The Catalina Eddy Event of July 1987: A Coastally Trapped Mesoscale Response to Synoptic Forcing

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  • 1 Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania
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Abstract

The Catalina eddy that existed from 5 July to 12 July 1987 during FIRE (First ISSCP Regional Experiment) over offshore California is analyzed. There were two stages to the eddy's lifecycle. During the first, from 5 July to 1200 UTC 9 July, the eddy formed just south of Santa Barbara and drifted southeastward parallel to the coastline. This motion is attributed to an equivalent β effect associated with gradients of marine layer depth perpendicular to the coast. The eddy's thermal structure was characterized by an elevated marine inversion with surface temperatures 2°–4°C higher than beyond the periphery. Over offshore regions a sharp edge to the eddy was noted with a sudden change in mixed layer depth, wind speed and direction, and temperature. The eddy's influence on coastal winds was most notable during the nighttime and early morning. The strong local sea-breeze circulation overwhelmed the coastal eddy circulation during daytime. A pronounced diurnal wind fluctuation was observed at San Nicolas Island during this period, associated with a perturbation wind parallel to the California coastline. We conclude that it is due to either an extended coastal sea-breeze influence (latitudes in this region are close to the critical latitude according to linear theory) or northward-propagating coastally trapped Kelvin waves. The eddy's second stage was initiated on 9 July by the formation of a cutoff low in the middle troposphere immediately above the eddy. During this period the eddy expanded horizontally, moved southwestward away from the coastline, and eventually weakened. For a brief time, a coherent meso-α structure existed from the surface to about 500 hPa.

Eddy formation was precipitated by the passage of a low-level trough that strengthened the northerly flow across the mountains north of Santa Barbara. Froude numbers at the time of eddy formation suggest considerable lee troughing as the airflow was forced over and possibly around the topography.

Abstract

The Catalina eddy that existed from 5 July to 12 July 1987 during FIRE (First ISSCP Regional Experiment) over offshore California is analyzed. There were two stages to the eddy's lifecycle. During the first, from 5 July to 1200 UTC 9 July, the eddy formed just south of Santa Barbara and drifted southeastward parallel to the coastline. This motion is attributed to an equivalent β effect associated with gradients of marine layer depth perpendicular to the coast. The eddy's thermal structure was characterized by an elevated marine inversion with surface temperatures 2°–4°C higher than beyond the periphery. Over offshore regions a sharp edge to the eddy was noted with a sudden change in mixed layer depth, wind speed and direction, and temperature. The eddy's influence on coastal winds was most notable during the nighttime and early morning. The strong local sea-breeze circulation overwhelmed the coastal eddy circulation during daytime. A pronounced diurnal wind fluctuation was observed at San Nicolas Island during this period, associated with a perturbation wind parallel to the California coastline. We conclude that it is due to either an extended coastal sea-breeze influence (latitudes in this region are close to the critical latitude according to linear theory) or northward-propagating coastally trapped Kelvin waves. The eddy's second stage was initiated on 9 July by the formation of a cutoff low in the middle troposphere immediately above the eddy. During this period the eddy expanded horizontally, moved southwestward away from the coastline, and eventually weakened. For a brief time, a coherent meso-α structure existed from the surface to about 500 hPa.

Eddy formation was precipitated by the passage of a low-level trough that strengthened the northerly flow across the mountains north of Santa Barbara. Froude numbers at the time of eddy formation suggest considerable lee troughing as the airflow was forced over and possibly around the topography.

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