Thermodynamic characteristics of marine atmospheric boundary layer across frontal regions of the Indian Ocean Sector of the Southern Ocean based on three field campaigns

Abstract

The Southern Ocean (SO) marine atmospheric boundary layer (MABL), regulated by continuous air-sea exchanges, plays a key role in transporting exchanges between tropics and poles. Among SO sectors, the Indian Ocean sector of SO (ISSO) remained least explored in terms of MABL characterization and is examined in this study. In ISSO, occurrence of sharp oceanic thermohaline fronts regulates the vertical thermodynamic structure of MABL, clouds, and inversions. In this study, these properties of ISSO MABL are investigated over three oceanic domains (Sub-Tropical Indian Ocean (STIO), ISSO, and High-Latitude SO (HLSO)). To achieve this, near-sea surface air-sea exchanges along with profiles of meteorological parameters generated between 25°S and 68°S and 57°E to78°E during three field campaigns conducted in the austral summers of 2017, 2018, and 2020 were utilized. Results showed strong SST-Tair variability across the study region. In STIO, positive SST-Tair indicated Low-Level Cold Air Advection (LLCAA)-induced destabilized and coupled MABLs capped by multiple inversions (>3INV's of strength∼0.35 K m−1 above∼1200 m) and thin mid/high-altitudes clouds (cloud-base∼830 m, cloud-top∼2309 m, and cloud-thickness∼758 m). Over ISSO and northern HLSO, weak and negative SST-Tair indicated Low-Level Warm Air Advection (LLWAA)-induced stratified and decoupled MABLs. Aided by advective mixing of multiple air-masses, low-level thick multilayered clouds (>2 layers, average cloud-base∼604 m, cloud-top∼2288 m, and cloud-thickness∼1314 m) and multiple strong high-level inversions (>2INV's, strength>0.4 K m−1,∼1836 m) was observed. In HLSO, weakly positive SST-Tair indicated LLCAA-induced weakly destabilized MABLs capped by mid-altitude inversions (strength∼0.22 K m−1,∼1632 m) and mid-altitude clouds (cloud-base∼979 m, cloud-top∼2465 m, and cloud-thickness∼1263 m) supported by sublimation leading to virga conditions.