Mesoscale processes regulating the upper layer dynamics of Andaman waters during winter monsoon

Abstract. The characteristic of cold core eddies and its influence on the hydrodynamics and biological production in Andaman waters were studied using in situ and satellite observations. The specific structure and patterns of the temperature–salinity (T–S) profiles, nutrients and chl a indicate the occurrence of the eddy, the spatial extent of which is well marked in sea surface height anomaly (SSHA). The Cyclonic Eddies are centered at 7° N and 86° E, 13° N and 88° E and 13° N and 93° E (CE1, CE2 and CE3 respectively). In situ measurements are done in the eastern flank CE1 along 8° N and 92.5–93.5° E. Vertical currents recorded using Acoustic Doppler Current Profiles (ADCP) shows northward flow along the track (0.3 m s⁻¹) while along the western flank, the flow is weak and southward. This evidence the occurrence of cyclonic eddy and the altimetry derived SSHA depicts the spatial extent. Analysis to explore the possible forcing to induce the occurrence of eddy, indicate baroclinic instability (Ri < 0.0001) in the water column due to vertical shear in the horizontal flow. Bay of Bengal (BoB) water evidenced from the T–S profiles and the semi-annual Rossby wave are the contributing factors of eddy formation. Whereas, the wind stress curl is not a major inductive of divergence in the region. The eddy influenced the nutrient pattern (NO₂, NO₃, PO₄ and SiO₄) and the biological production (chl a) in the region though the influence is less significant. CE1 and CE2 are similar in terms of forcing mechanisms while, CE3 is associated with convective mixing processes occurring along the northwest coast of Andaman due to the prevalent cold dry continental air from north east.