Ocean Surface Layer Response Under Madden-Julian Oscillation Convective Systems in the Equatorial Indian Ocean

Extensive surface heat flux, acoustics, and turbulence measurements, from a geostationary ship at 0,80.5E and from moorings at 0,80.5E and 0,90E equipped with moored temperature microstructure profilers [χpod], documented buoyancy and current responses to four Madden-Julian Oscillation (MJO) passages in the central-eastern equatorial Indian Ocean during the fall 2011 - spring 2012 DYNAMO experiment. Each MJO passage was marked with a net air-sea heat flux into the atmosphere, mainly controlled by attenuated shortwave radiation and increased evaporative cooling, and the occurrence of westerly wind bursts which lasted for a couple of days. In response to the prevailing strong eastward wind stress (τ_x> 0.2 N/m²) and buoyancy loss (J_b⁰>5x10^-7 m²s^-3), sea surface temperature dropped by about 0.5°-1.5°C, strong surface-forced mixing occurred (turbulent kinetic energy dissipation rate ε>10^-6 m²s^-3), homogeneous layer within the upper 40-60 m formed, pycnocline descended, and westerly-driven eastward currents accelerated. Although intermittent surface freshening due to rain squalls was evident, entrained subsurface salty water appeared to increase surface salinity by 0.3 - 0.5 psu during MJO passage. The swift 1-1.5 m/s eastward currents (the Yoshida Wyrtki jet), strongest across the upper 90 m and within 2° of the equator, persisted for several weeks and created highly sheared currents below the jet’s base inducing amplified shear instability-generated mixing beneath the surface mixed layer. Low R_i (<0.5) and strong turbulent kinetic energy dissipation rate (10^-7<ε<10^-6 m²s^-3) characterized the jet’s base between 60-90 m. This sustained subsurface vertical mixing attributed to the Yoshida-Wyrkti jet affects the surface mixed layer heat budget and potentially delays the sea surface temperature recovery following the MJO passage.