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, 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 blockingfavorable configuration indicates a significant strengthening of the subtropical jet and a reduction in midlatitude 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.

The Seasonal Cycle of Blocking and Associated Physical Mechanisms in the Australian Region and Relationship with Rainfall

“…Palmer et al (2008) have demonstrated that the frequency of winter blocking is systematically underestimated in the Northern Hemisphere (NH) by a selection of these climate models. Similar detailed studies of the behavior of climate models in relation to blocking in the Southern Hemisphere (SH) have been undertaken recently (e.g., Ummenhofer et al 2013;Marshall et al 2013).…”

Seasonal and regional signature of the projected southern Australian rainfall reduction

“…In the Australian region, Ummenhofer et al (2013) found that, while an atmospheric GCM (with no coupled ocean) could reproduce the preferred location and seasonal cycle of blocking, it also underestimated the magnitude, especially in late winter and spring. Grose et al (2012) showed that dynamical downscaling of CMIP3 models only partly improved the representation.…”

How did ocean warming affect Australian rainfall extremes during the 2010/2011 La Niña event?

“…CAM3 uses a spectral dynamical core, 26 vertical levels, and was run at a T85 horizontal resolution (∼1.4° latitude/longitude). Other model‐specific aspects relevant to this study are described in relation to the model's simulation of the hydrological cycle [ Hack et al , ], tropical Pacific variability [ Zelle et al , ], and specifically the climate of the Australian region [ Taschetto et al , ; Ummenhofer et al , ]. The following AGCM simulations were conducted: Control simulation: A 120 year run with a repeating monthly climatology of global SST [ Hurrell et al , ] (based on the 1951–2010 period); Hindcast simulation: A simulation forced by global observed SST [ Smith et al , ] for the period 1951–2010; as such, the SST forcing contains the seasonal cycle, interannual variability, and the long‐term trend; Observed 2010/2011 SST experiment: Observed SST during the period January 2010 to March 2011 is used to force the AGCM; 57 ensemble members of 15 months duration each, initialized on 1 January from different years of the control simulation.…”