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In the study authors analyzed the interannual relationship between the Arctic Oscillation (AO)/North Atlantic Oscillation (NAO) and the tropical Indian Ocean (TIO) precipitation in boreal winter for the period 1979–2009. A significant simultaneous teleconnection between them is found. After removing the El Niño/Southern Oscillation and Indian Ocean dipole signals, the AO/NAO and the TIO precipitation (0°–10°S, 60°–80°E) yield a correlation of +0.56, which is also consistent with the AO/NAO-outgoing longwave radiation correlation of −0.61. The atmospheric and oceanic features in association with the AO/NAO-precipitation links are investigated. During positive AO/NAO winter, the Rossby wave guided by westerlies tends to trigger persistent positive geopotential heights in upper troposphere over about 20°–30°N and 55°–70°E, which is accompanied by a stronger Middle East jet stream. Meanwhile, there are anomalous downward air motions, strengthening the air pressure in mid-lower troposphere. The enhanced Arabian High brings anomalous northern winds over the northern Indian Ocean. As a result the anomalous crossing-equator air-flow enhances the intertropical convergence zone (ITCZ). On the other hand, the anomalous Ekman transport convergence by the wind stress curl over the central TIO deepens the thermocline. Both the enhanced ITCZ and the anomalous upper ocean heat content favor in situ precipitation in the central TIO. The AO/NAO-TIO precipitation co-variations in the IPCC AR4 historical climate simulation (1850–1999) of Bergen Climate Model version 2 were investigated. The Indian Ocean precipitation anomalies (particularly the convective precipitation along the ITCZ), in conjunction with the corresponding surface winds and 200 hPa anticyclonic atmospheric circulation and upper ocean heat contents were well reproduced in simulation. The similarity between the observation and simulation support the physical robustness of the AO/NAO-TIO precipitation links.

High‐latitude winter warming was observed following strong tropical volcanism, which has long been believed to be due to the volcanic‐induced positive North Atlantic Oscillation (NAO) phase. However, recent works argue that this warming is caused by El Niño–Southern Oscillation (ENSO) variability instead of volcanoes. Moreover, some studies further argue that El Niño and volcanoes work together to produce this post‐volcanic NAO winter warming. To better understand these arguments on post‐volcanic high‐latitude winter warming, we conducted ENSO‐preconditioned volcanic experiments. Our simulations strongly suggest that the post‐eruption Eurasian winter warming is caused by a post‐eruption positive NAO phase and not by coexisting ENSO‐preconditioned variability. Additionally, we find that the El Niño‐preconditioned volcanic eruption enhances the El Niño phase; however, the neutral and La Niña‐preconditioned eruptions do not lead to an ENSO–like response. These findings are helpful to better understand volcanic‐induced circulation impacts and have important implications for the interpretation of model results and post‐volcanic prediction. Plain Language Summary The El Niño–Southern Oscillation (ENSO) and North Atlantic Oscillation (NAO) linkage to volcanism and associated post‐volcanic high‐latitude winter warming is a topic of great interest. However, the origin of this post‐volcanic winter warming is still controversial. Therefore, to resolve the controversies related to post‐volcanic ENSO and NAO variability and high‐latitude winter warming, we conducted a set of ENSO‐preconditioned volcanic experiments using a coupled atmosphere‐ocean model. The model simulations demonstrate that the post‐eruption high‐latitude Eurasian winter warming is mainly related to the post‐eruption positive NAO phase with no linkage to post‐volcanic ENSO variability. Moreover, the strong tropical volcanism initialized with the El Niño state enhances the El Niño phase, while the Neutral and La Niña initialized simulations do not lead to ENSO–like variability. These results strongly suggest that the high‐latitude post‐eruption Eurasian winter warming is caused by NAO pressure changes during volcanism, and not due to strengthened ENSO responses to volcanism. Key Points Volcano‐El Niño–Southern Oscillation (ENSO) sensitivity experiments are conducted to better understand the source of post‐eruption northern hemisphere high‐latitude winter warming We found that the post‐eruption Eurasian winter warming is caused by volcanic‐induced positive North Atlantic Oscillation (NAO)‐phase and not by ENSO teleconnection Coexistence of El Niño with volcanism is not essential to produce volcanic‐induced positive NAO and associated winter warming

The North Atlantic sea surface temperature exhibits fluctuations on the multidecadal time scale, a phenomenon known as the Atlantic Multidecadal Oscillation (AMO). This letter demonstrates that the multidecadal fluctuations of the wintertime North Atlantic Oscillation (NAO) are tied to the AMO, with an opposite-signed relationship between the polarities of the AMO and the NAO. Our statistical analyses suggest that the AMO signal precedes the NAO by 10-15 years with an interesting predictability window for decadal forecasting. The AMO footprint is also detected in the multidecadal variability of the intraseasonal weather regimes of the North Atlantic sector. This observational evidence is robust over the entire 20th century and it is supported by numerical experiments with an atmospheric global climate model. The simulations suggest that the AMO-related SST anomalies induce the atmospheric anomalies by shifting the atmospheric baroclinic zone over the North Atlantic basin. As in observations, the positive phase of the AMO results in more frequent negative NAO-and blocking episodes in winter that promote the occurrence of cold extreme temperatures over the eastern United States and Europe. Thus, it is plausible that the AMO plays a role in the recent resurgence of severe winter weather in these regions and that wintertime cold extremes will be promoted as long as the AMO remains positive.

A multi-model analysis of Atlantic multidecadal variability is performed with the following aims: to investigate the similarities to observations; to assess the strength and relative importance of the different elements of the mechanism proposed by Delworth et al. (J Clim 6:1993–2011, 1993) (hereafter D93) among coupled general circulation models (CGCMs); and to relate model differences to mean systematic error. The analysis is performed with long control simulations from ten CGCMs, with lengths ranging between 500 and 3600 years. In most models the variations of sea surface temperature (SST) averaged over North Atlantic show considerable power on multidecadal time scales, but with different periodicity. The SST variations are largest in the mid-latitude region, consistent with the short instrumental record. Despite large differences in model configurations, we find quite some consistency among the models in terms of processes. In eight of the ten models the mid-latitude SST variations are significantly correlated with fluctuations in the Atlantic meridional overturning circulation (AMOC), suggesting a link to northward heat transport changes. Consistent with this link, the three models with the weakest AMOC have the largest cold SST bias in the North Atlantic. There is no linear relationship on decadal timescales between AMOC and North Atlantic Oscillation in the models. Analysis of the key elements of the D93 mechanisms revealed the following: Most models present strong evidence that high-latitude winter mixing precede AMOC changes. However, the regions of wintertime convection differ among models. In most models salinity-induced density anomalies in the convective region tend to lead AMOC, while temperature-induced density anomalies lead AMOC only in one model. However, analysis shows that salinity may play an overly important role in most models, because of cold temperature biases in their relevant convective regions. In most models subpolar gyre variations tend to lead AMOC changes, and this relation is strong in more than half of the models.

Using fifth-generation ECMWF Atmospheric Reanalysis (ERA-5) we investigate the topical changes in global teleconnection with seasonal precipitation dynamics in the backdrop of climate change particularly for the recent 20 years (L20:2002 to 2021) with respect to the previous 20 years (F20: 1982–2001). It unveils a considerable decrease in both monsoon and pre-monsoon precipitation over the Northwest Himalaya (NWH) in the Indian region and a decline in winter precipitation over the Tibetan Plateau (TP). Statistically robust K-S test shows notable changes (5% significance level) in the regional distribution of the atmospheric parameters in parts of the High Mountain Asia (HMA) region in recent decades (L20). Further, a weakening of the North Atlantic Oscillation (NAO) and El-Nino Southern Oscillation (ENSO) influence on precipitation pattern is noted in the L20 (2002–2021) compared to the F20 (1982–2001) years over parts of the HMA region. A regional heterogeneity is perceptible among different parts of HMA i.e. NWH in the Indian region and the TP. The NWH precipitation shows a diametrically opposite relation with the NAO index for winter precipitation in L20 (correlation coefficient R = -0.18, sample size: 240) compared to a positive correlation in F20 (correlation coefficient R  = 0.15, sample size: 240) which is significant at 5% significance level. The plausible reasons for such a diametrical change in the relationship during winter are addressed by examining the changing signature of the subtropical jet stream and its relationship with NAO and ENSO. Sea surface temperature shows cooling of the tropical Pacific and the resulting weakening in El-Nino signatures in recent years. Our results point to the noteworthy changes in the atmospheric circulation pattern over the HMA region in recent decades which is attributed to the alteration in the global teleconnection patterns and subsequent their association with the precipitation distribution over the HMA region. This study would be useful for an improved understanding of the precipitation dynamics over the HMA region and global teleconnections in the future.

The sea ice growth (SIG) in the Kara Sea plays a crucial role in the Arctic climate system. Many studies have examined its long‐term trend, but whether its variability has changed is less clear. Using observations and reanalysis data, we observe an intensified interannual variability of the Kara Sea SIG during boreal late winter (December–March) since 2004/2005. This arises from the retreat of active ice production zones in response to the strengthened modulation of the westward‐shifted North Atlantic Oscillation (NAO). Using a sea ice concentration budget, we quantitatively partition the NAO's contribution to total SIG variability post‐2004 (∼63.3%), revealing comparably dominant roles of thermodynamic and dynamic processes. During this period, the negative NAO‐associated surface winds concurrently cool sea surface and export sea ice from the Kara Sea, thereby injecting freshwater into the lower latitudes. Our study advances the understanding of the regional air‐ice‐ocean climate feedbacks in recent Arctic.

The Summer North Atlantic Oscillation (SNAO) is the dominant climate pattern that affects heatwaves and droughts across Europe and downstream over Asia. However, its seasonal prediction remains challenging. While recent studies have identified stratospheric pathways for improving SNAO prediction, the role of tropospheric wave dynamics remains unclear. Using a latest‐generation large‐ensemble seasonal forecasting system, we find that the model exhibits limited SNAO prediction skill and underestimates downstream climate impacts. These limitations are associated with deficiencies in circumglobal waveguide dynamics. Subsampling ensemble members that best capture circulation anomalies near circumglobal teleconnection centers or tropical/monsoon rainfall that forces the teleconnection is accompanied by significantly improved SNAO prediction. Furthermore, deficiencies in the model circumglobal waveguide, particularly over the Mediterranean and North Atlantic with underestimated background zonal winds and erroneous curvature, may disrupt wave propagation. Refining these dynamical features could improve teleconnection representation and SNAO prediction, aiding summer climate risk mitigation.

The summertime variability of the extratropical storm track over the Atlantic sector and its links to European climate have been analysed for the period 1948-2011 using observations and reanalyses. The main results are as follows. (1) The dominant mode of the summer storm track density variability is characterized by a meridional shift of the storm track between two distinct paths and is related to a bimodal distribution in the climatology for this region. It is also closely related to the Summer North Atlantic Oscillation (SNAO). (2) A southward shift is associated with a downstream extension of the storm track and a decrease in blocking frequency over the UK and northwestern Europe. (3) The southward shift is associated with enhanced precipitation over the UK and northwestern Europe and decreased precipitation over southern Europe (contrary to the behaviour in winter). (4) There are strong ocean-atmosphere interactions related to the dominant mode of storm track variability. The atmosphere forces the ocean through anomalous surface fluxes and Ekman currents, but there is also some evidence consistent with an ocean influence on the atmosphere, and that coupled ocean-atmosphere feedbacks might play a role. The ocean influence on the atmosphere may be particularly important on decadal timescales, related to the Atlantic Multidecadal Oscillation (AMO).

Following strong tropical volcanism, the Middle East and North Africa (MENA) region witnessed significant winter cooling, conventionally attributed to volcanically forced positive phase of North Atlantic Oscillation (NAO) and direct volcanic effects. However, coexisting positive phase of El Niño–Southern Oscillation (ENSO) prompts that this enhanced winter cooling may stem from ENSO forcing rather than volcanic-induced NAO. To address this complexity, we analyzed ENSO-preconditioned volcanic sensitivity experiments. Our simulations assert that the post-eruption MENA amplified winter cooling is primarily driven by the subsequent positive NAO phase, diminishing the essentiality of El Niño-preconditioned eruptions for inducing forced NAO changes. These findings deepen our comprehension of volcanic-induced circulation impacts in MENA, providing valuable insights for model interpretation, and are useful for policy research on post-volcanic predictions.

There is evidence that the observed changes in winter North Atlantic Oscillation (NAO) drive a significant portion of Atlantic Multi Decadal Variability (AMV). However, whether the observed decadal NAO changes can be forced by the ocean is controversial. There is also evidence that artificially imposed multi-decadal stratospheric changes can impact the troposphere in winter. But the origins of such stratospheric changes are still unclear, especially in early to mid winter, where the radiative ozone-impact is negligible. Here we show, through observational analysis and atmospheric model experiments, that large-scale Atlantic warming associated with AMV drives high-latitude precursory stratospheric warming in early to mid winter that propagates downward resulting in a negative tropospheric NAO in late winter. The mechanism involves stratosphere/troposphere dynamical coupling, and can be simulated to a large extent, but only with a stratosphere resolving model (i.e., high-top). Further analysis shows that this precursory stratospheric response can be explained by the shift of the daily extremes toward more major stratospheric warming events. This shift cannot be simulated with the atmospheric (low-top) model configuration that poorly resolves the stratosphere and implements a sponge layer in upper model levels. While the potential role of the stratosphere in multi-decadal NAO and Atlantic meridional overturning circulation changes has been recognised, our results show that the stratosphere is an essential element of extra-tropical atmospheric response to ocean variability. Our findings suggest that the use of stratosphere resolving models should improve the simulation, prediction, and projection of extra-tropical climate, and lead to a better understanding of natural and anthropogenic climate change.