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On the non-parametric changepoint detection of flow regimes in cyclone Amphan

Reddem Venkat Shesu, Jampana Venkata, Muthalagu Ravichandran, Venkateswara Rao Bekkam, Eluri Pattabhi Rama Rao, Tummala Srinivasa Kumar

Oceanologia

2023

Vol. 65 (2)

310--317

The Bay of Bengal was witness to a severe cyclone named Amphan during the summer of the year 2020. The National Institute of Ocean Technology (NIOT), INDIA moorings BD08 and BD09 happened to be in the vicinity of the cyclone. The highly instrumented mooring recorded near-surface meteorological parameters like wind speed, sea surface temperature, and near-surface pressure. This article explores the possibility of using a non-parametric algorithm to identify different flow regimes using a one-month long time-series data of the near-surface parameters. The changes in the structure of the time series signal were statistically segmented using an unconstrained non-parametric algorithm. The non-parametric changepoint method was applied to time series of near-surface winds, sea surface temperature, sea level pressure, air temperature and salinity and the segmentations are consistent with visual observations. Identifying different data segments and their simple parameterization is a crucial component and relating them to different flow regimes is useful for the development of parametrization schemes in weather and climate models. The segmentations can considerably simplify the parametrization schemes when expressed as linear functions. Moreover, the usefulness of non-parametric automatic detection of data segments of similar statistical properties shall be more apparent when dealing with relatively long time series data.

Response of the Bay of Bengal to super cyclone Amphan examined using synergistic satellite and in-situ observations

Chacko Neethu, Jayaram Chiranjivi

Oceanologia

2022

No. 64 (1)

131--144

Tropical cyclone Amphan is the first super cyclone that happened in the north Indian Ocean in the last 20 years. In this work, multi-platform datasets were used to investigate the responses of the upper ocean to cyclone Amphan. The most striking response was the cold wake left by the cyclone spanning the entire Bay of Bengal with an amplitude up to ∼4°C. Satellite salinity observations revealed that the maximum increase in surface salinity was ∼1.5 PSU on the right side of the track of Amphan. Surface circulation was also observed to be modulated with the passage of a cyclone with a rightward bias in the change in its speed and direction. The currents observed from a moored buoy showed strong inertial oscillations. Argo observations showed that changes induced by the cyclone occurred up to 150 m depth of the cyclone and ocean heat content in the upper 150 m depth decreased due to the passage of the cyclone. There was an enhancement of surface chlorophyll concentration (∼1.5 mg/m3) after the passage of the cyclone, which was centred along the track of the cyclone where the winds were the highest. Mixed layer heat and salinity budget analysis showed that the sea surface cooling and increase in salinity was primarily driven by vertical mixing processes, though horizontal advection contributed meagrely. This study also brings forward the fact that regional differences exist in the responses of the ocean to the forcing of cyclones.