Supplementary material to "Impact of North Atlantic SST and Jet Stream anomalies on European Heat Waves"

Abstract. Amplified Arctic ice loss in recent decades has been linked to increased occurrence of extreme mid-latitude weather. The underlying dynamical mechanisms remain elusive, however. Here, we demonstrate a novel mechanism linking freshwater releases into the North Atlantic with summer weather in Europe. Combining remote sensing, atmospheric reanalyses and model simulations, we show that freshwater events in summer trigger progressively sharper sea surface temperature gradients in subsequent winters, destabilising the overlying atmosphere and inducing a northward shift in the North Atlantic Current. In turn, the jet stream over the North Atlantic is deflected northward in the following summers, leading to warmer and drier weather over Europe. Our results suggest that growing Arctic freshwater fluxes will increase the risk of heat waves and droughts over the coming decades, and could yield enhanced predictability of European summer weather, months to years in advance.

Download Full-text

Impact of North Atlantic SST and Jet Stream anomalies on European Heat Waves

10.5194/wcd-2020-32 ◽

2020 ◽

Author(s):

Julian Krüger ◽

Robin Pilch Kedzierski ◽

Karl Bumke ◽

Katja Matthes

Keyword(s):

North Atlantic ◽

Heat Waves ◽

Phase Speed ◽

Early Summer ◽

Jet Stream ◽

Transient Waves ◽

European Continent ◽

The North ◽

The North Atlantic ◽

Sst Anomalies

Abstract. European heat waves have increased during the two recent decades. Particularly 2015 and 2018 were characterized by a widespread area of cold North Atlantic sea surface temperatures (SSTs) in early summer as well as positive surface temperature anomalies across large parts of the European continent during later summer. The European heat wave of 2018 is further suggested to be induced by a quasi-stationary and high-amplified Rossby wave pattern associated with the so-called quasi-resonant amplification (QRA) mechanism. In this study, we evaluate the North Atlantic SST anomalies and the QRA theory as potential drivers for European heat waves for the first time in combination by using the ERA-5 reanalysis product. A composite and correlation study reveals that cold North Atlantic SST anomalies in early summer favour a more undulating jet stream and a preferred trough-ridge pattern in the North Atlantic–European sector. Further we found that cold North Atlantic SSTs promote a stronger double jet occurrence in this sector. Thus, favorite conditions for a QRA signature are evident together with a necessary preconditioning of a double jet. However, our wave analysis covering two-dimensional probability density distributions of phase speed and amplitude does not confirm a relationship between cold North Atlantic SSTs and the QRA theory, compositing cold SSTs, high double jet indices (DJIs) or both together. Instead, we can show that cold North Atlantic SST events enhance the dominance of transient waves. In the presence of a trough during cold North Atlantic events, we obtain a slow-down of the transient waves, but not necessarily an amplification or stationarity. The deceleration of the transient waves result in a longer duration of a trough over the North Atlantic accompanied by a ridge downstream over Europe, triggering European heat episodes. Although a given DJI preconditioning may also be subject to the onset of certain QRA events, our study found no general relation between cold North Atlantic SST events and the QRA diagnostics. Our study highlights the relevance of cold North Atlantic SSTs for the onset of high European temperatures by affecting travelling jet stream undulations (but without involving QRA in general). Further attention should be drawn not only to the influence of North Atlantic SST year-to-year variability, but also to the effect of the North Atlantic warming hole as a negative SST anomaly in the long term, which is projected to evolve through climate change.

Download Full-text

Downstream impact of the North Pacific subtropical sea surface temperature front on the North Atlantic westerly jet stream in winter

Atmospheric Research ◽

10.1016/j.atmosres.2021.105492 ◽

2021 ◽

Vol 253 ◽

pp. 105492

Author(s):

Leying Zhang ◽

Haiming Xu ◽

Jing Ma ◽

Jiuwei Zhao ◽

Xia Xu

Keyword(s):

Surface Temperature ◽

Sea Surface Temperature ◽

North Atlantic ◽

North Pacific ◽

Sea Surface ◽

Jet Stream ◽

The North ◽

Westerly Jet ◽

The North Atlantic ◽

The North Pacific

Download Full-text

Seasonal Predictability of Wintertime mid-latitude Cyclonic Activity over the North Atlantic and Europe

10.5194/egusphere-egu21-11134 ◽

2021 ◽

Author(s):

Alvise Aranyossy ◽

Sebastian Brune ◽

Lara Hellmich ◽

Johanna Baehr

Keyword(s):

North Atlantic ◽

Large Scale ◽

Jet Stream ◽

Cyclonic Activity ◽

Max Planck ◽

Pressure System ◽

Wind Speeds ◽

Average Lifespan ◽

The North ◽

The North Atlantic

<p>We analyse the connections between the wintertime North Atlantic Oscillation (NAO), the eddy-driven jet stream with the mid-latitude cyclonic activity over the North Atlantic and Europe. We investigate, through the comparison against ECMWF ERA5 and hindcast simulations from the Max Planck Institute Earth System Model (MPI-ESM), the potential for enhancement of the seasonal prediction skill of the Eddy Kinetic Energy (EKE) by accounting for the connections between large-scale climate and the regional cyclonic activity. Our analysis focuses on the wintertime months (December-March) in the 1979-2019 period, with seasonal predictions initialized every November 1st. We calculate EKE from wind speeds at 250 hPa, which we use as a proxy for cyclonic activity. The zonal and meridional wind speeds are bandpass filtered with a cut-off at 3-10 days to fit with the average lifespan of mid-latitude cyclones.&#160;</p><p>Preliminary results suggest that in ERA5, major positive anomalies in EKE, both in quantity and duration, are correlated with a northern position of the jet stream and a positive phase of the NAO. Apparently, a deepened Icelandic low-pressure system offers favourable conditions for mid-latitude cyclones in terms of growth and average lifespan. In contrast, negative anomalies in EKE over the North Atlantic and Central Europe are associated with a more equatorward jet stream, these are also linked to a negative phase of the NAO.&#160; Thus, in ERA5, the eddy-driven jet stream and the NAO play a significant role in the spatial and temporal distribution of wintertime mid-latitude cyclonic activity over the North Atlantic and Europe. We extend this connection to the MPI-ESM hindcast simulations and present an analysis of their predictive skill of EKE for wintertime months.</p>

Download Full-text

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

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.2104105118 ◽

2021 ◽

Vol 118 (38) ◽

pp. e2104105118

Author(s):

Matthew B. Osman ◽

Sloan Coats ◽

Sarah B. Das ◽

Joseph R. McConnell ◽

Nathan Chellman

Keyword(s):

21St Century ◽

North Atlantic ◽

Climate Model ◽

Ice Cores ◽

Natural Variability ◽

Jet Stream ◽

Early 21St Century ◽

The North ◽

North Atlantic Jet ◽

Water Isotope

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.

Download Full-text