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.