PFEL coastal upwelling indices are calculated based upon Ekman's theory of mass transport due to wind stress. 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. The standard west coast six-hourly upwelling indices are a product of the 3 pressure field interpolated from the 1 grid. 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 idea behind the upwelling indices was to develop simple time series that represent variations in coastal upwelling.