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International audience; The knowledge of offshore and coastal wave climate evolution towards the end of the twenty-first century is particularly important for human activities in a region such as the Bay of Biscay and the French Atlantic coast. Using dynamical downscaling, a high spatial resolution dataset of wave conditions in the Bay of Biscay is built for three future greenhouse gases emission scenarios. Projected wave heights, periods and directions are analysed at regional scale and more thoroughly at two buoys positions, offshore and along the coast. A general decrease of wave heights is identified (up to -20 cm during summer within the Bay of Biscay), as well as a clockwise shift of summer waves and winter swell coming from direction. The relation between those changes and wind changes is investigated and highlights a complex association of processes at several spatial scales. For instance, the intensification and the north-eastward shift of strong wind core in the North Atlantic Ocean explain the clockwise shift of winter swell directions. During summer, the decrease of the westerly winds in the Bay of Biscay explains the clockwise shift and the wave height decrease of wind sea and intermediate waves. Finally, the analysis reveals that the offshore changes in the wave height and the wave period as well as the clockwise shift in the wave direction continue toward the coast. This wave height decrease result is consistent with other regional projections and would impact the coastal dynamics by reducing the longshore sediment flux.

International audience; The response of the Atlantic Meridional Overturning Circulation (AMOC) to an increase in atmospheric CO2 concentration is analyzed using the IPSL-CM4 coupled ocean-atmosphere model. Two simulations are integrated for 70 years with 1%/year increase in CO2 concentration until 2×CO2, and are then stabilized for further 430 years. The first simulation takes land-ice melting into account, via a simple parameterization, which results in a strong freshwater input of about 0.13 Sv at high latitudes in a warmer climate. During this scenario, the AMOC shuts down. A second simulation does not include this land-ice melting and herein, the AMOC recovers after 200 years. This behavior shows that this model is close to an AMOC shutdown threshold under global warming conditions, due to continuous input of land-ice melting. The analysis of the origin of density changes in the Northern Hemisphere convection sites allows an identification as to the origin of the changes in the AMOC. The processes that decrease the AMOC are the reduction of surface cooling due to the reduction in the air-sea temperature gradient as the atmosphere warms and the local freshening of convection sites that results from the increase in local freshwater forcing. Two processes also control the recovery of the AMOC: the northward advection of positive salinity anomalies from the tropics and the decrease in sea-ice transport through the Fram Strait toward the convection sites. The quantification of the AMOC related feedbacks shows that the salinity related processes contribute to a strong positive feedback, while feedback related to temperature processes is negative but remains small as there is a compensation between heat transport and surface heat flux in ocean-atmosphere coupled model. We conclude that in our model, AMOC feedbacks amplify land-ice melting perturbation by 2.5.

Three- and four-dimensional variational assimilation (3DVAR and 4DVAR) systems have been developed for the Ocean Parallelise (OPA) ocean general circulation model of the Laboratoire d'Oceanographie Dynamique et de Climatologie. They have been applied to a tropical Pacific version of OPA and cycled over the period 1993–98 using in situ temperature observations from the Global Temperature and Salinity Pilot Programme. The assimilation system is described in detail in Part I of this paper. In this paper, an evaluation of the physical properties of the analyses is undertaken. Experiments performed with a univariate optimal interpolation (OI) scheme give similar results to those obtained with the univariate 3DVAR and are thus not discussed in detail. For the 3DVAR and 4DVAR, it is shown that both the mean state and interannual variability of the thermal field are improved by the assimilation. The fit to the assimilated data in 4DVAR is also very good at timescales comparable to or shorter than the 30-...

The 1997 El Nino was followed step by step by observation systems, both in situ and from space. This outstanding coverage described all the details of its development. Meanwhile this very large anomaly was not very well detected by seasonal forecasting systems. Some forecasts, in particular those made from a simplified approach to the ocean-atmosphere system built from the observations made during the 80s, did not capture the beginning of the anomaly and its development. On the contrary, more complex forecasting systems, built on coupled general circulation models with data assimilation, were more successful. These results show that our understanding of the ocean-atmosphere system is still limited and that observation systems are essential to improve our knowledge of physical mechanisms and their parameterization in models.

The thirty year simulation of the coupled global atmosphere-tropical Pacific Ocean general circulation model of the Laboratoire de Meterologie Dynamique and the Laboratoire d’Oceanographie Dynamique et de Climatologie presented in Part I is further investigated in order to understand the mechanisms of interannual variability. The model does simulate interannual events with ENSO characteristics; the dominant periodicity is quasi-biennial, though strong events are separated by four year intervals. The mechanism that is responsible for seasonal oscillations, identified in Part I, is also active in interannual variability with the difference that now the Western Pacific is dynamically involved. A warm interannual phase is associated with an equatorward shift of the ITCZ in the Western and Central Pacific. The coupling between the ITCZ and the ocean circulation is then responsible for the cooling of the equatorial subsurface by the draining mechanism. Cold subsurface temperature anomalies then propagate eastward along the mean equatorial thermocline. Upon reaching the Eastern Pacific where the mean thermocline is shallow, cold subsurface anomalies affect surface temperatures and reverse the phase of the oscillation. The preferred season for efficient eastward propagation of thermocline depth temperature anomalies is boreal autumn, when draining of equatorial waters towards higher latitudes is weaker than in spring by a factor of six. In that way, the annual cycle acts as a dam that synchronizes lower frequency oscillations.

The mechanisms responsible for the mean state and the seasonal and interannual variations of the coupled tropical Pacific-global atmosphere system are investigated by analyzing a thirty year simulation, where the LMD global atmospheric model and the LODYC tropical Pacific model are coupled using the delocalized physics method. No flux correction is needed over the tropical region. The coupled model reaches its regime state roughly after one year of integration in spite of the fact that the ocean is initialized from rest. Departures from the mean state are characterized by oscillations with dominant periodicites at annual, biennial and quadriennial time scales. In our model, equatorial sea surface temperature and wind stress fluctuations evolved in phase. In the Central Pacific during boreal autumn, the sea surface temperature is cold, the wind stress is strong, and the Inter Tropical Convergence Zone (ITCZ) is shifted northwards. The northward shift of the ITCZ enhances atmospheric and oceanic subsidence between the equator and the latitude of organized convention. In turn, the stronger oceanic subsidence reinforces equatorward convergence of water masses at the thermocline depth which, being not balanced by equatorial upwelling, deepens the equatorial thermocline. An equivalent view is that the deepening of the thermocline proceeds from the weakening of the meridional draining of near-surface equatorial waters. The inverse picture prevails during spring, when the equatorial sea surface temperatures are warm. Thus temperature anomalies tend to appear at the thermocline level, in phase opposition to the surface conditions. These subsurface temperature fluctuations propagate from the Central Pacific eastwards along the thermocline; when reaching the surface in the Eastern Pacific, they trigger the reversal of sea surface temperature anomalies. The whole oscillation is synchronized by the apparent meridional motion of the sun, through the seasonal oscillation of the ITCZ. This possible mechanism is partly supported by the observed seasonal reversal of vorticity between the equator and the ITCZ, and by observational evidence of eastward propagating subsurface temperature anomalies at the thermocline level.

During the Tropical Ocean-Global Atmosphere (TOGA) program substantial progress was made in the development of coupled general circulation models with regard to representation of the tropical mean state and climate variability. This paper provides a review of the main developments, focusing on the tropical Pacific region. Early coupled general circulation models were relatively crude; with coarse resolution and limited physical parameterizations and poor surface fluxes, the model drift from the observed mean state was often substantial, and their use for investigating climate variability such as El Nino-Southern Oscillation (ENSO) was limited. Improvements in resolution and parameterizations led to rapid progress. Through the TOGA program it has been possible to assess coupled model mean states and variability against high-quality observations, particularly for the tropical Pacific. Both components of the coupled system (ocean and atmosphere) have benefited from an improved understanding of the physics. Coupled experiments have revealed deficiencies in each model component that were concealed in separate forced runs. Several general systematic errors have yet to be eliminated, especially in the east Pacific. The variety of behavior obtained with coupled models provides evidence that more than one mechanism is active in the generation and evolution of ENSO events. There are also indications that interannual variability is linked to the mean structure of the equatorial Pacific.

A set of numerical simulations of the tropical Pacific Ocean during the 1985–94 decade is used to investigate the effects of haline stratification on the low-frequency equilibrium of the Coupled Ocean–Atmosphere Response Experiment region. The simulated sea surface salinity structure is found to be quite sensitive to the freshwater forcing and to the other fluxes. Despite this sensitivity, several robust features are found in the model. Sensitivity experiments illustrate the important role of the haline stratification in the western Pacific. This stratification is the result of a balance between precipitations and entrainment of subsurface saltier water. It inhibits the downward penetration of turbulent kinetic energy. This results notably in a trapping of the westerly wind burst momentum in the surface layer, giving rise to strong fresh equatorial jets. The model is able to produce a barrier layer between 5°N and 10°S in the western Pacific and under the intertropical convergence zone (as in the...

A set of OGCM experiments is used to investigate the processes responsible for barrier layer (BL) formation in the Pacific Ocean. As in existing datasets, BL appears in the present experiments both in the western Pacific (WP) and under the intertropical convergence zone (ITCZ). In the WP, the BL displays a strong interannual variability linked to ENSO variability, in qualitative agreement with the observations of Ando and McPhaden. In both the equatorial and 3°–8°S bands, a subduction process is responsible for BL formation. In the equatorial region, it results from a strong downwelling near the salinity front created by convergence between central Pacific salty water and WP freshwater. In the southern region, the subduction of the South Equatorial Current salty water involves mainly mixed layer thinning due to the freshening of the surface layer by rain and equatorial divergence of water from the eastward fresh equatorial jets. The formation of BL under the ITCZ is found to be mostly related to ...

An OGCM is used to investigate the importance of lateral mixing in the tropical Pacific Ocean circulation. Horizontal subgrid-scale physics is parameterized by the usual Laplacian operator. Three simulations are performed using three different orders of magnitude for lateral eddy viscosity and diffusivity coefficients: 104, 103, and 102 m2 s−1. The upper layer response is found sensitive to lateral diffusion as well as the rest of the general circulation. Decreasing lateral mixing coefficients raises the mean kinetic energy level and the input of energy by the wind, and enhances the vertical dissipation. This weakens the equatorial meridional cell and induces a reduction of 20 Sv in the transport of the equatorial upwelling. These results are due to the nonlinear interplay between horizontal and vertical diffusion. The nature of the Equatorial Undercurrent (EUC) is found particularly sensitive to the relative importance of the diffusive conditions. Lateral mixing dominates the different regimes o...

The large-scale processes preconditioning the winter deep-water formation in the northwestern Mediterranean Sea are investigated with a primitive equation numerical model where convection is parameterized by a non- penetrative convective adjustment algorithm. The ocean is forced by momentum and buoyancy fluxes that have the gross features of mean winter forcing found in the MEDOC area. The wind-driven barotropic circulation appears to be a major ingredient of the preconditioning phase of deep-water formation. After three months, the ocean response is dominated by a strong barotropic cyclonic vortex located under the forcing area, which fits the Sverdrup balance away from the northern coast. In the vortex center, the whole water column remains trapped under the forcing area all winter. This trapping enables the thermohaline forcing to drive deep-water formation efficiently. Sensitivity studies show that b effect and bottom topography play a paramount role and confirm that deep convection occurs only in areas that combine a strong surface thermohaline forcing and a weak barotropic advection so that water masses are submitted to the negative buoyancy fluxes for a much longer time. In particular, the impact of the Rhone Deep Sea Fan on the barotropic circulation dominates theb effect: the barotropic flow is constrained to follow the bathymetric contours and the cyclonic vortex is shifted southward so that the fluid above the fan remains quiescent. Hence, buoyancy fluxes trigger deep convection above the fan in agreement with observations. The selection of the area of deep-water formation through the deflection of the barotropic circulation by the topography seems a more efficient mechanism than those associated with the wind- driven barotropic vortex. This is due to its permanency, while the latter may be too sensitive to time and space variations of the forcing.

The seasonal cycle over the tropical Pacific simulated by 11 coupled ocean–atmosphere general circulation models (GCMs) is examined. Each model consists of a high-resolution ocean GCM of either the tropical Pacific or near-global means coupled to a moderate- or high-resolution atmospheric GCM, without the use of flux correction. The seasonal behavior of sea surface temperature (SST) and eastern Pacific rainfall is presented for each model. The results show that current state-of-the-art coupled GCMs share important successes and troublesome systematic errors. All 11 models are able to simulate the mean zonal gradient in SST at the equator over the central Pacific. The simulated equatorial cold tongue generally tends to be too strong, too narrow, and extend too far west. SSTs are generally too warm in a broad region west of Peru and in a band near 10°S. This is accompanied in some models by a double intertropical convergence zone (ITCZ) straddling the equator over the eastern Pacific, and in others by an ITCZ that migrates across the equator with the seasons; neither behavior is realistic. There is considerable spread in the simulated seasonal cycles of equatorial SST in the eastern Pacific. Some simulations do capture the annual harmonic quite realistically, although the seasonal cold tongue tends to appear prematurely. Others overestimate the amplitude of the semiannual harmonic. Nonetheless, the results constitute a marked improvement over the simulations of only a few years ago when serious climate drift was still widespread and simulated zonal gradients of SST along the equator were often very weak.

La dynamique sedimentaire littorale est controlee par un ensemble de facteurs hydrodynamiques qui sont susceptibles d’etre modifies par le changement climatique. Les vagues ont un role dominant dans la dynamique du littoral aquitain. Afin d’analyser l’impact du changement climatique, un modele de vagues est mis en place et des simulations sont realisees pour les climats actuel (1961-2000) et futurs (2061-2100). L’analyse des futures conditions de vague dans le Golfe de Gascogne met en evidence une diminution generalisee des hauteurs de vague et une rotation horaire de l’ensemble des vagues estivales et des houles hivernales par rapport au climat actuel. Le long de la cote aquitaine, la diminution des hauteurs de vague est toujours significative. Cependant, la refraction bathymetrique induit une attenuation de la rotation des vagues, en particulier des houles hivernales. Le calcul de la derive littorale (aussi appelee flux de sediment longshore ) au niveau de la plage de Biscarrosse a partir de formules empiriques indique que ces changements resulteraient en une diminution des flux longshore pouvant atteindre 10 %. Cette diminution est associee a la diminution des hauteurs de vague a proximite de la cote. Enfin, l’inventaire des incertitudes liees aux differentes etapes de cette etude suggere des pistes d’amelioration de la quantification des futurs flux de sediment, par exemple en considerant davantage de modeles climatiques ou en modelisant les vagues dans la zone de deferlement.

Sea Level variations of the Indian Ocean north of 20°S are analyzed from Geosat satellite altimeter data over April 1985–September 1989. These variations are compared and interpreted with numerical simulations derived from a reduced gravity model forced by FSU observed winds over the same period. After decomposition into complex empirical orthogonal function the low-frequency anomalies are described by the first two modes for observations as well as for simulations. The sums of the two modes contain 34% and 40% of the observed and simulated variances respectively. Averaged over the basin, the observed and simulated sea level changes are correlated by 0.92 over 1985–1988. The strongest change happens during the El Nino 1986–1987, between winter 1986 and summer 1987 the basin-averaged sea level rises by ∼1 cm. These low-frequency variations can partly be explained by changes in the Sverdrup circulation. The southern tropical Indian Ocean between 10° and 20°S is the domain where those changes are st...

Deep-water formation (DWF) in the northwestern Mediterranean Sea and the subsequent horizontal circulation are investigated in a rectangular basin with a three-dimensional primitive equation model. The basin is forced by constant climatological heat and salt fluxes. Convective motion is parameterized by a simple nonpenetrative convective adjustment process plus Richardson number–dependent vertical eddy viscosity and diffusivity. A homogeneous column of dense water is progressively formed in the forcing area. Meanders of 40-km wavelength develop at the periphery of the column. These features agree with observations. Energy studies show that the meanders are generated mainly through a baroclinic instability process. These meanders, and the associated cells of vertical motion, contribute to the process of DWF. They generate vertical advection, while the associated horizontal advection tends to restratify the surface water of the column, and thus to inhibit very deep convection. Just before the end o...

On the time scale of a century, the Atlantic thermohaline circulation (THC) is sensitive to the global surface salinity distribution. The advection of salinity toward the deep convection sites of the North Atlantic is one of the driving mechanisms for the THC. There is both a northward and a southward contributions. The northward salinity advection (Nsa) is related to the evaporation in the subtropics, and contributes to increased salinity in the convection sites. The southward salinity advection (Ssa) is related to the Arctic freshwater forcing and tends on the contrary to diminish salinity in the convection sites. The THC changes results from a delicate balance between these opposing mechanisms. In this study we evaluate these two effects using the IPSL-CM4 ocean-atmosphere-sea-ice coupled model (used for IPCC AR4). Perturbation experiments have been integrated for 100 years under modern insolation and trace gases. River runoff and evaporation minus precipitation are successively set to zero for the ocean during the coupling procedure. This allows the effect of processes Nsa and Ssa to be estimated with their specific time scales. It is shown that the convection sites in the North Atlantic exhibit various sensitivities to these processes. The Labrador Sea exhibits a dominant sensitivity to local forcing and Ssa with a typical time scale of 10 years, whereas the Irminger Sea is mostly sensitive to Nsa with a 15 year time scale. The GIN Seas respond to both effects with a time scale of 10 years for Ssa and 20 years for Nsa. It is concluded that, in the IPSL-CM4, the global freshwater forcing damps the THC on centennial time scales.

In the present study, we use a coupled model to evaluate the effect of shallow salinity stratification on the sea surface temperature (SST) and on the monsoon onset in the southeastern Arabian Sea (SEAS). A 100-year control experiment shows that the coupled model reproduces the main climatic features in this region in terms of SST, precipitation and barrier layer (BL). A 100-year sensitivity experiment (where BL effects have been suppressed in the SEAS) shows that BL enhances the spring SST warming by 0.5°C, and leads to a statistically significant increase of precipitation in May (3 mm/day) linked to an early (10 to 15 days) monsoon onset. This suggests that the BL extent may be a useful predictor of the summer monsoon onset in the area with a two-month lead-time. However the effect above is mostly concentrated in the SEAS, and there is no significant impact over continental India.

The focus of this study is to document the possible role of the southern subtropical Indian Ocean in the transitions of the monsoon-ENSO system during recent decades. Composite analyses of sea surface temperature (SST) fields prior to El Nino-Southern Oscillation (ENSO), Indian summer monsoon (ISM), Australian summer monsoon (AUSM), tropical Indian Ocean dipole (TIOD) and Maritime Continent rainfall (MCR) indices reveal the southeast Indian Ocean (SEIO) SSTs during late boreal winter as the unique common SST precursor of these various phenomena after the 1976–1977 regime shift. Weak (strong) ISMs and AUSMs, El Ninos (La Ninas) and positive (negative) TIOD events are preceded by significant negative (positive) SST anomalies in the SEIO, off Australia during boreal winter. These SST anomalies are mainly linked to subtropical Indian Ocean dipole events, recently studied by Behera and Yamagata (Geophys Res Lett 28:327–330, 2001). A wavelet analysis of a February–March SEIO SST time series shows significant spectral peaks at 2 and 4–8 years time scales as for ENSO, ISM or AUSM indices. A composite analysis with respect to February–March SEIO SSTs shows that cold (warm) SEIO SST anomalies are highly persistent and affect the westward translation of the Mascarene high from austral to boreal summer, inducing a weakening (strengthening) of the whole ISM circulation through a modulation of the local Hadley cell during late boreal summer. At the same time, these subtropical SST anomalies and the associated SEIO anomalous anticyclone may be a trigger for both the wind-evaporation-SST and wind-thermocline-SST positive feedbacks between Australia and Sumatra during boreal spring and early summer. These positive feedbacks explain the extraordinary persistence of the SEIO anomalous anticyclone from boreal spring to fall. Meanwhile, the SEIO anomalous anticyclone favors persistent southeasterly wind anomalies along the west coast of Sumatra and westerly wind anomalies over the western Pacific, which are well-known key factors for the evolution of positive TIOD and El Nino events, respectively. A correlation analysis supports these results and shows that SEIO SSTs in February–March has higher predictive skill than other well-established ENSO predictors for forecasting Nino3.4 SST at the end of the year. This suggests again that SEIO SST anomalies exert a fundamental influence on the transitions of the whole monsoon-ENSO system during recent decades.

The variability in the East African short rains is investigated using 41-yr data from the observation and 200-yr data from a coupled general circulation model known as the Scale Interaction Experiment-Frontier Research Center for Global Change, version 1 (SINTEX-F1). The model-simulated data provide a scope to understand the climate variability in the region with a better statistical confidence. Most of the variability in the model short rains is linked to the basinwide large-scale coupled mode, that is, the Indian Ocean dipole (IOD) in the tropical Indian Ocean. The analysis of observed data and model results reveals that the influence of the IOD on short rains is overwhelming as compared to that of the El Niño–Southern Oscillation (ENSO); the correlation between ENSO and short rains is insignificant when the IOD influence is excluded. The IOD–short rains relationship does not change significantly in a model experiment in which the ENSO influence is removed by decoupling the ocean and atmosphere in the tropical Pacific. The partial correlation analysis of the model data demonstrates that a secondary influence comes from a regional mode located near the African coast. Inconsistent with the observational findings, the model results show a steady evolution of IOD prior to extreme events of short rains. Dynamically consistent evolution of correlations is found in anomalies of the surface winds, currents, sea surface height, and sea surface temperature. Anomalous changes of the Walker circulation provide a necessary driving mechanism for anomalous moisture transport and convection over the coastal East Africa. The model results nicely augment the observational findings and provide us with a physical basis to consider IOD as a predictor for variations of the short rains. This is demonstrated in detail using the statistical analysis method. The prediction skill of the dipole mode SST index in July and August is 92% for the observation, which scales slightly higher for the model index (96%) in August. As observed in data, the model results show decadal weakening in the relationship between IOD and short rains owing to weakening in the IOD activity.

This paper uses recent gridded climatological data and a coupled general circulation model (GCM) simulation in order to assess the relationships between the interannual variability of the Indian summer monsoon (ISM) and the El Nino-Southern Oscillation (ENSO). The focus is on the dynamics of the ISM-ENSO relationships and the ability of the state-of-the-art coupled GCM to reproduce the complex lead-lag relationships between the ISM and the ENSO. The coupled GCM is successful in reproducing the ISM circulation and rainfall climatology in the Indian areas even though the entire ISM circulation is weaker relative to that observed. In both observations and in the simulation, the ISM rainfall anomalies are significantly associated with fluctuations of the Hadley circulation and the 200 hPa zonal wind anomalies over the Indian Ocean. A quasi-biennial time scale is found to structure the ISM dynamical and rainfall indices in both cases. Moreover, ISM indices have a similar interannual variability in the simulation and observations. The coupled model is less successful in simulating the annual cycle in the tropical Pacific. A major model bias is the eastward displacement of the western North Pacific inter-tropical convergence zone (ITCZ), near the dateline, during northern summer. This introduces a strong semiannual component in Pacific Walker circulation indices and central equatorial Pacific sea surface temperatures. Another weakness of the coupled model is a less-than-adequate simulation of the Southern Oscillation due to an erroneous eastward extension of the Southern Pacific convergence zone (SPCZ) year round. Despite these problems, the coupled model captures some aspects of the interannual variability in the tropical Pacific. ENSO events are phase-locked with the annual cycle as observed, but are of reduced amplitude relative to the observations. Wavelet analysis of the model Nino34 time series shows enhanced power in the 2–4 year band, as compared to the 2–8 year range for observations during the 1950–2000 period. The ISM circulation is weakened during ENSO years in both the simulation and the observations. However, the model fails to reproduce the lead-lag relationship between the ISM and Nino34 sea surface temperatures (SSTs). Furthermore, lag correlations show that the delayed response of the wind stress over the central Pacific to ISM variability is insignificant in the simulation. These features are mainly due to the unrealistic interannual variability simulated by the model in the western North Pacific. The amplitude and even the sign of the simulated surface and upper level wind anomalies in these areas are not consistent with observed patterns during weak/strong ISM years. The ISM and western North Pacific ITCZ fluctuate independently in the observations, while they are negatively and significantly correlated in the simulation. This isolates the Pacific Walker circulation from the ISM forcing. These systematic errors may also contribute to the reduced amplitude of ENSO variability in the coupled simulation. Most of the unrealistic features in simulating the Indo-Pacific interannual variability may be traced back to systematic errors in the base state of the coupled model.

Seasonal forecasting based on coupled general circulation models suffers from important errors. In order to provide insight into the causes of these errors in relation to ocean and atmosphere models, we carried out a comparison using a set of 10-yr ensemble hindcasts of four coupled climate models of the DEMETER project. The four models are based on two different atmosphere models and three different ocean models. This allows us to analyse the relative weight of the ocean and atmosphere components in the error of a coupled model. Using the hindcast climatologies over the years 1991 to 2000, we looked specifically at the sea surface and soil level temperature over the tropics with respect to the hindcast start date. Our results indicate that the monthly evolution of large mean deviations from the observations (> ±1 °C after 6 months) can be decomposed into two terms. One is the first month error, which results from the errors in the initial conditions plus the error introduced by the first month of coupling. It corresponds to the slowly varying component of the error, comparable to an initial shift that persists during the entire coupled experiment. The other term is the remaining time-evolving error, which is fast varying. We show that whereas the slowly varying term is strongly dependent upon the ocean and atmosphere component chosen for the coupling, the atmosphere generally controls the rapidly varying term to first order. The partition appears to be more balanced over some fractions of the Pacific warm pool and the east-equatorial coastal upwelling at certain seasons. These results, specific to seasonal forecasting and probably model dependent, can hardly be interpreted in terms of coupling mechanisms. The weak sensitivity of the mean error to the ocean component could in particular be related to the current limitations of state-of-the-art climate models.

Two ten-members ensemble experiments using a coupled ocean-atmosphere general circulation model are performed to study the dynamical response to a strong westerly wind event (WWE) when the tropical Pacific has initial conditions favourable to the development of a warm event. In the reference ensemble (CREF), no wind perturbation is introduced, whereas a strong westerly wind event anomaly is introduced in boreal winter over the western Pacific in the perturbed ensemble (CWWE). Our results demonstrate that an intense WWE is capable of establishing the conditions under which a strong El Nino event can occur. First, it generates a strong downwelling Kelvin wave that generates a positive sea surface temperature (SST) anomaly in the central-eastern Pacific amplified through a coupled ocean-atmosphere interaction. This anomaly can be as large as 2.5degreesC 60 days after the WWE. Secondly, this WWE also initiates an eastward displacement of the warm-pool that promotes the occurrence of subsequent WWEs in the following months. These events reinforce the initial warming through the generation of additional Kelvin waves and generate intense surface jets at the eastern edge of the warm-pool that act to further shift warm waters eastward. The use of a ten-members ensemble however reveals substantial differences in the coupled response to a WWE. Whereas four members of CWWE ensemble develop into intense El Nino warming as described above, four others display a moderate warming and two remains in neutral conditions. This diversity between the members appears to be due to the internal atmospheric variability during and following the inserted WWE. In the four moderate warm cases, the warm-pool is initially shifted eastward following the inserted WWE, but the subsequent weak WWE activity ( when compared to the strong warming cases) prevents to further shift the warm-pool eastwards. The seasonal strengthening of trade winds in June - July can therefore act to shift warm waters back into the western Pacific, reducing the central-eastern Pacific warming. This strong sensitivity of the coupled response to WWEs may therefore limit the predictability of El Nino events, as the high frequency wind variability over the warm pool region remains largely unpredictable even at short time lead.

[1] The role of salinity during the onset and growth of the 1997 Indian Ocean Dipole Mode event is explored with an ocean general circulation model thoroughly validated to observations. In fall 1997, anomalous easterlies drive an Indian Ocean equatorial circulation similar to that regularly observed in the Pacific Ocean: a South Equatorial Current (SEC) near the equator, an Equatorial Under Current (EUC) subsurface, and intense upwelling in the eastern part of the basin. The SEC transports westward the eastern Indian Ocean fresh pool creating, along the equator, a shallow salinity stratification favoring the creation of a barrier layer. In our experiment, the shallow top of the thermocline limits the barrier layer thickness to about 10 m; nevertheless, salinity has a significant impact on sea surface temperature (SST). Indeed, the shallow salinity stratification along the equator traps the wind forcing in a thin surface mixed-layer. The reduction of wind momentum penetration decreases the deceleration of the EUC, and the greater amplitude of wind momentum input in surface layers strengthens the SEC. This intensification of the equatorial zonal circulation increases the Sumatra upwelling and its associated meridional circulation. This strengthening of the whole equatorial circulation shifts upward the thermohaline structure and reinforces the cold SST anomaly off Sumatra by about 20%. Overall, the effect of salinity on the 1997 Indian Ocean Dipole is to reinforce the oceanic anomalies favoring a strengthening of the air-sea interactions.

A systematic modular approach to investigate the respective roles of the ocean and atmosphere in setting El Niño characteristics in coupled general circulation models is presented. Several state-of-the-art coupled models sharing either the same atmosphere or the same ocean are compared. Major results include 1) the dominant role of the atmosphere model in setting El Niño characteristics (periodicity and base amplitude) and errors (regularity) and 2) the considerable improvement of simulated El Niño power spectra—toward lower frequency—when the atmosphere resolution is significantly increased. Likely reasons for such behavior are briefly discussed. It is argued that this new modular strategy represents a generic approach to identifying the source of both coupled mechanisms and model error and will provide a methodology for guiding model improvement.

International audience; A multi-model ensemble-based system for seasonal-to-interannual prediction has been developed in a joint European project known as DEMETER (Development of a European Multimodel Ensemble Prediction System for Seasonal to Interannual Prediction). The DEMETER system comprises seven global atmosphere–ocean coupled models, each running from an ensemble of initial conditions. Comprehensive hindcast evaluation demonstrates the enhanced reliability and skill of the multimodel ensemble over a more conventional single-model ensemble approach. In addition, innovative examples of the application of seasonal ensemble forecasts in malaria and crop yield prediction are discussed. The strategy followed in DEMETER deals with important problems such as communication across disciplines, downscaling of climate simulations, and use of probabilistic forecast information in the applications sector, illustrating the economic value of seasonal-to-interannual prediction for society as a whole.

A four-dimensional variational scheme is described in which ocean observations are assimilated into an ocean general circulation model using wind stress forcing fields as control variables. Idealized (“twin”) experiments are performed to evaluate the possibility of reconstructing wind stress variability and its oceanic response from synthetic observations of the ocean state. Two types of wind stress errors are considered: time-varying errors associated with a wind burst and constant errors associated with a wind stress bias. Both sets of experiments demonstrate that the spatial structure of the wind stress variations is well reconstructed, while the estimation of their amplitude and time evolution is less accurate. Sparser equatorial sampling, similar to that of the Tropical Atmosphere–Ocean array, only slightly degrades the analysis. Omitting velocity and salinity observations leads to a less accurate amplitude and time evolution of the wind stress increment. Still, general features are captured...

[1] The hypothesis of the South Pacific origin of the decadal (7–35 years) ENSO-like variation is investigated based on 200-year outputs of the ECHAM4/OPA/OASIS coupled GCM. Associated with the decadal warm (cold) SST anomalies in the tropical Pacific, an anomalous cyclonic (anticyclonic) circulation, which is tilted in a southeast-northwest direction, appears in the South Pacific. This results in anomalous upward (downward) Ekman pumping along the northeastern edge of the anomalous circulation and, hence, shallowing (deepening) the oceanic thermocline there. Such an external source of heat content tends to slowly discharge/recharge the tropical ocean on the decadal timescale. The above result is consistent with the observational study of Luo and Yamagata [2001]. Besides, the model reproduces amazingly a high lagged-correlation between the global land surface temperature and the decadal Nino3 SST. Despite the model biases, this suggests a potential prediction skill of the global warming based on the ENSO-like decadal variation.

[1] The impact of salinity and the barrier layer on eastern Indian Ocean dynamics is investigated using the OPA ocean general circulation model for 1979–1993. The study focuses on processes involved in the eastern Indian Ocean circulation during the period of the fall Wyrtki Jet. An exploration of interactions between salinity and ocean dynamics is made using a set of salinity sensitivity experiments. The inclusion of salinity favors a stronger Wyrtki Jet that extends further eastward in two steps. First, a sporadic barrier layer along the jet increases the jet speed by trapping wind momentum in a thinner mixed layer. Second, zonal advection transports the current maximum eastward and participates in the creation of a strong current front at the eastern edge of the jet during the following month. Off Sumatra, the sea surface salinity gradient, bordering the Indian Ocean fresh pool, strengthens when the Wyrtki Jet reaches this region. The pressure gradient associated with the enhanced salinity front then becomes larger than the eastward wind stress forcing in the zonal momentum equation. This initiates a westward surface current opposed to the wind stress. The later penetration of the Wyrtki Jet into the Indian warm and fresh pool modifies the sea level and favors geostrophic westward currents around 4°N. A combination of these mechanisms additionally favors a northward shift of the Wyrtki Jet. All these phenomena affect the zonal displacement of the Indian fresh pool and create SST anomalies of 0.5°C, in a region of high ocean-atmosphere coupling.

International audience; This paper uses recent gridded and historical data in order to assess the relationships betweeninterannual variability of the Indian Summer Monsoon (ISM) and Sea Surface Temperature (SST)anomaly patterns over the Indian and Pacific oceans.Interannual variability of ISM rainfall and dynamical indices for the traditional summer monsoonseason (June-September) are strongly influenced by rainfall and circulation anomalies observedduring August and September, or the Late Indian Summer Monsoon (LISM). Anomalous monsoonsare linked to well-defined LISM rainfall and large-scale circulation anomalies. The east-westWalker and local Hadley circulations fluctuate during the LISM of anomalous ISM years. LISMcirculation is weakened and shifted eastward during weak ISM years. Therefore, we focus on thepredictability of the LISM in this study.Strong (weak) (L)ISMs are preceded by significant positive (negative) SST anomalies in thesoutheastern subtropical Indian Ocean, off Australia, during boreal winter. These SST anomaliesare mainly linked to south Indian Ocean dipole events, recently studied by Behera and Yamagata(2001), and to the El Niño-Southern Oscillation (ENSO) phenomenon. These SST anomalies arehighly persistent and affect the northwestward translation of the Mascarene high from austral toboreal summer. The southeastward (northwestward) shift of this subtropical high associated withcold (warm) SST anomalies off Australia causes a weakening (strengthening) of the wholemonsoon circulation through a modulation of the local Hadley cell during the LISM. Furthermore, itis suggested that the Mascarene high interacts with the underlying SST anomalies through apositive dynamical feedback mechanism, maintaining its anomalous position during the LISM.Our results also explain why a strong ISM is preceded by a transition in boreal spring from an ElNiño to a La Niña state in the Pacific and vice versa. An El Niño event and the associated warmSST anomalies over the southeastern Indian Ocean during boreal winter may play a key role in thedevelopment of a strong ISM by strengthening the local Hadley circulation during the LISM. On theother hand, a developing La Niña event in boreal spring and summer may also enhance the eastwestWalker circulation and the monsoon as demonstrated in many previous studies.

We describe the behaviour of 23 dynamical ocean-atmosphere models, in the context of comparison with observations in a common framework. Fields of tropical sea surface temperature (SST), surface wind stress and upper ocean vertically averaged temperature (VAT) are assessed with regard to annual mean, seasonal cycle, and interannual variability characteristics. Of the participating models, 21 are coupled GCMs, of which 13 use no form of flux adjustment in the tropics. The models vary widely in design, components and purpose: nevertheless several common features are apparent. In most models without flux adjustment, the annual mean equatorial SST in the central Pacific is too cool and the Atlantic zonal SST gradient has the wrong sign. Annual mean wind stress is often too weak in the central Pacific and in the Atlantic, but too strong in the west Pacific. Few models have an upper ocean VAT seasonal cycle like that observed in the equatorial Pacific. Interannual variability is commonly too weak in the models: in particular, wind stress variability is low in the equatorial Pacific. Most models have difficulty in reproducing the observed Pacific 'horseshoe' pattern of negative SST correlations with interannual Nino3 SST anomalies, or the observed Indian-Pacific lag correlations. The results for the fields examined indicate that several substantial model improvements are needed, particularly with regard to surface wind stress.

International audience; An ensemble of twenty four coupled ocean-atmosphere models has been compared with respect to their performance in the tropical Pacific. The coupled models span a large portion of the parameter space and differ in many respects. The intercomparison includes TOGA (Tropical Ocean Global Atmosphere)-type models consisting of high-resolution tropical ocean models and coarse-resolution global atmosphere models, coarse-resolution global coupled models, and a few global coupled models with high resolution in the equatorial region in their ocean components. The performance of the annual mean state, the seasonal cycle and the interannual variability are investigated. The primary quantity analysed is sea surface temperature (SST). Additionally, the evolution of interannual heat content variations in the tropical Pacific and the relationship between the interannual SST variations in the equatorial Pacific to fluctuations in the strength of the Indian summer monsoon are investigated. The results can be summarised as follows: almost all models (even those employing flux corrections) still have problems in simulating the SST climatology, although some improvements are found relative to earlier intercomparison studies. Only a few of the coupled models simulate the El Niño/Southern Oscillation (ENSO) in terms of gross equatorial SST anomalies realistically. In particular, many models overestimate the variability in the western equatorial Pacific and underestimate the SST variability in the east. The evolution of interannual heat content variations is similar to that observed in almost all models. Finally, the majority of the models show a strong connection between ENSO and the strength of the Indian summer monsoon.

In this study, the processes affecting sea surface temperature variability over the 1992–98 period, encompassing the very strong 1997–98 El Niño event, are analyzed. A tropical Pacific Ocean general circulation model, forced by a combination of weekly ERS1–2 and TAO wind stresses, and climatological heat and freshwater fluxes, is first validated against observations. The model reproduces the main features of the tropical Pacific mean state, despite a weaker than observed thermal stratification, a 0.1 m s−1 too strong (weak) South Equatorial Current (North Equatorial Countercurrent), and a slight underestimate of the Equatorial Undercurrent. Good agreement is found between the model dynamic height and TOPEX/Poseidon sea level variability, with correlation/rms differences of 0.80/4.7 cm on average in the 10°N–10°S band. The model sea surface temperature variability is a bit weak, but reproduces the main features of interannual variability during the 1992–98 period. The model compares well with the TAO current variability at the equator, with correlation/rms differences of 0.81/0.23 m s−1 for surface currents. The model therefore reproduces well the observed interannual variability, with wind stress as the only interannually varying forcing.\ud \ud This good agreement with observations provides confidence in the comprehensive three-dimensional circulation and thermal structure of the model. A close examination of mixed layer heat balance is thus undertaken, contrasting the mean seasonal cycle of the 1993–96 period and the 1997–98 El Niño. In the eastern Pacific, cooling by exchanges with the subsurface (vertical advection, mixing, and entrainment), the atmospheric forcing, and the eddies (mainly the tropical instability waves) are the three main contributors to the heat budget. In the central–western Pacific, the zonal advection by low-frequency currents becomes the main contributor. Westerly wind bursts (in December 1996 and March and June 1997) were found to play a decisive role in the onset of the 1997–98 El Niño. They contributed to the early warming in the eastern Pacific because the downwelling Kelvin waves that they excited diminished subsurface cooling there. But it is mainly through eastward advection of the warm pool that they generated temperature anomalies in the central Pacific. The end of El Niño can be linked to the large-scale easterly anomalies that developed in the western Pacific and spread eastward, from the end of 1997 onward. In the far-western Pacific, because of the shallower than normal thermocline, these easterlies cooled the SST by vertical processes. In the central Pacific, easterlies pushed the warm pool back to the west. In the east, they led to a shallower thermocline, which ultimately allowed subsurface cooling to resume and to quickly cool the surface layer.

In March 1997, a strong westerly wind event (WWE) occurred in the western equatorial Pacific prior to the 1997-1998 E1 Nifio event. It produced downwelling Kelvin waves that interacted non linearly with the surface temperature, salinity and zonal current fronts located at the eastern edge of the warm-fresh pool (EEWP). This non-linear interaction locally increased zonal currents by a factor of three compared to a theoretical linear response, and advected the EEWP at an unexpected rate (-l m/s) to which the ocean-atmosphere coupled system may have been responding rapidly to trigger E1 Nino conditions.

Uncertainties in the surface wind field have long been recognized as a major limitation in the interpretation of results obtained by oceanic circulation models. It is especially true in the tropical oceans, where the response to wind forcing is very strong on short time scales. The purpose of this paper is to show that these uncertainties can be greatly reduced by using spaceborne wind sensors that provide accurate measurements on a global basis.Surface winds over the global oceans have been measured by scatterometry since the launch of the European Remote Sensing Satellite (ERS-1) in August 1991 by the European Space Agency, Noordwijk, The Netherlands, and is currently provided by ERS-2, launched in April 1995. The ground track wind vectors are processed to compute mean weekly surface winds onto a 1 degrees square grid at the Institut Francais de Recherche pour 1'Exploitation de la Mer (IFREMER), Plouzane, France. These winds are validated by comparison with the buoy array in the tropical Pacific ocean, showing good agreement. In order to further evaluate this wind field, the three dimensional (3-D) ocean model OPA7 developed at Laboratoire d'Oceanographie Dynamique et de Climatologie, Paris, Prance, is forced over the tropical oceans by the ERS-derived wind stress fields and by fields from the atmospheric model Arpege/Climat. Selected ocean parameters are defined in order to validate the ocean model results with measurements of the tropical ocean and global atmosphere (TOGA) buoys in the Pacific ocean. The ability of the model to describe the short scale (a few weeks to a few years) oceanic variability is greatly enhanced when the satellite-derived surface forcing is used.In this paper, we present further comparison of the ocean model results with the TOPEX-Poseidon altimeter measurements. Simulated and measured sea level variability are described over the three tropical oceans. The annual and semi-annual signals, as well as the interannual variability, partly linked to the El Nino southern oscillation (ENSO) phenomenon, are well simulated by the OPA7 model when the satellite winds are used. Furthermore, it shows that the objective method, kriging technique, used to interpolate the mean ERS wind fields, dramatically reduces the effects of the satellite bandlike sampling. In the last part of this paper, we focus on the relationship between the wind stress anomalies and the sea level anomalies in the case of the 1997-1998 El Nino event. It clearly shows that sea level anomalies in the eastern and western parts of the Pacific are strongly linked to wind stress anomalies in the central Pacific. The forthcoming scatterometers aboard the METOP and ADEOS satellites will provide a much better coverage. It will enable the wind variability spatial and temporal scares to be resolved better, in order that wind uncertainties no longer blur the interpretation of ocean circulation numerical models results.

The thermocline depth anomalies during 1985–1992 and the 15-m current anomalies during 1988–1992 simulated by three oceanic general circulation models (OGCMs) of the tropical Pacific forced by different surface wind stress fields are compared to observations using a multivariate model testing procedure. The Hadley Centre model, OPA from LODYC, Paris, and HOPE from MPI, Hamburg, were forced by wind stress fields derived from the pseudo-wind stress produced from surface marine reports at the Florida State University (FSU) or from the response of atmospheric general circulation models (AGCMs) (ARPEGE, ECHAM3 and the Hadley Centre model) to the observed sea surface temperature (SST). It is found that the three oceanic models simulate the thermocline depth anomalies rather well, although large discrepancies with the observations remain, in particular, in the eastern part of the Pacific, and that they simulate them better than the surface current anomalies, without clear correspondence in model skill between the two variables. The simulated thermocline depth anomalies are also less sensitive to the choice of the wind stress. In general, the FSU wind stress leads to reliable simulations for both variables, while a given atmospheric model field may lead to a good simulation with one ocean model and to a bad one with another. Overall, the analysis suggests that the model parameters, which optimize a variable using a particular forcing, are not necessarily appropriate for another variable or another forcing, which suggests that some of the physics are not correctly represented in the oceanic models.

Numerous numerical simulations of basin-scale ocean circulation display a vast interior downwelling and a companion intense western boundary layer upwelling at midlatitude below the thermocline. These features, related to the so-called Veronis effect, are poorly rationalized and depart strongly from the classical vision of the deep circulation where upwelling is considered to occur in the interior. Furthermore, they significantly alter results of ocean general circulation models (OGCMs) using horizontal Laplacian diffusion. Recently, some studies showed that the parameterization for mesoscale eddy effects formulated by Gent and McWilliams allows integral quantities like the streamfunction and meridional heat transport to be free of these undesired effects. In this paper, an idealized OGCM is used to validate an analytical rationalization of the processes at work and help understand the physics. The results show that the features associated with the Veronis effect can be related quantitatively to ...

Satellite wind and wind stress fields at the sea surface, derived from the scatterometers on European Remote Sensing satellites 1 and 2 (ERS-1 and ERS-2) are used to drive the ocean general circulation model (OGCM) “OPA” in the tropical oceans. The results of the impact of ERS winds are discussed in terms of the resulting thermocline, current structures, and sea level anomalies. Their adequacy is evaluated on the one hand by comparison with simulations forced by the Arpege-Climat model and on the other hand by comparison with measurements of the Tropical Atmosphere-Ocean (TAO) buoy network and of the TOPEX/Poseidon altimeter. Regarding annual mean values, the thermal and current responses of the OGCM forced by ERS winds are in good agreement with the TAO buoy observations, especially in the central and eastern Pacific Ocean. In these regions the South Equatorial Current, the Equatorial Undercurrent, and the thermocline features simulated by the OGCM forced by scatterometer wind fields are described. The impact of the ERS-1 winds is particularly significant to the description of the main oceanic variability. Compared to the TAO buoy observations, the high-frequency (a few weeks) and the low-frequency of the thermocline and zonal current variations are described. The correlation coefficients between the time series of the thermocline simulated by ERS winds and that observed by the TAO buoy network are highly significant; their mean value is 0.73, over the whole basin width, while it is 0.58 between Arpege model simulation and buoy observations. At the equator the time series of the zonal current simulated by the ERS winds, at three locations (110°W, 140°W, and 165°E) and at two depths, are compared to the TAO current meter and acoustic Doppler current profiler (ADCP) measurements. The mean value of the significant correlation coefficients computed with the in situ measurements is 0.72 for ERS, while it is 0.51 for the Arpege-Climat model. Thus ERS wind fields through the OGCM generate more realistic current variations than those obtained with Arpege climate winds, and they are particularly efficient in capturing abrupt changes (“wind bursts”) which may be important regarding ocean dynamics.

Two transient CO 2 experiments using two coupled general circulation models developed by the French GASTON group have been realized using the same methodology. No flux corrections at the air-sea interface were used in these experiments. The main features of the present climate are reasonably well captured by both coupled models in the control simulations, although the biases are not the same, The transient CO 2 simulations show a global warming, ranging between 1.6 and 2.0 °C at the time of CO 2 doubling (+ 70 years). These values, and the main geographical characteristics of climate change, are in agreement with previous studies published by other research groups, using either flux corrected or non-flux corrected models.

A high-resolution oceanic general circulation model (OGCM) of the three tropical oceans is used to investigate long equatorial wave activity in the Pacific Ocean during the 1985–94 TOGA period. The ARPEGE atmospheric general circulation model simulated zonal wind stress forcing and the OPA OGCM simulated dynamic height are interpreted using techniques previously applied to data. Long equatorial waves of the first baroclinic mode (Kelvin and first-mode Rossby waves) are detected propagating in the model outputs during the entire period. A seasonal cycle and interannual anomalies are computed for each long equatorial wave. In the east Pacific basin, long equatorial wave coefficients are dominated by seasonal variations, while west of the date line they display strong interannual anomalies. Interannual long-wave anomalies are then compared to wave coefficients simulated by a simple wind-forced model. The results presented here indicate the major role played by wind forcing on interannual timescales ...

International audience; A primitive equation model is used to investigate the warm pool equilibrium of the tropical Pacific ocean. Attention is focused on the upper ocean. The oceanic response is described using an isothermal approach applied to warm waters contained in the TOGA-COARE domain. The heat balance shows that all the terms, atmospheric surface fluxes, advection and diffusion, operate in the heat budget with different time scales. Over long periods, diffusive heat fluxes transfer heat received from the atmosphere out of the warm pool trough the top of the main thermocline. Over short periods, the impact of westerly wind bursts modifies this balance: atmospheric heating is reversed, diffusion is enhanced and advective heat transports out of the warm pool operate through zonal and vertical contributions. We were able to relate the two latter processes to zonal jets and Ekman pumping, respectively. Conversely, the meridional contribution always represents a source of heat, mainly due to the tropical wind convergence. The modelling results clearly show that except during strong wind events, entrainment cooling is not an important component of the budget. The inability to remove heat is due to the salt stratification which needs to be first reduced or even destroyed by westerly wind bursts to activate heat entrainment into deeper layers. Finally, we suggest that the near zero estimate for the surface heat flux entering the warm pool may be extended to longer periods including seasonal to interannual time scale.

A 10-year simulation with a coupled ocean-atmosphere general circulation model (CGCM) is presented. The model consists of the climate version of the Meteo-France global forecasting model, ARPEGE, coupled to the LODYC oceanic model, OPA, by the CERFACS coupling package OASIS. The oceanic component is dynamically active over the tropical Pacific, while climatological time-dependent sea surface temperatures (SSTs) are prescribed outside of the Pacific domain. The coupled model shows little drift and exhibits a very regular seasonal cycle. The climatological mean state and seasonal cycle are well simulated by the coupled model. In particular, the oceanic surface current pattern is accurately depicted and the location and intensity of the Equatorial Undercurrent (EUC) are in good agreement with available data. The seasonal cycle of equatorial SSTs captures quite realistically the annual harmonic. Some deficiencies remain including a weak zonal equatorial SST gradient, underestimated wind stress over the Pacific equatorial band and an additional inter-tropical convergence zone (ITCZ) south of the equator in northern winter and spring. Weak interannual variability is present in the equatorial SST signal with a maximum amplitude of 0.5°C.

The warm event which spread in the tropical Atlantic during Spring-Summer 1984 is assumed to be partially initiated by atmospheric disturbances, themselves related to the major 1982–1983 El-Nino which occurred 1 year earlier in the Pacific. This paper tests such an hypothesis. For that purpose, an atmospheric general circulation model (AGCM) is forced by different conditions of climatic and observed sea surface temperature and an Atlantic ocean general circulation model (OGCM) is subsequently forced by the outputs of the AGCM. It is firstly shown that both the AGCM and the OGCM correctly behave when globally observed SST are used: the strengthening of the trades over the tropical Atlantic during 1983 and their subsequent weakening at the beginning of 1984 are well captured by the AGCM, and so is the Spring 1984 deepening of the thermocline in the eastern equatorial Atlantic, simulated by the OGCM. As assumed, the SST anomalies located in the El-Nino Pacific area are partly responsible for wind signal anomaly in the tropical Atlantic. Though this remotely forced atmospheric signal has a small amplitude, it can generate, in the OGCM run, an anomalous sub-surface signal leading to a flattening of the thermocline in the equatorial Atlantic. This forced oceanic experiment cannot explain the amplitude and phase of the observed sub-surface oceanic anomaly: part of the Atlantic ocean response, due to local interaction between ocean and atmosphere, requires a coupled approach. Nevertheless this experiment showed that anomalous conditions in the Pacific during 82–83 created favorable conditions for anomaly development in the Atlantic. DOI: 10.1034/j.1600-0870.1994.t01-1-00009.x

After a brief presentation of the ocean properties, the set of equations used in global ocean modelling is presented. Different technical choices can affect the solution: the system of coordinates, the grid, the numerical algorithms… The impact of physical choices is then presented. Present capacity of numerical modelling is discussed on two examples: a high resolution model of the tropical Pacific and a global circulation model.

The embedment of a 1.5 turbulence closure model in an ocean general circulation model of the equatorial Atlantic is presented. The eddy viscosity and diffusivity involved in the vertical mixing are defined as the product of a characteristic turbulent velocity—the root square of the turbulent kinetic energy—and a characteristic mixing length. The turbulent kinetic energy is defined through a prognostic equation while the turbulent length scales are defined by a diagnostic formulation. The results of an experiment that includes this closure scheme are compared to the results issued from another experiment that includes a Richardson number-dependent parameterization of the mixing coefficients. The two simulations were performed over the tropical Atlantic during the 1982–1984 period, which allows direct comparisons with data from the FOCAL and SEQUAL experiments. Obvious contrasts between the two experiments on the sea surface temperature and on the dynamics indicate that the turbulent vertical diffu...

[1] By modifying mixed layer thickness, the barrier layer (BL) may have a significant impact on air–sea interactions. In the tropical Indian Ocean, thermohaline observations are lacking over most of the basin, despite a few BL observations in the Bay of Bengal and off Sumatra. A 15-yearlong model simulation is used to investigate the seasonal variations and formation mechanisms of the BL. In the eastern equatorial area, the model thermohaline structure is in close agreement with observations and simulates a realistic seasonal variability. A robust and thick BL (of more than 40 m) is observed seasonally with a maximum horizontal extent (of about 1500 km) in November. This BL corresponds to the merging of two BLs that undergo different formation mechanisms. The first is formed as the Wyrtki Jet propagates eastward, deepening the thermocline while zonally advecting a salinity subsurface maximum. At the same time, northward wind stresses induce a northward advection of a thin fresh water layer toward the equator. This combination decouples salinity from temperature, and a BL is formed. Later, as the jet penetrates further toward the fresh pool, a second BL appears off Sumatra. This one is controlled by two different mechanisms. A westward surface current trapped in the thin mixed layer off Sumatra advects fresh water from the coastal precipitation area. Then, significant upwelling associated with the westward current further raises the halocline. In situ observations confirm the realism of these simulated formation mechanisms. From year to year, the timing, thickness, and shape of the BLs follow the strong interannual variability of the Wyrtki Jet. Despite these variations, the formation mechanisms remain unchanged.

An Ocean General Circulation Model is used to investigate the oceanic response to the March 1997 Westerly Wind Event that is suggested to have played an important role in the onset of the 1997–1998 El Nino. Our results point out three distinct impacts. First a strong wind-forced downwelling Kelvin wave propagates eastward generating sea surface temperature anomalies up to 1°C and large subsurface temperature and zonal current anomalies, mainly located in the core of the thermocline. Second the northward and westward extension of this wind event is responsible for a surface advection of cold waters from 130°E–5°N to the equator. Third it generates large zonal surface currents at the eastern edge of the warm and fresh pool by a nonlinear interaction between the wind-forced surface jet and the local thermohaline front. Salinity through both its contribution to the local zonal pressure gradient at the front and the barrier layer effect is crucial in the occurrence of this nonlinear mechanism. The fast displacement of the front (2000 km in a month) together with the cooling in the western Pacific is likely to be responsible for the eastward displacement of atmospheric deep convection and eastward winds observed in April–June 1997 and thus to have played a major role in initiating the El Nino of the century.