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<p>This study investigates the effect of depth-dependent salinity and potential temperature (PT) anomalies on the timing of monsoon onset over the Kerala (MoK) coast, utilizing the Estimating the Circulation and Climate of the Ocean Version 4 (ECCO4) reanalysis dataset for March-May (MAM) during 1992–2017. The monsoon onset years have been divided into four pentads—too early (P1), early (P2), normal (P3) and late (P4) onset years based on India Meteorological Department (IMD) reports. Analysis reveals a dominance of positive salinity anomalies up to ~ 0.90 PSU during P1 and P2 years across the northern Indian Ocean (IO), specifically, in the regions feeding the Kerala coast. These conditions are consistent with elevated evaporation levels coinciding with intense southwesterlies and enhanced west-east moisture convergence over the Arabian Sea (AS). However, localized contrasting patterns appear over upper levels along the western/eastern coasts of India. Conversely, the P4 years appear to show negative salinity anomalies (-0.2 to -0.4 PSU), significant surface cooling of approximately − 0.75&#xa0;°C, and weaker southwesterlies, particularly over the AS. Surface salinity anomalies exhibit a reversal over AS and adjoining regions across the onset pentads (from P1 to P4). A strong inverse relationship between salinity and PT at mixed layer depth (MLD) over the equatorial IO highlights the role of stratification in modulating onset timing. Furthermore, subsurface warming (~ 0.75&#xa0;°C) during P1-P2 years, while cooling of around − 0.90&#xa0;°C during P4 years in the Bay of Bengal (BoB) and eastern IO, is evident. The resulting stratification/thermal structure modifies vertical mixing, affecting atmospheric convection and moisture availability. Overall, the depth-dependent salinity anomalies influence near-surface ocean stability and ocean-atmospheric interactions, necessary for monsoon progression and MoK. These findings highlight the importance of thermal observations and subsurface salinity in enhancing monsoon onset diagnostics.</p>

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Pre-monsoon Indian Ocean subsurface salinity and potential temperature anomalies associated with monsoon onset over Kerala

  • 期刊论文

  • 发表日期：

    2026年5月20日

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  • Dinesh K. Yadav,
  • Manas Pant,
  • B. Mandal,
  • Kumar Abhijeet,
  • R. Bhatla

摘要

This study investigates the effect of depth-dependent salinity and potential temperature (PT) anomalies on the timing of monsoon onset over the Kerala (MoK) coast, utilizing the Estimating the Circulation and Climate of the Ocean Version 4 (ECCO4) reanalysis dataset for March-May (MAM) during 1992–2017. The monsoon onset years have been divided into four pentads—too early (P1), early (P2), normal (P3) and late (P4) onset years based on India Meteorological Department (IMD) reports. Analysis reveals a dominance of positive salinity anomalies up to ~ 0.90 PSU during P1 and P2 years across the northern Indian Ocean (IO), specifically, in the regions feeding the Kerala coast. These conditions are consistent with elevated evaporation levels coinciding with intense southwesterlies and enhanced west-east moisture convergence over the Arabian Sea (AS). However, localized contrasting patterns appear over upper levels along the western/eastern coasts of India. Conversely, the P4 years appear to show negative salinity anomalies (-0.2 to -0.4 PSU), significant surface cooling of approximately − 0.75 °C, and weaker southwesterlies, particularly over the AS. Surface salinity anomalies exhibit a reversal over AS and adjoining regions across the onset pentads (from P1 to P4). A strong inverse relationship between salinity and PT at mixed layer depth (MLD) over the equatorial IO highlights the role of stratification in modulating onset timing. Furthermore, subsurface warming (~ 0.75 °C) during P1-P2 years, while cooling of around − 0.90 °C during P4 years in the Bay of Bengal (BoB) and eastern IO, is evident. The resulting stratification/thermal structure modifies vertical mixing, affecting atmospheric convection and moisture availability. Overall, the depth-dependent salinity anomalies influence near-surface ocean stability and ocean-atmospheric interactions, necessary for monsoon progression and MoK. These findings highlight the importance of thermal observations and subsurface salinity in enhancing monsoon onset diagnostics.