Sensitivity of the Position and Variability of the Eddy-Driven Jet to Different SST Profiles in an Aquaplanet General Circulation Model

Abstract

Abstract The response of the storm track and eddy-driven jet to various steady sea surface temperature (SST) forcings is analyzed using the aquaplanet configuration of the atmospheric general circulation model Action de Recherche Petite Échelle Grande Échelle Climat (ARPEGE-Climat). The SST profiles are zonally homogeneous and piecewise-linear functions of the latitude. It allows for modifying the tropical component that controls the intensity of the subtropical jet without changing the midlatitude SST front and vice versa. Sensitivities to the position, width, and intensity of the midlatitude SST front as well as to the tropical SSTs are investigated. The baroclinicity and the storm-track eddy activity both intensify for a stronger, a wider, or a more equatorward SST front. The eddy-driven jet always lies on the poleward side of the SST front and its relative distance to the front is interpreted in terms of upper-tropospheric Rossby wave breakings. A focus is then made on the eddy-driven jet variability. The leading EOF of the vertical-average zonal-mean zonal wind is in most cases characterized by latitudinal shifting of the eddy-driven jet. For a more equatorward-shifted front, there is a more efficient positive eddy feedback. The mode has a greater persistence and explains a larger percentage of variance than for a more poleward-shifted front. However, the former case presents smaller latitudinal fluctuations than the latter because of the vicinity of the storm track to the subtropical jet. In the absence of positive eddy feedback, the pulsing of the eddy-driven jet intensity can dominate the variability such as for a high-latitude front having relatively small SSTs in the presence of an intense subtropical jet.