Surface Heat Fluxes Drive a Two-Phase Response in Southern Ocean Mode Water Stratification

Subantarctic mode waters have low stratification and are formed through subduction from thick winter mixed layers in the Southern Ocean. To investigate how surface forcing affects the stratification in mode water formation regions in the Southern Ocean, a set of adjoint sensitivity experiments are conducted. The objective function is the annual-average stratification over the mode water formation region, which is evaluated from potential temperature and salinity adjoint sensitivity experiments. The analysis of impacts, from the product of sensitivities and forcing variability, identifies the separate effects of the wind stress, heat flux, and freshwater flux, revealing that the dominant control on stratification is from surface heat fluxes, as well as a smaller effect from zonal wind stress. The adjoint sensitivities of stratification to surface heat flux reveal a surprising change in sign over 2 years lead time: surface cooling leads to the expected initial local decrease in stratification, but there is a delayed response leading to an increase in stratification. This delayed response in stratification involves effective atmospheric damping of the surface thermal contribution, so that eventually the oppositely-signed advective haline contribution dominates. This two-phase response of stratification is found to hold over mode water formation regions in the South Indian and Southeast Pacific sectors of the Southern Ocean, where there are strong advective flows linked to the Antarctic Circumpolar Current. The Southern Ocean, surrounding the Antarctic continent, plays an important role in the uptake and transport of heat and carbon. Subantarctic mode waters, which are characterized by their low stratification, play an important role in this uptake of heat and carbon, and therefore the factors impacting their properties need to be properly understood. To understand how surface forcing affects Subantarctic mode waters, sensitivity studies are conducted in an ocean state estimate, which consider the relative importance of surface heat flux, freshwater flux, and wind stresses on the stratification of mode waters. Surface heat flux has the largest impact on mode water formation both on seasonal and longer interannual timescales. Initially, surface heat loss leads to a decrease in stratification in the mode waters. However, there is a delayed response where the surface temperature response is effectively damped by the atmosphere and there is an opposing-signed salinity response advected into the region, leading to a subsequent increase in stratification in the mode waters. The sensitivity of Southern Ocean mode water stratification to surface heat flux changes sign over time Surface heat loss leads to an initial decrease in stratification in the mode waters Surface heat loss leads to a delayed restratification due to a haline contribution after a thermal contribution is effectively damped