Interannual variability of the Tropical Indian Ocean mixed layer depth

Abstract

International audienceIn the present study, interannual fluctuations of the mixed layer depth (MLD) in the tropical Indian Ocean are investigated from a long-term (1960-2007) eddy permitting numerical simulation and a new observational dataset built from hydrographic in situ data including Argo data (1969-2008). Both datasets show similar interannual variability patterns in relation with known climate modes and reasonable phase agreement in key regions. Due to the scarcity of the observational dataset, we then largely rely on the model to describe the interannual MLD variations in more detail. MLD interannual variability is two to four times smaller than the seasonal cycle. A large fraction of MLD interannual variations is linked to large-scale climate modes, with the exception of coastal and subtropical regions where interannual signature of small-scale structures dominates. The Indian Ocean Dipole is responsible for most variations in the 10°N-10°S band, with positive phases being associated with a shallow MLD in the equatorial and south-eastern Indian Ocean and a deepening in the south-central Indian Ocean. The El Niño signature is rather weak, with moderate MLD shoaling in autumn in the eastern Arabian Sea. Stronger than usual monsoon jets are only associated with a very modest MLD deepening in the southern Arabian Sea in summer. Finally, positive Indian Ocean Subtropical Dipoles are associated with a MLD deepening between 15 and 30°S. Buoyancy fluxes generally appear to dominate MLD interannual variations except for IOD-induced signals in the south-central Indian Ocean in autumn, where wind stirring and Ekman pumping dominate

Seasonal Surface Chlorophyll a Variability in the Seychelles–Chagos Thermocline Ridge

“…[28][29][30] . Other than winds, stratification also plays a major role in determining the depth mixed layer 31 . Strong stratification can be a result of high buoyancy at the surface layer, which is eventually determined by sea-surface temperature (SST) and sea-surface salinity (SSS).…”

Mixed layer variability and chlorophyll <i>a</i> biomass in the Bay of Bengal

“…Using a global general circulation model de Boyer Montegut et al (2007) studied the mixed layer heat budget of the Bay of Bengal as a part of the northern Indian Ocean and stated that the salinity stratification plays a clear role in maintaining a high winter SST (sea surface temperature) in the Bay of Bengal while the presence of freshwater near the surface allows heat storage below the surface layer that can later be recovered by entrainment warming during winter cooling. More recently, Keerthi et al (2012) studied the interannual variability of the mixed layer in the tropical Indian Ocean and its link to climate modes using eddy permitting numerical simulation and in situ hydrographic data.…”

Physical control of interannual variations of the winter chlorophyll bloom in the northern Arabian Sea

“…The ocean physical parameters are derived from an updated version of the dataset described in Keerthi et al (2013), with an extended temporal coverage (2002 to 2013) and an estimate of the TCD in addition to that of the MLD. These MLD and TCD datasets are built from a combination of Argo and historical temperature and salinity profiles.…”

“…The resolution of the data was then degraded to a regular 2 • monthly grid, by taking the median of all MLDs and TCDs in each grid mesh. A more detailed description of this procedure can be found in Keerthi et al (2013). An overview of the spatio-temporal coverage of this dataset over the NAS is provided in Fig.…”