ON THE NATURE OF DECADAL ANOMALIES IN NORTH-ATLANTIC SEA-SURFACE TEMPERATURE

North Atlantic sea surface temperature data from the Comprehensive Oce an-Atmosphere Data Set were used to investigate the behavior of temper ature anomalies on multiple-year timescales during the period 1948-199 2. Monthly anomaly time series for each 2 degrees square from the equa tor to 70 degrees N were low-pass filtered at 4 years and normalized b y the local standard deviation. Attention is focused on the extreme ev ents, the upper and lower deciles, of the anomaly time series. A 45-ye ar sequence of January maps shows the already familiar phenomena of ge nerally cold conditions prior to 1951, a long warm interval from 1951 through 1967, and again a cold period from 1968 through 1977. The year s 1978 through 1982 were largely devoid of persistent strong anomalies , but moderate cold conditions returned during 1983-1986. Warm Conditi ons dominated the North Atlantic from 1987 onward. Within these therma l epochs, however, a total of five cold anomaly features and nine warm anomaly features have been identified. These features have individual lifetimes of 3 to 10 years. A typical size is 20 degrees of latitude or longitude, but they range from barely detectable to spanning the wi dth of the basin, the latter especially in lower latitudes. Most of th e anomalies move long distances along certain preferred paths. These p aths generally follow the routes of the subarctic and subtropical gyre s. Anomalies originating off North America along the boundary between the gyres move northeastward toward the Norwegian Sea along the approx imate route of the North Atlantic Current. Midlatitude anomalies origi nating at the eastern boundary tend to spread both northward and south ward along the coast. The speed of these movements (1-3 km d(-1)) is g enerally less than the expected speed of the near-surface ocean circul ation. Simple ideas about the effects of beta dynamics and air-sea hea t exchanges are briefly considered but do not provide a satisfactory e xplanation for the movements of the anomalies. The long timescale of t hese extreme events and the continuity of their movements suggest a us eful degree of predictability of sea surface temperature based on pers istence and propagation of features.