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A coherent poleward displacement of the Hadley cell (HC) edge and eddy-driven jet latitude has been well documented in the Southern Hemisphere (SH) under the present and future climate changes. However, a recent study showed that during the Last Glacial Maximum (LGM) winter, an equatorward shift of the HC edge but a poleward shift of the jet latitude are found in the SH. These opposing circulation changes are investigated in this study by conducting the parameter sweep experiments using a dynamical core GCM. By systematically varying the amplitude of tropical upper-tropospheric and polar surface cooling, mimicking the LGM-like climate state, an opposing shift of circulation is reproduced when polar cooling is much stronger than tropical cooling. This is due to the higher sensitivity of the jet-latitude change compared to the HC-edge change in response to polar cooling. Eddy cospectra analysis reveals that the poleward jet shift is dominated by fast waves as the baroclinic zone expands poleward with increasing polar surface cooling. Instead, the HC-edge change is largely attributed to the activity of slow waves and the axisymmetric circulation change. They lead to the HC edge being weakly influenced by extratropical baroclinicity, resulting in an equatorward shift of the HC edge under a global cooling–like condition. Similar circulation changes but with an opposite sign are also found in global warming–like experiments. It suggests that a poleward HC shift together with an equatorward jet shift can occur even in a future climate if Arctic amplification is accelerated relative to tropical warming., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The impact of the quasi-biennial oscillation (QBO) on the surface air temperature (SAT) in the Northern Hemisphere extratropics is investigated. It is found that the QBO, defined as 70-hPa zonal wind in the deep tropics, is negatively correlated with the SAT over the western North Pacific in February and March. Cold temperature anomaly appears during the QBO westerly phase. Such relationship is likely mediated by the subtropical jet. During the QBO westerly phase, a horseshoe-shaped zonal wind anomaly forms in the upper troposphere and lower stratosphere and is connected to the equatorward shift of the Asia-Pacific jet. This equatorward jet shift is accompanied by a cyclonic circulation anomaly in the subtropical North Pacific and an anticyclonic circulation anomaly over northern Eurasia in the troposphere. The resultant temperature advection brings cold air to East Asia and the western North Pacific. This regional downward coupling in February and March, which is not sensitive to El Niño-Southern Oscillation, has become statistically significant in recent decades., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The El Niño–Southern Oscillation (ENSO) significantly affects the wintertime surface climate in western North America (WNA) via the Pacific/North American-like teleconnection pattern. Here, we reveal significant sub-seasonal variations of the ENSO teleconnections on WNA despite the persistence of ENSO-related tropical sea surface temperature anomalies throughout the entire winter. In particular, the ENSO–WNA teleconnections become substantially weakened in mid-winter and considerably weaker than those in early and late winter. This mid-winter weakening of the ENSO–WNA teleconnections stems from the tropical convection over the Indian Ocean. In mid-December of El Niño winters, the anomalously enhanced convection is developed over the equatorial Indian Ocean and excites the teleconnection pattern with a two-week lag that opposes the Pacific/North American-like teleconnection pattern that is induced by anomalous convection over the equatorial central Pacific. Such sub-seasonal variability of ENSO teleconnections is poorly predicted by multi-model hindcasts, affecting the mid-latitude prediction skill. It particularly results in a lower prediction skill of WNA surface air temperature in mid-winter than in late winter. This result highlights a need to better understand the ENSO-related tropical convection changes and the associated teleconnections for improving the long-lead prediction of the WNA surface climate., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

International audience; Extreme weather events have devastating impacts on human health, economic activities, ecosys tems, and infrastructure. It is therefore crucial to anticipate extremes and their impacts to allow for preparedness and emergency measures. There is indeed potential for probabilistic subseasonal prediction on timescales of several weeks for many extreme events. Here we provide an overview of subseasonal predictability for case studies of some of the most prominent extreme events across the globe using the ECMWF S2S prediction system: heatwaves, cold spells, heavy precipitation events, and tropical and extratropical cyclones. The considered heatwaves exhibit predictability on timescales of 3-4 weeks, while this timescale is 2-3 weeks for cold spells. Precipitation extremes are the least predictable among the considered case studies. Tropical cyclones, on the other hand, can exhibit probabilistic predictability on timescales of up to 3 weeks, which in the presented cases was aided by remote precursors such as the Madden-Julian Oscillation. For extratropical cyclones, lead times are found to be shorter. These case studies clearly illustrate the potential for event - dependent advance warnings for a wide range of extreme events. The subseasonal predictability of extreme events demonstrated here allows for an extension of warning horizons, provides advance information to impact modelers, and informs communities and stakeholders affected by the impacts of extreme weather events.