440 m that propagate along the pathway of the NAC in general agreement with the observations. The analysis of individual components of the ocean heat budget reveals that the anomalies are primarily generated by the wind stress anomaly-induced oceanic heat transport divergence. After their generation they are advected with the mean current. Surface heat flux anomalies account for only one-third of the total temperature changes. Along the pathway of the NAC temperature anomalies of opposite signs are formed in the first and second halves of the pathway, a pattern called here the North Atlantic dipole (NAD). The response of the ocean depends fundamentally on R t = (L/υ)/τ, the ratio between the time it takes for anomalies to propagate along the NAC [(L/υ) ∼ 10 years] compared to the forcing period τ. The authors find that for NAO periods shorter than 4 years (R t > 1) the response in the subpolar region is mainly determined by the local forcing. For NAO periods longer than 32 years (R t < 1); however, the SST anomalies in the northeastern part of the NAD become controlled by ocean advection. In the subpolar region maximal amplitudes of the temperature response are found for intermediate (decadal) periods (R t ∼ 1) where the propagation of temperature anomalies constructively interferes with the local forcing. A comparison of the NAO-generated propagating temperature anomalies with those found in observations will be discussed.

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