Latitudinal shifts versus pulses of the eddy-driven jet in an aquaplanet general circulation model

The response of the storm-track and eddy-driven jet to various steady sea surface temperatures (SST) forcings is analyzed using the aquaplanet configuration of the atmospheric general circulation model ARPEGE-Climat of Météo-France. The SST profiles are zonally homogeneous and piecewise linear functions of the latitude. It allows to modify the tropical component which controls the intensity of the subtropical jet without changing the mid-latitude SST front and vice-versa. The position of the eddy-driven jet is first analyzed for different position, width and intensity of the midlatitude SST front and for different tropical SSTs. It is interpreted in terms of upper-tropospheric Rossby wave breakings.

A focus is then made on the eddy-driven jet variability which has been much less studied in the context of aquaplanet simulations of a state-of-the-art climate model. 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 equatorward-shifted case presents smaller latitudinal fluctuations than the poleward-shifted one 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 more poleward-shifted front having relatively small SSTs.