Impact of Surface Forcing on Southern Hemisphere Atmospheric Blocking in the Australia-New Zealand Sector
Abstract
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. 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 explores 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. The combined effects of tropical heating and extratropical temperature gradients are further explored 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 (see figure). 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. Schematic of surface temperature anomalies employed in the AGCM simulations and resultant 500hPa zonal wind anomalies (m/s) at 160E in the experiments for the (a) BLmax case maximizing blocking and (b) BLmin case with unfavorable blocking conditions. Zonal wind anomalies are relative to the control simulation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.A43F0340U
- Keywords:
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- 3305 ATMOSPHERIC PROCESSES Climate change and variability;
- 3339 ATMOSPHERIC PROCESSES Ocean/atmosphere interactions;
- 3337 ATMOSPHERIC PROCESSES Global climate models;
- 9330 GEOGRAPHIC LOCATION Australia