Role of Intersecting Equatorial and Coastal Waveguides Near Sri Lanka on Intraseasonal Sea Level Variability Along the West Coast of India

Abstract

The sea level variations along the west coast of India (WCI) significantly affect the ecosystems and fisheries, because of their tight coupling with the oxycline depths in this region, which hosts the world's largest natural hypoxic system. Here, we investigate the main causes of the WCI sea level variability. Using idealized experiments with a linear, continuously stratified ocean model, we first demonstrate that there is a direct pathway between the equatorial waveguide and the WCI, in addition to the well‐documented pathway that aligns with the coastal waveguide in the Bay of Bengal. This direct connection results from the intersection of equatorial and coastal waveguides near Sri Lanka. The forced and reflected equatorial Rossby waves induce sea level variations at the Sri Lankan coast around 6°N, which propagate directly to the WCI as coastal Kelvin waves, without transiting through the Bay of Bengal coastal waveguide. Using model experiments with realistic coastline and forcing, we then illustrate that this direct pathway is the primary contributor (0.4 regression coefficient) to the WCI intraseasonal (20–150 days) sea level variability, followed by the Bay of Bengal coastal waveguide pathway (0.3) and the wind forcing in a small region near Sri Lanka (0.25). The remote forcing originating from the rest of the Bay of Bengal and the WCI local wind forcing have weaker influence. We conclude by discussing why this direct connection exhibits strong impact on sea level variability at the intraseasonal timescale, while its influence is considerably weaker at the longer seasonal and interannual timescales.