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Estimating depth of the 20°C isotherm from sea level in the Arabian Sea

For the tropical Indian Ocean (IO), depth of the 20°C isotherm (hereafter referred to as D20) is a reasonable proxy for the thermocline depth. Unfortunately, this important parameter cannot be measured by sensors onboard Earth-observing satellites. In this study, this parameter was estimated for the Arabian Sea using satellite altimeter observations. First, an empirical orthogonal function (EOF) analysis was performed separately on the fields of altimeter-observed sea level anomaly (SLA) and model-simulated D20. Then, the temporal amplitudes of D20 were regressed against those of SLA to find simple analytical relations which were used to generate basin-wide fields of D20 from altimeter data. A comparison of D20 fields estimated in this manner with in situ observed fields of D20 suggests that this method performs well. In quantitative terms, the root mean square errors (RMSEs) of the estimated D20 as well as the model-simulated D20 are both of the order of 24 m, which is also less than the standard deviation (SD) of the observed D20 (27 m). Hence, it seems that this method can serve as an attractive and cost-effective alternative for estimating a basin-wide field of D20 in the Arabian Sea.