Simulation of ENSO teleconnections to precipitation extremes over the US in the high resolution version of E3SM

We evaluate the simulated teleconnection of El Niño Southern Oscillation (ENSO) to winter season precipitation extremes over the United States in a long (98 years) 1950-control high resolution version (HR, 25 km nominal atmosphere model horizontal resolution) of US Department of Energy’s (DOE) Energy Exascale Earth System Model version 1 (E3SMv1). Model bias and spatial pattern of ENSO teleconnections to mean and extreme precipitation in HR overall are similar to the low-resolution model’s (LR, 110 km) historical simulation (4-member ensemble, 1925-1959). However, over the Southeast US (SE-US), HR produces stronger El Niño associated extremes, reducing upon LR’s model bias. Both LR and HR produce weaker than observed increase in storm track activity during El Niño events there. But, HR improves the ENSO associated variability of moisture transport over SE-US. During El Niño, stronger vertical velocities in HR produce stronger large-scale precipitation causing larger latent heating of the troposphere that pulls in more moisture from the Gulf of Mexico into the SE-US. This positive feedback also contributes to the stronger mean and extreme precipitation response in HR. Over the Pacific Northwest, LR’s bias of stronger than observed La Niña associated extremes is amplified in HR. Both models simulate stronger than observed moisture transport from the Pacific Ocean into the region during La Niña years. The amplified HR bias there is due to stronger orographically driven vertical updrafts that create stronger large scale precipitation, despite weaker La Niña induced storm track activity.

Meridional Position Changes of the Sea Surface Temperature Anomalies in the North Pacific

Changes in the meridional position of the sea surface temperature (SST) anomalies (SSTAs) associated with the interannual component (PC1-I) of the principal component 1 (PC1) of the first leading mode of the North Pacific SST (referred here as PC1-I-related SSTAs) are investigated using reanalysis products and climate model output. It is found that the PC1-I-related SSTAs (or PC1-I anomaly) significantly shift southward at a rate of 1.04°/decade and have moved southward by 4.4 degrees since the 1960s. Our further analysis indicates that the southward shift of the PC1-I-related SSTAs is due to changes in ENSO teleconnections. Compared to the 1950–1975 period (PRE era), the meridional width of the ENSO-induced tropical positive geopotential height (GH) anomaly is narrower during the 1991–2016 period (POST era), inducing a southward shift of the subtropical westerly anomaly over the North Pacific through geostrophic wind relations. This southward shift of the westerly anomaly favors the southward shift of the ENSO-induced negative GH anomaly (cyclonic circulation anomaly) over the North Pacific by positive vorticity forcing of the zonal wind shear. The southward-shifting GH anomaly associated with ENSO further forces the PC1-I anomaly to shift southward. Furthermore, the contraction of the ENSO-induced tropical positive GH anomaly is related to the contraction of the meridional width of ENSO. The modeling results support that the decrease in the ENSO meridional width favors the contraction of the ENSO-induced tropical positive GH anomaly and the southward shift of ENSO teleconnections over the North Pacific, contributing to the southward shift of the PC1-I anomaly.

Response of Global SSTs and ENSO to the Atlantic and Pacific Meridional Overturning Circulations

Consequences from a slowdown or collapse of the Atlantic Meridional Overturning Circulation (AMOC) could include modulations to El Niño-Southern Oscillation (ENSO) and development of the Pacific Meridional Overturning Circulation (PMOC). Despite potential ramifications to the global climate, our understanding of the influence of various AMOC and PMOC states on ENSO and global sea surface temperatures (SSTs) remains limited. Five multi-centennial, fully-coupled model simulations created with the Community Earth System Model were used to explore the influence of AMOC and PMOC on global SSTs and ENSO. We found that the amplitude of annual cycle SSTs across the tropical Pacific decreases and ENSO amplitude increases as a result of an AMOC shutdown, irrespective of PMOC development. However, active deep overturning circulations in both the Atlantic and Pacific basins reduce ENSO amplitude and variance of monthly SSTs globally. The underlying physical reasons for changes to global SSTs and ENSO are also discussed, with the atmospheric and oceanic mechanisms that drive changes to ENSO amplitude differing based on PMOC state. These results suggest that if climate simulations projecting AMOC weakening are realized, compounding climate impacts could occur given the far-reaching ENSO teleconnections to extreme weather and climate events. More broadly, these results provide us with insight into past geologic era climate states, when PMOC was active.

Recent weakening in the winter ENSO teleconnection over the North Atlantic-European region

<p>New observational evidence for variability of the atmospheric response to wintertime El Niño-Southern Oscillation (ENSO) is found. A weakening in the recent ENSO teleconnection over the North Atlantic-European (NAE) region is demonstrated by using various methods (e.g. composite analysis, running correlation, regression maps) applied onto different observational datasets and reanalyses (HadSLP, NOAA 20th Century reanalysis). Changes in both the spatial pattern and strength of the ENSO teleconnection indicate a turning point in the 1970s, with a shift from a response resembling the North Atlantic Oscillation (NAO) in late winter to an anomaly pattern with very weak or statistically non-significant values; and to nearly non-existent teleconnection in the most recent decades. Weakening of the ENSO signal is found at the surface (sea level pressure), but also at higher levels for different variables (geopotential height, temperature, zonal wind). To offer a possible reason behind the observed change, we have investigated the potential role of sea-ice and SST climatology in modulating the ENSO-NAE teleconnection. Sensitivity experiments made with a GCM of indermediate complexity (ICTP AGCM) using different combinations of sea-ice and sea surface temperature (SST) climatology conditions enabled the investigation of their respective roles. As indicated by the targeted simulations, recent change in the SST climatology in the Atlantic and Arctic has contributed to the weakening of the ENSO effect. Results highlight the importance of the background SST state and sea-ice climatology having opposite effects in modulating the ENSO-NAE teleconnection over the area of interest. The findings of this study could further our understanding of modulations of ENSO teleconnections and the role of ENSO as a source of predictability in the NAE sector.</p>

Emergent constraints on the large scale atmospheric circulation and regional hydroclimate: do they still work in CMIP6 and how much can they actually constrain the future?

An ‘emergent constraint’ (EC) is a statistical relationship, across a model ensemble, between a measurable aspect of the present day climate (the predictor) and an aspect of future projected climate change (the predictand). If such a relationship is robust and understood, it may provide constrained projections for the real world. Here, Coupled Model Intercomparison Project 6 (CMIP6) models are used to revisit several ECs that were proposed in prior model intercomparisons with two aims: (1) to assess whether these ECs survive the partial out-of-sample test of CMIP6 and (2) to more rigorously quantify the constrained projected change than previous studies. To achieve the latter, methods are proposed whereby uncertainties can be appropriately accounted for, including the influence of internal variability, uncertainty on the linear relationship, and the uncertainty associated with model structural differences, aside from those described by the EC. Both least squares regression and a Bayesian Hierarchical Model are used. Three ECs are assessed: (a) the relationship between Southern Hemisphere jet latitude and projected jet shift, which is found to be a robust and quantitatively useful constraint on future projections; (b) the relationship between stationary wave amplitude in the Pacific-North American sector and meridional wind changes over North America (with extensions to hydroclimate), which is found to be robust but improvements in the predictor in CMIP6 result in it no longer substantially constrains projected change in either circulation or hydroclimate; and (c) the relationship between ENSO teleconnections to California and California precipitation change, which does not appear to be robust when using historical ENSO teleconnections as the predictor.

Enhanced risk of concurrent regional droughts with increased ENSO variability and warming

Spatially compound extremes pose substantial threats to globally interconnected social-economic systems. We use an Earth system model large ensemble to examine the future risk of compound droughts during the boreal summer over ten global regions with highly seasonal climate. Relative to the late-20th century, the probability, mean extent and severity of compound droughts increase by ~60%, ~10% and ~20% respectively by the late-21st century, with a disproportionate increase in risk across North America and the Amazon. These changes result in a ~9-fold increase in exposure over agricultural areas and ~5 to 20-fold increase in population exposure depending on the shared socioeconomic pathway. ENSO is the predominant large-scale driver of compound droughts with 68% of historical events occurring during El Niño or La Niña conditions. ENSO teleconnections remain stationary in the future though an ~22% increase in ENSO extremes combined with projected warming, drive the elevated risk of compound droughts.

Enhanced interactions of Kuroshio Extension with tropical Pacific in a changing climate

Quasi-decadal climate of the Kuroshio Extension (KE) is pivotal to understanding the North Pacific coupled ocean–atmosphere dynamics and their predictability. Recent observational studies suggest that extratropical-tropical coupling between the KE and the central tropical Pacific El Niño Southern Oscillation (CP-ENSO) leads to the observed preferred decadal time-scale of Pacific climate variability. By combining reanalysis data with numerical simulations from a high-resolution climate model and a linear inverse model (LIM), we confirm that KE and CP-ENSO dynamics are linked through extratropical-tropical teleconnections. Specifically, the atmospheric response to the KE excites Meridional Modes that energize the CP-ENSO (extratropicstropics), and in turn, CP-ENSO teleconnections energize the extratropical atmospheric forcing of the KE (tropicsextratropics). However, both observations and the model show that the KE/CP-ENSO coupling is non-stationary and has intensified in recent decades after the mid-1980. Given the short length of the observational and climate model record, it is difficult to attribute this shift to anthropogenic forcing. However, using a large-ensemble of the LIM we show that the intensification in the KE/CP-ENSO coupling after the mid-1980 is significant and linked to changes in the KE atmospheric downstream response, which exhibit a stronger imprint on the subtropical winds that excite the Pacific Meridional modes and CP-ENSO.

Resampling of ENSO teleconnections: accounting for cold season evolution reduces uncertainty in the North Atlantic

We re-examine the uncertainty of the El Niño–Southern Oscillation teleconnection to the North Atlantic following the investigation of Deser et al. (2017) (DES2017). Our analyses are performed on the November–December (ND) and January–February (JF) means separately, and for a geographical area that covers a larger extent in the midlatitude North At- lantic than DES2017. The motivation for splitting the cold season in this way arises from the fact that the teleconnection patterns and underlying physical mechanisms are different in late fall compared to mid-winter. As in DES2017, our main technique in quantifying the uncertainty is bootstrap resampling. Amplitudes and spatial correlations of the bootstrap samples are presented together effectively using Taylor diagrams. In addition to the confidence intervals calculated from Student's t tests and the percentiles of anomalous sea-level pressure (SLP) values in the bootstrap samples, we also investigate additional confidence intervals using techniques that are not widely used in climate research, but have different advantages. In contrast to the interpretation by DES2017, our results indicate that we can have some confidence in the signs of the teleconnection SLP anomalies. The uncertainties in the amplitudes remain large, with the upper-percentile anomalies at up to 2 times those of the lower percentiles in the North Pacific, and 2.8 times in the North Atlantic.

Indian Ocean mediates the ENSO teleconnections to the Central Southwest Asia during the wet season

<p>Central Southwest Asia (CSWA) is a region with the largest number of glaciers, outside the polar regions in its northeast and the Arabian desert to its southwest. The region receives precipitation from November to April period also known as the wet season, which contributes to the regional freshwater resources. Mainly, El Niño–Southern Oscillation (ENSO) modulates the wet season precipitation over CSWA, with a positive relationship. However, the intraseasonal characteristics of ENSO influence are largely unknown, which may be important to understand the regional sub-seasonal to seasonal hydroclimate variability. We noted that the ENSO‐CSWA teleconnection varies intraseasonally and is a combination of direct and indirect positive influences. The ENSO direct influence is through a Rossby wave‐like pattern in the tail months of the wet season, while the indirect influence is noted through an ENSO‐forced atmospheric dipole of diabatic heating anomalies in the tropical Indian Ocean (TIO), which also generates a Rossby wave‐like forcing and persists throughout the wet season. The stronger ENSO influence is found when both direct and indirect modes are in phase, while the relationship breaks down when the two modes are out of phase. Moreover, the idealized numerical simulations confirm and reproduce the observed circulation patterns. This suggests that improvements in sub-seasonal to seasonal scale predictability requires the better representation of intraseasonal variability of ENSO teleconnection, as well as the role of interbasin interactions in its propagation.</p>

The Combined Influence of the Stratospheric Polar Vortex and ENSO on Zonal Asymmetries in the Southern Hemisphere Upper Tropospheric Circulation during Austral Spring and Summer

<p>The influence of El Niño Southern Oscillation (ENSO) and the Stratospheric Polar Vortex (SPV) on the zonal asymmetries in the Southern Hemisphere atmospheric circulation during spring and summer is examined. The main objective is to explore if the SPV can modulate the ENSO teleconnections in the extratropics. We use a large ensemble of seasonal hindcasts from the European Centre for Medium-Range Weather Forecasts Integrated Forecast System to provide a much larger sample size than is possible from the observations alone.</p><p>We find a small but statistically significant relationship between ENSO and the SPV, with El Niño events occurring with weak SPV and La Niña events occurring with strong SPV more often than expected by chance, in agreement with previous works. We show that the zonally asymmetric response to ENSO and SPV can be mainly explained by a linear combination of the response to both forcings, and that they can combine constructively or destructively. From this perspective, we find that the tropospheric asymmetries in response to ENSO are more intense when El Niño events occur with weak SPV and La Niña events occur with strong SPV, at least from September through December. In the stratosphere, the ENSO teleconnections are mostly confounded by the SPV signal. The analysis of Rossby Wave Source and of wave activity shows that both are stronger when El Niño events occur together with weak SPV, and when La Niña events occur together with strong SPV.</p>