Observed seasonal and intraseasonal variability of the East India Coastal Current on the continental slope

Abstract

We present data from three acoustic Doppler current profilers (ADCPs) moored off Cuddalore (12 • N), Kakinada (16.5 • N), and Gopalpur (19 • N) on the continental slope of the western Bay of Bengal and one mooring on the slope of the northern bay (89 • E, 19 • N; referred to as being located at Paradip). The data were collected during May 2009 to March 2013 and the observations show that the seasonal cycle, which includes the annual cycle, the semi-annual cycle, and a peak around 120 days, dominates the observed variability of the East India Coastal Current (EICC). Spectral analysis suggests that the 120-day peak dominates the seasonal variability at Paradip and is strong at Gopalpur and Kakinada. The annual cycle is coherent along the western boundary of the bay, i.e., the east coast of India, but with significant phase differences between moorings. At the semi-annual and 120-day periods, the alongshore coherence is weaker. Intraseasonal variability is weaker than the seasonal cycle, particularly at Cuddalore and Paradip, and it exhibits seasonality: the strongest intraseasonal variation is during spring (February-April). Peaks around 12 and 20-22 days are also seen at Gopalpur, Kakinada, and Cuddalore. A striking feature of the currents is the upward phase propagation, but there are also instances when phase propagates downward. The much lower vertical phase speed in the top ∼100 m at Cuddalore leads to a distinct undercurrent at this location; at other locations, the undercurrent, though it exists often, is not as striking. During spring, however, the EICC tends to flow poleward (eastward) at Cuddalore, Kakinada, and Gopalpur (Paradip) over the top ∼300 m, which is the maximum depth to which observations were made. The cross-shore component of the EICC is much weaker than the alongshore component at Cuddalore and, except for a few bursts during spring, at Kakinada and Gopalpur. It is only at Paradip, on the slope of the northern boundary, that significant cross-shore flows are seen during spring and the summer monsoon (June-August) and these flows are seen to be associated with eddy-like circulations in the altimeter data. We use the ADCP data to validate popular current data products like OSCAR (Ocean Surface Currents Analyses Real-time), ECCO2 (Estimating the Circulation and Climate of the Ocean, Phase II), and GODAS (Global Ocean Data Assimilation System). The OSCAR currents at Paradip match the observed currents well, but the correlation is much weaker at the other three locations. Both ECCO2 and GODAS fair poorly, particularly the latter because its variability in this boundary-current regime is extremely weak. Though it performs badly at Paradip, ECCO2 does capture the observed variability on occasions at the other locations.

An undercurrent off the east coast of Sri Lanka

“…During the southwest monsoon, the current along the Sri Lankan south coast (Schott et al, 1994), Sri Lankan east coast ( Fig. 2d-f; Table 2), and Indian east coast (Mukherjee et al, 2014) reverses its direction below approximately 100-150 m. The subsurface current flows eastward along the southern coast of Sri Lanka (Schott et al, 1994), in agreement with the circulation pattern derived from the POP velocity fields (Fig. 4b) and poleward along the Sri Lankan and the Indian east coast Mukherjee et al, 2014).…”

“…Circulation in the subsurface layer along the Sri Lankan east coast is characterized by reversed flows relative to the surface during boreal spring and summer, similar to the subsurface circulation along the Indian east coast and Sri Lankan south coast described by previous studies (Schott et al, 1994;McCreary et al, 1996;Mukherjee et al, 2014). During the southwest monsoon, the current along the Sri Lankan south coast (Schott et al, 1994), Sri Lankan east coast ( Fig.…”

“…Mooring observations show a reversing subsurface current occurring off the southern coast of Sri Lanka; it is most distinct during boreal spring and summer (Schott et al, 1994). In addition, a reversal of the EICC along the east coast of India is observed below 100 m that is southward during the southwest monsoon and northward during the northeast monsoon, with the winter undercurrent being a more permanent feature (Mukherjee et al, 2014). The direction of this undercurrent is in good agreement with findings from a continuously stratified linear ocean model (SLM) .…”

“…3) implying the influence of equatorial waves on the subsurface flow during this time (Luyten and Roemmick, 1982). Equatorial waves are also known to impact currents along the Indian east coast (Mukherjee et al, 2014) and Sri Lankan south coast (Schott et al, 1994;Shankar et al, 2002). Ekman pumping can also significantly impact the circulation along the eastern coast of Sri Lanka Vinayachandran et al, 1999).…”

ASIRI: An Ocean–Atmosphere Initiative for Bay of Bengal

“…At the basin scale, circulation and advection of salinity are strongly controlled by wind and remote equatorial forcing [Schott and McCreary (2001), Shankar et al (2002), Jensen (2001Jensen ( , 2003, Vialard et al (2009), Durand et al (2009), Girishkumar et al (2013), Vinayachandran et al (2013), Yu et al (1991), Potemra et al (1991), and Yu and McPhaden (2011) are a few references out of many]. The monsoon-driven, seasonally reversing currents alternately export low-salinity BoB water into the Arabian Sea (AS) via the East India Coastal Current (EICC) and Winter Monsoon Current (WMC) and import saltier Arabian Sea water into the BoB via the Summer Monsoon Current (SMC; e.g., Murty et al 1992;Schott et al 1994;Shetye et al 1996;McCreary et al 1996;Schott and McCreary 2001;Jensen 2001Jensen , 2003Durand et al 2009;Vinayachandran et al 2013;Mukherjee et al 2014). Monsoonal forcing also induces energetic mesoscale and submesoscale features, which complicate the regional oceanographic circulation, and, consequently, it is not completely clear how the BoB and Arabian Sea interact with each other and with the Indian Ocean equatorial region in distributing freshwater.…”