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 layer in which an appreciable amount of this offshore transport occurs is termed the surface Ekman layer, and is generally 20 to 50 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 deep water upwelled from depths below 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 large negative anomalies of January and February of this year were very extreme, while last summer's values, although generally negative, were quite moderate. There appears to have been a sudden moderation in offshore temperatures in recent months. The very deep warm water reported in our 1997 Annual Report persisted through winter and early spring. According to Bob Hannah, on April 20 the warm anomalies still extended well below the surface.