Dynamics of a Catalina Eddy revealed by numerical simulation

Through numerical simulations with the Pennsylvania State University-NCAR M esoscale 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 clim atological northwesterlies to northerlies. results in a more pronounced eff ect of the coastal orography around the California bight region. In particu lar, relative vorticity formed by Row over the coastal terrain remains offs hore. Prior to the formation of a quasi-steady eddy within the bight, the n ortherlies are strong enough to advect anomalously high vorticity out of th e region. The formation of a mature Catalina eddy relies on a rapid deceler ation of the synoptic-scale northerlies on 30 June. such that vorticity, on ce formed, remains in the bight. The eddy is also strongly modulated by the diurnal cycle. Northwesterly flo w 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 l and-sea thermal contrast. This strengthened Row overlaps temporally with a minimum in low-level stratification due to surface heating. The result is a ir characterized by a relatively high Froude number, which traverses over t he 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 f low becomes more stable, resulting in flow around, rather than over, the co astal mountains. This regime transition yields a wake with little depressio n of the marine layer and an absence of vorticity generation on the scale o f the bight region. Given strong ambient how, vorticity generated in the ev ening is swept southward past the bight the following day, but with weak am bient Row, the eddy persists in the bight during daytime, weakening slowly.