Anticyclonic Suppression of the North Pacific Transient Eddy Activity in Midwinter

Dynamical understandings of midlatitude transient eddy activity, especially its midwinter minimum over the North Pacific, are still limited, partly because conventional Eulerian eddy statistics are incapable of separating cyclonic and anticyclonic contributions. Here we evaluate the two contributions separately based on local curvature of instantaneous flow fields to compare their seasonality between the North Pacific and North Atlantic storm-tracks. The anticyclonic contribution is found crucial for the midwinter minimum of the North Pacific transient eddy activity. Eddy energetics reveals that the net efficiency of the anticyclonic contribution in replenishing total transient eddy energy over the North Pacific exhibits a pronounced midwinter minimum leading to net energy loss, while that of its cyclonic counterpart does not, in harmony with a precipitation peak around midwinter. This study suggests that more attention should be paid to anticyclones in studying midlatitude storm-track dynamics. Our understanding of the dynamics of midlatitude transient eddy activity, especially its midwinter minimum over the North Pacific, is still limited. This is partly because conventional local statistics based on temporal filtering, which are commonly used as a measure of transient eddy activity, are unable to treat contributions from cyclones and anticyclones separately. Here we evaluate cyclonic and anticyclonic contributions to local eddy statistics separately based on local curvature of instantaneous flow fields, to compare their seasonality between the North Pacific and North Atlantic storm-tracks. The anticyclonic contribution is found crucial for the midwinter minimum of the North Pacific transient eddy activity. We then apply eddy energetics to assess the relative importance of various processes relevant to the seasonality of eddy activity. The net efficiency of the relevant processes associated with the anticyclonic contribution in replenishing total transient eddy energy over the North Pacific exhibits a pronounced midwinter minimum leading to net energy loss. By contrast, that of the cyclonic counterpart does not, in harmony with a precipitation peak around midwinter. This study suggests that more attention should be paid to anticyclones in studying midlatitude storm-track dynamics. Anticyclonic contribution is crucial for the midwinter minimum of the North Pacific transient eddy activityThis minimum is consistent with net energy loss for anticyclonic regions in midwinter in energy conversion/generation termsMore attention should be paid to anticyclones in studying midlatitude storm-track activity and their interaction with a time-mean flow