The monsoon circulation of the Indian Ocean

In this paper, we review observations, theory and model results on the monsoon circulation of the Indian Ocean. We begin with a general overview, discussing wind-stress forcing fields and their anomalies, climatological distributions of stratification, mixed-layer depths, altimetric sea-level distributions, and seasonal circulation patterns (Section 2). The three main monsoon circulation sections deal with the equatorial regime (Section 3), the Somali Current and western Arabian Sea (Section 4), and the Bay of Bengal, seasonally reversing monsoon currents south of India and Sri Lanka, and the eastern and central Arabian Sea (Section 5). For the equatorial regime, we discuss equatorial jets and undercurrents, their interactions with the eastern and western boundaries, and intraseasonal and vertically propagating signals. In the Somali Current section, we describe the ocean's responses to the summer and winter monsoon winds, and outline the modelling efforts that have been carried out to understand them. In the Bay of Bengal section, we present observational and modeling evidence showing the importance of remote forcing from the east, which to a large extent originates along the equator. In the following three sections, we review the southern-hemi sphere subtropical regime and its associated boundary currents (Section 6), the Indonesian Throughflow (Section 7), the Red Sea and Persian Gulf circulations (Section 8), and discuss aspects of their interactions with other Indian-Ocean circulations. Next, we describe the Indian Ocean's deep and shallow meridional overturning cells (Section 9). Model results show large seasonal variability of the meridional overturning streamfunction and heat flux, and we discuss possible physical mechanisms behind this variability. While the monsoon-driven variability of the deep cell is mostly a sloshing motion affecting heat storage, interesting water-mass transformations and monsoonal reversals occur in the shallow cross-equatorial cell. In the mean, the shallow cell connects the subduction areas in the southern subtropics and parts of the Indonesian Throughflow waters with the upwelling areas of the northern hemisphere via the cross-equatorial Somali Current. Its near-surface branch includes a shallow equatorial roll that is seasonally reversing. We close by looking at coupled ocean-climate anomalies, in particular the large events that were observed in the tropical and subtropical Indian Ocean in 1993/94 and 1997/98. These events have been interpreted as an independent Indian-Ocean climate mode by some investigators and as an ENSO-forced anomaly by others

Water resources variability in Africa during the 20th century = Variabilité des ressources en eau en Afrique au 20ème siècle

The tropical Atlantic Ocean exhibits two primary modes of interannual climate variability : an "equatorial mode" analogous to, but weaker than, the Pacific El Nino phenomenon, and a "dipole" mode that does not have a Pacific counterpart. The equatorial mode is responsible for warm (and cold) sea surface temperature (SST) events in the Gulf of Guinea, and is identifiable with changes in the equatorial thermocline slope resulting from zonal-wind anomalies in the western tropical Atlantic. The dipole mode is characterised by SST anomalies of opposite sign on either side of the mean position of the Intertropical Convergence Zone (ITCZ). To date, the dipole mode has been detected in the ocean only from SST. Here it is shown, using surface and subsurface oceanic temperatures obtained from observations as well as from a numerical solution, that the dipole mode is linked to changes in the equatorial thermocline slope occurring at interannual time scales. Thus, the two main interannual climatic modes appear to be dynamically linked in this frequency band. Futhermore, the dominant pattern of variability in both modes involves North-South displacements of the ITCZ, as in the annual response. (Résumé d'auteur

Dynamics of the Atlantic meridional overturning circulation. Part 2: Forcing by winds and buoyancy

AbstractRecently, Schloesser et al. (2012) explored the dynamics of the descending branch of meridional overturning circulations (MOCs), by obtaining analytic solutions to a variable-density, 2-layer model (VLOM) forced only by a surface buoyancy flux. Key processes involved are the poleward thickening of the upper layer along the eastern boundary due to Kelvin-wave adjustments, the westward propagation of that coastal structure by Rossby waves, and their damping by mixing; the resulting zonal pressure gradient causes the surface MOC branch to converge into the northern basin near the eastern boundary.In this paper, we extend the Schloesser et al. (2012) study to include forcing by a zonal wind stress τx(y). Much of the paper is devoted to the derivation and analysis of analytic solutions to VLOM; for validation, we also report corresponding numerical solutions to an ocean general circulation model (OGCM). Solutions are obtained in a flat-bottom, rectangular basin confined to the northern hemisphere. The buoyancy forcing relaxes upper-ocean density to a prescribed profile ρ∗(y) that increases polewards until it becomes as large as the deep-ocean density at latitude y2; north of y2, then, the ocean is homogeneous (a 1-layer system). The wind stress τx drives Subtropical and Subpolar Gyres, and in our standard solution the latter extends north of y2. Vertical diffusion is not included in VLOM (minimized in the OGCM); consequently, the MOC is not closed by upwelling associated with interior diffusion, but rather by flow through the southern boundary of the basin (into a southern-boundary sponge layer in the OGCM), and solutions are uniquely determined by specifying the strength of that flow or the thermocline depth along the tropical eastern boundary.Solutions forced by τx and ρ∗ differ markedly from those forced only by ρ∗ because water flows across y2 throughout the interior of the Subpolar Gyre, not just near the eastern boundary. In some of our solutions, the strength of the MOC’s descending branch is determined entirely by this wind-driven mechanism, whereas in others it is also affected by Rossby-wave damping near the eastern boundary. Upwelling can occur in the interior of the Subpolar Gyre and in the western-boundary layer, providing “shortcuts” for the overturning circulation; consequently, there are different rates for the convergence of upper-layer water near y2,Mn, and the export of deep water south of the Subpolar Gyre, M, the latter being a better measure of large-scale MOC strength. When western-boundary upwelling occurs in our solutions, M is independent of the diapycnal processes in the subpolar ocean

Relationship between the equatorial and meridional modes of climatic variability in the tropical Atlantic

The tropical Atlantic Ocean exhibits two primary modes of interannual climate variability : an equatorial mode analogous to, but weaker than, the Pacific El Nino phenomenon, and a meridional mode that does not have a Pacific counterpart. The equatorial mode is responsible for warm (and cold) sea surface temperature (SST) events, mainly in the Gulf of Guinea, and is identifiable by abnormal changes in the equatorial thermocline slope resulting from zonal-wind anomalies in the western tropical Atlantic. The meriodional mode is characterized by a north-south interhemispheric gradient of SST anomalies. Here it is shown, using observed surface and subsurface oceanic temperatures, that the meridional mode is linked to the equatorial mode, at both decadal and short-interannual (1-2 years) time scales. Both modes involve North-South displacements of the ITCZ, as in the annual response. (Résumé d'auteur

Observing Systems in the Indian Ocean, in Proceedings of OceanObs’09

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