Tim Woollings

Abstract:

Purpose of Review: Extratropical jets and associated storm tracks significantly influence weather and regional climate across various timescales. Understanding jet responses to climate change is essential for reliable regional climate projections. This review serves two main purposes: (1) to provide an accessible overview of extratropical jet dynamics and a comprehensive examination of current challenges and uncertainties in predicting jet responses to greenhouse gas increases and (2) to suggest innovative experiments to advance our understanding of these responses. Recent Findings: While successive generations of climate model ensembles consistently project a mean poleward shift of the midlatitude zonal-mean maximum winds, there remains considerable intermodel spread and large uncertainty across seasonal and regional jet responses. Of particular note is our limited understanding of how these jets respond to the intricate interplay of multiple concurrent drivers, such as the strong warming in polar and tropical regions, and the relative importance of each factor. Furthermore, the difficulty of simulating processes requiring high resolution, such as those linked to sharp sea surface temperature gradients or diabatic effects related to tropical convection and extratropical cyclones, has historically hindered progress. Summary: We advocate for a collaborative effort to enhance our understanding of the jet stream response to climate change. We propose a series of new experiments that take advantage of recent advances in computing power and modelling capabilities to better resolve small-scale processes such as convective circulations, which we consider essential for a good representation of jet dynamics.

Abstract:

<jats:p>Abstract. Latent heating modifies the jet stream by modifying the vertical geostrophic wind shear, thereby altering the potential for baroclinic development. Hence, correctly representing diabatic effects is important for modelling the mid-latitude atmospheric circulation and variability. However, the direct effects of diabatic heating remain poorly understood. For example, there is no consensus on the effect of latent heating on the cross-jet temperature contrast. We show that this disagreement is attributable to the choice of spatio-temporal averaging. Jet representations relying on averaged wind tend to have the strongest latent heating on the cold flank of the jet, thus weakening the cross-jet temperature contrast. In contrast, jet representations reflecting the two-dimensional instantaneous wind field have the strongest latent heating on the warm flank of the jet. Furthermore, we show that latent heating primarily occurs on the warm flank of poleward directed instantaneous jets, which is the case for all storm tracks and seasons. </jats:p>

Abstract:

<jats:title>Abstract</jats:title><jats:p>An ocean memory framework is proposed to reveal the atmosphere's influence on ocean temperatures. Anomalous atmospheric forcing alters the ocean state through two mechanisms: short‐term, local effects involving airsea heat fluxes and Ekman circulation, and long‐term, far‐field effects involving changes from overturning and gyre circulations. The framework employs the Green function's method to incorporate both effects, enabling the quantification of ocean memory and the contribution of atmospheric forcing to ocean thermal variability. The framework is employed to examine the North Atlantic Oscillation's (NAO) influence on the North Atlantic Ocean variability, including the Atlantic Multidecadal Variability, with its memory estimated to be years. The NAO and variability in the North Atlantic jet speed explain up to 30% of ocean decadal variability, primarily driven by temporal changes in ocean heat transport. Therefore, decadal fluctuations in ocean temperatures cannot be accurately modeled solely as a passive response to stochastic atmospheric forcing.</jats:p>

Abstract:

The projected winter changes of the North Atlantic eddy-driven jet (EDJ) under climate change conditions have been extensively analysed. Previous studies have reported a squeezed and elongated EDJ. However, other changes present large uncertainties, specifically those related to the intensity and latitude. Here, the projections of the EDJ in a multimodel ensemble of CMIP6 are scrutinised by using a multiparametric description of the EDJ. The multimodel mean projects non-stationary responses of the EDJ latitude through the winter, characterised by a poleward shift in early winter and equator migration in late winter. These intraseasonal shifts (rather than a genuine narrowing) explain the previously established squeezing of the EDJ and are linked to the future changes in different drivers: the 200 hPa meridional temperature gradient and Atlantic warming hole in early winter, and the stratospheric vortex in late winter. Model biases also influence EDJ projections, contributing to the poleward shift in early winter.

Abstract:

The reality of human-induced climate change is unequivocal and exerts an ever-increasing global impact. Access to the latest scientific information on current climate change and projection of future trends is important for planning adaptation measures and for informing international efforts to reduce emissions of greenhouse gases (GHGs). Identification of hazards and risks may be used to assess vulnerability, determine limits to adaptation, and enhance resilience to climate change. This article highlights how recent research programs are continuing to elucidate current processes and advance projections across major climate systems and identifies remaining knowledge gaps. Key findings include projected future increases in monsoon rainfall, resulting from a changing balance between the rainfall-reducing effect of aerosols and rainfall-increasing GHGs; a strengthening of the storm track in the North Atlantic; an increase in the fraction of precipitation that falls as rain at both poles; an increase in the frequency and severity of El Niño Southern Oscillation (ENSO) events, along with changes in ENSO teleconnections to North America and Europe; and an increase in the frequency of hazardous hot-humid extremes. These changes have the potential to increase risks to both human and natural systems. Nevertheless, these risks may be reduced via urgent, science-led adaptation and resilience measures and by reductions in GHGs.