Abstract
Fourteen years of monthly repeated expendable conductivity–temperature–depth sections across the northeastern Bay of Bengal were used to examine the interannual variability of temperature inversions during Indian Ocean dipole (IOD) years. Data averaged over three regions at the latitude band of 15°–19°N along the Port Blair–Kolkata shipping transect reveal occurrences of temperature inversions during November–February. Notably, inversions with higher thickness (∼92 m) and intensity (3.24°C) occurred at relatively shallow depths (∼50 m) during the 2012 positive IOD compared to the 2016 negative IOD (∼25-m thickness, 1.91°C intensity, at 86-m depth). The intensity of the inversion layer gradually weakened from Kolkata to Port Blair during the 2012 and 2019 positive IOD years and vice versa during the co-occurrence of IOD with El Niño–Southern Oscillation (ENSO) in 2015. The observed stratification was higher during 2012 and 2019 positive IOD years. Salt budget analysis revealed that the advection of freshwater plumes was the primary reason for this observed high stratification. Interestingly, on average, the inversion layer occupied 73%–86% of the barrier layer during the positive IOD years (2012 and 2019) and 40%–55% during the 2016 negative IOD (nIOD) year, from Kolkata to Port Blair. Heat budget analysis revealed that the net surface heat flux and penetrative shortwave radiation were the governing factors responsible for the observed inversion characteristics in the study region during different IOD phases. Our analysis indicates that undulations from westward-propagating Rossby waves were the driving mechanism behind the shallow and deep occurrences of inversions in our study region during the positive (2012 and 2019) and negative (2016) phases of IOD. The changes in mixed layer temperature caused by these wave processes were notably more dominant than the effect of the heat trapped in the inversion layer.
Significance Statement
The increase of temperature with depth in the ocean, known as temperature inversion, forms over the northern Bay of Bengal regularly during winter. The heat trapped in the inversion layer is believed to impact the sea surface temperature and, hence, the region’s climate. Therefore, using 14 years of expendable conductivity–temperature–depth data collected along the Kolkata–Port Blair transect through passenger ships, the year-to-year variability of temperature inversion is studied. Due to excess freshwater plumes, weak downwelling Rossby wave propagation, and conducive heat fluxes, intense temperature inversions formed near the surface in the 2012 positive Indian Ocean dipole year compared to the 2016 negative Indian Ocean dipole year. The heat trapped in the inversion does not seem to impact sea surface temperature as it is eroded due to the propagation of the upwelling Rossby waves.
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