5:15 PM - 6:45 PM
[ACG32-P01] Anticyclonic suppression of the North Pacific transient eddy activity in midwinter
Keywords:storm track, migratory cyclones and anticyclones, North Pacific, North Atlantic, midwinter suppression
Storm tracks typically occur coincident with the climatological oceanic fronts, under the anchoring effect of those oceanic fronts by cross-frontal differential heat supply from the ocean. The climatological baroclinic growth of transient eddies has shown to peak where the baroclinicity of the flow is largest around oceanic fronts. Nevertheless, 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 the 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.