Interpretation of Recent Southern Hemisphere Climate Change

Thompson, David W. J.; Solomon, Susan

doi:10.1126/science.1069270

Cited by 1,702 publications

(1,720 citation statements)

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“…A better understanding of factors influencing SH blocking characteristics is desirable, even more so in light of strengthened westerlies due to a robust upward trend in the southern annular mode (e.g., Thompson and Solomon 2002), a weakening of the split jet in the region in recent decades (Bals-Elsholz et al 2001), reduced blocking activity , and decreases in cutoff low rainfall across southern regions of Australia Risbey et al 2013). To usefully assess future projected changes in midlatitude blocking and, by inference, rainfall projections in those regions where blocking is an important driver, improvement in the representation of atmospheric blocking in state-of-theart GCMs is thus crucial, especially given that blocking can be considered an ''emergent phenomenon created indirectly by dynamical and physical processes in the model.…”

Section: Resultsmentioning

confidence: 99%

Impact of Surface Forcing on Southern Hemisphere Atmospheric Blocking in the Australia–New Zealand Sector

et al. 2013

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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.

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Section: Resultsmentioning

confidence: 99%

Impact of Surface Forcing on Southern Hemisphere Atmospheric Blocking in the Australia–New Zealand Sector

et al. 2013

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“…Model simulations provide a way to help elucidate the link between climate variability and sea-ice variability and trends. As previous studies have suggested that the Southern Annular Mode (SAM) 21,22 , the El Niño Southern Oscillation (ENSO) 23,23 and the weather 24,25 may all be important factors in sea-ice variability and trends, a study to assess the dominant drivers is needed.…”

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confidence: 99%

Sources of heterogeneous variability and trends in Antarctic sea-ice

et al. 2015

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While the Northern Hemisphere sea-ice has uniformly declined over the past several decades, the observed sea-ice in the Southern Hemisphere has exhibited regions of increase and decrease. Here we use a comprehensive set of ocean-sea-ice simulations (1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007) to elucidate the drivers of the observed heterogeneous sea-ice trends. We show wind variability is an important determinant of the heterogeneous pattern of the variability and trends in Southern Hemisphere sea-ice. Only in the West Pacific region does Southern Annular Mode wind forcing contribute significantly to the trend in sea-ice duration. El Niño Southern Oscillation wind forcing contribution to the sea-ice duration trend is confined to the Atlantic and Pacific. In the Indian Ocean, weather is a significant driver of the sea-ice duration trend. Only in the East Pacific region is wind forcing alone insufficient to give rise to the observed sea-ice decline and must be augmented by warming to reproduce the observations.

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“…Recent warming over the Antarctic Peninsula has been explained in part by a positive trend in the Southern Annular Mode (SAM), manifested as a strengthening of the circumpolar westerlies, and attributed to stratospheric ozone depletion and increasing greenhouse gases 6,7 . The SAM does not readily account for warming in continental West Antarctica, however, because increased westerlies tend to cool, rather than warm, the Antarctic interior 8,9 .…”

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confidence: 99%