Mean and Seasonal Eddy Heat Fluxes Estimated from a 20-year Time Series of Upper-Ocean Observations in Drake Passage

Drake Passage is an identified eddy hot spot in the Antarctic Circumpolar Current (ACC), corresponding to large eddy heat flux (EHF) and eddy kinetic energy (EKE). We estimate EHF and EKE from a unique 20-year time series of upper ocean temperature and velocity transects with unprecedented along-track resolution in Drake Passage. The EHF across the ACC relative to the positions of the major fronts is quantified, and its seasonality is described. The EHF across the ACC is calculated using two frames of reference: the time-mean geostrophic streamlines estimated from the transect data and the time-varying (synoptic) geostrophic streamlines that use altimetry to estimate the time-varying component. The synoptic streamlines provide the best estimate of the component of EHF that crosses the ACC because they provide the best tracking of the shifting and meandering ACC fronts. The across-stream EHF integrated over the upper 900-m is maximum poleward (-0.10 $\pm$ 0.05 GW m${}^{-1}$) in the Subantarctic Front (SAF) and remains poleward but is reduced towards the south, becoming statistically insignificant in the Polar Front, resulting in heat convergence south of the SAF. EKE is maximum in the SAF and the Polar Frontal Zone (PFZ) and exhibits a seasonal cycle. The EKE in the SAF/northern PFZ is reduced (enhanced) during the austral fall (spring). The southern PFZ lags the northern PFZ, resulting in a seasonal shift in the location of the peak EKE. The seasonal cycle of the across-stream EHF indicates maximum poleward eddy heat advection during the austral fall-winter. Since eddies provide the closure for the Southern Ocean meridional overturning circulation, the seasonal cycle of the across-stream EHF suggests that the eddy-driven upwelling of deep waters has a seasonal component.