Response of the Southern California current system to the mid‐latitude North Pacific coastal warming events of 1982–1983 and 1940–1941

Abstract

ABSTRACTThe large‐scale structure of the California Current during the 1982–1983 mid‐latitude North Pacific coastal warming event showed several persistent (> 16 months), anomalous conditions: positive sea surface temperature anomalies (1–2.5oC), depression of the inshore thermocline by as much as 50 m greater than normal, anomalous high inshore steric heights (˜ 1.05 dyn. m) compared with normal values, anomalous high sea levels (˜ 25 cm), positive subsurface temperature anomalies (3–4oC), negative salinity anomalies (0.1‐0.3%), positive dissolved oxygen anomalies (0.5‐1.5 ml/1), and negative inshore nutrient anomalies. The magnitudes of the subsurface anomalies generally are much larger than those of the surface anomalies. The cross‐shelf length scales of the subsurface anomalies vary between 300 and 500 km. During this same period, pronounced negative sea surface temperature anomalies (2–3oC) developed in the central mid‐latitude North Pacific. The characteristic diagrams and sign reversals in the salinity, oxygen, and nutrient anomalies are consistent with enhanced onshore transport of Pacific subarctic water from the offshore California Current. The source of this water is primarily from the west‐northwest. The subsurface anomalies were produced dynamically by a depression of the inshore thermocline, which, at least in part, resulted from convergence of mass at the coastal boundary. The surface anomalies, however, were produced by a combination of dynamical and local thermodynami‐cal processes. Observed anomalous atmospheric forcing, as reflected in the 700 mb height anomaly and in negative upwelling indices, is consistent with enhanced onshore transport from offshore California Current. All the data support the conclusion that the expansion and intensification of the Aleutian Low and the decrease in strength of the North Pacific High produced an anomalous basin‐wide atmospheric circulation that, coupled directly with the large‐scale wind‐driven oceanic circulation, produced a major component of the North Pacific coastal warming response observed during 1982–1983. Nearly identical mid‐latitude atmospheric forcing and oceanic responses were observed during the 1940–1941 analog event. Anomalous phytoplankton and zooplankton distributions also were observed, and fish catch during this period decreased by about 40%, with nearshore pelagic species particularly affected. Not only was the abundance of single species affected, but also the community structure (i.e., rank order of abundance) was changed in response to the changing environmental conditions. The biological response appears to have been driven by the increased static stability, deeper thermocline, and reduced nutrient concentrations, which, in turn, resulted from the anomalous atmospheric circulation.