The Atmospheric Circulation over the North Atlantic as Induced by the SST Field

Spectral analyses of the sea surface temperature (SST) in the Simple Ocean Data Analysis (SODA) reanalysis for the past half-century identify prominent and statistically significant interannual oscillations in two regions along the Gulf Stream front over the North Atlantic. A model of the atmospheric marine boundary layer coupled to a baroclinic quasigeostrophic model of the free atmosphere is then forced with theSSThistoryfromtheSODAreanalysis.Twoextremestatesarefoundintheatmosphericsimulations:1)an eastward extensionofthe westerly jetassociatedwith thefront, which occurs mainly during boreal winter, and 2) a quiescent state of very weak flow found predominantly in the summer. This vacillation of the oceanicfront-induced jet in the model is found to exhibit periodicities similar to those identified in the observed Gulf Stream SSTfront itself. In addition, a close correspondence is found between interannualspectralpeaksin the observed North Atlantic Oscillation (NAO) index and the SODA-induced oscillations in the atmospheric model. In particular, significant oscillatory modes with periods of 8.5, 4.2, and 2.8 yr are found in both observedandsimulatedindicesandareshowntobehighlysynchronizedandofsimilarenergyinbothtimeseries. Theseoscillatorymodesinthesimulationsareshowntobesuppressedwheneither(i)theGulfStream frontor (ii) its interannual oscillations are omitted from the SST field. Moreover, these modes also disappear when (iii) the SST front is spatially smoothed, thus confirming that they are indeed induced by the oceanic front.