Atmospheric Blocking

Background

Atmospheric blocking represents an important feature for mid-latitude regional climate. During a blocking situation, the large-scale zonal flow is impeded and meridional anomalies occur at upper levels. Blocking not only influences mid-latitude mean climate but also extreme events, such as heat waves and droughts. However, despite its importance, our understanding of the factors that affect intensity and frequency of atmospheric blocking and the exact physical mechanism behind these modulations is still limited, especially in the Southern Hemisphere. Furthermore, marked biases exist in the representation of blocking in climate models, but only a few modeling studies address specific factors for their potential in modulating blocking in the Southern Hemisphere.

Summary

Characteristics of atmospheric blocking in the SH are explored in atmospheric general circulation model (AGCM) simulations, 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, midlatitude sea surface temperatures, 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. 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 weakening subtropical jet, enhanced midlatitude flow, and significantly reduced blocking.