North Atlantic jet stream projections in the context of the past 1,250 years

Reconstruction of the North Atlantic jet stream (NAJ) presents a critical, albeit largely unconstrained, paleoclimatic target. Models suggest northward migration and changing variance of the NAJ under 21st-century warming scenarios, but assessing the significance of such projections is hindered by a lack of long-term observations. Here, we incorporate insights from an ensemble of last-millennium water isotope–enabled climate model simulations and a wide array of mean annual water isotope (δ18O) and annually accumulated snowfall records from Greenland ice cores to reconstruct North Atlantic zonal-mean zonal winds back to the 8th century CE. Using this reconstruction we provide preobservational constraints on both annual mean NAJ position and intensity to show that late 20th- and early 21st-century NAJ variations were likely not unique relative to natural variability. Rather, insights from our 1,250 year reconstruction highlight the overwhelming role of natural variability in thus far masking the response of midlatitude atmospheric dynamics to anthropogenic forcing, consistent with recent large-ensemble transient modeling experiments. This masking is not projected to persist under high greenhouse gas emissions scenarios, however, with model projected annual mean NAJ position emerging as distinct from the range of reconstructed natural variability by as early as 2060 CE.

Tropical cloud-radiative changes contribute to robust DJF jet exit strengthening over Europe under global warming

<p>During boreal winter (December to February, DJF), the North Atlantic jet stream and storm track are expected to extend eastward over Europe in response to climate change. This will affect future weather and climate over Europe, for example by steering storms which are associated with strong winds and heavy precipitation towards Europe. The jet stream and storm track responses over Europe are robust across coupled climate models of phases 3, 5, and 6 of the Coupled Model Intercomparison Project (CMIP; Harvey et al., 2020, JGR-A, https://doi.org/10.1029/2020JD032701). We show that the jet stream response is further robust across CMIP5 models of varying complexity ranging from coupled climate models to atmosphere-only General Circulation Models (GCMs) with prescribed sea-surface temperatures (SSTs) and sea-ice cover. In contrast to the jet stream response over Europe, the jet stream response over the North Atlantic is not robust in the coupled climate models and the atmosphere-only GCMs.</p><p>In addition to the CMIP5 simulations, we investigate Amip-like simulations with the atmospheric components of ICON-NWP, and the CMIP5 models MPI-ESM-LR and IPSL-CM5A-LR that apply the cloud-locking method to break the cloud-radiation-circulation coupling and to diagnose the contribution of cloud-radiative changes on the jet stream response to climate change. In the simulations, SSTs are prescribed to isolate the impact of cloud-radiative changes via the atmospheric pathway, i.e., via changes in atmospheric cloud-radiative heating, and global warming is mimicked by a uniform 4K SST increase (cf. Albern et al., 2019, JAMES, https://doi.org/10.1029/2018MS001592 and Voigt et al., 2019, J. Climate, https://doi.org/10.1175/JCLI-D-18-0810.1). In all three models, cloud-radiative changes contribute significantly and robustly to the eastward extension of the North Atlantic jet stream towards Europe. At the same time, cloud-radiative changes contribute to the model uncertainty over the North Atlantic. In addition to the jet stream response, we investigate the impact of cloud-radiative changes on the storm track response in ICON-NWP and discuss similarities and differences between the jet stream and storm track responses over the North Atlantic-European region.</p><p>In ICON-NWP, the impact of cloud-radiative changes on the jet stream response is dominated by tropical cloud-radiative changes while midlatitude and polar cloud-radiative changes have a minor impact. A further division of the tropics into four smaller tropical regions that cover the western tropical Pacific, the eastern tropical Pacific, the tropical Atlantic, and the Indian Ocean shows that cloud-radiative changes over the western tropical Pacific, eastern tropical Pacific, and Indian Ocean all contribute about equally to the eastward extension of the North Atlantic jet stream towards Europe because these regions exhibit substantial upper-tropospheric cloud-radiative heating in response to climate change. At the same time, cloud-radiative changes over the tropical Atlantic hardly contribute to the jet response over Europe because changes in atmospheric cloud-radiative heating under climate change are small in this region. As for the impact of global cloud-radiative changes, we also discuss the impact of the regional cloud-radiative changes on the storm track response over the North Atlantic-European region to climate change.</p>

Impact of the North Atlantic Warming Hole on Sensible Weather

In future climate projections there is a notable lack of warming in the North Atlantic subpolar gyre, known as the North Atlantic warming hole (NAWH). In a set of large-ensemble atmospheric simulations with the Community Earth System Model, the NAWH was previously shown to contribute to the projected poleward shift and eastward elongation of the North Atlantic jet. The current study investigates the impact of the warming hole on sensible weather, particularly over Europe, using the same simulations. North Atlantic jet regimes are classified within the model simulations by applying self-organizing maps analysis to winter daily wind speeds on the dynamic tropopause. The NAWH is found to increase the prevalence of jet regimes with stronger and more-poleward-shifted jets. A previously identified transient eddy-mean response to the NAWH that leads to a downstream enhancement of wind speeds is found to be dependent on the jet regime. These localized regime-specific changes vary by latitude and strength, combining to form the broad increase in seasonal-mean wind speeds over Eurasia. Impacts on surface temperature and precipitation within the various North Atlantic jet regimes are also investigated. A large decrease in surface temperature over Eurasia is found to be associated with the NAWH in regimes where air masses are advected eastward over the subpolar gyre prior to reaching Eurasia. Precipitation is found to be locally suppressed over the warming hole region and increased directly downstream. The impact of this downstream response on coastal European precipitation is dependent on the strength of the NAWH.

Eight Hundred Years of North Atlantic Jet Stream Variability and its Influence on European Climate Extremes

<p><strong>A recent increase in mid-latitude extreme weather events has been linked to anomalies in the position, strength, and waviness of the Northern Hemisphere polar jet stream. The latitudinal position of the North Atlantic Jet (NAJ) in particular drives climatic extremes over Europe, </strong>by controlling the location of the Atlantic storm track and by influencing the occurrence and duration of atmospheric blocking. <strong>To put recent NAJ trends in a historical perspective and to investigate non-linear relationships between jet stream position, mid-latitude extreme weather events, and anthropogenic climate change, long-term records of NAJ variability are needed. Here, we combine two tree-ring based summer temperature reconstructions from Scotland and from the Balkan Peninsula to reconstruct inter-annual variability in the latitudinal position of the summer NAJ back to 1200 CE. We find that over the past centuries, a northward summer NAJ position has resulted in heatwaves in northwestern Europe, whereas a southward position has promoted wildfires in southeastern Europe and floods in northwestern Europe. The great famine of 1315-1317 in northwestern Europe, for instance, was associated with prolonged flooding and cold summers that resulted in failed grain harvest and were related to a southern NAJ position. We further find an unprecedented increase in NAJ anomalies since the 1960s, which supports more sinuous jet stream patterns and quasi-resonant amplification as potential dynamic pathways for Arctic warming to influence midlatitude weather.</strong></p>

Impact of the North Atlantic Warming Hole on Sensible Weather

<p>In future climate projections there is a notable lack of warming in the North Atlantic subpolar gyre, known as the North Atlantic warming hole (NAWH). The NAWH has been previously shown to contribute to a poleward shift and eastward elongation of the North Atlantic jet that constitutes an additional important driver of future changes in the North Atlantic jet using a set of large-ensemble atmosphere simulations with the Community Earth System model.  The current study investigates the impact of the warming hole on sensible weather, particularly over Europe using the same simulations. North Atlantic jet regimes are classified within the model simulations by applying self-organizing maps to winter daily wind speeds on the dynamic tropopause. The NAWH is found to increase the prevalence of jet regimes with stronger and more poleward jets.  A previously identified transient eddy-mean response to the NAWH that leads to downstream enhancements of wind speeds is found to be dependent on the jet regimes. These localized regime-specific changes vary by latitude and strength, combining to form the broad increase in seasonal mean wind speeds over Eurasia. Impacts on surface temperature and precipitation within the various North Atlantic jet regimes are also investigated. A large decrease in surface temperature over Eurasia is found to be associated with the NAWH in regimes where air masses are advected over the subpolar gyre.  Precipitation is found to be locally suppressed over the warming hole region and increased directly downstream. The impact of this downstream response on coastal European precipitation is dependent on the strength of the NAWH.</p>

Extratropical Cyclogenesis Changes in Connection with Tropospheric ENSO Teleconnections to the North Atlantic: Role of Stationary and Transient Waves

This study investigates mechanisms for changes in wintertime extratropical cyclogenesis over North America and the North Atlantic during different phases of El Niño–Southern Oscillation (ENSO). Insights into the relationship between the ENSO–North Atlantic teleconnection and the cyclogenesis changes are provided by diagnosing the relative roles of stationary wave propagation and transient eddies in setting cyclogenesis-conducive large-scale circulation anomalies. During La Niña winters, Rocky Mountain and Greenland cyclogenesis are enhanced, while Gulf Stream cyclogenesis is reduced. Diagnostics suggest that stationary waves of tropical origin work in tandem with transient eddies to amplify the ridge over the northeastern Pacific, establishing background flow anomalies that favor Rocky Mountain cyclogenesis; downstream, more transient eddies with an anticyclonic tilt push the North Atlantic jet poleward, favoring cyclogenesis near Greenland, while contributions from stationary waves are small. During central Pacific El Niño winters, the cyclogenesis situation is essentially the opposite: Rocky Mountain and Greenland cyclogenesis are reduced, while Gulf Stream cyclogenesis is enhanced. The analyses are consistent with stationary waves and transient eddies acting to weaken the climatological ridge over the northeastern Pacific, creating a more zonal Pacific jet; downstream, transient eddies with a cyclonic tilt push the North Atlantic jet equatorward, favoring Gulf Stream cyclogenesis. Anomalies in cyclogenesis frequencies, and the relative roles of transient and stationary waves, during eastern Pacific El Niño winters are associated with larger uncertainties.