The structure and variability of the marine atmosphere around the santa Barbara channel

The Santa Barbara Channel is a region characterized by coupled interaction between the lower-level atmosphere, the underlying ocean, and the elevated topography of the coastline. The nature of these interactions and the resul ting weather patterns vary between summer and winter. During summer, synoptic winds are largely controlled by the combined effect of the North Pacific anticyclone and the thermal low located over the sout hwestern United States, resulting in persistent northwesterly winds. A well -defined marine atmospheric boundary layer (MABL) with properties distinct from the free atmosphere above is a conspicuous feature during the summer. The wind has different characteristics in each of three zones. Maximum wind s occur in the area extending south and east from Pt. Conception (zone 1), where they initially increase as they turn to follow the coast, then decrea se farther east. Winds are usually weak in zone 2, located in the easternmo st part of the channel, offshore from the Oxnard plain. Winds are also weak in zone 3, sometimes reversing to easterly at night, in a narrow band loca ted along the mainland coast. Summer air temperature at the surface follows the SST closely and varies significantly with location. Summer sea level p ressure gradients are large, with the lowest pressure occurring on the nort heast end of the Santa Barbara Channel. Diurnal variations are strongest in summer, although the modulation is weakest in zone 1. The diurnal variatio n is parallel to the coast in all of zone 3 but the Oxnard plain, where it is perpendicular to the coast. The height of the marine layer varies betwee n 300 m in late afternoon and 350 m in late morning. In winter, synoptic conditions are driven by traveling cyclones and sometim es accompanied by fronts. These are usually preceded by strong southeast wi nds and followed by strong northwest winds. Atmospheric parameters are dist ributed more uniformly than in summer, and diurnal variations are greatly r educed. Sea level air temperature and pressure are more spatially uniform t han in the summer. Spatial variations in the observed fields in the summer are consistent with a hydraulic model of the MABL as a transcritical expansion fan. The summer time situation is governed by a coupled interaction between the atmosphere and the underlying water. The ocean influences the density of the MABL to t he extent that it behaves distinctly from the free atmosphere above, result ing in strong winds polarized in the direction parallel to the coast. In tu rn, these winds provoke an upwelling response in the coastal ocean, which i n part determines the surface properties of the water.