Dynamics of a Catalina Eddy Revealed by Numerical Simulation

Abstract

Through numerical simulations with the Pennsylvania State University?NCAR Mesoscale Model the dynamics of a Catalina Eddy event that formed during the period 26?30 June 1988 off the coast of southern California is examined. A strengthening and veering of the low-level synoptic-scale winds, from climatological northwesterlies to northerlies, results in a more pronounced effect of the coastal orography around the California bight region. In particular, relative vorticity formed by flow over the coastal terrain remains offshore. Prior to the formation of a quasi-steady eddy within the bight, the northerlies are strong enough to advect anomalously high vorticity out of the region. The formation of a mature Catalina eddy relies on a rapid deceleration of the synoptic-scale northerlies on 30 June, such that vorticity, once formed, remains in the bight. The eddy is also strongly modulated by the diurnal cycle. Northwesterly flow around 500 m above mean sea level impinging on the mountains north of the bight is enhanced during the late afternoon, mainly as a response to the land?sea thermal contrast. This strengthened flow overlaps temporally with a minimum in low-level stratification due to surface heating. The result is air characterized by a relatively high Froude number, which traverses over the coastal mountains and strongly depresses the marine layer over the bight. The depression in the marine layer results in a warm anomaly and cyclonic circulation. Later at night, the incident northwesterlies weaken and the flow becomes more stable, resulting in flow around, rather than over, the coastal mountains. This regime transition yields a wake with little depression of the marine layer and an absence of vorticity generation on the scale of the bight region. Given strong ambient flow, vorticity generated in the evening is swept southward past the bight the following day, but with weak ambient flow, the eddy persists in the bight during daytime, weakening slowly.