Climatology of Surface Heat Fluxes 

 Over the California Current Region 



CRAIG S. NELSON and DAVID M. HUSBY 1 

 ABSTRACT 



Historical surface marine weather observations are used to compute large-scale atmosphere-ocean heat 

 exchange components over the California Current region. Heat exchange components are summarized by 1 ° 

 square areas and long-term months, and major features of the monthly distributions are described. The accu- 

 racy of the derived air-sea interaction variables and methods of computation are discussed. 



The region off the west coast of the United States and Baja California is characterized by net annual heat 

 transfer from atmosphere to ocean. Net oceanic heat gain reaches a maximum during summer off Cape Men- 

 docino. Near the coast, surface heat flux is determined by a balance between incoming solar radiation and 

 effective back radiation. In the offshore regions, high cloudiness reduces the magnitude of the short-wave radi- 

 ative flux, and latent heat flux produces the largest heat loss. The principal seasonal and spatial variations in 

 air-sea heat transfer are a consequence of coastal upwelling which contributes to relatively low cloudiness and 

 high incident solar radiation near the coast, suppression of evaporative heat loss, and reversal of the sensible 

 heat flux. Simplified heat budget calculations demonstrate the importance of advective processes in maintain- 

 ing the seasonal heat balance in coastal upwelling regions. Nonseasonal fluctuations are evident in time series 

 of heat exchange processes, but low frequency components are not well described by the surface marine data 

 used in this study. 



INTRODUCTION 



Eastern boundary current regions, such as the area off the west coast 

 of the United States and Baja California, are characterized by high 

 organic production which supports large stocks of commercially 

 important fishes, e.g., Pacific sardine. Sardinops sagax; northern 

 anchovy, Engraulis mordax; and Pacific mackeral. Scomber japoni- 

 cus. High productivity is favored by vertical and horizontal transfer of 

 nutrients and by a shallow thermocline that typically lies above the 

 compensation depth in these temperate zones. Wind-forced divergence 

 of surface flow is the principal driving mechanism for vertical nutrient 

 exchange near the coast (Woosterand Reid 1963). Thermohaline proc- 

 esses modify the density structure and vertical stability of the upper 

 ocean and partially condition these waters for high primary productiv- 

 ity. Internal readjustment of mass, in response to both thermal and wind 

 forcing processes, may contribute to large-scale changes in circulation 

 offshore and may alter the normal seasonal patterns of alongshore flow 

 near the coast. 



Comparative studies of surface circulation and reproductive 

 strategies of pelagic fish stocks in the California Current suggest 

 that nonseasonal fluctuations in atmosphere-ocean exchanges of 

 momentum, heat, and mass lead to wide variations in recruitment 

 to coastal fish stocks of the region (Parrish et al . 1981). Anomalous 

 wind forcing may adversely affect larval survival by inducing loss 

 of epipelagic eggs and larvae through strong seaward surface trans- 

 port. Recent laboratory and field experiments have also suggested 

 that reproductive success of the northern anchovy is sensitive to 

 dispersion of food strata by wind mixing events during early larval 

 feeding CLasker 1978). The corollary implies that a precondition 

 for enhanced larval survival may be strong vertical stability in the 

 upper water column and the absence of vigorous mechanical wind 

 mixing at the surface. Because stability is strongly affected by ther- 



'Southwest Fisheries Center Pacific Environmental Group. National Marine 

 Fisheries Service, NOAA, c/o Fleet Numerical Oceanography Center. Monterey. 

 CA 93940. 



mohaline processes, analyses of seasonal and nonseasonal 

 atmosphere-ocean heat exchange processes may provide valuable 

 indices to map favorable areas for larval survival. 



Effects of climatic variability on stock recruitment have been 

 observed in pelagic species in other eastern boundary currents, the 

 North Sea, and the North Atlantic (Cushing 1975). Climatic 

 change may be in the form of long-term fluctuations or year-to-year 

 differences in the atmospheric forcing of the ocean. An important 

 first step in modeling the fisheries-environment relationships in the 

 California Current region is to determine the normal seasonal and 

 spatial variability of the atmospheric forcing processes from which 

 changes can be measured. 



The California Current flows equatorward along the west coast 

 of the United States between a cell of high atmospheric pressure to 

 the west and a continental thermal low located over central Califor- 

 nia. Seasonal variations in the California Current appear to be 

 related to fluctuations in wind stress and wind stress curl (Munk 

 1950; Reid et al. 1958; Hickey 1979). Nelson (1977) described the 

 seasonal and spatial variations in wind stress and wind stress curl 

 for the area off the west coast of the United States and Baja Califor- 

 nia. In addition to the wind-driven component of flow, a complete 

 description of the California Current System must include the 

 effects of atmosphere-ocean exchanges of heat and mass (evapora- 

 tion - precipitation) on upper ocean circulation. 



Summaries of large-scale heat exchange processes over the Cali- 

 fornia Current have been made by Roden (1959) and Clark et al. 

 (1974) based on monthly mean atmospheric properties within 5° 

 latitude-longitude quadrilaterals. Wyrtki (1965) discussed the aver- 

 age annual values of heat exchange over the North Pacific Ocean 

 north of lat. 20°S, computed from 2° square monthly mean atmo- 

 spheric properties. The climate and heat exchange in a coastal 

 upwelling region adjacent to the Pacific Northwest have been 

 described by Lane (1965), who analyzed 11 yr of surface marine 

 weather observations and computed monthly values of heat 

 exchange components from 1953 to 1962. These summaries 

 described the gross features of the large-scale heat exchange proc- 



