Javier García-Serrano

The winter North Atlantic Oscillation (NAO), key driver of climate variability in the Euro-Atlantic sector, remains unsettled as to its spatio-temporal characteristics in the Northern Hemisphere. A wavenumber decomposition of NAO-related circulation anomalies is performed using reanalysis data to explore its hemispheric signature, which exhibits a non-annular, wave-like structure at all levels. In the troposphere-stratosphere system, the NAO variability yields a wavenumber 2 pattern tilting westward and increasing in amplitude with height, suggesting upward propagation. Wavenumber 1, instead, shows no statistical significance in the middle stratosphere, unlike the El Niño-Southern Oscillation teleconnection where is dominant. In the upper troposphere, the NAO variability displays a wavenumber 5 pattern at middle-subtropical latitudes reminiscent of the Circumglobal Waveguide Pattern, and a wavenumber 3 pattern at middle-subpolar latitudes which is phase-locked with climatology in the lower troposphere. These findings emphasize that interannual NAO dynamics not only include transient, synoptic-scale eddy activity in the North Atlantic but also stationary, large-scale wave activity far beyond, having marked implications for climate predictability.

The Quasi‐Biennial Oscillation (QBO) is the dominant mode of tropical stratospheric variability that can influence tropospheric circulation and convection. El Niño‐Southern Oscillation (ENSO), which is the primary source of interannual variability in the tropical troposphere, can modulate both the QBO and its impact on the tropical troposphere. However, separating the tropospheric QBO teleconnection from the dominant influence of ENSO remains challenging. In a companion article, we used an atmosphere‐only experiment under climatological conditions to isolate the QBO impact and found that the QBO modifies convection over the Maritime Continent, particularly during summer and autumn. Here, we use two additional atmosphere‐only experiments under strong El Niño and La Niña conditions to explore how ENSO modulates the QBO impact. The results show that the QBO affects summer tropical convection in all experiments. For La Niña, the QBO impact resembles that under climatological conditions, but for El Niño, this impact shifts equatorward in early summer following the ENSO‐driven change in mean convection. Our findings also show that the QBO period is shorter (longer) under El Niño (La Niña) conditions. The faster downward phase propagation during El Niño leads to a sign reversal of the tropospheric QBO impact from early to late summer, evident across several dynamical and thermodynamical fields, including temperature, lapse rate, vertical motion and the divergent circulation. The QBO influence on tropical convection is therefore modulated by ENSO through shifts in the location of deep ascending motion and changes in the QBO downward‐propagating characteristics.

This study reveals distinct co‐variability between the North Pacific and the North Atlantic‐European (NAE) region at multi‐decadal and interannual timescales using reanalysis data. On interannual to decadal timescales, sea surface temperature (SST) anomalies in the North Pacific appear in phase with the traditional Pacific Decadal Oscillation (PDO), while in the tropics resemble El Niño‐Southern Oscillation (ENSO). The extratropical signal is the ENSO‐forced wave train in the troposphere correlating with a weaker stratospheric polar vortex. On multi‐decadal timescales (20–25 years), SST anomalies are stronger over the Kuroshio‐Oyashio Extension, and turbulent heat flux anomalies suggest a potential oceanic influence. The tropospheric teleconnection evolves differently through North America projecting onto the Pacific North American pattern, resulting in a stronger polar vortex. Consistently, the corresponding impacts on NAE surface climate are mostly out of phase. Results highlight the challenge of interpreting past and future trends in the Pacific‐NAE teleconnection.

The Quasi‐Biennial Oscillation (QBO), the leading mode of tropical stratospheric variability, is thought to influence the tropical troposphere. Disentangling this influence from the dominant effects of El Niño‐Southern Oscillation (ENSO) remains challenging. In this study, we use an atmosphere‐only experiment with climatological sea surface temperature to isolate the tropospheric impact of the QBO, while a companion article examines how ENSO further affects this impact. The analysis focuses on the QBO modulation of temperature and zonal wind in the upper troposphere‐lower stratosphere (UTLS) and the accompanying effects on static stability, wind shear and vorticity. Our results show that the QBO modifies deep tropical convection over the Maritime Continent region, and affects both the zonal Walker circulation and, more notably, the meridional Hadley circulation. These impacts are highly seasonal, with the strongest effects in summer and autumn. A zonal asymmetry in the vertical structure of the QBO signal and its influence on tropical circulation is identified, with anomalies descending into the upper troposphere over the Indo‐Pacific region. Our results also suggest that the timing of this teleconnection is primarily associated with QBO‐induced changes in static stability, which exhibit a strong correlation with precipitation and arrive first at the UTLS, followed by changes in wind shear and vorticity.

The Quasi‐Biennial Oscillation (QBO) of descending zonal winds is the leading mode of tropical stratospheric variability. Numerous studies have explored its connection with the troposphere, including its sensitivity to El Niño‐Southern Oscillation (ENSO). While it is accepted that an upward ENSO impact on the QBO exists, little investigation has been devoted to the potential downward influence of the QBO. Observational and model evidence show that the QBO modulates upper‐tropospheric divergence, with reduced outflow over the Maritime Continent during the westerly phase. It can also impact the warm phase of ENSO, El Niño, characterized by a weakened Walker circulation. Results show that the westerly phase of the QBO further suppresses tropical convection in the western Pacific and thus accentuates the weakening of the Walker circulation during El Niño. These results suggest that considering the QBO state could improve El Niño prediction and projection, particularly for extreme events.

Using the extended ERA5 reanalysis and three state‐of‐the‐art models, this study explores how El Niño‐Southern Oscillation (ENSO) can influence the total frequency, seasonal cycle and preconditioning of sudden stratospheric warmings (SSWs). Reanalysis data shows that in the last seven decades, winters with SSWs were more common than winters without, regardless El Niño (EN) or La Niña (LN) occurrence or the ENSO/SSW definitions. In agreement with previous studies, our models tend to simulate a linear ENSO‐SSW relationship, with more SSWs for EN, around mid‐winter (January–February) as in reanalysis, and less for LN when compared to neutral conditions. Independently of ENSO, the main tropospheric precursor of SSWs appears to be an anomalous wave‐like pattern over Eurasia, but it is dominated by wavenumber 1 (WN1) for EN and shows an enhanced wavenumber 2 (WN2) for LN. The differences in this Eurasian wave pattern, which is largely internally generated, emerge from the distinct configuration of the background, stationary wave pattern induced by ENSO in the North Pacific, favoring a stronger WN1 (WN2) component during EN (LN). Our results suggest that the ENSO‐forced signal relies on modulating the seasonal‐mean polar vortex strength, becoming weaker and more displaced (stronger and more stable) for EN (LN), while ENSO‐unforced wave activity represents the ultimate trigger of SSWs. This supports the view that ENSO and SSWs are distinct sources of variability of the winter atmospheric circulation operating at different time‐scales and may reconcile previous findings in this context.

The performance of the European Consortium Earth-system model (EC-EARTH) in the boreal winter stratosphere is comprehensively assessed for the first time, in particular its version 3.3 that contributes to CMIP6. A 100-year long simulation with prescribed climatological boundary conditions and fixed radiative forcing, representative of present-day climate, is used to evaluate the simulation of the climatological stratospheric circulation and to identify model biases. Results show that EC-EARTH has a large issue with the vertical distribution of stratospheric temperature from the tropics to mid-latitudes, seemingly linked to radiative processes of ozone, leading to a biased warm middle-upper stratosphere. Associated with this model bias, EC-EARTH simulates a stronger polar vortex at upper-stratospheric levels while the Brewer-Dobson circulation at middle/lower levels is weaker than reanalysis. The amplitude of the climatological planetary waves is overall underestimated, but the magnitude of the background wave injection from the troposphere into the stratosphere is overestimated, related to a weaker polar vortex at lower-stratospheric levels and, thus, a less effective wave filtering. This bias in the westerly flow could have a contribution from parameterized waves. The overestimation of background wave driving is maximum in early-winter, and may explain the overestimated frequency of sudden stratospheric warmings at this time, as compared to reanalysis. The spatial distribution of wave injection climatology has revealed a distinctive role of the climatological planetary waves: while large-scale waves (wavenumbers 1–2) dominate the eddy heat flux over the North Pacific, small-scale waves (wavenumbers 3–4) are responsible for the doubled-lobe structure of the eddy heat flux over Eurasia. EC-EARTH properly simulates this climatological feature, although overestimates its amplitude over central Eurasia.

Prediction skill of continental climate in the Northern Hemisphere (NH) midlatitudes is generally limited throughout the year in dynamical seasonal forecast systems. Such limitations narrow the range of possible applications by different stakeholders. Improving the predictive capacity in these regions has been a challenging task. Sea ice is a central component of the Arctic climate system and a local source of climate predictability, yet its state is often not fully constrained in dynamical forecast systems. Using the EC-Earth3 climate model, we study the added value of assimilating observed Arctic sea ice concentration on the NH extratropical climate in retrospective forecasts of summer and fall, initialized every spring over 1992–2019. Predictions in the North Atlantic and Eurasia benefit from better initialization of sea ice in the Atlantic sector of the Arctic in a two-step mechanism. Initially, sea ice influences the central North Atlantic Ocean through an atmospheric bridge that develops in the first forecast weeks, subsequently leading to preserved skill in the sea surface temperatures (SSTs) throughout summer and early fall. Secondly, these long-lasting SST improvements provide better surface boundary conditions for the atmosphere and lead to more skillful predictions of circulation and surface climate in the Euro-Atlantic and Asian regions. In addition, our findings suggest that fully coupled ocean-atmosphere-sea ice models are likely necessary to study linkages between Arctic sea ice and midlatitudes, by better representing the interactions and feedbacks between the different components of the climate system.

El Niño-Southern Oscillation (ENSO) is known to affect the Northern Hemisphere tropospheric circulation in late-winter (January–March), but whether El Niño and La Niña lead to symmetric impacts and with the same underlying dynamics remains unclear, particularly in the North Atlantic. Three state-of-the-art atmospheric models forced by symmetric anomalous sea surface temperature (SST) patterns, mimicking strong ENSO events, are used to robustly diagnose symmetries and asymmetries in the extra-tropical ENSO response. Asymmetries arise in the sea-level pressure (SLP) response over the North Pacific and North Atlantic, as the response to La Niña tends to be weaker and shifted westward with respect to that of El Niño. The difference in amplitude can be traced back to the distinct energy available for the two ENSO phases associated with the non-linear diabatic heating response to the total SST field. The longitudinal shift is embedded into the large-scale Rossby wave train triggered from the tropical Pacific, as its anomalies in the upper troposphere show a similar westward displacement in La Niña compared to El Niño. To fully explain this shift, the response in tropical convection and the related anomalous upper-level divergence have to be considered together with the climatological vorticity gradient of the subtropical jet, i.e. diagnosing the tropical Rossby wave source. In the North Atlantic, the ENSO-forced SLP signal is a well-known dipole between middle and high latitudes, different from the North Atlantic Oscillation, whose asymmetry is not indicative of distinct mechanisms driving the teleconnection for El Niño and La Niña.