The potential impact of intensified westerlies on the Lorenz energy cycle for the Southern Ocean is examined by employing a global eddy‐permitting ocean‐sea ice model. Two idealized sensitivity experiments are designed for this purpose: one is driven by 1992 forcing with weaker westerlies and the other driven by 1998 forcing with stronger westerlies. The intensified westerlies lead to the most significant increase of about 30% in the eddy kinetic energy (EKE) reservoir, followed by the mean kinetic energy (MKE) reservoir increase (17.9%), eddy available potential energy (EAPE) reservoir increase (8.6%), and mean available potential energy (MAPE) reservoir increase (6.5%). In contrast, the increases in the generations of kinetic energy and available potential energy are quite similar, ranging from 21% for EAPE generation to 26% for MKE generation. There are considerably increased energy transfers from MKE to MAPE (about 75%) and from MAPE to EAPE (about 78%), reflecting greatly enhanced baroclinic instability pathway. The conversion rates are strongly influenced by large topography; in particular, a relatively large energy conversion from EKE to MKE exists in the regions associated with large topography, in contrast to the energy flow from MKE to EKE over the broad Southern Ocean. Under stronger wind forcing, all energy conversions are enhanced, and the increases in the conversion rates from EAPE to EKE and from EKE to MKE are more prominent than the increases from MKE to MAPE and from MAPE to EAPE near large topography.

25The impact of synoptic atmospheric forcing on the mean ocean circulation is

Analyses of hydrographic observation data sets revealed that modified Circumpolar Deep Water (mCDW) tends to flood the western flank (73–78°E, 67–68°S) of the Four Ladies Bank (FLB) in Prydz Bay. In this study, we investigated the mechanism responsible for mCDW upwelling over the FLB based on an eddy‐resolving coupled ocean‐sea ice‐ice shelf model in conjunction with the latest high‐accuracy bathymetry. It was found that zonal step‐like declines in the seabed over the FLB are crucial for the mCDW onshore upwelling, through topographic dynamic effects on the alongshore Antarctic Slope Current. In the presence of meridional cross‐shelf σθ isopycnals, warm mCDW at ~500‐m depth in the deep sea can make its approach to the shallower continental shelf (~200‐ to 500‐m depths), characterized as a warm layer (> −1.8 °C) bounded by the cross‐shelf σθ isopycnal surfaces of 27.5 and 27.7 kg/m3. The simulated outflow of the Prydz Bay gyre is concentrated to the eastern flank of the Prydz Channel. These results suggest that, in addition to the depressions and troughs around the Antarctic, smaller topographic features such as step‐like declines in the seabed near the Antarctic Slope Current are also favorable for onshore mCDW intrusion. This study demonstrates that mCDW onshore transport around the Antarctic continental shelf might be significantly underestimated by numerical models with coarse spatial representations of the topography.

Between 2014 and 2016 the annual mean total extent of Antarctic sea ice decreased by a record, unprecedented amount of 1.6 3 10 6 km 2 , the largest in a record starting in the late 1970s. The mechanisms behind such a rapid decrease remain unknown. Using the outputs of a high-resolution, global ocean-sea ice model we show that the change was predominantly a result of record atmospheric low pressure systems over sectors of the Southern Ocean in 2016, with the associated winds inducing strong sea ice drift. Regions of large positive and negative sea ice extent anomaly were generated by both thermal and dynamic effects of the wind anomalies. Although the strong wind forcing also generated the warmest ocean surface state from April to December 2016, we show that enhanced northward sea ice drift and hence increased melting at lower latitudes driven by strong winds made the dominant contribution to the large decrease in total Antarctic sea ice extent between 2014 and 2016.