Suresh I., Vialard Jérôme, Izumo Takeshi, Lengaigne Matthieu. (2024). Role of intersecting equatorial and coastal waveguides near Sri Lanka on intraseasonal sea level variability along the west coast of India. Journal of Geophysical Research : Oceans, 129 (3), p. e2023JC020198 [16 p.]. ISSN 2169-9275.
Titre du document
Role of intersecting equatorial and coastal waveguides near Sri Lanka on intraseasonal sea level variability along the west coast of India
Suresh I., Vialard Jérôme, Izumo Takeshi, Lengaigne Matthieu
Source
Journal of Geophysical Research : Oceans, 2024,
129 (3), p. e2023JC020198 [16 p.] ISSN 2169-9275
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 degrees 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. Sea level reflects variations in subsurface oceanic heat content and structure. Along the west coast of India (WCI), it acts as a warning sign for sudden strong decrease in seawater oxygen content that has deleterious impacts on ecosystems and fisheries. It is thus important to understand the causes of sea level variations in this region. Previous studies emphasized that equatorial wind variations force sea level anomalies that travel eastward, reach the Indonesian coast, and then propagate counter clockwise around the Bay of Bengal rim, around Sri Lanka and to the WCI. Here, we demonstrate that there is also a much shorter, direct pathway from the equatorial band to Sri Lankan coast and then to the WCI, bypassing the pathway through the Bay of Bengal. This newly discovered direct connection is in fact the primary contributor to WCI intraseasonal (20-150 days) sea level variations. The "classical" Bay of Bengal pathway is the second contributor. We also identify a forcing "hotspot" over a small region east of Sri Lanka as the third contributor. Finally, we demonstrate that the "direct" connection that we have highlighted here does not operate efficiently for sea level variations that span over a season or longer. This study highlights a new pathway for propagation of signals from equatorial band directly to west coast of India, bypassing Bay of Bengal This direct connection results from intersection of equatorial and coastal waveguides near Sri Lanka This pathway contributes to the west coast of India sea level primarily at intraseasonal timescale, but weakly at longer timescales
