Northern hemisphere atmospheric response to changes of atlantic ocean SST on decadal time scales: a GCM experiment

Abstract

Analyses indicate that the Atlantic Ocean sea- surface temperature (SST) was considerably colder at the beginning than in the middle of the century. In pa- rallel, a systematic change in the North Atlantic sea- level pressure (SLP) pattern was observed. To find out whether the SST and SLP changes analyzed are consis- tent, which would indicate that the SST change was real and not an instrumental artifact, a response experiment with a low-resolution (T21) atmospheric GCM was per- formed. Two perpetual January simulations were ducted, which differ solely in the Atlantic Ocean (40 ° S-60 ° N) SST: the cold simulation utilizes the SSTs for the period 1904-1913; the warm simulation uses the SSTs for the period 1951-1960. Also, a con- trol run with the model's standard SST somewhat be- tween the cold and warm SST was made. For the response analysis, a rigorous statistical approach was taken. First, the null hypothesis of identical horizontal distributions was subjected to a multivariate signifi- cance test. Second, the level of recurrence was esti- mated. The multivariate statistical approaches are based on hierarchies of test models. We examined three different hierarchies: a scale-dependent hierarchy based on spherical harmonics (S), and two physically motivated ones, one based on the barotropic normal modes of the mean 300 hPa flow (B) and one based on the eigenmodes of the advection diffusion operator at 1000 hPa (A). The intercomparison of the cold and warm experiments indicates a signal in the geostrop-