Jérôme Vialard

RAMA: The research moored array for African-Asian-Australian monsoon analysis and prediction

The Indian Ocean is unique among the three tropical ocean basins in that it is blocked at 25°N by the Asian landmass. Seasonal heating and cooling of the land sets the stage for dramatic monsoon wind reversals, strong ocean–atmosphere interactions, and intense seasonal rains over the Indian subcontinent, Southeast Asia, East Africa, and Australia. Recurrence of these monsoon rains is critical to agricultural production that supports a third of the worlds population. The Indian Ocean also remotely influences the evolution of El Nino–Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), North American weather, and hurricane activity. Despite its importance in the regional and global climate system though, the Indian Ocean is the most poorly observed and least well understood of the three tropical oceans. This article describes the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA), a new observational network designed to address outstanding scientific questions related to Indian Ocean variability and the monsoons. RAMA is a multinationally supported element of the Indian Ocean Observing System (IndOOS), a combination of complementary satellite and in situ measurement platforms for climate research and forecasting. The article discusses the scientific rationale, design criteria, and implementation of the array. Initial RAMA data are presented to illustrate how they contribute to improved documentation and understanding of phenomena in the region. Applications of the data for societal benefit are also described.

An OGCM Study for the TOGA Decade. Part I: Role of Salinity in the Physics of the Western Pacific Fresh Pool

Abstract 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 Model Study of Oceanic Mechanisms Affecting Equatorial Pacific Sea Surface Temperature during the 1997–98 El Niño

In this study, the processes affecting sea surface temperature variability over the 1992-98 period, encompassing the very strong 1997-98 El Nino event, are analyzed. A tropical Pacific Ocean general circulation model, forced by a combination of weekly ERS1-2and 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 s21 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 108N-108S 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 21 for surface currents. The model therefore reproduces well the observed interannual variability, with wind stress as the only interannually varying forcing. 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 Nino. 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 Nino. 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 Nino 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.

An OGCM Study for the TOGA Decade. Part II: Barrier-Layer Formation and Variability

Abstract 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 ...

Influence of the state of the Indian Ocean Dipole on the following year’s El Niño

El Nino-Southern Oscillation (ENSO) consists of irregular episodes of warm El Nino and cold La Nina conditions in the tropical Pacific Ocean(1), with significant global socio-economic and environmental impacts(1). Nevertheless, forecasting ENSO at lead times longer than a few months remains a challenge(2,3). Like the Pacific Ocean, the Indian Ocean also shows interannual climate fluctuations, which are known as the Indian Ocean Dipole(4,5). Positive phases of the Indian Ocean Dipole tend to co-occur with El Nino, and negative phases with La Nina(6-9). Here we show using a simple forecast model that in addition to this link, a negative phase of the Indian Ocean Dipole anomaly is an efficient predictor of El Nino 14 months before its peak, and similarly, a positive phase in the Indian Ocean Dipole often precedes La Nina. Observations and model analyses suggest that the Indian Ocean Dipole modulates the strength of the Walker circulation in autumn. The quick demise of the Indian Ocean Dipole anomaly in November-December then induces a sudden collapse of anomalous zonal winds over the Pacific Ocean, which leads to the development of El Nino/La Nina. Our study suggests that improvements in the observing system in the Indian Ocean region and better simulations of its interannual climate variability will benefit ENSO forecasts.

The oceanic zone of convergence on the eastern edge of the Pacific warm pool : A synthesis of results and implications for El Niño-Southern Oscillation and biogeochemical phenomena

The eastern edge of the western Pacific warm pool corresponds to the separation between the warm, rainfall-induced low-salinity waters of the warm pool and the cold, high-salinity upwelled waters of the cold tongue in the central-eastern equatorial Pacific. Although not well defined in sea surface temperature (SST), this eastern edge is characterized by a sharp salinity front that is trapped to the equator. Several studies, using numerous in situ and satellite data and three classes of ocean models, indicate that this front is the result of the zonal convergence of the western and central Pacific water masses into the eastern edge of the warm pool. This occurs through the frequent encounter of the eastward jets in the warm pool and the westward South Equatorial Current in the cold tongue. The notable and alternate variations of these wind-driven zonal currents are trapped to the equator and are chiefly interannual in the vicinity of the edge. Consequently, the Eastern Warm Pool Convergence Zone (EWPCZ) is subject to eastward or westward displacements over several thousands of kilometers along the equatorial band, in synchrony with the warm phase (El Nino) and the cold phase (La Nina) of the El Nino-Southern Oscillation (ENSO) phenomenon. Zonal advection appears to be the predominant mechanism for the ENSO displacements of the eastern edge of the warm and fresh pool. The existence of the EWPCZ and its ENSO displacements have significant effects on the physics of the tropical Pacific and on related biogeochemical phenomena. The EWPCZ is important for the formation of the barrier layer in the isothermal layer of the warm pool. Its zonal displacements control SST in the central equatorial Pacific, which in turn drives the surface winds and atmospheric convection (and vice versa). Hence the central equatorial Pacific is a key region for ENSO coupled interactions. All these findings from several studies and additional analyses lead to a revision of the delayed action oscillator theory of ENSO. The existence of the EWPCZ and its zonal displacements are also reasons for the ENSO variations in production and exchange of CO2 with the atmosphere over the equatorial Pacific. The zone of one-dimensional convergence seems to congregate the worlds most important tuna fishery in the western equatorial Pacific, and its displacements are likely the reason for this fishery to move zonally over thousands of kilometers in phase with ENSO.

Cirene: Air—Sea Interactions in the Seychelles—Chagos Thermocline Ridge Region

The Vasco-Cirene program explores how strong air-sea interactions promoted by the shallow thermocline and high sea surface temperature in the Seychelles-Chagos thermocline ridge results in marked variability at synoptic, intraseasonal, and interannual time scales. The Cirene oceanographic cruise collected oceanic, atmospheric, and air-sea flux observations in this region in January–February 2007. The contemporaneous Vasco field experiment complemented these measurements with balloon deployments from the Seychelles. Cirene also contributed to the development of the Indian Ocean observing system via deployment of a mooring and 12 Argo profilers. Unusual conditions prevailed in the Indian Ocean during January and February 2007, following the Indian Ocean dipole climate anomaly of late 2006. Cirene measurements show that the Seychelles-Chagos thermocline ridge had higher-than-usual heat content with subsurface anomalies up to 7°C. The ocean surface was warmer and fresher than average, and unusual eastward cur...

Impact of temperature inversions on SST evolution in the South-Eastern Arabian Sea during the pre-summer monsoon season

Temperature inversions are known to occur in the near-surface ocean regime where salinity stratification is large enough to influence the density field. However, they have not been known as features that alter near-surface processes significantly to influence the sea surface temperature (SST). From the analysis of new observed datasets as well as of state-of-the-art numerical model outputs, this paper shows that heat trapped within a temperature inversion makes significant contribution to warming of the SST in the South-Eastern Arabian Sea during the pre-southwest monsoon season.

Intraseasonal response of the northern Indian Ocean coastal waveguide to the Madden‐Julian Oscillation

A new observational record of upper-ocean currents at 15°N on the western coast of India is dominated by intraseasonal (55-110 day) variations of alongshore currents, whereas sea level at the same location has a clear seasonal signal. These observations can be interpreted within the framework of linear wave theory. At 15°N, the minimum period for planetary waves is ~90 day, meaning that intraseasonal energy is largely trapped at the coast in the form of poleward-propagating Kelvin waves, while lower-frequency signals associated with the annual cycle can radiate offshore as planetary waves. This dynamical difference results in a steeper offshore slope of sea level at intraseasonal timescale, and thus stronger geostrophic alongshore currents. A consequence is that the alongshore currents are in-phase with intraseasonally-filtered sea level near the coast, and a gridded satellite product is shown to reproduce the current variations reasonably well. The intraseasonal current variations along the west coast of India are part of basin-scale sea-level fluctuations of the Northern Indian Ocean equatorial and coastal waveguides. The wind forcing associated with this basin scale circulation closely matches surface wind signals associated with the Madden-Julian Oscillation Copyright 2009 by the American Geophysical Union.

Indo-Pacific Sea Surface Temperature Perturbations Associated with Intraseasonal Oscillations of Tropical Convection

Abstract Since the ISV of the convection is an intermittent phenomenon, the local mode analysis (LMA) technique is used to detect only the ensemble of intraseasonal events that are well organized at large scale. The LMA technique is further developed in this paper in order to perform multivariate analysis given patterns of SST and surface wind perturbations associated specifically with these intraseasonal events. During boreal winter, the basin-scale eastward propagation of the convective perturbation is present only over the Indian Ocean Basin. The intraseasonal SST response to convective perturbations is large and recurrent over thin mixed layer regions located north of Australia and in the Indian Ocean between 5° and 10°S. By contrast, there is little SST response in the western Pacific basin and no clear eastward propagation of the convective perturbation. During boreal summer, the SST response is large over regions with thin mixed layers located north of the Bay of Bengal, in the Arabian Sea, and in ...