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Abstract:

<jats:title>Abstract</jats:title> <jats:p>In contrast to global warming trends, much of Eurasia experienced a winter cooling trend over 1990–2014. Some studies have proposed a causal link between this regional cooling, particularly strong over Siberia, to coincident reductions in Arctic sea-ice extent. However, free-running historical climate models overwhelmingly simulate a forced Eurasian warming signal, leading other studies to suggest that internal variability explains the observed cooling. Here, we use retrospective seasonal climate predictions to highlight a robust dynamical link between Siberian cooling and upstream north-east Atlantic atmospheric circulation changes. Examining the interannual predictability of these circulation patterns, we find spuriously weak but skilful model signals. When these weak dynamical signals are corrected, stronger low-frequency variability in downstream Siberian temperature also emerges, with half of the observed 1990–2014 cooling simulated. Our results suggest that Eurasian decadal climate variability is at least partly driven by a predictable atmospheric circulation response to slowly evolving boundary conditions.</jats:p>

Abstract:

The eddy-driven jet streams, which are regions of strong westerly wind in the mid-latitudes of both hemispheres, exert a leading influence on regional climate. In this Review, we outline the seasonally and regionally varying drivers, characteristics and changes in the jet streams. State-of-the-art models commonly predict a future polewards shift of the zonal-mean and annual-mean jet streams, typically ranging between 0° and 2° latitude by the end of the century under a high-emissions scenario, but with large model-to-model uncertainty. Furthermore, regional and seasonal projections can deviate substantially from the annual-mean and zonal-mean picture, and the drivers of these projected changes are not fully understood. Jet trends have emerged in the reanalysis record since 1979, of which a polewards shift of the summertime austral jet of ~0.3°/ decade is the trend most clearly attributable to anthropogenic forcing. Although other trends have been observed, potentially large internal variability and incomplete understanding of the drivers of these trends precludes clear anthropogenic attribution at this point. Research is unevenly distributed across regions and seasons, with winter receiving the most attention, particularly in the North Atlantic. To 91̽�� physical understanding and impact assessments, future research should provide a more complete picture of the seasonally and regionally varying jet stream drivers, and their changes, especially in spring and autumn.

Abstract:

Plain Language Summary: Midlatitude storms transport water vapor poleward and upward. When ascending, the air cools, causing the vapor to condense, releasing latent heat. The latent heating boosts the ascent in which it occurs and amplifies the storms originally responsible for the heating. This circular chain of events couples latent heating and storms in a nonlinear relationship we call the latent heating feedback. We simulate an atmosphere where latent heating is static and not a consequence of warm, moist air ascending. Comparing this to an atmosphere with realistic latent heating, we show that realistic latent heating leads to more intense storms traveling further poleward, especially west of North America and Europe. Simultaneously, the longitudinally averaged jet streams and storms respond by retracting toward the equator, leaving reduced westerlies and a double jet tendency over North America and Europe. Previous works tend to focus on the effect of latent heating on the average atmospheric state. Our work shows that this effect is only part of the story and that the latent heating effect on storms directly causes regional differences that climate models struggle with.

Abstract:

Plain Language Summary: The Pacific Northwest (PNW) experienced a record‐breaking heatwave during the summer of 2021, resulting in significant adverse effects on both human society and ecosystems. A heavy rainfall band was observed stretching from south China to south of Japan 1 week prior to the heatwave, fueling the debate over whether the monsoon activity contributed to this event. Our study found that while the monsoon activity typically has a cooling effect on the PNW's climate, in this particular year, it had a warming effect and thus contributed to this specific heatwave. This unusual warming effect was driven by a stronger and more northward‐shifted Pacific jet stream, which altered the extratropical response to the monsoon, resulting in an anticyclonic pattern over the PNW instead of the typical cyclonic response seen under average climatic conditions. Therefore, it is important to distinguish between the general question of whether monsoon can influence such events on average, and the specific question of whether it did in any specific case. We argue that when discussing the influence of large‐scale climate drivers on extremes, it is crucial to clearly state whether the focus is on the general potential for influence or on the specific role in a particular event.

Abstract:

Evidence has emerged of a discrepancy in tropical Pacific sea surface temperature (SST) trends over the satellite era, where most coupled climate models struggle to simulate the observed La Niña-like SST trends. Here we highlight wider implications of the tropical Pacific SST trend discrepancy for global circulation trends during boreal winter, using two complementary methods to constrain coupled model SST trends: conditioning near-term climate prediction (hindcast) simulations, and pacemaking coupled climate simulations. The robust circulation trend response to constraining the tropical Pacific SST trend resembles the interannual La Niña response. Constraining tropical Pacific SST robustly reduces tropical tropospheric warming, improving agreement with reanalyses, and moderately shifts the zonal-mean jets poleward. It also improves surface air temperature and precipitation trends in ENSO-sensitive regions, such as the Americas, South Asia, and southern Africa. Our results underline the importance of tropical Pacific SST for achieving confidence in multidecadal model projections.