Eastward propagating surface anomalies at ocean gyre boundaries

Abstract

Inspection of animations and time‐distance (Hovmoller) plots of unfiltered data from a state‐of‐the‐art coupled climate model (HadCM3) reveals eastward propagating surface temperature‐salinity anomalies at subpolar/subtropical gyre boundaries throughout the world ocean. Maximum anomalies are ∼4°C and ∼0.3 practical salinity units. Horizontal length scales are ∼1000 km. The anomalies travel at a speed consistent with advection by mean near‐surface currents. Generation regions are located at western boundaries and possess an order of magnitude higher variability than all other regions. In some cases, surface heat fluxes are shown to cause anomaly generation; in other cases, surface current anomalies are likely to be responsible. Eastward anomaly propagation also occurs in the Estimating the Circulation and Climate of the Ocean model. The coarser resolution Fast Ocean Rapid Troposphere Experiment (FORTE) coupled model shows no propagating anomalies. Sea surface anomalies in the instrumental record are investigated, using ship‐based sea surface temperature (SST) and heat fluxes, and satellite SST and altimetry. Unlike the models, Hovmoller plots and animations of the unfiltered observational data showed no evidence of eastward propagation. By applying a decadal‐timescale band‐pass filter to the SST time series, we identify underlying propagation of basin‐scale anomalies in both HadCM3 and observations. Maximum anomalies are ∼1°C and length scales are ∼2500 km. These anomalies travel at speeds significantly slower than mean near‐surface currents. Ten anomalies are identified during the period 1870–2002, mostly in the eastern Atlantic basin (east of 45°W). HadCM3 displayed only one propagating anomaly of this type in a typical 100 years of data. We discuss possible reasons for the range of results in models and observations.