Anomalous Anomalies in Averaged Hydrographic Data

Abstract

A comparison of a recently assembled hydrographic database for the North Atlantic with the Lovitus atlas shows striking differences in the vicinity of the Gulf Stream and the North Atlantic Current. On isopycnal surfaces in the main thermocline, isolated pools of warm, saline water are found in the Levitus database but are absent in the new database. Using synoptic data as a proxy for temporally averaged climatological data, it is shown that the anomalous features can be accounted for by the differences in the averaging process. To produce a gridded database from irregularly spaced station data, Levitus averaged the data on pressure surfaces while the new database was prepared with averaging an potential density surfaces. It is shown that averaging on a pressure surface in an area of sharply sloping isopycnals produces a water mass with a θ–S signature uncharacteristic of the local water mass(es). The anomalous potential temperatures and salinities that result are compared to the large-scale water mass anomalies of the North Atlantic and are shown to be of comparable strength. Finally, the consequences of having sizable averaging artifacts are discussed.