The State of the California Current, 1997-1998: Transition to El Niño Conditions

Abstract

This report, part of a continuing series of annual reports describing oceanographic conditions in the coastal waters of the Californias, emphasizes the 1997-98 period. The coastal waters of the Californias were strongly influenced by El Niiio conditions beginning late in the summer of 1997 and continuing into the summer of 1998. Timely prediction of the onset of this event made it possible for several research programs to augment their observation programs. We review the pattern of atmospheric forcing and changes in the tropical ocean and note the initial impacts upon the California Current system. Sampling being done by the CalCOFI (California Cooperative Oceanic Fisheries Investigations) program is described, and recent data are summarized and interpreted. Data from several other programs including oceanographic sampling off Baja California and central California, and coastal data from buoys, shore stations, and diving programs in kelp forests are reported. There were large and rapid changes in atmospheric forcing and in the upper ocean temperature and salinity distribution and circulation pattern. The pelagic ecosystem was strongly influenced; cruise mean macrozooplankton abundance during the spring of 1998 was the lowest in the 50-year CalCOFI time series. Large changes in the range and abundance of plankton and fish populations were observed. El Nifio-induced changes must also be considered in the context of changes on other space-time scales, and the relation of El Niiio-related changes and secular trends seen since the mid-1970s regime shift will merit particular attention. INTRODUCTION This is the fifth in an annual series of reports (Hayward et al. 1994, 1995, 1996; Schwing et al. 1997) that describe and interpret oceanographic and related environmental data from the coastal region off the Californias. Physical data series are updated and intercompared to explain the prevailing forcing, and biological series and their anomalies are examined in relation to the physical structure. Our intent is to describe observational programs in the region and to provide a preliminary summary and interpretation of their results. The emphasis is on CalCOFI. We have, however, included information from several other programs in order to place the CalCOFI observations in a larger regional context and to provide a brief summary of other programs and a point of contact for additional information about them. The list of additional programs is by no means complete. In an effort to consider the most timely information that is available, we have included preliminary data in this report; some values may change as the final steps of data processing are completed. We have also had to balance our goals of interpreting and summarizing observations for a timely report with the need to use preliminary data and the brief lead time for their analyses. LYNN ET AL.: THE CALIFORNIA CURRENT, 1997-1 998 CalCOFl Rep., Vol. 39, 1998 We have chosen to include some more speculative interpretation of the most recent data while recognizing that subsequent reevaluation may lead to revision of some of the ideas. We start by examining the large-scale atmospheric and oceanic conditions that force much of the variability in the California Current. Last year's report (Schwing et al. 1997) described the initial development of strong El Nifio conditions in the eastern tropical Pacific Ocean during the first five months of 1997 while in the same period the California Current system was affected by regional forcing. El Nifio conditions became the dominant forcing process in the latter months of 1997. This report continues the description of the tropical El Niiio and its subsequent effect on the California Current region. However, events on other space-time scales also strongly influence this system, and the effects of El Niho must be considered in the context of longer-term trends, particularly the secular trends of warming of the upper layers and decline in macrozooplankton biomass that have been evident since the mid-1970s regime shift (Roemmich and McGowan 1995). Our emphasis is upon oceanographic conditions and pelagic ecosystem structure, but trends in coastal kelp forest communities and pelagic seabirds are also described. A few fisheries issues are considered as well. DATA SETS AND METHODS Coastal data include temperature and salinity at shore stations (Walker et al. 1994). La Jolla ( S I 0 Pier) and Pacific Grove daily temperatures and their anomalies from the long-term harmonic mean (1916-93 for La Jolla and 1919-93 for Pacific Grove) are shown as time series. Coastal sea-level data from San Diego and San Francisco are shown as monthly anomalies from the 1975-95 mean, corrected for atmospheric pressure. Monthly upwelling indices and their anomalies, relative to 1948-67, for the western North American coast are presented. From six representative buoys throughout the California Current region, time series of the daily alongshore wind component and sea-surface temperature (SST; data courtesy NOAA National Data Buoy Center) are plotted against the harmonic mean of each record; the location and base period of each buoy is given in table 1, and the position of all but the most northern one is plotted in figure 1. Data from quarterly CalCOFI surveys in 1997 and 1998 are described. The CalCOFI monitoring program started in 1949. The present program consists of quarterly (normally January, April, July, October) cruises that occupy a grid of 66 stations off southern California (fig. 1). The core time-series data set now collected at each station includes a CTD/rosette cast with sensors for pressure, temperature, salinity, dissolved oxygen, photosynthetically active radiation, fluorescence, and transmissivity. Water samples are collected at 20-24 depths in 1997 . . . .. . _.. . .. . . .. . . ... . .