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:

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.

Abstract:

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 air − ${-}$ sea 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 10 – 20 $10\mbox{--}20$ 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.