Tim Woollings

Abstract:

Abstract. Recent studies have highlighted the link between upper-level jet stream dynamics, especially the persistence of certain jet configurations, and extreme summer weather in Europe. The weaker and more variable nature of the jets in summer makes it difficult to apply the tools developed to study them in winter, at least not without modifications. Here, to further investigate the link between jets and persistent summer weather, we present two complementary approaches to characterize the jet dynamics in the North Atlantic sector and use them primarily on the Northern Hemisphere summer circulation. First, we apply the self-organizing map (SOM) clustering algorithm to create a 2D distance-preserving discrete feature space for the tropopause-level summer wind field over the North Atlantic. The dynamics of the tropopause-level summer wind can then be described by the time series of visited SOM clusters, in which a long stay in a given cluster relates to a persistent state and a transition between clusters that are far apart relates to a sudden considerable shift in the configuration of upper-level flow. Second, we adapt and apply a jet core detection and tracking algorithm to extract individual jets and classify them into the canonical categories of eddy-driven and subtropical jets (EDJs and STJs, respectively). Then, we compute a wide range of jet indices for each jet category for the entire year to provide easily interpretable scalar time series representing upper-tropospheric dynamics. This work will focus on the characterization of historical trends, seasonal cycles, and persistence properties of the jet stream dynamics, while ongoing and future work will use the tools presented here and apply them to the study of connections between jet dynamics and extreme weather. The SOM allows the identification of specific summer jet configurations, each one representative of a large number of days in historical time series, whose frequency or persistence had increased or decreased in the last few decades. Detecting and categorizing jets adds a layer of interpretability and precision to previously and newly defined jet properties, allowing for a finer characterization of their trends and seasonal signals. Detecting jets at pressure levels of maximum wind speed at each grid point instead of in the dynamical tropopause is more reliable in summer, and finding wind-direction-aligned subsets of 0 contours in a normal wind shear field is a fast and robust way to extract jet cores. Using the SOM, we isolate persistent circulation patterns and assess if they occur more or less frequently over time. Using properties of the jets, we confirm that the Northern Hemisphere summer subtropical jet is weakening, that both jets get wavier, and that these jets overlap less frequently over time. We find no significant trend in jet latitude or in jet persistence. Finally, both approaches agree on a rapid shift in the subtropical jet position between early and late June.

Abstract:

Tropical cloud feedbacks are an important source of uncertainty in estimates of climate sensitivity. The extent to which changes in atmospheric circulation contribute to these feedbacks remains an open question. Here, all-sky radiative flux observations and an atmospheric reanalysis are used to estimate tropical cloud feedbacks from multi-decadal trends (1985 – 2020) in cloud radiative effect and surface temperature. We decompose the observed feedbacks into dynamic and non-dynamic components to quantify the impact of circulation trends. Narrowing and strengthening of tropical ascent lead to substantial dynamic feedbacks on regional scales that are similar in magnitude to the non-dynamic feedbacks. However, as previously shown for high- and low-resolution climate models, large dynamic feedbacks in different circulation regimes are connected by the atmospheric mass budget and approximately cancel when averaged across the tropics due the quasi-linear relationship between cloud radiative effect and vertical velocity. This results in small dynamic contributions to the tropical-mean net, longwave and shortwave feedbacks. We suggest that this result will hold in future and thus that isolating the non-dynamic components associated with individual cloud types can provide important insights into the processes controlling the tropical-mean cloud feedback and its uncertainty. Additionally, we show that feedbacks estimated from multi-decadal trends differ from those estimated from inter-annual variability. We demonstrate that, for dynamic feedbacks, this is because changes are controlled by different mechanisms and this leads to a differing spatial distribution of temperature sensitivity. Finally we provide new estimates of the uncertain combined tropical anvil area and albedo feedback using both multi-decadal trends and inter-annual variability.

Abstract:

Many Mediterranean‐type climates (MCs) have experienced wintertime drying trends since 1979. Using a dynamical adjustment method, we separate the effects of circulation‐induced drying trends from other residual trends. Our analysis reveals that circulation trends are the leading cause of the observed drying in Central Chile and the US Southwest, and that models show the drying across Southern Hemisphere MCs is independent of trends in the Southern Annular Mode. All Mediterranean‐type climates have exhibited residual drying trends from both internal variability and externally forced thermodynamic processes. Large ensembles suggest internal variability contributes significantly to the observed drying. However, in many regions the observed drying lies outside the ensemble distribution, raising questions about model accuracy.

Abstract:

<jats:title>Abstract</jats:title> <jats:p>The winter North Atlantic Oscillation (NAO) is the dominant pattern of atmospheric circulation variability over the North Atlantic region. It influences climate and weather such as surface air temperatures (SAT) downstream over Eurasia through establishing a large-scale teleconnection, but past studies on the NAO’s downstream teleconnection have been largely limited to observational data, and further evidence of downstream impacts and associated mechanisms from comprehensive climate modelling is desirable. This study quantifies and analyzes this teleconnection on an interannual timescale by using both ERA5 reanalysis, and five large ensembles from four climate simulation models. A particular focus is placed on dynamical pathways, as well as variability among ensemble members that modulates the teleconnection strength. Results suggest that NAO signals are propagated downstream by Rossby waves, efficiently transmitted through waveguides along both the polar and subtropical jet streams to Eastern Eurasia; while heat can be advected weakly from upstream, advection plays a rather local effect inducing temperature anomalies from the Pacific Ocean onshore. Multiple linear regression shows that internal climate variability significantly modulates the teleconnection: a more locally dominant NAO pattern, and narrower waveguides could strengthen the teleconnection. These two factors combine to explain up to 70% of variance in the teleconnection strength, with each contributing almost equally. Reanalysis data marginally agree with the regression model (1.9 standardized residuals higher in strength), suggesting potential model biases in jets and the NAO variability. Monitoring these modulating factors would be crucial to understanding downstream climate predictability and improving climate prediction models linked to the NAO.</jats:p>