Comparison of variability in air-sea heat fluxes and Subantarctic Mode Water formation from concurrent mooring observations in the Southeast Indian and Southeast Pacific

Subantarctic Mode Water (SAMW) formation and subduction north of the Subantarctic Front in the Southern Ocean plays an important role in global budgets of heat, carbon and nutrients. Air-sea heat fluxes are a key process driving the formation of SAMW, but there are few direct observations of fluxes, particularly during the winter. The Ocean Observatories Initiative (OOI) Southern Ocean mooring (in the Southeast Pacific) and the Southern Ocean Flux Site (SOFS, in the Southeast Indian) provide the first concurrent, multi-year, time series of air-sea fluxes in the Southern Ocean, and are well-placed in two key SAMW formation regions. In this work we compare and contrast characteristics and variability of air-sea heat fluxes, mixed-layer depths and SAMW formation from observations at these two mooring location, and combine these with Argo float data and atmospheric reanalyses to provide temporal and spatial context for the mooring observations.

We show that stronger ocean heat loss events measured at the SOFS site than at the OOI site result from larger latent heat losses associated with warmer sea surface temperatures and stronger winds. The interannual variability of wintertime ocean heat loss is larger at OOI than SOFS site. inter mixed-layer depth anomalies tend to be in phase at the two moorings, where anomalously deep mixed layers are associated with anomalous advection of cold air from the south, and conversely shallow mixed layers correspond to warm air from the north. However, both the winter heat flux and mixed- layer depth anomalies show a complex spatial pattern, with a zonal dipole pattern in both the Indian and Pacific basins that we relate to the leading modes of climate variability in the Southern Ocean. Finally, we use the Walin water mass transformation framework to estimate SAMW formation rates and relate their variability to variability in air-sea heat flux.