A shallow cyclonic circulation is found to occur in the summer time over the Monterey Bay. Since it is often centered offshore from the city of Santa Cruz and it has never been studied before, it is named "Santa Cruz Eddy" in this study. With its horizontal size of 10-20 km, the Santa Cruz eddy represents the only non-severe weather example of a meso-gamma circulation in the atmosphere. It forms in the late afternoons and evenings and it can either last a few hours or continue all night. The Santa Cruz eddy is important for local weather because it causes surface winds along the Santa Cruz coast to blow from the east instead of from the north-west, which represents the climatological summer pattern for this area. As a consequence, cool and moist air is advected from the south and south-east into the Santa Cruz area, bringing both relief from the heat and fog to the city.

Data were collected during the summer of 2000 from three buoys and eight inland stations, for a total of eleven sites. They showed that: (1) a strong pressure difference (i.e., up to 2 mb) exists during the day between the southern and the north-eastern parts of the bay; (2) a southerly wind starts forming in the eastern part of the bay in the early afternoon before the eddy forms, advecting cooler air northward; (3) this cooler air takes about four hours to reach the Santa Cruz area and to close the circulation associated with the eddy, which therefore forms at the end of the day; (4) the eddy seems to dissipate and then reform once or twice during the night, which suggests that two different mechanisms act to form the eddy, one during the day and one at night.

The MM5 model is used as a tool to better understand the mechanism of formation of the Santa Cruz Eddy, in particular to study the difference between the vorticity formation during day and night. The test case chosen is the eddy event of August 24th 2000, which is simulated with two one-way nested domains with 150x120 grid points each and with horizontal resolutions of 5 and 1 km.