One of the greatest uncertainties when it comes to future projections of regional climate is how the large-scale atmospheric circulation will change. While there is a general consensus among models on a zonal mean poleward shifting of the mid-latitude westerlies and associated storm tracks. For the Northern Hemisphere we discuss the impact of uncertainty in future changes in the stratospheric polar vortex on tropospheric climate change. For the Southern Hemisphere we discuss the relative roles of stratospheric ozone depletion and
changing greenhouse gas concentrations on the future evolution of the Southern Hemisphere mid-latitude jet stream.
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The role of the stratosphere in future mid-latitude climate projections
One of the greatest uncertainties when it comes to future projections of regional climate is how the large-scale atmospheric circulation will change (Shepherd 2014). While there is a general consensus among models on a zonal mean poleward shifting of the mid-latitude
westerlies and associated storm tracks (Yin 2005; Kidston and Gerber 2010; Chang et al. 2012; Swart and Fyfe 2012; Wilcox et al. 2012; Barnes and Polvani 2013), there is a large spread in the magnitude of this response. In addition to this zonal mean, poleward shifting view, there are more localized changes in the circulation associated with altered stationary wave patterns (Stephenson and Held 1993; Joseph et al. 2004; Simpson et al. 2014). For many of these predicted changes, we do not have a good physical understanding of the mechanisms that produce them, or the factors that govern their uncertainty. The stratosphere and how it is expected to change in the future is one source of uncertainty, among many, in future tropospheric mid-latitude circulation change. There are a variety of ways in which the stratosphere’s mean state, variability and composition may impact on tropospheric climate change. Instead of providing an exhaustive review of this topic, we focus on the role of changes in the extra-tropical mean state of the stratosphere in future projections of tropospheric mid-latitude climate by considering two particular aspects. For the Northern Hemisphere we discuss the impact of uncertainty in future changes in the stratospheric polar vortex on tropospheric climate change. For the Southern Hemisphere we discuss the relative roles of stratospheric ozone depletion and changing greenhouse gas concentrations on the future evolution of the Southern Hemisphere mid-latitude jet stream.
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- Award ID(s):
- 1734760
- NSF-PAR ID:
- 10183312
- Date Published:
- Journal Name:
- US CLIVAR VARIATINS
- Volume:
- 17
- Issue:
- 1
- Page Range / eLocation ID:
- 33 - 39
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The joint influence of the stratospheric quasi‐biennial oscillation (QBO) and the El Niño Southern Oscillation (ENSO) on the polar vortex, subtropical westerly jets (STJs), and wave patterns during boreal winter is investigated in 40 years (1979–2018) of monthly mean ERA‐Interim reanalyses. The method of Wallace et al. (1993),
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Abstract Two decades into the 21st century there is growing evidence for global impacts of Antarctic and Southern Ocean climate change. Reliable estimates of how the Antarctic climate system would behave under a range of scenarios of future external climate forcing are thus a high priority. Output from new model simulations coordinated as part of the Coupled Model Intercomparison Project Phase 6 (CMIP6) provides an opportunity for a comprehensive analysis of the latest generation of state‐of‐the‐art climate models following a wider range of experiment types and scenarios than previous CMIP phases. Here the main broad‐scale 21st century Antarctic projections provided by the CMIP6 models are shown across four forcing scenarios: SSP1‐2.6, SSP2‐4.5, SSP3‐7.0 and SSP5‐8.5. End‐of‐century Antarctic surface‐air temperature change across these scenarios (relative to 1995–2014) is 1.3, 2.5, 3.7 and 4.8°C. The corresponding proportional precipitation rate changes are 8, 16, 24 and 31%. In addition to these end‐of‐century changes, an assessment of scenario dependence of pathways of absolute and global‐relative 21st century projections is conducted. Potential differences in regional response are of particular relevance to coastal Antarctica, where, for example, ecosystems and ice shelves are highly sensitive to the timing of crossing of key thresholds in both atmospheric and oceanic conditions. Overall, it is found that the projected changes over coastal Antarctica do not scale linearly with global forcing. We identify two factors that appear to contribute: (a) a stronger global‐relative Southern Ocean warming in stabilisation (SSP2‐4.5) and aggressive mitigation (SSP1‐2.6) scenarios as the Southern Ocean continues to warm and (b) projected recovery of Southern Hemisphere stratospheric ozone and its effect on the mid‐latitude westerlies. The major implication is that over coastal Antarctica, the surface warming by 2100 is stronger relative to the global mean surface warming for the low forcing compared to high forcing future scenarios.
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Abstract Changes in midlatitude clouds as a result of shifts in general circulation patterns are widely thought to be a potential source of radiative feedbacks onto the climate system. Previous work has suggested that two general circulation shifts anticipated to occur in a warming climate, poleward shifts in the midlatitude jet streams and a poleward expansion of the Hadley circulation, are associated with differing effects on midlatitude clouds. This study examines two dynamical cloud‐controlling factors, mid‐tropospheric vertical velocity, and the estimated inversion strength (EIS) of the marine boundary layer temperature inversion, to explain why poleward shifts in the Southern Hemisphere midlatitude jet and Hadley cell edge have varying shortwave cloud‐radiative responses at midlatitudes. Changes in vertical velocity and EIS occur further equatorward for poleward shifts in the Hadley cell edge than they do for poleward shifts of the midlatitude jet. Because the sensitivity of shortwave cloud radiative effects (SWCRE) to variations in vertical velocity and EIS is a function of latitude, the SWCRE anomalies associated with jet and Hadley cell shifts differ. The dynamical changes associated with a poleward jet shift occur further poleward in a regime where the sensitivities of SWCRE to changes in vertical velocity and EIS balance, leading to a near‐net zero change in SWCRE in midlatitudes with a poleward jet shift. Conversely, the dynamical changes associated with Hadley cell expansion occur further equatorward at a latitude where the sensitivity of SWCRE is more strongly associated with changes in mid‐tropospheric vertical velocity, leading to a net shortwave cloud radiative warming effect in midlatitudes.
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