Intra‐seasonal sea level variability along the west coast of India

Abstract

The importance of local versus distant forcing is studied for the wind‐driven intra‐seasonal (30–120 day) sea level anomaly (SLA) variations along the west coast of India. Significant correlations of altimeter‐derived SLA on the west coast are found with the mid‐basin SLA east of Sri Lanka and SLA as far as Sumatra and the equator, with increased lags, connecting with the remote forcing from the equator in the form of reflected Rossby waves. The highest correlations between SLA on the west coast and winds are found with the winds at the southern tip of India. Coherence calculations help to identify the importance of a narrow band (40–60 day) for the interactions of winds with the intra‐seasonal SLA variations. A multivariate regression model, along with the coherences within this narrower band, suggest the lags of SLA on the west coast with winds to range from 0 to 2 days with the local forcing to 11–13 days with the forcing along south east coast of India. Hovmöller diagrams illustrate the propagation of signals by estimating phase speed for Rossby waves (57 cm/s) across the Indian Ocean from Sumatra and Coastal Trapped Waves (CTWs) along the west coast of India (178 cm/s). Propagation from the south‐east coast of India is not as robust as Rossby waves from Sumatra.

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

“…As a result, the contribution of RWs progressively diminishes, leading to a reduction in the overall magnitude of EQD at the seasonal and interannual timescales. validate the previous hypotheses by Amol et al (2012) and Dhage and Strub (2016), which proposed that the WCI intraseasonal sea level is influenced by wind variations in the vicinity of Sri Lanka (regression coefficient to the total sea level is ∼0.25), as the case for the seasonal (Suresh et al, 2016) and interannual (Suresh et al, 2018) timescale.…”

“…(2013) regarding the existence of the EQD connection at the intraseasonal timescale. Dhage and Strub (2016) inferred from observations that this direct connection plays an important role in the WCI sea level ISV. Our results not only validate their hypothesis but also provide a quantitative assessment of its contribution, with regression coefficient of ∼0.4 and ∼0.3 for EQD and EQB, respectively, when regressed against the total intraseasonal sea level.…”

“…More specifically, we first investigated whether the "direct connection" (EQD) between the equatorial waveguide and the WCI near Sri Lanka, proposed by Suresh et al (2013) and Dhage and Strub (2016), exists. We also investigated if the "forcing hotspot" near Sri Lanka, highlighted at seasonal and interannual timescales by Suresh et al (2016Suresh et al ( , 2018, does operate effectively at the intraseasonal timescale, as suggested by Amol et al (2012) and Dhage and Strub (2016).…”

Climatic Drivers of Extreme Sea Level Events Along the Coastline of Western Australia

“…This contribution of sea level variability arises from remote generation sources of the Leeuwin Current in northern Australia that are primarily driven by the seasonal reversal of monsoon winds that initially create a large pulse of elevated sea level within the Gulf of Carpentaria that then propagates counter-clockwise around Australia toward Tasmania; as well as a secondary contribution from the large seasonal cycle of surface heat fluxes on the northwest shelf of Australia (Ridgway & Godfrey, 2015). The amplitude of the seasonal sea level cycle along WA (0.10-0.15 m) is thus relatively large when placed in a global context; however, there are also other regions that experience seasonal cycles of similar magnitude; for example, off India (Dhage & Strub, 2016) and in areas within the South China Sea (Amiruddin et al, 2015). However, for the southwestern region of Australia, where the equivalent mean tidal amplitude (i.e., half of the range) is only ∼0.2 m, these seasonal water level fluctuations make a significant contribution to the occurrence and timing of ESL events each year.…”

Progress in understanding of Indian Ocean circulation, variability, air–sea exchange, and impacts on biogeochemistry

“…Recently, a modelling study suggested that interception of the intraseasonal equatorial Rossby waves by the southern tip of India and Sri Lanka excites coastal Kelvin waves that contribute significantly (∼ 60 %-70 %) to the intraseasonal variability along the west coast (Suresh et al, 2013). A satellite sea level study by Dhage and Strub (2016) confirmed the model-based findings of Suresh et al (2013) and revealed that large-scale winds from the south of India and Sri Lanka also contribute to the coastal signals along the west coast of India.…”