The sensitivity of the Seychelles – Chagos thermocline ridge to large-scale wind anomalies (original) (raw)
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Supplement to Cirene: air-sea interactions in the Seychelles-Chagos thermocline ridge region
2009
Indian Ocean provides new insights into ocean-atmosphere interactions in a key climatic region. W hile easterly trade winds blow year-round over the southern Indian Ocean, surface winds experience a striking reversal north of 10°S. During boreal summer, the low-level easterly flow penetrates northward, is deflected when crossing the equator, and forms the strong Indian monsoon jet. During boreal winter, northeasterly winds also bend while crossing the equator southward and form a weak low-level westerly jet between the equator and 10°S (Fig. la). The cyclonic circulation at the meeting point of these two wind regimes is responsible for the formation of a peculiar oceanic structure: the "Seychelles Chagos thermocline ridge" (SCTR; Hermes and Reason 2008; see the "Seychelles-Chagos thermocline ridge" sidebar for a more complete explanation of the formation of this feature). This region has attracted attention because it is home to distinct oceanic and atmospheric variability at multiple time scales, each time with significant climatic consequences. Anomalously warm sea surface temperature (SST) in the SCTR region is associated with increased • ^Pacific Marine Environment Laboratory Publication Number 3179 ^ir Underwater photograph of AS I P. See "The Air-Sea Interaction profiler" sidebar for more information.
Modeling the Wind-Driven Variability of the South Indian Ocean
Journal of Physical Oceanography, 1999
This article describes the results of numerical experiments carried out with a general circulation ocean model to investigate the effect of the seasonal cycle of the wind forcing on the Agulhas transport. Two cases are described. The first was initialized with temperature and salinity values obtained by horizontally averaging Levitus climatology. The second experiment was designed to isolate the spatial and temporal structure of the barotropic mode. The model, therefore, was initialized with constant values of temperature and salinity. Both experiments were started from rest, forced at their surface with Hellerman and Rosenstein wind stress climatology, and spun up until dynamical equilibrium. According to the experiments there are two distinct modes of variability in the south Indian Ocean. These modes appear to be separated by the topographic ridges that run south of Madagascar. On the western side of the basin there is a dominant mode with a maximum during spring-summer and a minimum during fall-winter. East of Madagascar there is a marked decrease of the circulation in fall and relative maximums during late summer and late winter. The midlatitude time variability, east of 45ЊE, appears to be dominated by advection and wave propagation. West of 45ЊE there is dominance by local wind forcing. A comparison between baroclinic and barotropic experiments indicates that although their annual mean structure is markedly different, their monthly anomalies, south of 30ЊS, are quite similar. This result, which agrees with previous theoretical and experimental studies, indicates that the seasonal adjustment in the south Indian Ocean is mostly accomplished by the westward propagation of barotropic planetary waves. This propagation is inhibited by the bottom topography of the Madagascar Ridge and Southwest Indian Ridge (ϳ45ЊE). These topographic features appear to isolate the Agulhas Current in the western region from the large-scale gyre farther east at seasonal timescales.
Cirene: Air—Sea Interactions in the Seychelles—Chagos Thermocline Ridge Region
Bulletin of the American Meteorological Society, 2009
W hile easterly trade winds blow year-round over the southern Indian Ocean, surface winds experience a striking reversal north of 10°S. During boreal summer, the low-level easterly flow penetrates northward, is deflected when crossing the equator, and forms the strong Indian monsoon jet. During boreal winter, northeasterly winds also bend while crossing the equator southward and form a weak low-level westerly jet between the equator and 10°S . The cyclonic circulation at the meeting point of these two wind regimes is responsible for the formation of a peculiar oceanic structure: the "Seychelles Chagos thermocline ridge" (SCTR; Hermes and Reason 2008; see the "Seychelles-Chagos thermocline ridge" sidebar for a more complete explanation of the formation of this feature). This region has attracted attention because it is home to distinct oceanic and atmospheric variability at multiple time scales, each time with significant climatic consequences. Anomalously warm sea surface temperature (SST) in the SCTR region is associated with increased ! Under water photograph of ASIP. See "The Air-Sea Interaction profiler" sidebar for more information. FIG. 1. (a) Climatological surface winds and 0-300-m average ocean temperature in Jan-Feb. (See "The Seychelles-Chagos thermocline ridge" sidebar for explanation.) The thick black arrows indicate the surface flow induced by wind that promotes upwelling and leads to the SCTR formation. The arrows marked SEC and SECC indicate the south equatorial current and south equatorial countercurrent. (b) Meridional section of the ocean temperature along 67°E, indicated by a dashed line in (a). (c) Blowup of the region framed in (a) summarizing the Cirene cruise. The trajectory of R/V Suroît is shown in black (first leg) and red (second leg). The orange rectangles indicate the locations where groups of three Argo profilers were deployed. The blue circle indicates the location of the ATLAS and ADCP moorings. The red circle indicates the location of the long CTD station (12 days during each leg). The climatological temperature data come from World Ocean Atlas 2005 (Locarnini et al. 2006), while the wind is the Quick Scatterometer (QuikSCAT) wind gridded product produced by the Center for Satellite Exploitation and Research (CERSAT).
Interannual variability of the South Indian Ocean in observations and a coupled model
2008
The mean state, annual cycle, and interannual variability of the coupled ocean-atmosphere in the South Indian Ocean produced by a 300-year simulation of a coupled ocean-atmosphere general circulation model (CGCM) are compared with those from 51-year (1950-2000) observational datasets. The CGCM simulates realistically the mean annual cycles for both the sea surface temperature (SST) and lower atmospheric circulation, including the seasonal positions of the 10 o C and 20 o C SST isotherms, the zonal and meridional migration of the South Indian Ocean subtropical high, and the fluctuation of the southeast trade winds and mid-latitude westerly winds.
The shallow overturning circulation of the Indian Ocean
2002
The Indian Ocean differs from the other two large oceans in not possessing an annual-mean equatorial upwelling regime. While the subtropical cells (STCs) of the Atlantic and Pacific Oceans connect subtropical subduction regimes with tropical upwelling via equatorward thermocline flows and coastal undercurrents, much of the upwelling in the Indian Ocean occurs in the coastal regimes of the northern hemisphere. Consequently, the counterpart of the STCs of the other oceans has to be a cross-equatorial cell connecting the southern subtropical subduction zone via the Somali Current with the upwelling areas off Somalia and Oman. The southward return flow is by interior Ekman transports. This annual-mean picture is accomplished by a dominance of the summer monsoon, during which only northern upwelling occurs, over the winter monsoon. Pathways of the thermocline flows related to the shallow overturning circulations are investigated here and estimates of subduction and upwelling are presented. From the observed mean northward flow of thermocline waters within the Somali Current and the interior southward cross-equatorial return flow the magnitude of the cross-equatorial cell is estimated at 6 Sv, with part of the thermocline waters being supplied by the Indonesian Throughflow. From observations we estimate that the northern upwelling occurs dominantly through the offshore outflows of the Somali Current by the Southern Gyre and Great Whirl and to a lesser degree off Oman. However, we also present model results suggesting a much lower role of Somali upwelling and a significant contribution from openocean upwelling in cyclonic domes around India and Sri Lanka. An interesting aspect of the Indian Ocean crossequatorial cell is the mechanism by which the Ekman transport crosses the equator. Typically, Ekman transports during the summer (winter) monsoon are southward (northward) on both sides of the equator, while mean meridional winds on the equator are in the respective opposite direction. Earlier model evidence had suggested that this type of forcing should lead to an equatorial roll with northward surface flow and southward subsurface flow during the summer monsoon and reverse orientation during the winter monsoon. Observational evidence is presented here, based on shipboard ADCP sections, moored stations and surface drifters, confirming the existence of the equatorial roll. It is strongly developed in the western Indian Ocean during the SW monsoon where the wind conditions for the roll are best met. While in the central Indian Ocean and during the winter monsoon the roll appears to be a more transient phenomenon, superimposed by equatorial-wave currents. The evidence further suggests that the roll is mostly confined to the surfacemixed layer and is, therefore, of little consequence for the meridional heat transport.
Philosophical Transactions of The Royal Society B: Biological Sciences, 2005
The variability in the southwest Indian Ocean is connected to the basin-scale and global-scale ocean circulation. Two bands of enhanced variability stretch across the Southern Indian Ocean east of Madagascar around 12° S and 25° S, respectively. They mark the preferred routes along which anomalies, generated by varying forcing over the central basin, near the eastern boundary or in the equatorial region, propagate westward as baroclinic Rossby waves. Sea-surface height anomalies pass along the northern tip of Madagascar and are observed by satellite altimetry to propagate into the central Mozambique Channel. There, eddies are subsequently formed that propagate southward into the Agulhas retroflection region. The anomalies along the southern band trigger the formation of large dipolar vortex pairs in the separation region of the East Madagascar Current at the southern tip of the island. South of Africa these eddies and dipoles trigger the shedding of Agulhas Rings that feed the Atlantic meridional overturning circulation with warm, salty, Indian Ocean water. Interannual variability of the forcing over the Indian Ocean, such as that associated with the Indian Ocean Dipole/El Niño climate modes, propagates along these pathways and leads to associated modulations of the eddy transports into the South Atlantic.
Long-term observations indicate that the Indian Ocean displays significant low-frequency variability in mean sea-level pressure, near-surface wind, cloud and sea-surface temperature (SST). A general circulation model is used to study the response of the atmosphere to an idealized SST anomaly pattern (warm in southern mid-latitudes, cool in southern tropics) that captures the essence of observed multidecadal SST variability as well as that associated with ENSO in the South Indian Ocean. The major objectives are to investigate air -sea interaction mechanisms potentially associated with the variability and whether the atmospheric response to the SST is likely to lead to maintenance or damping of the original SST anomaly pattern, and on what time scale. Two types of experiment are performed to tackle these objectives.