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|Cloud study: weaker Walker circulation
The paper is published in the Quarterly Journal of the Royal Meteorological Society. The authors, Michael Previdi of the Lamont-Doherty Earth Observatory of Columbia University and Lorenzo M. Polvani of Columbia University, state that in their review paper, they present the following new contributions:
Up-to-date synthesis of current scientific understanding of the climate system response to stratospheric ozone depletion and recovery, with separate sections on the responses of the atmosphere, ocean and cryosphere;
Examples of certain aspects of the climate system response to stratospheric ozone forcing that cannot be understood in terms of the ever-popular Southern Annular Mode;
Outlook section describing promising new avenues of research in this area.
(See also: "A Resolution Of The Antarctic Sea Ice Paradox" here.)
Anthropogenic climate change has become synonymous in many circles with the secular rise in atmospheric greenhouse gas concentrations resulting from human activity. For the SH, however, GHG-induced climate change is only one part of the story. In this review, we have documented the significant and widespread climate changes that have occurred in response to a very different anthropogenic forcing: stratospheric ozone depletion.
The climate impacts of ozone depletion are pervasive, extending from the stratosphere down to the surface (and possibly into the Southern Ocean), and from the Antarctic continent to the SH subtropics. This fact is only beginning to be truly appreciated. Although significant and widespread climate changes have been observed in recent decades in the SH, and many of these changes have been linked to changes in the winds (notably, to the SAM (Southern Annular Mode)), what has been missing is the following critical connection: since ozone depletion was the primary driver of the observed changes in the winds, it too, therefore, was the primary driver of the other observed climate changes that were a response to changes in the winds. In this review, we have sought to better emphasize this connection, which is particularly relevant for linking ozone depletion with changes in the Southern Ocean and possibly sea ice, and with changes at the surface in general.
The impact of ozone depletion on the SH climate system begins in the Antarctic lower stratosphere, where a substantial springtime cooling of ~8 K has been observed over the last few decades of the twentieth century in response to the formation of the ozone hole (Figure 3). This cooling was the trigger for a series of atmospheric circulation changes, including a strengthening of the stratospheric polar vortex and tropospheric midlatitude jet, and a poleward shift of the tropospheric jet and Hadley cell. The changes in the tropospheric circulation, which are commonly described as a positive trend in the SAM, have been linked to changes in tropospheric and surface temperatures, clouds and cloud radiative effects, and precipitation. The seasonality of these climate changes in observations (with significant changes largely confined to austral summer) clearly suggests that they are primarily a response to stratospheric ozone depletion, rather than increases in GHGs. This conclusion is substantiated by very robust climate modeling results, based on a whole hierarchy of models.
The response of the Southern Ocean to ozone depletion is less clear than the atmospheric response. Oceanographic observations are ambiguous with regard to whether or not the Antarctic Circumpolar Current and Southern Ocean overturning have strengthened in recent decades as a result of the poleward intensification of the surface westerly winds. Some strengthening of the ACC/MOC is expected as a consequence of enhanced wind-driven Ekman transport; however, this effect may have been largely offset by increases in mesoscale eddy activity. Determining, precisely, how the Southern Ocean circulation has responded to ozone depletion, and the associated poleward intensification of the westerly winds, is critical since climate models project that the recent wind trends will continue throughout the twenty-first century (albeit with a different seasonality and potentially a different magnitude than were observed in the past). The circulation response to these future changes in the winds will affect the rates of oceanic uptake of heat and anthropogenic carbon, with possible implications for the rate and magnitude of global warming.
The story of Antarctic sea ice is also complicated because the observed sea ice expansion during the satellite era is inconsistent with expectations based on the positive trend in the SAM. While it has been suggested that the sea ice response to ozone depletion may not be mediated entirely through the SAM (Turner et al., 2009), thus offering the possibility of reconciling the observational data, more recent modeling evidence indicates that ozone depletion acts to decrease Antarctic SIE rather than increase it. It has been argued that the observed sea ice expansion might not be a response to external forcing, but instead could be a reflection of internal variability within the climate system. However, alternative hypotheses to explain the observed sea ice changes have also been proposed. For example, Bintanja et al. (2013) have suggested that increased meltwater from Antarctic ice shelves has led to a cooling and freshening of the ocean surface layer in recent decades. This could have effectively shielded Antarctic sea ice from the upwelling of warmer water from depth, contributing to the observed increase in SIE. This proposed mechanism has recently been called into question, however (Swart and Fyfe, 2013). Clearly, there is no simple answer when it comes to explaining the observed sea ice changes. Whatever the answer may be, though, it almost certainly involves ozone depletion at some level.
Finally, the climate impacts of stratospheric ozone depletion are expected to reverse in the coming decades as stratospheric ozone recovery becomes a reality. Ozone recovery will play an important role in the SH climate system since its impacts will largely offset the impacts of increasing GHGs. While we have focused in section 5 on how this is likely to affect the atmospheric circulation, we emphasize that the competing effects of ozone recovery and GHG increases will be crucial for understanding other aspects of future climate change as well. For example, ozone recovery is expected to mitigate a substantial portion of the Antarctic sea ice loss brought about by increasing GHGs over the next fifty years (Smith et al., 2012). Our ability to anticipate the future climate impacts of ozone recovery, however, is somewhat limited by an incomplete understanding of what has happened in the recent past. To this end, we stress that further work is needed in order to better understand how the climate system responded to ozone depletion in the late twentieth century. In particular, it will be critical to resolve how the Southern Ocean circulation has changed, as this is key to understanding past changes in sea ice, oceanic CO2 uptake, and ice sheet mass balance.
The path forward should be rooted firmly in observational data. This means both acquiring new observations, and finding novel ways to analyze existing data. Additionally, it will be valuable to examine the climate effects of ozone depletion and recovery using models that have not traditionally been employed, such as Earth system models and stand-alone ice sheet models. This will allow for quantification of the impacts of ozone forcing on Southern Ocean CO2 uptake and Antarctic ice sheet mass balance. Mass balance changes due to ozone depletion and recovery have yet to be quantified, and this should be set as a high priority for future work. These changes may have important implications for past and future trajectories of global sea level rise.
We review what is presently known about the climate system response to stratospheric ozone depletion and its projected recovery, focusing on the responses of the atmosphere, ocean and cryosphere. Compared to well-mixed greenhouse gases (GHGs), the radiative forcing of climate due to observed stratospheric ozone loss is very small: in spite of this, recent trends in stratospheric ozone have caused profound changes in the Southern Hemisphere (SH) climate system, primarily by altering the tropospheric midlatitude jet, which is commonly described as a change in the Southern Annular Mode. Ozone depletion in the late twentieth century was the primary driver of the observed poleward shift of the jet during summer, which has been linked to changes in tropospheric and surface temperatures, clouds and cloud radiative effects, and precipitation at both middle and low latitudes. It is emphasized, however, that not all aspects of the SH climate response to stratospheric ozone forcing can be understood in terms of changes in the midlatitude jet.
The response of the Southern Ocean and sea ice to ozone depletion is currently a matter of debate. For the former, the debate is centered on the role of ocean eddies in possibly opposing wind-driven changes in the mean circulation. For the latter, the issue is reconciling the observed expansion of Antarctic sea ice extent during the satellite era with robust modeling evidence that the ice should melt as a result of stratospheric ozone depletion (and increases in GHGs).
Despite lingering uncertainties, it has become clear that ozone depletion has been instrumental in driving SH climate change in recent decades. Similarly, ozone recovery will figure prominently in future climate change, with its impacts expected to largely cancel the impacts of increasing GHGs during the next half-century.
Read the abstract and get the paper here.
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