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[1] Sea surface salinity (SSS) measurements from the Aquarius/SAC-D satellite during September–December 2011 provide the first satellite observations of the salinity structure of tropical instability waves (TIWs) in the Pacific. The related SSS anomaly has a magnitude of approximately ±0.5 PSU. Different from sea surface temperature (SST) and sea surface height anomaly (SSHA) where TIW-related propagating signals are stronger a few degrees away from the equator, the SSS signature of TIWs is largest near the equator in the eastern equatorial Pacific where salty South Pacific water meets the fresher Inter-tropical Convergence Zone water. The dominant westward propagation speed of SSS near the equator is approximately 1 m/s. This is twice as fast as the 0.5 m/s TIW speed widely reported in the literature, typically from SST and SSHA away from the equator. This difference is attributed to the more dominant 17-day TIWs near the equator that have a 1 m/s dominant phase speed and the stronger 33-day TIWs away from the equator that have a 0.5 m/s dominant phase speed. The results demonstrate the important value of Aquarius in studying TIWs.

[1] Observations are used to develop metrics for interannual tropical instability wave (TIW) variability in the Atlantic and to relate that variability to larger scale processes. The analysis is partitioned into different latitude bands to distinguish between off-equatorial (5°S, 2°N, and 5°N) and near-equatorial (2°S and 0°) TIWs. TIW metrics based on sea surface temperature (SST) and sea level anomaly (SLA) fluctuations are compared against interannual anomalies of SST in the cold tongue region. To examine the role of barotropic shear instabilities in modulating the intensity of a TIW season, wind stress and near-surface current indices are developed in regions where the shear between the Equatorial Undercurrent (EUC) and the northern branch of the South Equatorial Current (nSEC) and between the nSEC and the North Equatorial Countercurrent (NECC) are expected to be largest. Good agreement is found between the SST and SLA TIW metrics along the off-equatorial latitude bands, and interannual variations of both metrics can largely be attributed to barotropic shear instabilities. In particular, years with low (high) TIW variance along the off-equatorial latitude bands are associated with anomalously warm (cold) SSTs in the cold tongue region, weak (strong) wind stress divergence and curl in the EUC-nSEC region, and weak (strong) zonal current shear in the nSEC-NECC region. In contrast, in the near-equatorial latitude bands, poor agreement is found between interannual TIW activity based on the SST and SLA metrics, and near-equatorial TIW variability cannot be explained by the large-scale SST, wind stress divergence and curl, and current shear indices.

The Kalman filter is widely used in data assimilation for operational oceanography, in particular for forecasting problems. Yet, now that data assimilation applications tend to diversify, with reanalysis problems for instance, the three-dimensional (3D) formulation of the filter doesn't allow an optimal use of the observations. The four-dimensional extention of the 3D methods, called smoothers, allows a better use of the observations, assimilating them on a retrospective way. We study in this work the implementation and the effects of a reduced-rank smoother on reanalysis, with a realistic tropical Atlantic ocean circulation model. First we expose some sensitive steps required for the smoother implementation, most notably the covariances evolution parametrisation of the filter. The smoother's benefits for reanalysis are then exposed, compare to a 3D reanalysis. It shows that the global error can be reduced by 15% on assimilated variables (like temperature). The smoother also leads to an analyzed solution dynamically closer to the reference (compare to the filter), as we can observe with phasing of Brazil rings for instance. Finally, we studied a case of smoothing based on optimal interpolation (instead of the filter). This case is inconsistent with the theory but often used in operational centers. Results shows that the smoother can improve the reanalysis solution in an OI case (reducing the global error from 10 to 15%), but still the dynamical evolution of error covariances (filter) are needed to get a correction according with the real error structures.; Le filtre de Kalman est largement utilisé pour l'assimilation de données en océanographie opérationnelle, notamment dans le cadre de prévisions. Néanmoins, à l'heure où les applications de l'assimilation de données tendent à se diversifier, notamment avec les réanalyses, la formulation tridimensionnelle (3D) du filtre n'utilise pas de façon optimale les observations. L'extension de ces méthodes 3D (filtre) à une formulation 4D (appelés lisseurs), permet de mieux tirer partie des observations en les assimilant de façon rétrograde. Nous étudions dans cette thèse la mise en place et les effets d'un lisseur de rang réduit sur les réanalyses, dans le cadre d'une configuration réaliste de la circulation océanique en Atlantique tropical. Ce travail expose dans un premier temps les aspects sensibles mais nécessaires de l'implémentation du lisseur, avec notamment la paramétrisation des statistiques d'erreur et leur évolution temporelle. Les apports du lissage sur les réanalyses sont ensuite étudiés, en comparant la qualité de la solution lissée par rapport à la solution filtrée. Ces résultats permettent d'exposer les bienfaits d'une assimilation 4D. On observe notamment une diminution de l'erreur globale de environ 15% sur les variables assimilées, ainsi qu'une bonne capacité du lisseur à fournir une solution cohérente avec la dynamique de référence. Ce point est illustré par le rephasage de certaines structures sensibles comme les anneaux du Brésil. Enfin, un cas moins en accord avec la théorie mais plus facile à mettre en pratique (et plus souvent utilisé dans les centres opérationnels), l'interpolation optimale, a permis d'étudier les apports du lissage et ses limites dans une telle configuration. L'évolution temporelle des erreurs pour le lissage s'est ainsi révélée nécessaire pour garder un maximum de cohérence avec les erreurs réelles. Néanmoins, le lisseur montre tout de même des résultats encourageant avec l'interpolation optimale en abaissant le niveau global d'erreur (de 10 à 15%).

International audience; This study investigated the seasonality of tropical instability waves (TIWs) and its feedback to the seasonal cycle in the tropical eastern Pacific using a high-resolution ocean model covering 1958-2007. The climatological mean of the TIWs featured intraseasonal fluctuations, implying that TIWs are not occurring randomly, but their amplitude is partly in phase from one year to another. This seasonality of TIW activity is attributed to their dependency on the seasonal mean variation of current and temperature. Energy conversion analysis confirmed that the strong variability of TIWs near 4 • N was due to the barotropic energy conversion associated with the large meridional shear of NECC and SEC and that at another pole near 2 • N was due to the baroclinic energy conversion associated with the temperature front in the mixed layer. The former and latter poles are somehow largely responsible for amplifying the dynamic and thermal eddies of TIWs, respectively. The intensified TIWs during a boreal fall increase the tropical eastern Pacific SST by associating the warm thermal advection by anomalous currents, with a rate of up to 1 • C/month in September. Therefore, this leads to interactive feedback between seasonal and intraseasonal variations, that is, TIWs in the tropical eastern Pacific.

THE ocean has considerable spatial and temporal heterogeneity in biomass and productivity owing in part to the effects of ocean circulation and mixing1,2. Water mass boundaries (fronts) in coastal waters are well-known sites of enhanced biological activity3,4. Comparatively little is known of open-ocean fronts, and one of the few biological studies of an oceanic front showed phytoplankton biomass at only slightly higher densities than in surrounding waters5. Here we present photographs and measurements from satellites, aircraft, ships and the Space Shuttle Atlantis which show dramatic biological responses to circulation and mixing processes associated with an open-ocean front. Breaking waves (whitecaps) caused by water turbulence and mixing, and very dark green water caused by extremely high concentrations (>20 mg of chlorophyll a per m3) of buoyant diatoms (Rhizosolenia sp.) made a distinct line in the sea visible for hundreds of kilometres. The line traced the northern edge of a westward-progagating (50 km per day) tropical instability wave (1,000-km wavelength) delineating the boundary between cold, upwelled waters and warmer waters to the north. High phytoplankton biomass and primary production associated with the extensive diatom patches may explain anecdotal observations of high animal abundance along this frontal boundary.

Tropical instability waves have been shown to have a major impact on the variability of temperature and nutrients along the equatorial wave guide. In order to assess the impact of these features on carbon species distributions during an ENSO event, sea surface temperature, salinity, sigma-t, nitrate, CO[sub 2] fugacity, total inorganic carbon, total alkalinity, and pH along the equator were measured from 130[degrees]W to 100[degrees]W during 8-15 May 1992. Concurrent moored measurements of surface currents and temperature were also made at 0[degrees], 110[degrees]W. Results indicate that tropical instability waves, with periods of 15 to 20 days and zonal wavelengths of 700-800 km, controlled the observed spatial variability of the CO[sub 2] species, nitrate and hydrographic parameters at the equator. 37 refs., 3 figs.

[1] Sea surface temperature images and surface drifter observations are compared to the results from a high-resolution numerical simulation to study the properties of cyclonic eddies generated at the density front of the tropical instability waves in the tropical Pacific Ocean. These cyclonic eddies, of which the diameter is about 30–100 km and the vertical extent is limited to the upper 100 m in depth, have physical characteristics similar to those of smaller submesoscale eddies at the midlatitudes according to the model. They have highly coherent structures below the surface, carrying cold and salty upwelled equatorial water probably rich in marine life. The stretching and tilting of the upper layer of the ocean provides the main mechanism responsible for the intense cyclonic vorticity of the eddies, involving complex evolution of the density field into occluded fronts.

[1] A regional coupled model is used for a dynamic downscaling over the tropical Atlantic based on a global warming simulation carried out with the Geophysical Fluid Dynamics Laboratory CM2.1. The regional coupled model features a realistic representation of equatorial ocean dynamical processes such as the tropical instability waves (TIWs) that are not adequately simulated in many global coupled climate models. The coupled downscaling hence provides a unique opportunity to assess their response and impact in a changing climate. Under global warming, both global and regional models exhibit an increased (decreased) rainfall in the tropical northeast (South) Atlantic. Given this asymmetric change in mean state, the regional model produces the intensified near-surface cross-equatorial southerly wind and zonal currents. The equatorial cold tongue exhibits a reduced surface warming due to the enhanced upwelling. It is mainly associated with the increased vertical velocities driven by cross-equatorial wind, in contrast to the equatorial Pacific, where thermal stratification is suggested to be more important under global warming. The strengthened upwelling and zonal currents in turn amplify the dynamic instability of the equatorial ocean, thereby intensifying TIWs. The increased eddy heat flux significantly warms the equator and counters the effect of enhanced upwelling. Zonal eddy heat flux makes the largest contribution, suggesting a need for sustained monitoring of TIWs with spatially denser observational arrays in the equatorial oceans. Overall, results suggest that eddy heat flux is an important factor that may impact the mean state warming of equatorial oceans, as it does in the current climate.

We examine the variability of the sea surface temperatures in the eastern equatorial Atlantic during 2005-2007 by using Argo profiling floats, Prediction and Research Moored Array in the Atlantic buoys and satellite, in situ, and atmospheric data sets. The eastern equatorial Atlantic, characterized by shallow mixed layers all yearlong, is divided into nine boxes of nearly equal surface area, with respect to the dynamics and thermodynamics in this region. Monthly mixed layer heat budgets are computed in each box from 10 day Argo profiles. In all the boxes, the net surface heat flux is one of the main causes of the seasonal evolution of sea surface temperatures for the 3 studied years. The amount of short-wave radiation penetrating through the base of the mixed layer and horizontal heat advection may locally contribute to the temperature variability, while entrainment has a weaker contribution. To balance the heat budget, a residual term exists which includes all processes that cannot be calculated with observations as well as the possible errors in the other terms. This residual is more intense in the cold tongue and the northern region and exhibits a clear seasonal cycle, with minimum (negative) values in boreal summer and maximum values in winter. This residual compares well with available observations of vertical turbulent mixing collected during Etude de la Circulation Oceanique et des Echanges Ocean-Atmosphere dans le Golfe de Guinee campaigns (2005-2007) in the eastern equatorial Atlantic. When assuming that the residual is mostly associated with vertical turbulent mixing, it can be conjectured that turbulent mixing is a significant cooling source in the cold tongue and north of the equator.

International audience; Submesoscale dynamics in tropical instability waves (TIWs) of the Pacific ocean are analyzed from nested numerical simulations of increasing resolution (36 km, 12 km, and 4 km). It is shown that numerical convergence, as determined by an invariance of the kinetic energy (KE) spectrum with resolution, can be obtained for grid spacing around 10 km. This finding contrasts with mid-latitude simulations of submesoscale processes that do not converge even for grid spacing less than 1 km. The difference is associated with the larger Rossby radius of deformation at low latitudes due to the weaker Coriolis parameter. For the same reason, the mixed layer Rossby radius is larger as well, resulting in submesoscale mixed layer eddies (MLEs) with scales of 50-200 km. Similar to MLEs at mid-latitudes, those eddies that form at TIW fronts restratify the mixed layer by releasing available potential energy. They lead to an additional source of KE at submesoscale that is larger in the finer resolution simulations. At wavelengths smaller than the mesoscale peak of KE injection, a forward cascade of KE is evident in the simulations where three dynamical ranges are observed: a quasi-inertial range of slope k−2, a pre-dissipation range and a far-dissipation range. Numerical dissipation is evaluated and significant dissipation is found to occur in a pre-dissipation range, i.e., for wavenumbers well below the range where the KE spectrum tails off exponentially.

A nonlinear, 2 1 2 - layer model is used to study the dynamics of wind-driven equatorial ocean circulation, including the generation of mean flows and instabilities. The model allows water to entrain into, and detrain from, the upper layer, and as a consequence the temperatures of the two active layers can vary. The model ocean basin is rectangular, extends 100° zonally, and for most solutions has open boundaries at 15°S and 15°N. All solutions are forced by a switched-on wind field that is an idealized version of the Pacific trades: the wind is westward, uniform in the meridional direction (so it has no curl), located primarily in the central and eastern oceans, and in most cases it has an amplitude of 0.5 dyn cm−2. For reasonable choices of parameters, solutions adjust to have a realistic equatorial circulation with a westward surface jet, an eastward undercurrent, and with upwelling and cool sea surface temperature in the eastern ocean. Most of the meridional circulation (81% of the transport) is part of a closed tropical circulation cell, in which water upwells in the eastern, equatorial ocean and downwells elsewhere in the basin; the rest participates in a mid-latitude circulation cell with lower-layer water entering the basin and upper-layer water leaving it through the open boundaries. Three basic types of unstable disturbances are generated in the eastern ocean: two of them are antisymmetric about the equator, one being surface-trapped with a period of about 21 days (f1), and the other predominantly a lower-layer oscillation with periods ranging from 35 to 53 days (f2) that causes the undercurrent to meander; the third is symmetric with a period of about 28 days (f0) and a structure like that of a first-meridional-mode Rossby wave. The amplitudes of the disturbances are sensitive to model parameters, and as parameter values are varied systematically solutions appear to follow variations of the quasi-periodic route to turbulence, one of the common transitions to chaotic behavior. Realistic mean flows develop only when detrainment and lower-layer cooling are present in the model physics, processes that are necessary for the generation of a tropical circulation cell: without detrainment, water accumulutes in the upper layer until entrainment ceases and the model adjusts to Sverdrup balance, which is a state of rest for a wind without curl; without cooling, the temperature of the lower layer slowly rises until it approaches that of the upper layer. The mean-momentum budget for the upper layer shows that the model's Reynolds-stress terms are not a significant part of the momentum balance, having a maximum amplitude only about 19% of the wind stress. In contrast, the mean-heat budget demonstrates that eddy heating warms the cold tongue significantly, with an amplitude as large as the heating through the surface. Interestingly, the time-averaged continuity equations indicate that the instabilities tend to increase the upward tilt of the upper-layer interface toward the equator. When layer temperatures are kept fixed only a weak version of disturbance f1 develops, indicating that the equatorial temperature front is an important aspect of instability dynamics. In fact, a frontal instability does exist in the model; it involves the conversion of mean to eddy potential energy, but it is the mean energy associated with the variable upper-layer temperature field, rather than with tilted layer interfaces, as is the case for traditional baroclinic instability. Perturbation-energy budgets suggest that frontal, barotropic and Kelvin-Helmholtz instabilities are energy sources for the disturbances, whereas traditional baroclinic instability is an energy sink. The two, fastest growing, antisymmetric, unstable-wave solutions to a linearized version of the model correspond closely to disturbances f1 and f2 from the nonlinear model, and perturbation-energy budgets for these waves indicate that their energy sources are primarily frontal instability and lower-layer barotropic instability, respectively.

(1) This study documents the variability of noise in the equatorial Pacific associated with the El Nino and La Nina events based on the ensemble retrospective forecasts of the Climate Forecast System. It is found that the noise (measured by the ensemble spread) in the western equatorial Pacific zonal wind stress is enhanced around and before the peak of El Nino events and weakened around 2-3 months after the peak of El Nino events. The change in the wind stress noise is communicated to the thermocline depth in the eastern equatorial Pacific with about 1 month time lag. The eastern equatorial Pacific SST noise, however, decreases during the decay stages of El Nino events while the corresponding noise in the surface zonal wind and heat flux increases. This decrease in the SST noise is related to the weakening of the SST front and the suppression of the tropical instability waves along the flanks of the equatorial Pacific cold tongue that is associated with the SST warming due to El Nino events. Similar relations are seen during La Nina events except that the changes are in opposite sense. As such, the signal-to-noise ratio and, thus, the predictability for the eastern equatorial Pacific SST is relatively high during warm events compared to cold events.

[1] The nonlinearity of Tropical Instability Waves (TIWs) is studied using shallow-water equations with various mean states from the equatorial Pacific Ocean. In the early linear stage, unstable TIWs, centered near 5°N with a wavelength about 1000 km and a period about one month, dominate the whole domain. However neutral Yanai waves with periods about 15―22 days emerge near the equator when the unstable TIWs grow into fully nonlinear vortices and begin to rotate, which stabilizes the mean states substantially. The strength of these Yanai waves are sensitive to the instability of the initial mean flow. Meanwhile the TIWs centered near 5°N are slowed down and weakened. The external forcing terms are found to be important for them to retain their dominance from 3°N to 7°N and also be able to suppress the late emerging Yanai waves if strong enough.

The tropical Indian and Pacific oceans share the largest span of warm water and deep atmospheric convection on our planet, which is also the ascending part of the Walker circulation, an essential component of the climate system. This region is also home to strong atmospheric and oceanic intraseasonal to interannual variability, with strong climatic consequences. The El Niño phenomenon, and, to a lesser extent, the Indian Ocean dipole (IOD) indeed have strong climatic consequences around these two basins, but also at global scale. Intraseasonal variability linked to the Madden-Julian oscillation (MJO) is also important, with its modulation of Indian and Australian monsoons and its potentially important role in triggering El Niño. In this document, I describe my research on intraseasonal to interannual oceanic and atmospheric variability in the Indian and Pacific basins. El Niño or the IOD are by essence coupled ocean-atmosphere phenomena: they result from positive feeadbacks arising from air-sea interactions (the "Bjerknes feedback"). At intraseasonal timescale, the coupling seems less important and only partially modifies the characteristic of a dominantly atmospheric or oceanic mode of variability. For example, Tropical Instability Waves in the eastern Pacific are the results of internal oceanic instabilities. However, they influence the stability of the atmospheric boundary layer and the surface windstress, and this feedback tends to reduce slightly their variability. On the other hand, the MJO is predominantly an atmospheric process, resulting from the coupling between atmospheric dynamics and deep atmospheric convection. But I will show that this phenomenon induce astrong oceanic response in the Indian ocean, both in terms of dynamics and surface temperature. The degree to which air-sea coupling influences properties of the MJO however remains an open question. I will also discuss interactions between those various modes of variability. I will for example illustrate how intraseasonal atmospheric variability can trigger an El Niño; how an El Niño can suppress tropical instability waves and be affected in return; how the IOD modulates the MJO activity and the interactions between the IOD and El Niño. I will then discuss a region of the Indian ocean which is emblematic of these scale interactions and in which I have developed past (Cirene cruises from 2005 and 2008) and hopefully future (TRIO cruise) observational programs. The 5°S-10°S band in the Indian Ocean has an elevated thermocline and shallow mixed layer due to climatological Ekman pumping, but high SST close to the threshold of deep atmospheric convection. These two factors increase air-sea coupling in this region, which has clear variability at synoptic (cyclones), intraseasonal (MJO) and interannual (IOD) timescales. This region has strong climatic consequences and impacts the quality of the following Indian monsoon, the number of cyclones in the Madagascar-La Réunion area and even atmospheric patterns over the Maritime continent and Northern Pacific ocean. I will conclude by presenting my future research plans and my projects in terms of cruises and observational networks.; Les océans Pacifique et Indien tropicaux se partagent la plus grande étendue d'eau chaude et de convection profonde de la planète. Cette région est le siège de la branche ascendante de la circulation de Walker, circulation atmosphérique d'échelle planétaire parfois décrite comme la " machine thermique " de la Terre. Cette région, dont les répercussions sur le climat sont importantes, est aussi source de variabilité océanique et atmosphérique aux échelles intrasaisonnières et interannuelles . En effet, la variabilité interannuelle associée à El Niño, dans l'océan Pacifique, et - dans une moindre mesure - au dipôle de l'Océan Indien (DOI) ont des conséquences climatiques marquées sur les pourtours de ces bassins et à l'échelle du globe. La variabilité intrasaisonnière liée à l'oscillation de Madden-Julian (OMJ) a également des conséquences climatiques marquées : modulations des moussons indiennes et australiennes et un rôle potentiellement important dans le déclenchement d'ENSO. Dans ce mémoire, je vais décrire mes travaux de recherche sur la variabilité océanique et atmosphérique aux échelles intrasaisonnière et interannuelle dans les Océans Indien et Pacifique. El Niño ou le DOI sont des modes couplés : c'est la rétroaction positive découlant des interactions océan-atmosphère qui est source de variabilité (le " Bjerknes feedback "). À l'échelle intrasaisonnière, le rôle du couplage océan-atmosphère semble moins primordial, et modifie seulement des modes de variabilité essentiellement atmosphériques ou océaniques. Par exemple, les ondes d'instabilité dans le Pacifique Est sont le résultat d'une instabilité interne océanique. Cependant, elles affectent la stabilité atmosphérique, les vents de surface, et cela tend à réduire légèrement leur activité. À l'inverse, l'OMJ est un phénomène dont la source est atmosphérique, naissant du couplage entre dynamique et convection dans les tropiques. Toutefois, nous verrons que ce phénomène a une réponse océanique forte dans l'Océan Indien, à la fois en termes de dynamique et de thermodynamique. Le degré d'influence du couplage dans les propriétés de l'OMJ reste toutefois une question largement ouverte. Nous nous intéresserons aussi à la question des interactions entre ces différents modes de variabilité. Nous verrons par exemple comment la variabilité intrasaisonnière atmosphérique peut déclencher un El Niño, comment El Niño peut supprimer l'activité des ondes tropicales d'instabilité et l'effet retour, comment le DOI module l'activité de l'OMJ et enfin, quelles sont les interactions entre DOI et El Niño. Je présenterai alors une région de l'Océan Indien assez emblématique de ces interactions d'échelle, et dans laquelle j'ai développé une activité d'observations (campagnes océanographiques Cirene de 2005 à 2008 et projet de campagne TRIO). La bande 5°S-10°S dans l'Océan Indien est une région très particulière. En raison de la structure des vents, la thermocline y est proche de la surface et la couche de mélange est peu profonde, ce qui induit une forte réactivité de la température de surface aux sollicitations de l'atmosphère. De plus, la température de surface en hiver boréal est proche du seuil de convection, impliquant une sensibilité accrue de l'atmosphère à de petites variations de température. Ces deux facteurs augmentent le couplage océan atmosphère dans cette région qui a une variabilité très marquée aux échelles synoptiques (cyclones), intrasaisonnières (OMJ) et interannuelle (réponse à El Niño, mais aussi au DOI). Cette région a enfin des conséquences climatiques marquées (sur l'intensité des pluies de la mousson suivante, sur le nombre de cyclones dans le secteur La Réunion-Madagascar, sur la convection au-dessus du continent maritime, et même sur l'Amérique du Nord). Pour conclure, je présenterai ma réflexion sur mes axes de recherches futurs, ainsi que mes projets en termes de campagnes et réseaux d'observations.

The tropical Indian and Pacific oceans share the largest span of warm water and deep atmospheric convection on our planet, which is also the ascending part of the Walker circulation, an essential component of the climate system. This region is also home to strong atmospheric and oceanic intraseasonal to interannual variability, with strong climatic consequences. The El Niño phenomenon, and, to a lesser extent, the Indian Ocean dipole (IOD) indeed have strong climatic consequences around these two basins, but also at global scale. Intraseasonal variability linked to the Madden-Julian oscillation (MJO) is also important, with its modulation of Indian and Australian monsoons and its potentially important role in triggering El Niño. In this document, I describe my research on intraseasonal to interannual oceanic and atmospheric variability in the Indian and Pacific basins. El Niño or the IOD are by essence coupled ocean-atmosphere phenomena: they result from positive feeadbacks arising from air-sea interactions (the "Bjerknes feedback"). At intraseasonal timescale, the coupling seems less important and only partially modifies the characteristic of a dominantly atmospheric or oceanic mode of variability. For example, Tropical Instability Waves in the eastern Pacific are the results of internal oceanic instabilities. However, they influence the stability of the atmospheric boundary layer and the surface windstress, and this feedback tends to reduce slightly their variability. On the other hand, the MJO is predominantly an atmospheric process, resulting from the coupling between atmospheric dynamics and deep atmospheric convection. But I will show that this phenomenon induce astrong oceanic response in the Indian ocean, both in terms of dynamics and surface temperature. The degree to which air-sea coupling influences properties of the MJO however remains an open question. I will also discuss interactions between those various modes of variability. I will for example illustrate how intraseasonal atmospheric variability can trigger an El Niño; how an El Niño can suppress tropical instability waves and be affected in return; how the IOD modulates the MJO activity and the interactions between the IOD and El Niño. I will then discuss a region of the Indian ocean which is emblematic of these scale interactions and in which I have developed past (Cirene cruises from 2005 and 2008) and hopefully future (TRIO cruise) observational programs. The 5°S-10°S band in the Indian Ocean has an elevated thermocline and shallow mixed layer due to climatological Ekman pumping, but high SST close to the threshold of deep atmospheric convection. These two factors increase air-sea coupling in this region, which has clear variability at synoptic (cyclones), intraseasonal (MJO) and interannual (IOD) timescales. This region has strong climatic consequences and impacts the quality of the following Indian monsoon, the number of cyclones in the Madagascar-La Réunion area and even atmospheric patterns over the Maritime continent and Northern Pacific ocean. I will conclude by presenting my future research plans and my projects in terms of cruises and observational networks.; Les océans Pacifique et Indien tropicaux se partagent la plus grande étendue d'eau chaude et de convection profonde de la planète. Cette région est le siège de la branche ascendante de la circulation de Walker, circulation atmosphérique d'échelle planétaire parfois décrite comme la " machine thermique " de la Terre. Cette région, dont les répercussions sur le climat sont importantes, est aussi source de variabilité océanique et atmosphérique aux échelles intrasaisonnières et interannuelles . En effet, la variabilité interannuelle associée à El Niño, dans l'océan Pacifique, et - dans une moindre mesure - au dipôle de l'Océan Indien (DOI) ont des conséquences climatiques marquées sur les pourtours de ces bassins et à l'échelle du globe. La variabilité intrasaisonnière liée à l'oscillation de Madden-Julian (OMJ) a également des conséquences climatiques marquées : modulations des moussons indiennes et australiennes et un rôle potentiellement important dans le déclenchement d'ENSO. Dans ce mémoire, je vais décrire mes travaux de recherche sur la variabilité océanique et atmosphérique aux échelles intrasaisonnière et interannuelle dans les Océans Indien et Pacifique. El Niño ou le DOI sont des modes couplés : c'est la rétroaction positive découlant des interactions océan-atmosphère qui est source de variabilité (le " Bjerknes feedback "). À l'échelle intrasaisonnière, le rôle du couplage océan-atmosphère semble moins primordial, et modifie seulement des modes de variabilité essentiellement atmosphériques ou océaniques. Par exemple, les ondes d'instabilité dans le Pacifique Est sont le résultat d'une instabilité interne océanique. Cependant, elles affectent la stabilité atmosphérique, les vents de surface, et cela tend à réduire légèrement leur activité. À l'inverse, l'OMJ est un phénomène dont la source est atmosphérique, naissant du couplage entre dynamique et convection dans les tropiques. Toutefois, nous verrons que ce phénomène a une réponse océanique forte dans l'Océan Indien, à la fois en termes de dynamique et de thermodynamique. Le degré d'influence du couplage dans les propriétés de l'OMJ reste toutefois une question largement ouverte. Nous nous intéresserons aussi à la question des interactions entre ces différents modes de variabilité. Nous verrons par exemple comment la variabilité intrasaisonnière atmosphérique peut déclencher un El Niño, comment El Niño peut supprimer l'activité des ondes tropicales d'instabilité et l'effet retour, comment le DOI module l'activité de l'OMJ et enfin, quelles sont les interactions entre DOI et El Niño. Je présenterai alors une région de l'Océan Indien assez emblématique de ces interactions d'échelle, et dans laquelle j'ai développé une activité d'observations (campagnes océanographiques Cirene de 2005 à 2008 et projet de campagne TRIO). La bande 5°S-10°S dans l'Océan Indien est une région très particulière. En raison de la structure des vents, la thermocline y est proche de la surface et la couche de mélange est peu profonde, ce qui induit une forte réactivité de la température de surface aux sollicitations de l'atmosphère. De plus, la température de surface en hiver boréal est proche du seuil de convection, impliquant une sensibilité accrue de l'atmosphère à de petites variations de température. Ces deux facteurs augmentent le couplage océan atmosphère dans cette région qui a une variabilité très marquée aux échelles synoptiques (cyclones), intrasaisonnières (OMJ) et interannuelle (réponse à El Niño, mais aussi au DOI). Cette région a enfin des conséquences climatiques marquées (sur l'intensité des pluies de la mousson suivante, sur le nombre de cyclones dans le secteur La Réunion-Madagascar, sur la convection au-dessus du continent maritime, et même sur l'Amérique du Nord). Pour conclure, je présenterai ma réflexion sur mes axes de recherches futurs, ainsi que mes projets en termes de campagnes et réseaux d'observations.

[1] The breeding method of Toth and Kalnay finds the perturbations that grow naturally in a dynamical system like the atmosphere or the ocean. Here breeding is applied to a global ocean model forced by reanalysis winds in order to identify instabilities of weekly and monthly timescales. This study extends the method to show how the energy equations for the bred vectors can be derived with only very minimal approximations and used to assess the physical mechanisms that give rise to the instabilities. Tropical Instability Waves in the tropical Pacific are diagnosed, confirming the existence of bands of both baroclinic and barotropic energy conversions indicated earlier by Masina et al. and others. In the South Atlantic Convergence Zone, the bred vector energy analysis shows that there is kinetic to potential ocean eddy energy conversion, suggesting that the growing instabilities found in this area are forced by the wind.

On the basis of integrations of an eddy-permitting coupled physical-biological model of the tropical Pacific we explore changes in the simulated mean circulation as well as its intraseasonal to interannual variability driven by the biologically modulated vertical absorption profiles of solar radiation. Three sensitivity ocean hind-cast experiments, covering the period from 1948 to 2003, are performed. In the first one, simulated chlorophyll affects the attenuation of light in the water column, while in the second experiment, the chlorophyll concentration is kept constant in time by prescribing an empirically derived spatial pattern. The third experiment uses a spatially and temporally constant value for the attenuation depth. The biotically induced differential heating is generated by increased absorption of light in the surface layers, leading to a surface warming and subsurface cooling. The effect is largest in the eastern equatorial Pacific. However, the initial vertical redistribution of heat leads to considerable changes of the near-surface ocean circulation subsequently influencing the near-surface temperature structure. In general, including biophysical coupling improves the model performance in terms of temperature and ocean circulation patterns. In particular, the upwelling in the eastern equatorial Pacific is enhanced, the mixed layer becomes shallower, the warm bias in the eastern Pacific is reduced, and the zonal temperature gradient increases. This leads to stronger La Niña events and an associated increase in the variability of the Niño3 SSTA time series. Furthermore, the eddy kinetic energy (EKE) associated with mesoscale eddies in the eastern equatorial Pacific increases by almost 100% because of enhanced EKE production due to enhanced horizontal and vertical shear of the mean currents.

[1] Ocean eddies and fronts affect surface stress via two mechanisms: (1) ocean surface currents altering the relative motion between air and sea and, hence, the stress fields and (2) ocean sea surface temperature (SST) gradients forcing changes in stability and near-surface winds. In this paper, we quantify the first effect and how it impacts Tropical Instability Waves (TIW) in the eastern Pacific. High-resolution satellite data and a regional coupled model are used to distinguish between stress changes due to the surface currents and those due to the changes in stability and near-surface winds. It is found that both mechanisms affect the surface stress curl, but they do so at different latitudes, allowing for their effect on Ekman pumping to be distinguished. The Ekman pumping due to the surface current effect alone, leads to significant damping of the TIWs. In terms of the eddy kinetic energy, the inclusion of surface current in the stress leads to decay with an e-folding time comparable with the period of the TIWs. It is, thus, an important damping mechanism to be included in ocean and coupled ocean-atmosphere models.

[1] Tropical instability waves (TIWs) are 20- to 40-day waves that appear along the sea surface temperature (SST) front of the equatorial cold tongues. In this study, we investigate the atmospheric planetary boundary layer response to TIW-induced SST variations in the tropical Atlantic Ocean using Community Atmosphere Model version 3 (CAM3) developed at the National Center for Atmospheric Research and compare the model simulation with available satellite measurements. CAM3 is used to simulate the response to Atlantic TIWs with T85 and T42 horizontal resolutions forced with daily Tropical Rainfall Measuring Mission microwave imager (TMI) SSTs and T85 horizontal resolution forced with weekly TMI and Reynolds SSTs. All four runs exhibit an atmospheric response to the SST anomalies, but the amplitude of the response is different in each case. The model simulates winds, convergence, column-integrated water vapor, and precipitation perturbations associated with TIWs that are similar in many respects to the satellite observations. However, the wind convergence and divergence anomalies in the satellite observations are located west of the SST anomalies. In the CAM3 simulations the wind convergence is more in phase with SST anomalies. Differences between the observations and the model simulations appear to be due to both differences in the boundary layer response and differences in the horizontal advection process.

[1] The spatial and temporal distributions of tropical instability waves (TIWs) in the Atlantic Ocean are investigated using a combination of current observations with moored instruments deployed at the equator at 23 degrees W and a realistic eddy-resolving (1/12 degrees) general circulation model of the Atlantic Ocean. The meridional and vertical shears of the zonal current system contribute to the eddy production rates and thus to the generation of TIWs in the central tropical Atlantic Ocean. In the Southern Hemisphere, TIWs are forced only by baroclinic instability associated with the vertical shear of the central part of the South Equatorial Current (SEC). In the Northern Hemisphere, baroclinic instability due to the vertical shear of the northern SEC (nSEC) as well as barotropic instabilities due to horizontal shears of the Equatorial Undercurrent (EUC)/nSEC and nSEC/North Equatorial Countercurrent (NECC) contribute to the generation of the TIWs. Since seasonal changes of the instability production rates related to the EUC/nSEC are comparable low while the rates related to the nSEC/NECC are high, we suggest that the seasonality of the NECC dominates the seasonal modulation of the TIWs.

[1] The impacts of parameterized lateral ocean viscosity on climate are explored using three 120-year integrations of a fully coupled climate model. Reducing viscosity leads to a generally improved ocean circulation at the expense of increased numerical noise. Five domains are discussed in detail: the equatorial Pacific, where the emergence of tropical instability waves reduces the cold tongue bias; the Southern Ocean, where the Antarctic Circumpolar Current increases its kinetic energy but reduces its transport; the Arctic Ocean, where an improved representation of the Atlantic inflow leads to a better sea-ice distribution; the North Pacific, where the more realistic path of the Kuroshio leads to more realistic temperatures across the midlatitude Pacific; and the northern marginal seas, where stronger boundary currents lead to significantly less sea-ice. Although the ocean circulation and sea-ice distribution improve, the oceanic heat uptake, the poleward heat transport, and the large scale atmospheric circulation are not changed significantly. In particular, the improvements to the equatorial cold tongue did not lead to better representation of tropical precipitation or El Nino.

[1] Using a database of roughly 300 basin-wide conductivity-temperature-depth (CTD) stations per year from 1997 to 2003, we examined interannual and seasonal variability of nutrients and chlorophyll across the equatorial Pacific. During this period, chlorophyll concentrations exceeded the range of previous measurements for this region. Nitrate, silicic acid, and phosphate also varied widely but not necessarily coherently with each other or with chlorophyll. Across the La Nina to El Nino continuum there was nonmonotonic variability in chlorophyll, particulate backscatter (a proxy for phytoplankton carbon), and large size fraction (diatom) chlorophyll. In general, while El Nino was associated with decreased phytoplankton biomass, there was no corresponding increase in phytoplankton or diatoms during La Nina. However, there were increases in macronutrients in response to La Nina. We suggest that the lack of a biological response to these nutrient increases is due to their decoupling from iron supply. Multiple linear regression analysis of the physical factors responsible for vertical nutrient fluxes emphasized the importance of winds in the central and western Pacific, and thermocline depth in the east. We suggest that the role of the winds is not limited to enhancement of upwelling, but perhaps more importantly to increased vertical mixing of nutrients. Seasonal patterns were weak but consistent with previous work which has suggested that enhanced productivity in the second half of the calendar year causes reduced surface pCO2. Enhanced coverage of nutrient sensors on the tropical atmosphere ocean (TAO) array would help to quantify seasonal signals associated with processes such as tropical instability waves and lead to a better understanding of the links between productivity, carbon export, and air-sea CO2 exchange.

n n Recent high-resolution space-based observations reveal significant two-way air-sea interactions associated with tropical instability waves (TIWs) in the tropical oceans. But in most large-scale climate modeling studies, the atmospheric wind response to TIW-induced sea surface temperature (SSTTIW) anomalies has not been taken into account realistically and thus the corresponding feedbacks to the ocean are not known. Here we apply a singular value decomposition (SVD) analysis to satellite data to derive an empirical model for the SSTTIW induced wind stress (tau(TIW)) variability in the eastern tropical Pacific. The derived SSTTIW-tau(TIW) relationship is then nested into a basin-scale ocean and a hybrid coupled ocean-atmosphere model of the tropical Pacific to take into account the TIW-induced wind SST coupling. It is demonstrated that TIW-induced wind feedback to the ocean can have a rectified effect on large-scale mean ocean state and interannual variability, with an asymmetric difference in SST (a cooling) and a significant modulation of ENSO amplitude.

Tropical instability waves and tropical instability vortices (TIVs) exert major controls on ocean dynamics, thermodynamics, and biology on intraseasonal to seasonal time scales. To understand the fundamental mechanisms at play, a Lagrangian analysis of the 3D circulation of westward-propagating TIVs was performed in a high-resolution Atlantic Ocean simulation. The model reproduces the main temperature and velocity features of the tropical Atlantic mean state and the TIVs. Lagrangian diagnostics were used to track the water masses transported in vortices and exchanged with surrounding waters. The 3D circulation within vortices is consistent with previous observations and dominated by anticyclonic rotation with downwelling and upwelling near the leading and trailing edges of the vortex, respectively. This convergent flow creates sharp gradients at the TIV southwestern edge, where vertical mixing is most efficient. While TIVs remain highly dynamically coherent throughout their lifetime, significant exchanges occur with their surroundings, with 50% of their water being renewed over one rotation cycle. A detailed investigation of the eddies’ sources and sinks reveals that they mostly transport southern water zonally, while northern waters are mostly passing through or fluxed southward in their lee. A notable source of entrained water is the Equatorial Undercurrent.

International audience; The spatial and temporal distributions of tropical instability waves (TIWs) in the Atlantic Ocean are investigated using a combination of current observations with moored instruments deployed at the equator at 23°W and a realistic eddy-resolving (1/12°) general circulation model of the Atlantic Ocean. The meridional and vertical shears of the zonal current system contribute to the eddy production rates and thus to the generation of TIWs in the central tropical Atlantic Ocean. In the Southern Hemisphere, TIWs are forced only by baroclinic instability associated with the vertical shear of the central part of the South Equatorial Current (SEC). In the Northern Hemisphere, baroclinic instability due to the vertical shear of the northern SEC (nSEC) as well as barotropic instabilities due to horizontal shears of the Equatorial Undercurrent (EUC)/nSEC and nSEC/North Equatorial Countercurrent (NECC) contribute to the generation of the TIWs. Since seasonal changes of the instability production rates related to the EUC/nSEC are comparable low while the rates related to the nSEC/NECC are high, we suggest that the seasonality of the NECC dominates the seasonal modulation of the TIWs.

The strong response of the atmospheric troposphere to the oceanic tropical instability waves (TIWs) results in a signature of wavelike in the free-tropospheric temperatures T850-300. A positive lag exists between the atmospheric temperature and the SST anomaly, the phasic lags change with the TIWs position and the atmospheric adjustment speed. Over warm water, the mixed boundary layer is unstable, air temperature is higher and the turbulence and convection enhances the exchange of momentum from winds aloft to the surface, and higher surface wind flux result.

International audience; Intraseasonal variability (10-50 days) in the equatorial Atlantic Ocean is analyzed from multiyear (1999-2005) satellite gridded products of sea-level anomalies (SLA) and sea-surface temperature (SST). Two regions with distinct intraseasonal variability have been identified. The first one, west of 10 degrees W, is dominated by westward-propagating anomalies, with maximum values in SLA along 5 degrees N and in SST along 2 degrees N: They occur in boreal summer with periods of 25-50 days and are known to correspond to tropical instability waves (TIWs). We show that TIWs have also a signature, though weaker, south of the equator, especially along 5 degrees S, in SLA. Northern and southern anomalies propagate together westward, being mostly out of phase, suggesting that equatorial wave dynamics is involved in TIWs variability. An SST signature of TIWs is also observed near 2 degrees S, in quadrature with SST anomalies detected in the Northern Hemisphere. The interannual modulations of the TIW signature in SLA and SST are compared and discussed. The second dominant intraseasonal signal is only seen east of 10 degrees W in SST and corresponds to an equatorially trapped variability, confined to the Gulf of Guinea with periods between 10 and 20 days. This signal is present in boreal summer when an intense SST front is observed just north of the equator. Intraseasonal variability with comparable periods is also observed in the meridional wind stress throughout the year. Comparison of SST and meridional wind stress anomalies suggests that the 10- to 20-day variability in SST is forced by the wind stress but seasonally modulated by the presence of the SST front.

Eddy energetics in the central equatorial Pacific Ocean is examined using Acoustic Doppler Current Profiler velocities and CTD densities collected during the Hawaii-to-Tahiti Shuttle Experiment, in 1979–80. Three distinct sources of eddy energy are identified with varying degrees of statistical reliability, and are interpreted as evidence for three separate instabilities of the mean flow field. An instability at and just north of the equator occurs primarily in boreal summer and fall. It arises from the cyclonic shear between the Equatorial Undercurrent and the South Equatorial Current (SEC) north of the equator. The instability is present only when and where both currents are well developed, and there is little involvement of the shear between the SEC and the North Equatorial Countercurrent (NECC). The instability is characterized by local maxima in zonal and meridional eddy velocity variance, strong U*V* Reynolds stress, and large mean flow to eddy kinetic energy conversion. Despite seasonal va...

[1] High-resolution satellite measurements from the Tropical Rainfall Measuring Mission (TRMM), Quick Scatterometer (QuikSCAT), and Special Sensor Microwave Imager (SSM/I) are used to study the variability of sea surface temperature (SST), surface wind velocity, water vapor, cloud liquid water, and precipitation associated with westward moving tropical instability waves (TIWs) in the Atlantic Ocean from 1998 to 2005. Coherent ocean-atmosphere patterns are shown during these 8 years. Southeasterly trades strengthen and water vapor increases over warm SST anomalies associated with TIWs. The opposite is true over cold TIW SST anomalies. The cloud liquid water and rain response to the SSTs follows a similar pattern, appearing generally downstream of SST anomalies in the central tropical Atlantic. The atmospheric response to the TIW SST anomalies extends north of the TIW active region, suggesting a remote response to the TIWs. The atmospheric response to the Atlantic TIWs also exhibits interannual variations. In 1999, owing to the southward movement of the Atlantic Intertropical Convergence Zone (ITCZ), the rainfall response to the TIW SST anomalies is much larger than in other years. When the Atlantic ITCZ moves south, it is more susceptible to TIW influence.

[1] Velocity data from moored current meters is combined with satellite sea surface temperature (SST) to compute oceanic mixed layer temperature advection by tropical instability waves (TIWs). For the years 2002 to 2005 it is found that this process heats the equatorial mixed layer at an annual mean rate of +0.8°C/month at 0°N, 140°W; and +2.8°C/month at 0°N, 110°W. At 0°N, 110°W, approximately 25% of the heating is contributed by zonal temperature advection, a process that has often been assumed to be negligible. From a nine month segment of data (May 2004–February 2005), the zonal temperature advection at 2°N, 140°W has been estimated to be approximately 0.7°C/month, much larger than the equatorial value for the same time period. Thus, the data supports a recent hypothesis that tropical instability waves contribute a significant mean zonal temperature advection with off-equatorial maxima to the equatorial mixed-layer heat budget. Comparisons with numerical model results suggest that current ocean general circulation models can realistically simulate important aspects of tropical eddy-mixed layer interactions.

[1] Eight years of microwave data from the Tropical Rainfall Measuring Mission (TRMM) satellite are used to study the interannual variations of tropical instability waves (TIWs) in the Pacific and Atlantic Oceans. SST signatures of TIWs in the Pacific Ocean are strongest during the cold phase of ENSO, when the cold tongue is most pronounced. The waves are weak during the warm phase of ENSO. A low-frequency Atlantic air-sea coupled equatorial mode affects TIW activity in the Atlantic in a similar fashion. Interannual variations in sea-surface temperatures due to TIW activity can produce interannual variations in the atmospheric mesoscale response.

[1] Near-surface current velocity data from the Pilot Research Moored Array in the Tropical Atlantic at 23°W in the year 2002 were re-examined and compared to simultaneous unpublished velocity data at 10°W, near the surface. Strong 7-day-period fluctuations were observed in the zonal velocity component and in temperature records at 23°W. The large temperature fluctuations near the thermocline depth suggest a vertical velocity component at the same 7-day period. The 7-day-period signals occurred in June/July simultaneously with the seasonal upwelling. The meridional velocity component presented similar spectral contents at 23°W and at 10°W. Two distinguishable quasiperiodic signals were observed in the meridional component at both locations: The first signal occurred in the spring, had a periodicity of about 14 days, was subsurface intensified, and had the characteristics of a mixed Rossby-gravity wave. The second signal occurred three times in the year (once in winter 2001–2002, another in summer, and another during fall 2002), had a periodicity of about 20 days, was strongest close to the surface, and was the result of the passage of tropical instability waves (TIWs). Satellite sea surface temperature (SST) data show the same periodicities as the meridional velocity records, and present a spatial structure in accordance with the TIW-SST signature. The summer of 2002 showed different TIW-SST patterns depending upon location, suggesting the existence of different types of TIWs. Overall, variability in the current meter data for the year 2002 was mainly due to current instabilities, except around boreal spring, where intraseasonal variability in surface winds was strongest.

[1] The controlling mechanism of the seasonal cycle of surface partial pressure of CO2 (pCO2) in the central/eastern equatorial Pacific is studied using a coupled physical-biogeochemical model. The model results indicate that the individual components of physical transport of total CO2 (TCO2) in the Nino3 area are out of phase with each other such that the net TCO2 transport is an order smaller. This net transport is further nearly balanced by the biological drawdown and air-sea CO2 flux combined, resulting in a weak seasonality of surface TCO2. As a consequence, the seasonal cycle of sea surface temperature (SST) controls the surface pCO2 cycle with a maximum in boreal spring-summer and a minimum in boreal fall-winter. In turn, the seasonality of air-sea CO2 flux is largely controlled by SST and surface winds via the piston velocity of gas exchange. Perturbations to the subtle balance of TCO2 transport by dynamical processes on other timescales such as El Nino, La Nina and tropical instability waves, could induce significant change to the surface pCO2, in addition to the change due to SST variability.

International audience; This paper presents a comparison of two reduced-order, sequential, and variational data assimilation methods: the singular evolutive extended Kalman filter (SEEK) and the reduced 4D-Var (R-4D-Var). A hybridization of the two, combining the variational framework and the sequential evolution of covariance matrices, is also preliminarily investigated and assessed in the same experimental conditions. The comparison is performed using the twin-experiment approach on a model of the tropical Pacific domain. The assimilated data are simulated temperature profiles at the locations of the TAO/TRITON array moorings. It is shown that, in a quasilinear regime, both methods produce similarly good results. However, the hybrid approach provides slightly better results and thus appears as potentially fruitful. In a more nonlinear regime, when tropical instability waves develop, the global nature of the variational approach helps control model dynamics better than the sequential approach of the SEEK filter. This aspect is probably enhanced by the context of the experiments in that there is a limited amount of assimilated data and no model error.

A numerical simulation is used to investigate the mixed layer heat balance of the tropical Pacific Ocean including the equatorial cold tongue and the region of vortices associated with tropical instability waves (TIWs). The study is motivated by a need to quantify the effects that TIWs have on the climatological heat budget of the cold tongue mixed layer; there has been some discrepancy between observations indicating very large equatorward heat transport by TIWs and models that disagree on the full three-dimensional budget. Validation of the model reveals that the TIW-induced circulation patterns are realistic but may have amplitudes about 15% weaker than those in the observations. The SST budget within tropical instabilities is first examined in a frame of reference moving with the associated tropical instability vortices (TIVs). Zonal advection of temperature anomalies and meridional advection of temperature by current anomalies dominate horizontal advection. These effects strongly heat the cold cusps and slightly cool the downwelling areas located at the leading edge of the vortices. Cooling by vertical mixing is structured at the vortex scale and almost compensates for horizontal advective heating in the cold cusps. In contrast to some previous studies, TIW-induced vertical advection is found to be negligible in the SST budget. Cooling by this term is only significant below the mixed layer. The effect of TIWs on the climatological heat budget is then investigated for the region bounded by 2 degrees S - 6 degrees N, 160 degrees - 90 degrees W, where instabilities are most active. TIWinduced horizontal advection leads to a warming of 0.84 degrees C month(-1), which is of the same order as the 0.77 degrees C month(-1) warming effect of atmospheric fluxes, while the mean currents and vertical mixing cool the upper ocean by -0.59 degrees C month(-1) and -1.06 degrees C month(-1), respectively. The cooling effect of TIW-induced vertical advection is also negligible in the long-term surface layer heat budget and only becomes significant below the mixed layer. The results above, and in particular the absence of cancellation between horizontal and vertical TIW-induced eddy advection, are robust in three other sensitivity experiments involving different mixing parameterizations and increased vertical resolution.

From a numerical simulation of the Atlantic Ocean, Jochum and Malanotte-Rizzoli provide evidence that the equatorial subsurface countercurrents can be triggered by tropical instability waves through eddy–mean flow interactions in a low-Rossby-number regime. Adapting the transformed Eulerian mean formalism to a shoaling jet, they propose eddy heat fluxes to be the driving mechanism for the subsurface countercurrents. Here it is shown that such a formalism relying on the existence of a residual meridional streamfunction cannot be applied to a shoaling jet, so that the eddy heat fluxes term in the zonal momentum equation cannot be rigorously justified. Moreover, the role of the zonal pressure gradient that was dropped in their study needs to be reassessed. Despite this mathematical questioning of Jochum and Malanotte-Rizzoli’s framework, the authors agree with them that eddy heat fluxes may contribute to the dynamics of the subsurface countercurrents.

Temperature, salinity, velocity, and wind from a mooring at 0°N, 23°W are used along with satellite data for sea surface temperature and sea level to examine the contribution of tropical instability waves (TIWs) to the energy and heat balance of the equatorial Atlantic mixed layer. The TIWs appear as periodic 20-30 day fluctuations of currents, temperature, and salinity, which intensify beginning in June and peak in late boreal summer. The intensification occurs in phase with strengthening of the southeasterly trade winds and the seasonal appearance of the equatorial tongue of cold mixed layer temperatures. In 2002 these waves, which warm the mixed layer by 0.35°C during summer months, are maintained by both barotropic and baroclinic conversions that are of comparable size. Salinity fluctuations, previously neglected, increase the magnitude of baroclinic energy conversion.

Tropical Instability Waves (TIW) have been suggested to fertilize the equatorial Pacific in iron leading to enhanced ecosystem activity. Using a coupled dynamical-biogeochemical model, we show that contrary to this suggestion, TIWs induce a decrease of iron concentration by 20% at the equator and by about 3% over the "TIW box" [90°W- 180, 5°N-5°S]. Chlorophyll decreases by 10% at the equator and 1% over the "TIW box". This leads to a decrease of new production up to 10% at the equator (4% over the "TIW box"). TIW-induced horizontal advection brings more iron-depleted water to the equator than it exports iron-rich equatorial water to the north. Additional iron decrease is caused by TIW-induced iron vertical diffusion. These two mechanisms are partly counter balanced by a decrease of iron biological uptake, driven by weaker phytoplankton concentration, and to a lesser extend by TIW- induced iron vertical advection.

[1] During 1998, five episodes of elevated chlorophyll concentration are observed in the Pacific around the Marquesas Islands (140°W, 10°S) in SeaWiFS ocean color observations. Daily positions of geostationary satellite sea surface temperature fronts reveal the influence of the westward propagating tropical instability waves as narrow temperature fronts that extend far to the south of the equator and intersect the nutrient rich waters of the Marquesas. Because the upwelled equatorial waters are rich in iron, phytoplankton blooms are produced and are advected downstream by the South Equatorial Current. Previous studies attributed the blooms to local sources of iron and the interaction of the Marquesas with the South Equatorial Current. The satellite observations suggest that the currents that produced the phytoplankton blooms may have diverted the Kon-Tiki southward in 1947 as described by Thor Heyerdahl.

[1] GasEx-2001 is a study of air-sea gas exchange in a region of CO2 outgassing. The bulk of the experiment followed a drifting array of near-surface instruments deployed during the second half of February 2001 just south of the equator in the central Pacific Ocean. Physical oceanographic conditions including local currents, the seasonal cycle, Kelvin waves, and tropical instability waves are described using shipboard data and a variety of other data sources to set the large-scale oceanographic context for GasEx-2001. Local physical oceanographic conditions during GasEx-2001 are then analyzed using shipboard data and a simple one-dimensional mixed layer model. The thermocline shoals about 13 m over the 15-day experiment, implying an upwelling rate of 1 × 10−5 m s−1. Zonal velocity is surface-intensified and westward, with vertical shear mostly through the thermocline. Meridional velocity is also strongly sheared with a maximum equatorward flow in the thermocline that is much reduced by 17-m depth. The mixed layer model exhibits more near-surface warming over the course of the experiment than is observed. Prescribing upwelling in the model closes the heat budget within error estimates. Entrainment at the base of the mixed layer plays a limited role in the mixed layer budgets of carbon and other water properties. Vertical shear of horizontal velocity within the mixed layer and slippage of the array through the surface water also have small (but uncertain) roles in these budgets.

[1] Using a mass-conserving (non-Boussinesq) Pacific Ocean model and TOPEX/POSEIDON sea-surface height measurements, we document previously unreported bottom pressure waves (BPWs), associated with large-amplitude eddies (vortices) both north and south of the Equator. The bottom pressure signals, defined as the vertically integrated water-mass anomalies, are found to propagate westward across the Pacific Ocean with an approximate 4-mbar amplitude (1 mbar = 100 Newton/m2, approximately equivalent to 1 cm of sea surface height), continuously interacting with topographic features. Their wavelength is about 1100 kilometers, phase speed is about 0.5 m/s, and period is about 30-days, all consistent with the known properties of tropical instability waves (TIWs). These results suggest that the constantly evolving tropical instability eddies might have strong bottom pressure signals, therefore, likely alias the U.S.-German Gravity Recovery and Climate Experiment (GRACE) measurements, which have a monthly-sampling period.

[1] In this study we investigate how the modulation of surface wind-stress by tropical instability waves (TIWs) feeds back onto TIWs and plays a role in their fundamental properties. An ocean general circulation model is used, that reproduces qualitatively well the properties of TIWs when forced by climatological winds, although with a 30% underestimated amplitude. The ocean model is coupled to the atmosphere through a simple parameterization of the wind stress response to SST. The properties of the TIWs in the coupled simulations are compared with those without active coupling. Active coupling results in a negative feedback on TIWs, slightly reducing their temperature and meridional current variability, both at the surface and sub-surface. This reduced activity modulates the meridional heat and momentum transport, resulting in modest changes to the mean state, with a cooler cold tongue and stronger equatorial currents.

[1] The Galapagos Islands provide a topographic barrier for the Southern Equatorial Current (SEC) and the Equatorial Undercurrent (EUC). An island wake effect can be diagnosed from the difference of an ocean general circulation model simulation which includes the Galapagos Islands and one which ignores their presence. Cold thermocline water upwells on the western side of the islands, and only during boreal winter season these cold waters can linger around the Islands at a depth of about 80 m and affect the far eastern equatorial Pacific surface waters. This effect is partly offset by the westward transport of cold surface waters by the SEC which creates a wake on the western side of the Islands. It is furthermore shown that changes in horizontal current shear, induced by the presence of the Galapagos Islands modify the generation of tropical instability waves and lead to a basin scale SST anomaly pattern.

The interannual variations of tropical instability waves (TIWs) in the eastern Pacific Ocean and their relationships with ENSO intensity are studied using a 28-year long coupled atmosphere-ocean general circulation model (CGCM) simulation. The activity of TIWs is measured by the root-mean-square value of the 50-day high-pass filtered sea surface temperature (SST) perturbations produced in the simulation. The CGCM realistically produces two branches of large TIW activity near the equator: one at 2°N and the other at 2°S. It is found that along both branches there are linear relationships between the year-to-year variations of TIW activity and those of ENSO intensity. TIW activity is enhanced or reduced in proportion to the NINO3 SST anomalies, with larger activity in La Nina years and smaller activity in El Nino years. It is found that ENSO modulates TIW activity in both branches primarily by changing the latitudinal SST gradient associated with the SST front immediately north of the equator. Weaker correlations are found between the year-to-year variations of TIW activity and those of the ocean current shears between the south equatorial current and the equatorial undercurrent or the north equatorial countercurrent. This CGCM study suggests that the baroclinic instability associated with the northern SST front is a major generation mechanism for both the northern and southern TIW branches in the eastern Pacific Ocean.

[1] Time series from the PIRATA 10-minute measurements are investigated in order to study the upper-ocean thermal field time-scales of variability using the serial covariance power spectra method and wavelet analysis. This is one of the few studies that analyze in time and space the PIRATA buoy data in the region. Analysis of the depth of the 20°C isotherm show distinct patterns for the northwestern Atlantic at 8N–38W and the southeastern Atlantic, at 10S–10W. In the west, warm surface waters are accompanied by a shallower thermocline while in the east warm surface waters are associated with its deepening, consistent with Houghton [1991]. At higher frequencies the dominant signals in sea surface temperature are the diurnal and semi-diurnal cycles. Spectral analysis of the depth of the 20°C isotherm show a predominance of the semi-diurnal period. In the eastern Atlantic there are two additional dominant bands in the spectra: 10–14 days, 25–27 days and for the depth of the 20°C isotherm there is also a lower frequency band at 20–40 days associated with tropical instability waves at the equator. The first band can be associated with atmospherically forced motions and the lower frequency band with barotropic instability waves.

[1] Numerical experiments are performed using an ocean general circulation model (OGCM) to assess the impact of the form of lateral mixing of momentum and tracers on the state of an equatorial ocean. It is found that the large-scale structure of both the velocity and temperature fields are very sensitive to the imposed parameterization of lateral mixing in the model. With uniform values for the viscosity and diffusion coefficient across the domain, a decrease in these coefficients increases the activity of tropical instability waves (TIW), resulting in an increase in the sea surface temperature of the cold tongue, and thus overcoming the cold bias often found in ocean models of the equatorial Pacific. However, there is an associated increase in the strength of the Equatorial Under-Current (EUC) to unrealistic levels. It is found that the strength of the EUC can be limited, while little affecting the TIW activity, by applying an enhanced level of mixing in the vicinity of the equator. This enhanced mixing is used to model the effect of the observed interleaving of water masses across the equator. Both the level and distribution of eddy kinetic energy (associated with TIWs) varies with the level and orientation of the lateral mixing. An examination of the main energy conversion terms of mean to eddy energy reveals that both the barotropic and baroclinic energy conversion terms are important for the production of TIWs. The relative role of each of these conversion terms changes as the lateral mixing is decreased. At low levels of lateral mixing, barotropic instability is found to dominate the production of total eddy energy. Including enhanced mixing at the equator decreases this barotropic production, but surprisingly increases the baroclinic production away from the equator.

[1] Global sea surface height anomaly signals from the TOPEX/Poseidon satellite altimeter from 1992 to 2000 are filtered into several spectral bands. These bands include propagating (Rossby, Kelvin, and tropical instability waves) and nonpropagating (annual and interannual basin-scale variability and eddies) signals. These signals are decomposed through a series of finite impulse response band-pass filters. Phase speed, period, wavelength, and amplitude are estimated. Signal-to-noise ratio, methodological errors, and natural variability are also estimated. Results are shown for the Pacific, Atlantic, and Indian basins for four Rossby wave components with periods between 3 and 24 months. Equatorial Kelvin waves and tropical instability waves are shown as peripheral results. The clearest and most energetic wave signals are generally observed for annual Rossby waves. Semiannual Rossby waves often have the largest amplitudes near the tropics. The Rossby wave phase speeds are in agreement with the linear theory, except for a ∼25% bias toward high speeds found in mid to high latitudes. This revitalizes the idea that on the average the Rossby wave signal in the ocean is in the form of free waves.

[1] This paper addresses the atmospheric and oceanic causes of the seasonal cycle of sea surface temperature (SST) in the tropical Atlantic on the basis of direct observations. Data sets include up to 4 years (September 1997 to February 2002) of measurements from moored buoys of the Pilot Research Array in the Tropical Atlantic (PIRATA), near-surface drifting buoys, and a blended satellite in situ SST product. We analyze the mixed layer heat balance at eight PIRATA mooring locations and find that the seasonal cycles of latent heat loss and absorbed shortwave radiation are responsible for seasonal SST variability in the northwest basin (8°N–15°N along 38°W). Along the equator (10°W–35°W), contributions from latent heat loss are diminished, while horizontal temperature advection and vertical entrainment contribute significantly. Zonal temperature advection is especially important during boreal summer near the western edge of the cold tongue, while horizontal eddy temperature advection, which most likely results from tropical instability waves, opposes temperature advection by the mean flow. The dominant balance in the southeast (6°S–10°S along 10°W) is similar to that in the northwest, with both latent heat loss and absorbed solar radiation playing important roles.