SEASONAL VARIABILITY OF NEAR-SURFACE ISOTHERMAL LAYER AND THERMOCLINECHARACTERISTICS OF THE TROPICAL INDIAN-OCEAN

A subset of world ocean monthly mean temperature climatology generated by Levitus and Boyer (1994), is utilised to describe the observed sea sonal variability of the characteristics of the near-surface isotherma l layer and thermocline for the entire tropical Indian Ocean (TIG). Th e most salient features of the observed annual cycle are described in terms of amplitude and phase of the annual and semi-annual frequencies employing Fourier analysis technique. On the annual mode, the near-su rface isothermal layer depth (ILD). exhibits larger variability away f rom the equator with peak values in the northernmost Arabian Sea: the northernmost Bay of Bengal and the southern TIG, while on the semi-ann ual mode, it shows larger variability in the central Arabian Sea. The variability of the near-surface isothermal layer temperature (ILT), on the annual mode, is very weak in the warmpool region, and increases w ith latitude, while on the semi-annual mode, it shows larger variabili ty in the northwestern Arabian Sea. The variability of 20 degrees C is otherm topography (D20), on the annual mode, is weakest in the equator ial region and largest in the coastal regions of the Arabian Sea and t he Bay of Bengal and in the southern TIO, while on the semi-annual mod e, it is prominent in the eastern and western equatorial regions. The thermocline gradient (TG) is very sharp below the warmpool region and diffuses meridionally. On the annual mode, it shows larger variability in the southern TIG, off Somalia and northernmost Arabian Sea, while on the semiannual mode, it shows larger variability in the southwester n Arabian Sea and eastern equatorial Indian Ocean. The relationship be tween near-surface isothermal layer and thermocline characteristics ov er an annual cycle are explored through correlation analysis. The corr elation between ILD and ILT is strong over much of the basin with the exception of the equatorial and coastal upwelling downwelling zones wh ere internal ocean dynamics are important. In the southern TIG, entrai nment of colder waters appears to be important in maintaining the annu al cycle of ILT as strong correlation is noticed between ILT and TG. I n the Indo-Pacific throughflow region and another region west of it, t he annual Rossby waves appear to control D20, as correlations between D20 and other fields are strong in these regions. A similar strong cor relation between D20 and ILD is also noticed in the southeastern Arabi an Sea where mode-2 Rossby wave is identified in numerical model solut ions.