27.10.2014: FB1-Seminar

Dr. Caroline Ummenhofer, Woods Hole Oceanographic Institution (USA): "Impact of Surface Forcing on Southern Hemisphere Atmospheric Blocking"


11:00 h, Hörsaal, Düsternbrooker Weg 20



Characteristics of atmospheric blocking in the Southern Hemisphere (SH) are explored in atmospheric general circulation model (AGCM) simulations with the Community Atmosphere Model, version 3, with a particular focus on the Australia-New Zealand sector. We show that preferred locations of blocking in SH observations and the associated seasonal cycle are well represented in the AGCM simulations, but the observed magnitude of blocking is underestimated throughout the year, particularly in late winter and spring. This is related to overly zonal flow due to an enhanced meridional pressure gradient in the model, which results in a decreased amplitude of the longwave trough/ridge pattern. A range of AGCM sensitivity experiments explored the effect on SH blocking of tropical heating, mid-latitude sea surface temperatures (SST), and land–sea temperature gradients created over the Australian continent during austral winter. We further assessed the combined effects of tropical heating and extratropical temperature gradients in a configuration that is favorable for blocking in the Australia-New Zealand sector with warm SST anomalies to the north of Australia, cold to the southwest of Australia, warm to the southeast, and cool Australian land temperatures. The blocking favorable configuration indicates a significant strengthening of the subtropical jet and a reduction in mid-latitude flow, which results from changes in the thermal wind. While these overall changes in mean climate, predominantly forced by the tropical heating, enhance blocking activity, the magnitude of atmospheric blocking compared to observations is still underestimated. The blocking-unfavorable configuration with surface forcing anomalies of opposite sign results in a weakened subtropical jet, enhanced mid-latitude flow, and significantly reduced blocking. The role of regional SST in modulating SH atmospheric blocking on decadal timescales and implications for the representation of atmospheric blocking in climate models will also be discussed.




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