Wind relaxations and poleward flow events in a coastal upwelling system on the central California coast

Abstract

[1] When upwelling favorable winds weaken or relax in coastal upwelling systems, prevailing equatorward currents often weaken and then reverse to form propagating poleward currents. Here the statistics of wind relaxations and poleward flow events along the south central California coast are derived using meteorological and oceanographic times series during [2000][2001][2002][2003][2004][2005][2006]. Over the 7-year record, 169 wind relaxations were observed (about 1 every 2 weeks) and 127 of these were followed by poleward flow events. Thermistor moorings and current profilers along the 15 m isobath at Alegria, Point Arguello, Point Purisima, and Point Sal recorded the poleward flows. The poleward flows propagate northward at 10-30 km dÀ1 and appear as sequential temperature increases at the moorings. Wind relaxations occur throughout the year but are most frequent in September and least frequent in April when upwelling winds are strong and persistent. Poleward flows follow wind relaxations frequently during May through November and rarely in December and January. Sea level differences between Santa Monica and Port San Luis, California, decreased as winds relaxed, consistent with forcing by alongshore pressure gradients. Temperature distributions at Point Arguello, Point Purisima, and Point Sal were skewed toward higher values because of the poleward flows. The alongshore distance traveled by the poleward flows increased with duration of the wind relaxations and magnitudes of alongshore temperature and sea level differences prior to the relaxations.

Wind relaxation and poleward flow events in the upwelling region off Baja California, Mexico

“…Winant et al (2003) reportaron varios patrones de circulación en la frontera entre la parte central y la parte sur de la costa de California, y mostraron que cuando los vientos favorables para las surgencias cesan o se relajan, se desarrolla un flujo hacia el polo. Melton et al (2009), con base en series de tiempo de vientos medidos por anemó-metros en boyas, identificaron eventos de relajación sobre las aguas frente a la parte central de California y los asociaron con corrientes cálidas hacia el polo. Torres y Gomez-Valdes (2015) también encontraron que los flujos hacia el polo ocurren después de eventos de relajación frente a la costa de Baja California.…”

“…Se implementó primero el método para identificar eventos de relajación del viento descrito por Melton et al (2009). Este método se basa en un análisis de funciones empíricas ortogonales (FEO) aplicado al campo de viento.…”

“…The method described by Melton et al (2009) was the first attempt to implement an objective method for the identification of wind relaxation events. The agreement between the results obtained with the Melton et al (2009) method and our method is a remarkable result.…”

“…Winant et al (2003) reported several circulation patterns at the boundary between the central and southern California coasts, showing that when upwelling-favorable winds cease or relax a poleward flow develops. Melton et al (2009), using wind time series from buoys, identified wind relaxation events over central California waters and associated them with warm poleward currents. Torres and GomezValdes (2015) also found that poleward flow events follow wind relaxation events off the northern Baja California coast.…”

“…The results obtained from the complex correlation technique were validated using 2 independent methods. The method to identify wind relaxation events described in Melton et al (2009) was implemented first. This method is based on an empirical orthogonal function (EOF) analysis of the wind field.…”