Tim Woollings

Abstract:

Climate models predict a weak poleward shift of the jets in response to continuing climate change. Here we revisit observed jet trends using 40 years of satellite-era reanalysis products and find evidence that general poleward shifts are emerging. The significance of these trends is often low and varies between datasets, but the similarity across different seasons and hemispheres is notable. While much recent work has focused on the jet response to amplified Arctic warming, the observed trends are more consistent with the known sensitivity of the circulation to tropical warming. The circulation trends are within the range of historical model simulations but are relatively large compared to the models when the accompanying trends in upper tropospheric temperature gradients are considered. The balance between tropical warming and jet shifts should therefore be closely monitored in the near future. We hypothesise that the sensitivity of the circulation to tropical heating may be one factor affecting this balance.

Abstract:

Previous studies have found inconsistent responses of the North Atlantic jet to Arctic sea-ice loss. The response of wintertime atmospheric circulation and surface climate over the North Atlantic-European region to future Arctic sea-ice loss under 2°C global warming is analyzed, using model output from the Polar Amplification Model Intercomparison Project. The models agree that the North Atlantic jet shifts slightly southward in response to sea-ice loss, but they disagree on the sign of the jet speed response. The jet response induces a dipole anomaly of precipitation and storm track activity over the North Atlantic-European region. The changes in jet latitude and speed induce distinct regional surface climate responses, and together they strongly shape the North Atlantic-European response to future Arctic sea-ice loss. Constraining the North Atlantic jet response is important for reducing uncertainty in the North Atlantic-European precipitation response to future Arctic sea-ice loss.

Abstract:

It is now well established that the Arctic is warming at a faster rate than the global average. This warming, which has been accompanied by a dramatic decline in sea ice, has been linked to cooling over the Eurasian subcontinent over recent decades, most dramatically during the period 1998-2012. This is a counterintuitive impact under global warming given that land regions should warm more than ocean (and the global average). Some studies have proposed a causal teleconnection from Arctic sea ice retreat to Eurasian wintertime cooling; other studies argue that Eurasian cooling is mainly driven by internal variability and the relationship to sea ice is coincidental. Overall, there is an impression of strong disagreement between those holding the “ice-driven” versus “internal variability” viewpoints. Here, we offer an alternative framing showing that the sea ice and internal variability views can be compatible. Key to this is viewing Eurasian cooling through the lens of dynamics (linked primarily to internal variability with a small contribution from sea ice; cools Eurasia) and thermodynamics (linked to sea ice retreat; warms Eurasia). This approach, combined with recognition that there is uncertainty in the hypothesized mechanisms themselves, allow both viewpoints (and others) to co-exist and contribute to our understanding of Eurasian cooling. A simple autoregressive model shows that Eurasian cooling of this magnitude is consistent with internal variability, with some periods being more susceptible to strong cooling than others. Rather than posit a “yes-or-no” causal relationship between sea ice and Eurasian cooling, a more constructive way forward is to consider whether the cooling trend was more likely given the observed sea ice loss, as well as other sources of low-frequency variability. Taken in this way both sea ice and internal variability are factors that affect the likelihood of strong regional cooling in the presence of ongoing global warming.