The Structure and Variability of the Marine Atmosphere around the Santa Barbara Channel

Abstract

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 resulting 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 southwestern 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 winds 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 decrease farther east. Winds are usually weak in zone 2, located in the easternmost 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 located along the mainland coast. Summer air temperature at the surface follows the SST closely and varies significantly with location. Summer sea level pressure gradients are large, with the lowest pressure occurring on the northeast end of the Santa Barbara Channel. Diurnal variations are strongest in summer, although the modulation is weakest in zone 1. The diurnal variation 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 between 300 m in late afternoon and 350 m in late morning. In winter, synoptic conditions are driven by traveling cyclones and sometimes accompanied by fronts. These are usually preceded by strong southeast winds and followed by strong northwest winds. Atmospheric parameters are distributed more uniformly than in summer, and diurnal variations are greatly reduced. Sea level air temperature and pressure are more spatially uniform than 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 summertime situation is governed by a coupled interaction between the atmosphere and the underlying water. The ocean influences the density of the MABL to the extent that it behaves distinctly from the free atmosphere above, resulting in strong winds polarized in the direction parallel to the coast. In turn, these winds provoke an upwelling response in the coastal ocean, which in part determines the surface properties of the water.