Dr Scott Osprey FRMetS

Abstract:

Significant changes have occurred during the last few decades across the North Atlantic climate system, including in the atmosphere, ocean, and cryosphere. These large-scale changes play a vital role in shaping regional climate and extreme weather events across the UK and Western Europe. This review synthesizes the characteristics of observed large-scale changes in North Atlantic atmospheric and oceanic circulations during past decades, identifies the drivers and physical processes responsible for these changes, outlines projected changes due to anthropogenic warming, and discusses the predictability of these circulations. On multi-decadal time scales, internal variability, anthropogenic forcings (especially greenhouse gases), and natural forcings (such as solar variability and volcanic eruptions) are identified as key contributors to large-scale variability in North Atlantic atmospheric and oceanic circulations. However, there remain many uncertainties regarding the detailed characteristics of these various influences, and in some cases their relative importance. We therefore conclude that a better understanding of these drivers, and more accurate quantification of their relative roles, are crucial for more reliable decadal predictions and projections of regional climate for the North Atlantic and Europe.<h4>Supplementary information</h4>The online version contains supplementary material available at 10.1007/s00382-025-07591-1.

Abstract:

<jats:title>Abstract</jats:title> <jats:p>Circumglobal teleconnections from wave-like patterns in the mid-latitude jets can lead to synchronized weather extremes in the mid-latitudes of Northern and Southern hemispheres. The simultaneous occurrence of record breaking and persistent northern hemisphere temperature anomalies in Summer 2018 were previously discussed in the context of a persistent zonally elongated wave-7 pattern that stretched over large parts of the northern hemisphere over an extended time and let to considerable societal impacts. Various diagnostics have been put forward to quantify and detect such wave patterns, many of which incorporate low-pass time filtering to separate signal from noise. In this response we argue that advancing our understanding of the large-scale circulation’s response to anthropogenic climate change and reducing associated uncertainties in future climate risk requires a diverse range of perspectives and diagnostics from both the climate and weather research communities.</jats:p>

Abstract:

<jats:p>Abstract. Model representations of the stratospheric semi-annual oscillation (SAO) show a common easterly bias, with a weaker westerly phase and stronger easterly phase compared to observations. Previous studies have shown that both resolved and parameterized tropical waves in the upper stratosphere are too weak. These waves propagate vertically through the underlying region dominated by the stratospheric quasi-biennial oscillation (QBO) before reaching the SAO altitudes. The influence of biases in the modelled QBO on the representation of the SAO is therefore explored. Correcting the QBO biases helps to reduce the SAO easterly bias through improved filtering of resolved and parameterized waves that contribute to improving both the westerly and the easterly phases of the SAO. The time-averaged zonal-mean zonal winds at SAO altitudes change by up to 25 % in response to the QBO bias corrections. The annual cycle in the equatorial upper stratosphere is improved as well. Most of the improvements in the SAO occur during the QBO easterly phase, coinciding with the period when the model's QBO exhibits the largest bias. Nevertheless, despite correcting for the QBO bias, there remains a substantial easterly bias in the SAO, suggesting that westerly wave forcing in the upper stratosphere and lower mesosphere is still severely under-represented. </jats:p>

Abstract:

The Quasi-Biennial Oscillation (QBO) is one of the most cyclic phenomena in the atmosphere except for the annular and diurnal cycles, which provide the predictability source for subseasonal-to-seasonal forecasts on the globe. The QBO is generated by the interaction between the background circulation and the equatorial waves, which cover a wide spectrum consisting of those that are eastward- and westward-propagating. The QBO can affect the climate in both the Northern and Southern Hemispheres through at least three dynamic pathways, including the stratospheric polar vortex pathway, the subtropical downward-arching zonal wind pathway, and the tropical convection pathway. The impact of the QBO on the extratropics is projected to strengthen in future scenario experiments, although the maximum QBO wind magnitude gradually decreased in recent decades. As a newly emerging feature, the QBO disruption during the westerly phase is mainly caused by the extremely active Rossby waves from the extratropics. The QBO disruptions are likely to increase in a warmer climate background.