Observed intra-seasonal to interannual variability of the upper ocean thermal structure in the southeastern Arabian Sea during 2002–2008

Abstract

The southeastern Arabian Sea (SEAS), located in the Indian Ocean warm pool, is a key-region of the regional climate system. It is suspected to play an important role in the dynamics of the Asian summer monsoon system. The present study reports the salient features derived from a newly harvested observational dataset consisting of repeated fortnightly XBT transects in the SEAS over the period [2002][2003][2004][2005][2006][2007][2008]. The fortnightly resolution of such a multi-year record duration is unprecedented in this part of the world ocean and provides a unique opportunity to examine the observed variability of the near-surface thermal structure over a wide spectrum, from intra-seasonal to interannual time scales. We find that most of the variability is trapped in the thermocline, taking the form of upwelling and downwelling motions of the thermal stratification. The seasonal variations are consistent with past studies and confirm the role of the monsoonal wind forcing through linear baroclinic waves (coastally-trapped Kelvin and planetary Rossby waves). Subseasonal variability takes the form of anomalous events lasting a few weeks to a few months and occurs at two preferred time-scales: in the 30-110 day band, within the frequency domain of the Madden-Julian Oscillation and in the 120-180 day band. While this subseasonal variability appears fairly barotropic in the offshore region, the sign of the anomaly in the upper thermocline is opposite to that in its lower part on many occasions along the coast. Our dataset also reveals relatively large interannual temperature variations of about 1°C from 50 m to 200 m depth that reflect a considerable year-to-year variability of the magnitude of both upwelling and downwelling events. This study clearly demonstrates the necessity for sustained long-term temperature measurements in the SEAS.Keywords: XBT data, Arabian Sea, Lakshadweep Sea, Inter-annual variability, Intra-seasonal variability, WICC. IntroductionThe Lakshadweep Sea (henceforth LS), in the southeastern Arabian Sea (SEAS), is of prime climatic importance, being situated in the Indian Ocean warm pool (Fig. 1). It is also an area of marked oceanic dynamical activity. Several hydrographic cruises carried out during different seasons revealed striking contrasts between summer monsoon and winter monsoon seasons (Shetye et al., 1990(Shetye et al., , 1991. The advent of altimetry in the early 1990s allowed documentation of a clear seasonal cycle in the SEAS, with positive sea level anomaly (SLA) during winter and negative SLA during summer (Shankar and Shetye, 1997;Bruce et al., 1998). This feature was termed the Lakshadweep High and Low (henceforth LH/LL), after the name of the island chain (see Fig. 1 Arabian Sea) takes place over timescales of a few weeks to a few months for the lowest-order baroclinic modes (McCreary et al., 1993). Over the last 15 years, a hierarchy of numerical studies (McCreary et al., 1993;Shankar and Shetye 1997; led to the conclusion that the seasonal variability of LS hy...

Sea Level Interannual Variability Along the West Coast of India

“…A positive IOD thus induces positive SLA in late summer and fall and negative SLA during the following winter and spring in southeastern Arabian Sea. This modeling result is in line with the XBT observations of strong negative subsurface temperature anomalies in this region from December 2006 to summer 2007 (Gopalakrishna et al, 2010), in both timing and sign, following the strong positive IOD in fall 2006. Besides its potential instantaneous impact on late summer rainfall, the delayed IOD influence could also potentially modulate subsurface conditions in the Arabian Sea mini-warm pool (Vinayachandran et al, 2007) in the following spring, which could in turn feedback onto Indian summer monsoon characteristics.…”

Processes of 30–90 days sea surface temperature variability in the northern Indian Ocean during boreal summer

“…Part of the mismatch between the model and observation near the coast may also be related to observational issues. Indeed, using repeated XBT transects in the South Eastern Arabian Sea over 2002-2008 period, Gopalakrishna et al (2009) …”