Comparison Between Climate Projections with Parameterized and Super-Parameterized Convection

Monday, May 12, 2014 - 07:00
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The vertical wind shear over the tropical Atlantic and its relationship with ENSO, and the summer precipitation over eastern US are analyzed in the Super Parameterized Community Climate System Model version 4 (SP-CCSM4) and in the conventional CCSM4 in control simulations and a climate change scenario. The climatology of vertical wind shear over the tropical Atlantic and the ENSO-shear relationship are well simulated in the control runs of SP-CCSM4 and CCSM4. However, due to different representations of cloud processes, in a warmer climate such as the RCP8.5 scenario, the suppression of convection over the western Africa in SP-CCSM4 is stronger than in CCSM4. As a result, SP-CCSM4 projects increased mean westerlies at 200hPa during July through October. The changes in the upper level wind further contribute to an increased climatological vertical wind shear over the equatorial Atlantic. The two models simulate a different response of the tropical Atlantic SST to ENSO. These differences are also present in the response of tropical Atlantic precipitation and convection to ENSO in warmer climate. In the RCP8.5 scenario, projection of the ENSO-shear relationships by SP-CCSM4, with and without the linear trend, retains similar features as in observations. Conversely, when excluding the linear trend, the ENSO-shear relationship projected by CCSM4 indicates an increase in the vertical wind shear dominating the tropical Atlantic during El Nino events. Sp-CCSM4 and CCSM4 simulate consistent climatologies of the summer precipitation (JJA) over the eastern US. Conversely, under the RCP8.5 climate change scenario the model with conventional representation of cloud processes, CCSM4, projects an increase in the summer precipitation, whereas the model with explicit representation of cloud processes, SP-CCSM4, projects a decrease of summer precipitation over the eastern US. In CCSM4 the change in precipitation is influenced mainly by local factors and is driven by a positive soil-moisture precipitation feedback. In SP-CCSM4 the precipitation changes in response to both local and large-scale effects, and the influence of the Atlantic subtropical high competes with a local negative soil-moisture precipitation feedback.