Variability of the depth of the 20°C isotherm along 6°N in the Bay of Bengal: its response to remote and local forcing and its relation to satellite SSH variability

The depth of the 20°C isotherm (hereafter referred to as D20) derived from expendable bathythermograph (XBT) measurements along 6°N in the southern Bay of Bengal (hereafter referred to as the Bay) shows pronounced seasonal-to-interannual variability and profound westward propagation. But not all these features are well depicted by satellite observations of sea-surface height (SSH). The cause of the thermocline variability for the period of 1988–1999 and the relationship to SSH variability were analyzed in this study with the addition of wind and salinity data sets. Ekman pumping and remote Kelvin/Rossby wave propagation are the two leading forcing mechanisms for the thermocline variability at the chosen XBT section. On seasonal timescales, the phase of the D20 semiannual variations is determined by a remote forcing signal, but the amplitude is determined by interaction with the annual background that is preconditioned by the local Ekman pumping. The local background has the phase oscillating in a way that it enhances the westward propagation of the remote semiannual signal during the first half of the year but suppresses the semiannual signal during the second half of the year. On interannual timescales, the D20 at 6°N is most sensitive to the remote equatorial wind variability, with local forcing variability being a secondary contributor. This study shows that the calculation of the Ekman pumping velocity should include the contribution from not only wind-stress curl but also zonal wind stress due to the β-effect at 6°N. Omitting the latter would augment the effect of local forcing. lationship between satellite SSH and D20 is not linear in the Bay, as large salinity variation contributes considerably to the SSH variability. Compared to the structure of seasonal D20 anomalies, the SSH anomalies are more zonally aligned, less westward propagating, and have a weak semiannual signal. A relationship between the sea level and thermocline variation is derived by using a two-layered system that includes seasonal variations of temperature and salinity in upper-ocean density. This allows the structure of the satellite SSH to be examined from the XBT temperature and the salinity climatology and enables salinity effect to be quantified. At 6°N the amplitude of the SSH variations induced by the salinity effect is about 3 cm, equivalent to the amplitude induced by the temperature effect.