Midlatitude atmospheric circulation responses under 1.5 and 2.0 °C warming and implications for regional impacts

Abstract

Abstract. This study investigates the global response of the midlatitude atmospheric circulation to 1.5 and 2.0 °C of warming using the HAPPI (Half a degree Additional warming, Prognosis and Projected Impacts) ensemble, with a focus on the winter season. Characterising and understanding this response is critical for accurately assessing the near-term regional impacts of climate change and the benefits of limiting warming to 1.5 °C above pre-industrial levels, as advocated by the Paris Agreement of the United Nations Framework Convention on Climate Change (UNFCCC). The HAPPI experimental design allows an assessment of uncertainty in the circulation response due to model dependence and internal variability. Internal variability is found to dominate the multi-model mean response of the jet streams, storm tracks, and stationary waves across most of the midlatitudes; larger signals in these features are mostly consistent with those seen in more strongly forced warming scenarios. Signals that emerge in the 1.5 °C experiment are a weakening of storm activity over North America, an inland shift of the North American stationary ridge, an equatorward shift of the North Pacific jet exit, and an equatorward intensification of the South Pacific jet. Signals that emerge under an additional 0.5 °C of warming include a poleward shift of the North Atlantic jet exit, an eastward extension of the North Atlantic storm track, and an intensification on the flanks of the Southern Hemisphere storm track. Case studies explore the implications of these circulation responses for precipitation impacts in the Mediterranean, in western Europe, and on the North American west coast, paying particular attention to possible outcomes at the tails of the response distributions. For example, the projected weakening of the Mediterranean storm track emerges in the 2 °C warmer world, with exceptionally dry decades becoming 5 times more likely.

Role of climate model dynamics in estimated climate responses to anthropogenic aerosols

“…In June-August, the monsoon precipitation decreased by 12.8 % in the ECHAM6-MACSP and 15.3 % in the NorESM1-MACSP experiments. Reduction of monsoon precipitation due to the anthropogenic aerosols has also been reported in several previous studies (Ganguly et al, 2012;Li et al, 2018b;Polson et al, 2014;Bollasina et al, 2011). In contrast with the seasonal cycle in temperature response, the largest precipitation response occurs in Northern Hemisphere summer during the Asian monsoon season.…”

Development of Future Heatwaves for Different Hazard Thresholds

“…However, internal variability is found to dominate the multimodel mean response of large‐scale circulations, including jet stream and stationary waves (Li et al, ). This is also supported by studies showing that an increase in the number of heatwave days and duration is mainly driven by the shift in the mean and not changes in variability or duration of anomalies (Ballester et al, ; Fischer & Schär, ; Lustenberger et al, ), for example, due to changes in synoptic conditions (Baldwin et al, ; King, ; Schaller et al, ).…”

Linking midlatitudes eddy heat flux trends and polar amplification

“…[17][18][19] To better understand the behavior of midlatitude eddies, previous studies have tried to relate eddy fluxes to the gradient of the mean temperature. 20,21 For example, using arguments from linear baroclinic instability theory, several studies [22][23][24][25] have tried to relate changes in Eady growth rate to changes in the fluxes. The relation between the eddies and the mean gradient has also been studied using simple diffusive closures, 19,[26][27][28] where the poleward eddy fluxes are assumed to be proportional to the mean meridional gradient, for example, v 0 T 0 / ÀT y , where v is meridional velocity, T is temperature, the subscript y denotes meridional derivative, and over-bar and prime denote mean and eddy terms, respectively.…”