Mid-Pliocene westerlies from PlioMIP simulations

Abstract

The midlatitude westerlies are one of the major components of the global atmospheric circulation. They play an important role in midlatitude weather and climate, and are particularly significant in interpreting aeolian sediments. In this study, we analyzed the behavior and the possible mechanism involved in the change of the westerlies, mainly in terms of the jet stream position, in the mid-Pliocene warm period (3.3 to 3.0 million years ago) using simulations of 15 climate models from the Pliocene Model Intercomparison Project (PlioMIP). Compared to the reference period, the mid-Pliocene midlatitude westerlies generally shifted poleward (approximately 3.6 • of latitude in the Northern Hemisphere and 1.9 • of latitude in the Southern Hemisphere at 850 hPa level) with a dipole pattern. The dipole pattern of the tropospheric zonal wind anomalies was closely related to the change of the tropospheric meridional temperature gradient as a result of thermal structure adjustment. The poleward shift of the midlatitude westerly jet corresponded to the poleward shift of the mean meridional circulation. The sea surface temperatures and sea ice may have affected the simulated temperature structure and zonal winds, causing the spread of the westerly anomalies in the mid-Pliocene between the atmosphere-only and coupled atmosphere-ocean general circulation model simulations.

Sensitivity of Pliocene climate simulations in MRI-CGCM2.3 to respective boundary conditions

“…In the PlioMIP1, the respective roles of the atmospheric and oceanic heat transports in the latitudinal warming gradient were not evaluated sufficiently. The intensified southern Hadley cell and a northward shift of the tropical cells can be confirmed in most of the PlioMIP1 models (Sun et al, 2013;Li et al, 2015). Both the PlioMIP1 multi-models and the MRI-CGCM2.3 PlioMIP2 run show the predominant latitudinal contrast of surface warming over the Northern Hemisphere and the meridionally asymmetric change in the Hadley cells.…”

“…Both the PlioMIP1 multi-models and the MRI-CGCM2.3 PlioMIP2 run show the predominant latitudinal contrast of surface warming over the Northern Hemisphere and the meridionally asymmetric change in the Hadley cells. However, the anomalous Hadley circulation transports heat southward, indicating that the change in the Hadley circulation is not a factor but can be understood as a result of the change in atmospheric meridional warming gradient (Li et al, 2015). Further analyses of the surface processes over land and ocean and three-dimensional atmospheric and oceanic processes (e.g.…”

“…6a), similar to results of PlioMIP1 AOGCMs (Figs. 6 and 7 in Li et al, 2015). Such temperature changes imply a possible intensification of mid-latitude eddy activity (e.g.…”

“…Figure 9 shows changes in atmospheric mean meridional circulation (MMC) determined by mass stream function (MSF). The MMC, one of the important factors for the Pliocene climate anomaly (Chandler et al, 1994;Brierley et al, 2009;Brierley and Fedorov, 2010;Sun et al, 2013;Li et al, 2015), shows a larger change compared with the PlioMIP1 (Fig. 6d).…”

“…Changes in meridional temperature gradient in the upper troposphere and near the surface (e.g. Li et al, 2015) are possible factors for the anomalous mid-latitude eddy activity (e.g. Ulbrich et al, 2009).…”