ERD coastal upwelling indices are calculated based upon Ekman's theory
of mass transport due to wind stress. Assuming homogeneity, uniform wind
and steady state conditions, the mass transport of the surface water due
to wind stress is 90° to the right of the wind direction in the
Northern Hemisphere. Ekman mass transport is defined as the wind stress
divided by the Coriolis parameter (a function of the earth's rotation and
latitude). The depth to which an appreciable amount of this offshore
transport occurs is termed the surface Ekman layer, and is generally 50
to 100 meters deep.
Ekman transports are resolved into components parallel and normal to the
local coastline orientation. The magnitude of the offshore component is
considered to be an index of the amount of water upwelled from the base
of the Ekman layer. Positive values are, in general, the result of
equatorward wind stress. Negative values imply downwelling, the onshore
advection of surface waters accompanied by a downward displacement of water.
Historically, the indices were computed from monthly mean pressure fields
prepared by FNMOC on a 3° mesh grid. After providing ERD with several
alternate pressure field grids over time, FNMOC currently produces six-hourly
fields of surface pressure on a global spherical 1° mesh (a 180 x 360 grid).
The standard west coast six-hourly upwelling indices are a product of the 3°
pressure field interpolated from the 1° grid. Monthly indices are derived from
a 3° mesh that is interpolated from the monthly-averages of the six-hourly 1°
pressures.
On a monthly basis, the Pacific Fisheries Environmental Laboratory generates
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. Geostrophic winds are derived from six-hourly
synoptic and monthly mean surface atmospheric pressure fields. The
pressure fields are provided by the U.S. Navy Fleet Numerical
Meteorological and Oceanographic Center (FNMOC), Monterey, CA. The idea
behind the upwelling indices was to develop simple time series that
represent variations in coastal upwelling. Daily and monthly index time
series are provided regularly to scientists and managers concerned with
marine ecosystems and their biota, and have been used in scores of studies
and scientific publications.
Monthly Processing
Changes to Processing for Upwelling Indices and Related Data Products
(December 1996)