FY 2019

Seasonal evolution of the surface layer heat balance in the subtropical Indian Ocean

Cyriac, A., M.J. McPhaden, H.E. Phillips, N.L. Bindoff, and M. Feng

J. Geophys. Res., 124(9), 6459–6477, doi: 10.1029/2018JC014559, View online (2019)

Abstract. The South Indian Ocean (SIO) is a region of strong air‐sea heat loss due to the unique ocean circulation pattern influenced by the Indonesian Throughflow. In this study, the seasonal variation of the surface layer heat budget in the eastern SIO is investigated using 2 years of measurements from a mooring at 25°S, 100°E, the only colocated upper ocean and surface meteorology time series in the subtropical Indian Ocean. The mooring data are combined with other in situ and satellite data to examine the role of air‐sea fluxes and ocean heat transport on the evolution of mixed layer temperature using heat budget diagnostic models. Results show that on seasonal time scales, mixed layer heat storage in the eastern SIO is mostly balanced by a combination of surface fluxes and turbulent entrainment with a contribution from horizontal advection at times. Solar radiation dominates the seasonal cycle of net surface heat flux, which warms the mixed layer during austral summer (67 W/m2) and cools it during austral winter (−44 W/m2). Entrainment is in good agreement with the heat budget residual for most of the year. Horizontal advection is spatially variable and appears to be dominated by the presence of mesoscale eddies and possibly annual and semiannual Rossby waves propagating from the eastern boundary. Results from the 2‐year mooring‐based data analysis are in reasonably good agreement with a 12‐year regional heat budget analysis around the mooring location using ocean reanalysis products.

Plain Language Summary. The southeast Indian Ocean is a region where the ocean loses a lot of heat to the atmosphere. However, until now there have not been any direct measurements of the heat flux from ocean to atmosphere and the other data sets that we use to understand this exchange do not agree on its size. In this study, we use 2 years of measurements from a flux mooring deployed near 25°S, 100°E, together with satellite data and model outputs to understand the seasonal changes in air‐sea fluxes and the role of ocean currents in controlling ocean surface temperatures in the southeast Indian Ocean. We found that the amount of heat stored in the surface mixed layer of the ocean is primarily the result of a balance between heat fluxes across the air‐sea interface and cooling of the surface ocean by mixing with deep water below. The heat transported by the ocean currents is highly impacted by eddies and waves propagating from the coast of Western Australia and at times also contributes to the heat balance in this region. The results of this study improve our understanding of how heat moves between the ocean and atmosphere to affect our climate and will help refine computer model projections of future climate change.