Surface fluxes of sensible heat, moisture and momentum over the oceans have long been thought to play a significant role in the modulation of the evolution of maritime extratropical cyclones and of the precipitation processes within these storms. In particular, the surface moisture flux provides a direct source of the moisture needed for precipitation, while the sensible heat flux can affect the stability of the storm environment, thereby modulating the precipitation process and precipitation. Various studies, most relying on numerical models, have shown that the degree to which surface sensible heat and moisture fluxes affect storm evolution and precipitation depends on the location and timing of these fluxes relative to the cyclone and its associated structures. In addition, the magnitude and even the sign of the impact depends on the relative temperatures of the ocean and the atmosphere. Hence, storms of comparable magnitude and thermal characteristics will be impacted differently over different oceanic regions. Storms passing over the relatively high SSTs of the Gulf Stream in the western Atlantic Ocean will be enhanced to a much greater extent than a storm of similar magnitude occurring over the relatively cooler waters off the west coast of the United States. The SST can vary temporally as well as spatially, and this can occur on much larger time scales than that of the individual storms. A well-known example of temporal SST changes is that occurring with the El Niņo-Southern Oscillation (ENSO). During the warm phase of a strong ENSO, such as occurred in 1983 and again in 1998, the SSTs in the central Pacific Ocean are anomalously cold, while the SSTs within 300 km of the California coast are anomalously warm. Hence, if the coastal surface sensible heat and moisture fluxes are important for storm development and precipitation, one might expect a different impact during a strong warm ENSO phase compared to other phases of the ENSO.

The California Landfalling Jets Experiment conducted its field program over the coastal and offshore waters of California during the strong warm ENSO phase of 1998. This paper will present the case of February 3, 1998, for which the observed surface sensible and latent heat fluxes within 300 km of the shore within a moderate low-level jet are shown to significantly contribute to the destabilization of the air just before it is forced to ascend the steep coastal terrain. As the air ascends, deep convection occurs with significant coastal flooding as result. A bulk surface flux parameterization is shown to match direct surface flux measurements reasonably well in this case, and is used to extend the results to suggest that the coastal fluxes only enhance the coastal precipitation during years with anomalously warm SSTs (ENSO years) and may even help suppress coastal convection during years of more normal colder SSTs.