Thermocline dynamics and salinity effects in the Indian Ocean in 1997-98
Andreas Schiller
CSIRO Marine Research
An ocean general circulation model (OGCM) coupled to an atmospheric boundary layer model is used to investigate mechanisms of ocean/atmosphere and ocean thermocline response during the 1997-98 ENSO event.
The Indian Ocean, despite producing large hydrographic and dynamic anomalies during 1997-98, seemed to react in a two way mode: while ENSO-induced wind changes due to the breakdown of the Walker circulation in the Pacific forced an initial response in the interior Indian Ocean, later a reversal of the thermocline response developed independent of ENSO which can be ascribed to internal ocean dynamics. We present some evidence for the hypothesis of an independent Indian Ocean climate mode as suggested by Webster et al. (1999).
Experiments have been performed to study the sensitivity of the tropical Indian Ocean to variations in freshwater fluxes. Salinity anomalies had a significant impact on upper oceanstratification during the 1997-98 dipole mode event in the Indian Ocean. The westward shift in tropical rainfall caused anomalous barrier layer formation in the westernequatorial Indian Ocean and reduced the stability of the mixed-layer in the east, allowing for strong entrainment cooling there. The appearance of the predominant east - west mode in barrier layer anomalies seems to be linked to "dipole - years" with excess (less) precipitation in the western (eastern) tropical Indian Ocean. In the central and eastern parts of the Indian Ocean anomalous advection of low salinity water, salinity entrainment as well as anomalous freshwater flux determined anomalies in barrier layer formation, whereas in the western areas and the southern tropical Indian Ocean a balance between anomalous entrainment and freshwater flux dominated the mixed-layer salinity budget.
Changes in low-latitude SSTA caused by anomalous surface freshwater fluxes were small over the whole integration period (about 0.1 Deg. C), because anomalies of thermal buoyancy fluxes were much larger than fluxes from haline buoyancy anomalies. This result suggests that at least on monthly to interannual timescales anomalies in tropical Indian Ocean freshwater fluxes have a diminishing effect on SST anomalies.
