A study of inter-annual variability of U.S. West Coast upwelling zones with ENSO and PDO indices using high-resolution regional climate modeling

The U.S. coastal offshore region marine atmospheric boundary layer was simulated for a period of 25 years using the 12-km resolution WRF model with initial and boundary conditions from the NCEP/NCAR reanalysis products. The optimum physical parameterizations were selected after multi-annual sensitivity tests. The simulations during the upwelling periods were able to reproduce the observed characteristics of the sequences of wind maxima in the lees of major capes and minima on the upwind sides as well as the positive wind stress curl in the offshore belt along the coast. These features show multi-year persistence throughout the simulation period. Three major upwelling areas were defined over the northern, middle, and southern California coastal waters. The wind stress curl was computed for major upwelling areas in the lees of Point Arena, Point Sur, Cape Mendocino and Cape Blanco and the results were analyzed with respect to the El Nino Southern Oscillation (ENSO) index. The comparison showed that the wind stress curl is inversely proportional to the ENSO index. This implies that larger wind stress and positive wind stress curl generating coastal upwelling can be expected during El Nino periods and the opposite effect during La Nina conditions. The comparison of the wind stress curl in the upwelling regions with the PDO index shows similar behavior, but with somewhat smaller correlations. According to the results, the strongest climate signal was in the lees of Point Sur, Point Arena, and Point Conception.