Since the early 1970's, the Pacific Fisheries Environmental Group (PFEG) has generated indices of the intensity of large-scale, wind-induced coastal upwelling at 15 standard locations along the west coast of North America. The indices are based on estimates of offshore Ekman transport driven by geostrophic wind stress, which is derived from six-hourly synoptic and monthly mean surface atmospheric pressure fields. The pressure fields are interpolated from Northern Hemisphere surface observations and are provided by the U.S. Navy, Fleet Numerical Meteorology and Oceanography Center (FNMOC), Monterey, CA.
Two sets of upwelling index time series are calculated. One set (1967-present) is comprised of daily and weekly means of six-hourly upwelling indices. Availability of synoptic data prior to 1967 is much more limited. The second set of indices are monthly time series derived from monthly-mean pressure fields, and extend back to 1946. The synoptic fields used to construct the monthly mean fields prior to July 1962 were acquired from a variety of sources, rather than FNMOC. Four NOAA Technical Memoranda describe the methods used to calculate the indices, and summarize the upwelling time series. The latest in this series, describing the history and derivation of the upwelling index and highlighting fifty years of the series, has just been published and is now available.
PFEG regularly produces and provides index time series to scientists and managers concerned with marine ecosystems and their biota. The indices are currently distributed to about 50 regular users each month, and each year another 40-50 individuals from several organizations request upwelling data or information on how they are derived. These series have been used in scores of studies and scientific publications; over 400 publications refer to the technical memorandum by Andrew Bakun that initially described the upwelling indices. Examples include studies to describe coastal circulation, ENSO, and climate change, as well as studies understanding linkages between environmental and biological variability of zooplankton, crabs, groundfish, small pelagics, salmon, and marine birds.
In the latest report, the shape of the annual cycle is estimated at each of the 15 standard upwelling locations by a least-squares regression of the daily data from 1967-91 to an annual and semiannual harmonic signal. The characteristics of the annual upwelling cycle at each point are summarized. The North American west coast appears to break down into three distinct upwelling regions; Baja California (21-30°N), continental US (30-48°N), and British Columbia and Alaska (48-60°N). The annual range off Baja California is relatively small, with wintertime values of less than 50 m3/s/100m and summer maxima of 75-150 m3/s/100m. This region is also characterized by secondary minima and maxima in August and October, respectively (more technically a pair of inflection points in the annual curves at 27°N and 30°N). In contrast, annual cycles north of 30°N have a strong sinusoidal (12-month harmonic) signature. The greatest upwelling rates occur at 33°N, with a linear decrease in the maximum upwelling indices north to about 54°N. The greatest annual range occurs at 39°N. Maximum values north of Washington are about zero, and occur in June-July. Minimum indices of about-100 m3/s/100m occur in ca. 1 January.
The time of minimum upwelling occurs within about 15 days of 1 January along the entire coast. The time of strongest upwelling increases from late April off Baja to mid-July off Oregon and Washington, a lag of about 80 days (ca. 30 km/d). The phase of maximum upwelling corresponds with climatological surface ocean conditions along the coast, developed by Ron Lynn from CalCOFI data. Maximum salinity occurs in May off northern Baja, June-July off southern California, and July and August along central California. Salinity maxima along southern Baja occur in fall, coinciding with the secondary upwelling maximum. The temperature mimima found by Lynn in upwelling regions, typically in April-May, is impacted substantially by the seasonal surface heat flux and agrees less well with the time of greatest upwelling. The relationship between the annual cycles of upwelling and temperature and salinity is also complicated by advection along the coast, and is influenced by the surface water masses of the California Current.
Annual cycles for the daily and monthly data sets are compared. It appears that upwelling indices derived from monthly pressure fields overestimate the magnitude of upwelling and downwelling based on monthly averages of the daily values. This discrepancy is most evident during the months of the highest absolute values at each location (e.g., monthly indices overestimate winter downwelling at the northern points, and summer upwelling at the southern points). Monthly stress calculations overestimate the daily upwelling rates off Alaska by as much as 130%, and off southern and Baja California by a factor of two. With 25 years of simultaneous daily and monthly data now available, our preliminary interpretation is that the relative discrepancy between the two data sets varies with season as well as latitude. We are currently developing a more complete analysis that will allow the monthly indices to be adjusted temporally and spatially to reflect more realistic estimates of coastal upwelling.
Rapid advancements in computing and communications since the initial development of the upwelling index over two decades ago have led to more accurate and efficient calculation and distribution of the upwelling products. New telecommunication methods and procedures such as e-mail, file transfer protocol, and the Internet, allow large volumes of data and model output to be transferred between computers, stored more efficiently and securely, and processed in a more portable fashion. Hard copy delivery is rapidly giving way to electronic transmission of the upwelling indices and products, which provides scientists and managers much quicker access to information than in the past. Within the next year, we envision having the upwelling indices and many other PFEG data products available on our world-wide web home page (http://www.pfeg.noaa.gov). This will make information easily accessible to a wider spectrum of users as soon as it is generated. We will continue to interact with researchers using the indices to link environmental and fisheries variability, to learn where these series may not be biologically relevant and, more generally, what type of information may be most useful. All these improvements will contribute to the long-term goal of producing an environmental index that has greater utility for biologists, fisheries scientists and resource managers.
While the upwelling indices described here are by far the most popular product that PFEG derives from the surface pressure fields, they are but one of several atmospheric and oceanic products presently generated from the six-hourly pressures. A future technical memorandum is planned that will summarize some of the more important products. In the interim, they are available to interested parties, and we encourage their use. (Franklin B. Schwing [831]648-9034)