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| Titel |
Seasonal variation of the South Indian tropical gyre |
| VerfasserIn |
Borja Aguiar-González, Leandro Ponsoni, Herman Ridderinkhof, Hendrik M. van Aken, Will P. M. de Ruijter, Leo R. M. Maas |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250132143
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| Publikation (Nr.) |
 EGU/EGU2016-12622.pdf |
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| Zusammenfassung |
| The South Indian tropical gyre receives and redistributes water masses from the Indonesian
Throughflow (ITF), a source of Pacific Ocean water which represents the only low-latitude
connector between the world oceans and, therefore, a key component in the global ocean
circulation and climate system.
We investigate the seasonal variation of the South Indian tropical gyre and its associated
open-ocean upwelling system, known as the Seychelles-Chagos Thermocline Ridge (SCTR),
based on satellite altimeter data (AVISO) and global atlases of temperature and salinity
(CARS09), wind stress (SCOW) and wind-driven circulation.
Two novel large-scale features governing the upper geostrophic circulation of the South
Indian tropical gyre are revealed. First, the seasonal shrinkage of the ocean gyre. This occurs
when the South Equatorial Countercurrent (SECC) recirculates before arrival to Sumatra
from winter to spring, in apparent synchronization with the annual cycle of the ITF. Second,
the open-ocean upwelling is found to vary following seasonality of the overlying
geostrophic ocean gyre, a relationship that has not been previously shown for this
region.
An analysis of major forcing mechanisms suggests that the thermocline ridge results from
the constructive interaction of basin-scale wind stress curl, local-scale wind stress forcing and
remote forcing driven by Rossby waves of different periodicity: semiannual in the west,
under the strong influence of monsoonal winds; and, annual in the east, where the
southeasterlies prevail. One exception occurs during winter, when the well-known westward
intensification of the upwelling core, the Seychelles Dome, is shown to be largely a response
of the wind-driven circulation.
Broadly speaking, the seasonal shrinkage of the ocean gyre (and the SCTR) is the one
feature that differs most when the geostrophic circulation is compared to the wind-driven
Sverdrup circulation. From late autumn to spring, the eastward SECC recirculates early in the
east on feeding the westward South Equatorial Current, therefore closing the gyre
before arrival to Sumatra. We find this recirculation longitude migrates over 20∘ and
collocates with the westward advance of a zonal thermohaline front emerging from the
encounter between (upwelled) Indian Equatorial Water and relatively warmer and
fresher Indonesian Throughflow Water. We suggest this front, which we call the
Indonesian Throughflow Front, plays an important role as forcing to the tropical gyre,
generating southward geostrophic flows that contribute to the early recirculation of the
SECC at longitudes more westward than predicted from the barotropic wind-driven
circulation.
Because our findings are based on time-averaged seasonal fields from 22 years
of satellite altimeter data and from about 60 years of non-systematic sampling of
ocean temperature and salinity data (CARS09), we stress the importance of further
study on the possibility that interanual variability in the seasonal ITF may cause
changes in the seasonal resizing of the ocean gyre and its associated upwelling
ridge. |
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