An examination of atmospheric mechanisms that may be responsible for the annual reversal of the beaufort sea ice field

Abstract

AbstractThe late‐summer reversal of the Beaufort Sea Gyre and the overlying sea ice cover is an intriguing phenomenon that may reveal some insight into atmosphere‐cryosphere interactions through a detailed diagnostic analysis.In this study we examine the atmospheric processes that may give rise to this reversal and maintain it through a case by case study of selected days during the late‐August‐early‐September reversal in 1980. We inspect the fields of total vertical motion and the contributions by advective, diabatic, and frictional mechanisms, as well as the isentropic potential vorticity, in order to identify the forcing functions.This episode is an example of the classic cold low with a central cold pool, the surface and 500‐mbar fields aligned vertically, yet indications in the ageostrophic field of significant potential for baroclinic development, and a depression that remains anchored in the region for a significant period (27 days).The episode occurs during a transition period from the summer regime with sluggish upper air anticyclonic circulation to the vigorous winter regime of a cyclonic vortex extending up through the stratosphere with concomitant increase of potential vorticity available for surface development. This transition, which appears to occur during a period that typically experiences rapid upper atmosphere cooling, may be the trigger for the creation of the low.The surface heat flux appears to be a significant factor in the maintenance of the low although there are two factors at work. The first is the anomalous heat flow from the ocean along the ice‐ocean boundary and from open water in the diverging ice pack, but this primarily appears to result in a regional modulation of the synoptic‐scale effect. At the synoptic scale, there is significant heat flux associated with the very cold core of the depression and the associated pattern of the lower atmospheric heating moves with the wandering migration of the system within the region. In addition, there is an important dynamic contribution to development owing to the ageostrophic advection and convergence of cold air out of the core. The long duration of this feature within the Canada Basin must be traced to a complex interplay of heat flow processes related to the nature of the surface as well as the formation of the cold core of the depression, dynamic properties of mid‐tropospheric air flow, and the increase of upper air potential vorticity.