Absence of the annual cycle in shelf current inshore of the East Indian Coastal Current

Abstract

A ubiquitous feature of the winds over the North Indian Ocean (NIO), which are dominated by monsoons, is the occurrence of variability with the annual period. It is equally pervasive in the ocean's wind-driven circulation. Here we report observations from the shelf off the east coast of India where this periodicity is absent even though local alongshore wind stress has it prominently, and so does the East India Coastal Current (EICC) that flows along the slope off the shelf only about 40 km away. Our observations are based on a high-frequency coastal radar (HF-R) installed at approximately 11.7°N on the east coast of India. It provided surface currents up to 200 km offshore. We use hourly data from two years, January 2017 to December 2018, to compare the alongshore current over the depth contour 50 m (taken to represent the shelf current, Sh-C) with that over the depth contour 1700 m (taken to represent the slope current, Sl-C). Wavelet analysis shows that Sh-C did not have the annual cycle and had periods primarily less than about 50 days. In contrast, Sl-C, i.e., the EICC, shows the annual period prominently and other lower periods from days to months. The two time-series when low-passed with a 100-day filter are uncorrelated. Theoretical models (Brink (2006), for example) attribute the absence of long periods on the shelf to finite friction on the shelf. It prompts longer-period shelf-wave modes to be weak near the coastline and stronger in deeper waters, making the shelf a high pass filter. Most marine processes (including biogeochemistry and fishery) in the NIO have been assumed to have an annual cycle due to a monsoon driven annual cycle in large-scale physical processes. Our observations show that this need not be the case on the shelf. Hence, a re-evaluation of existing ideas on shelf processes is needed.