Tropical Extratropical Teleconnections Research Articles

Unconventional cold vortex as precursor to historic early summer heatwaves in North China 2023

In mid-June to July 2023, North China witnessed extreme heatwaves, marked by intense near-surface warming with an advanced seasonal cycle of local air temperature. An unconventional upper-tropospheric cold vortex in early June, deviating from conventional “heat dome” patterns, preceded the heatwave extremes. The zonal SSTA gradient in Indo-Pacific warm pool initially suppressed Indian summer monsoon convection, which stimulated the cold vortex around North China via a tropical-extratropical teleconnection. This anomaly intensified the air-land thermal contrast, leading to increased sensible heating and reduced soil moisture in situ. The drier soil conditions maintained and further augmented sensible heating, escalating surface air temperature, and culminating in extraordinary heatwaves. The air column was then destabilized to mitigate the upper-level cold vortex. Historical records corroborate the extremity of the air-sea interactions in 2023. The ECMWF real-time subseasonal-to-seasonal (S2S) forecasts successfully capture the air-land feedback in both cold vortex and heatwave stages, albeit with an underestimation of heatwave intensity due to biases in soil moisture anomalies. Consequently, the initial cold vortex condition and air-land-sea interactions yield S2S predictability to the historic 2023 heatwaves in North China.

Interdecadal shift of the North Pacific Oscillation and its nonstationary relationship with East Asian climate

A pronounced shift of the North Pacific Oscillation (NPO), including its teleconnection with the East Asian winter monsoon, has been observed in recent decades, but the underlying mechanism remains unclear. In this study, we found that interannual variation of the NPO and its relationship with the dominant modes of winter mean and extreme temperatures over East Asia (EA) experienced a significant interdecadal shift in the mid-1980s. Before the shift (during 1950–1985), the NPO emerged and was maintained as a strong regional north-south dipole, manifesting a remarkable tropical-extratropical teleconnection. It significantly modulated the second mode of air temperature over EA (depicted by a north-south dipole) and extreme temperature events over southern EA, mainly through horizontal advection and adiabatic heating induced by the anomalous circulation in the southern lobe of the NPO. After the shift (during 1986–2021), the NPO was characterized by an intensified northern lobe and a weakened and northwestward shifted southern lobe, and by a close teleconnection with the Rossby wave trains at the mid-higher latitudes. In the meantime, tropical-extratropical teleconnections became apparently weak. Correspondingly, the NPO was mainly linked to the interannual variation of air temperature and extreme events over northern EA via horizontal advection. Importantly, the decadal shift of the relationships of the NPO with the Arctic Oscillation and El Niño–Southern Oscillation played an apparent role in modulating EA climate.

Warm conveyor belt activity over the Pacific: modulation by the Madden–Julian Oscillation and impact on tropical–extratropical teleconnections

Abstract. Research in the last few decades has revealed that rapidly ascending airstreams in extratropical cyclones – so-called warm conveyor belts (WCBs) – play an important role in extratropical atmospheric dynamics. However on the subseasonal timescale, the modulation of their occurrence frequency, henceforth referred to as WCB activity, has so far received little attention. Also, it is not yet clear whether WCB activity may affect tropospheric teleconnection patterns, which constitute a source of predictability on this subseasonal timescale. Using reanalysis data, this study analyzes the modulation of WCB activity by the Madden–Julian Oscillation (MJO). A key finding is that WCB activity increases significantly over the western North Pacific when the convection of the MJO is located over the Indian Ocean. This increased WCB activity, which is stronger during La Niña conditions, is related to enhanced poleward moisture fluxes driven by the circulation of subtropical Rossby gyres associated with the MJO. In contrast, when the convection of the MJO is located over the western North Pacific, WCB activity increases significantly over the eastern North Pacific. This increase stems from a southward shift and eastward extension of the North Pacific jet stream. However, while these mean increases are significant, individual MJO events exhibit substantial variability, with some events even exhibiting anomalously low WCB activity. Individual events of the same MJO phase with anomalously low WCB activity over the North Pacific tend to be followed by the known canonical teleconnection patterns in the Atlantic–European region; i.e., the occurrence frequency of the positive phase of the North Atlantic Oscillation (NAO) is enhanced when convection of the MJO is located over the Indian Ocean and similarly for the negative phase of the NAO when MJO convection is over the western North Pacific. However, the canonical teleconnection patterns are modified when individual events of the same MJO phase are accompanied by anomalously high WCB activity over the North Pacific. In particular, the link between MJO and the negative phase of the NAO weakens considerably. Reanalysis data and experiments with an idealized general circulation model reveal that this is related to anomalous ridge building over western North America favored by enhanced WCB activity. Overall, our study highlights the potential role of WCBs in shaping tropical–extratropical teleconnection patterns and underlines the importance of representing them adequately in numerical weather prediction models in order to fully exploit the sources of predictability emerging from the tropics.

Comprehensive Representation of Tropical–Extratropical Teleconnections Obstructed by Tropical Pacific Convection Biases in CMIP6

Abstract The central role of tropical sea surface temperature (SST) variability in modulating Northern Hemisphere (NH) extratropical climate has long been known. However, the prevailing pathways of teleconnections in observations and the ability of climate models to replicate these observed linkages remain elusive. Here, we apply maximum covariance analysis between atmospheric circulation and tropical SST to reveal two coexisting tropical–extratropical teleconnections albeit with distinctive spatiotemporal characteristics. The first mode, resembling the Pacific–North American (PNA) pattern, favors a tropical–Arctic in-phase (warm Pacific–warm Arctic) teleconnection in boreal spring and winter. However, the second mode, with a slight seasonal preference of summer, is manifested as an elongated Rossby wave train emanating from the tropical eastern Pacific that features an out-of-phase relationship (cold Pacific–warm Arctic) between tropical central Pacific SSTs and temperature variability over the Arctic (referred to as the PARC mode). While climate models participating in phase 6 of the Coupled Model Intercomparison Project (CMIP6) appear to successfully simulate the PNA mode and its temporal characteristics, the majority of models’ skill in reproducing the PARC mode is obstructed to some extent by biases in simulating low-frequency SST and rainfall variability over the tropical eastern Pacific and the climatological mean flow over the North Pacific during boreal summer. Considering the contribution of the PARC mode in shaping low-frequency climate variations over the past 42 years from the tropics to the Arctic, improving models’ capability to capture the PARC mode is essential to reduce uncertainties associated with decadal prediction and climate change projection over the NH. Significance Statement This study focuses on the skill of models in phase 6 of the Coupled Model Intercomparison Project (CMIP6) in simulating two leading observed Northern Hemisphere (NH) teleconnections that show distinctive spatial and temporal characteristics. The first one, the Pacific–North American (PNA) mode, exhibits a warm Pacific–warm Arctic pattern in boreal spring and winter, and the second one, the Pacific–Arctic (PARC) mode, features a cold Pacific–warm Arctic out-of-phase relationship. We find that models are skillful in simulating the PNA mode but not the PARC mode. This limitation may be rooted in unrealistic simulations of the mean state of winds and the low-frequency sea surface temperature variability in the tropical eastern Pacific. These biases call for caution when interpreting current models’ projections of extratropical circulations on multidecadal time scales.

Euro-Atlantic Weather Regimes and Their Modulation by Tropospheric and Stratospheric Teleconnection Pathways in ECMWF Reforecasts

Abstract This study combines operational reforecasts (2001–21) with results from a lower-resolution 41-yr reforecast (1980–2020) to provide a robust assessment of wintertime Euro-Atlantic regimes and their modulation by tropospheric and stratospheric teleconnection pathways in the European Centre for Medium-Range Weather Forecasts (ECMWF) Subseasonal to Seasonal Prediction project (S2S). In both operational and lower-resolution reforecasts, the climatological properties of wintertime Euro-Atlantic regimes, including regime structures, frequencies, and transition probabilities, are accurately simulated at S2S lead times. However, the 41-yr reforecasts allow us to diagnose substantial errors in regime statistics when conditioned on modes of intraseasonal-to-interannual variability. In particular, ECMWF reforecasts underestimate the response of the North Atlantic Oscillation (NAO) to the Madden–Julian oscillation (MJO) and fail to reproduce the modulation of MJO–NAO teleconnections by El Niño–Southern Oscillation (ENSO). Teleconnection and atmospheric wave diagnostics highlight two specific issues that are likely to contribute to these conditional errors in ECMWF reforecasts: (i) insufficient propagation of Rossby wave activity from the Pacific to the Atlantic following MJO phase 3 during El Niño conditions, when the direct tropospheric teleconnection pathway is most active; and (ii) an underestimated response of the stratospheric polar vortex following MJO phase 8 during La Niña conditions, when the indirect stratospheric teleconnection pathway is most active. Improving the representation of tropospheric and stratospheric teleconnection pathways is thus a priority for improving ECMWF forecasts of extratropical weather regimes and their associated surface impacts. Significance Statement Subseasonal to Seasonal Prediction project (S2S) forecasts are used operationally at ECMWF to provide early warning of cold conditions in Europe associated with persistent large-scale circulation patterns known as weather regimes. On average, ECMWF reforecasts accurately simulate wintertime Euro-Atlantic regime structures and frequencies at S2S lead times. However, regime forecasts show substantial errors when we restrict our analysis to certain phases of intraseasonal and interannual variability, such as El Niño–Southern Oscillation (ENSO). These errors are related to deficiencies in the simulated response of weather regimes to well-predicted variability in the tropics. Improving the representation of such tropical–extratropical teleconnections will improve predictions of extratropical weather regimes and their associated surface impacts.

Assessing decadal variability of subseasonal forecasts of opportunity using explainable AI

Identifying predictable states of the climate system allows for enhanced prediction skill on the generally low-skill subseasonal timescale via forecasts with higher confidence and accuracy, known as forecasts of opportunity. This study takes a neural network approach to explore decadal variability of subseasonal predictability, particularly during forecasts of opportunity. Specifically, this work quantifies subseasonal prediction skill provided by the tropics within the Community Earth System Model Version 2 (CESM2) Large Ensemble and assesses how this skill evolves on decadal timescales. Utilizing the networks’ confidence and explainable artificial intelligence, physically meaningful sources of predictability associated with periods of enhanced skill are identified. Using these networks, we find that tropically-driven subseasonal predictability varies on decadal timescales during forecasts of opportunity. Further, we investigate the drivers of the low frequency modulation of the tropical-extratropical teleconnection and discuss the implications. Analysis is extended to ECMWF Reanalysis v5 data, revealing that the relationships learned within the CESM2-Large Ensemble holds in modern reanalysis data. These results indicate that the neural networks are capable of identifying predictable decadal states of the climate system within CESM2 that are useful for making confident, accurate subseasonal precipitation predictions in the real world.

Seasonal Tropical–Extratropical Teleconnections Originating from Tropical Rainfall Modes Independent of the Niño-3.4 Index in Northern Winters

Abstract This study, based on an analysis with observational and reanalysis data, highlights seasonal tropical–extratropical atmospheric teleconnections originating from tropical rainfall modes unrelated to the Niño-3.4 index for northern winters. The mode decomposition for tropical rainfall is done by first removing the Niño-3.4 index–related variability from the tropical rainfall and then applying rotated empirical orthogonal function (REOF) analysis to the residual. The corresponding teleconnection patterns are obtained by regressing global atmospheric fields against the time series of the rainfall modes. Analyses of the tropical heating–atmospheric circulation relationship indicate that the circulation anomalies corresponding to the rainfall modes are forced responses to the corresponding rainfall mode. The teleconnection patterns reveal some new features and show that some intrinsic midlatitude patterns can be triggered by tropical forcing with different rainfall patterns. Results from this study are relevant to seasonal climate attribution and prediction analyses and climate model evaluation. As an illustration, the teleconnections from the rainfall modes, together with that related to the Niño-3.4 index and linear trend, are applied to the attribution analyses for the global circulation anomalies of 2019/20 winter and the California dry condition during the strong El Niño winter of 2015/16. The overall impact of these modes in the period of 1980–2021 is also discussed. Significance Statement This study highlights the seasonal tropical–extratropical atmospheric teleconnections independent of the Niño-3.4 index using tropical rainfall modes for northern winters. The reason for using rainfall rather than SST in the mode decomposition is that rainfall represents vertically integrated latent heat, which is the direct forcing of the tropical atmosphere, while SST may have no definite relationship with rainfall in the Indo-Pacific warm pool region. The results of this study are applicable to the analysis of climate attribution and prediction and climate model evaluation, and further, may also have the potential to help improve seasonal climate forecasts.

Validation of boreal summer tropical–extratropical causal links in seasonal forecasts

Abstract. Much of the forecast skill in the mid-latitudes on seasonal timescales originates from deep convection in the tropical belt. For boreal summer, such tropical–extratropical teleconnections are less well understood compared to winter. Here we validate the representation of boreal summer tropical–extratropical teleconnections in a general circulation model in comparison with observational data. To characterise variability between tropical convective activity and mid-latitude circulation, we identify the South Asian monsoon (SAM)–circumglobal teleconnection (CGT) pattern and the western North Pacific summer monsoon (WNPSM)–North Pacific high (NPH) pairs as the leading modes of tropical–extratropical coupled variability in both reanalysis (ERA5) and seasonal forecast (SEAS5) data. We calculate causal maps based on the Peter and Clark momentary conditional independence (PCMCI) causal discovery algorithm, which identifies causal links in a 2D field, to show the causal effect of each of these patterns on circulation and convection in the Northern Hemisphere. The spatial patterns and signs of the causal links in SEAS5 closely resemble those seen in ERA5, independent of the initialisation date of SEAS5. By performing a subsampling experiment (over time), we analyse the strengths of causal links in SEAS5 and show that they are qualitatively weaker than those in ERA5. We identify those regions for which SEAS5 data well reproduce ERA5 values, e.g. the southeastern USA, and highlight those where the bias is more prominent, e.g. North Africa and in general tropical regions. We demonstrate that different El Niño–Southern Oscillation phases have only a marginal effect on the strength of these links. Finally, we discuss the potential role of model mean-state biases in explaining differences between SEAS5 and ERA5 causal links.

Dynamical Drivers of the Exceptional Warmth over Siberia during the Spring of 2020

Abstract Much of Siberia experienced exceptional warmth during the spring of 2020, which followed an unusually warm winter over the same region. Here, we investigate the drivers of the spring warmth from the perspective of atmospheric dynamics and remote influences, focusing on monthly-time-scale features of the event. We find that the warm anomalies were associated with separate quasi-stationary Rossby wave trains emanating from the North Atlantic in April and May. The wave trains are shown to be extreme manifestations of the dominant modes of spring subseasonal meridional wind variability over the Northern Hemisphere. Using a large ensemble of simulations from NASA’s GEOS atmospheric model, in which the model is constrained to remain close to observations over selected regions, we further elucidate the remote drivers of the unusual spring temperatures in Siberia. In both April and May, the wave trains were likely forced from an upstream region including eastern North America and the western North Atlantic. Analysis with a stationary wave model shows that transient vorticity flux forcing over and downwind of the North Atlantic, which is strongly related to storm activity caused by internal variability, is key to generating the wave trains, suggesting limited subseasonal predictability of the Rossby waves and hence the exceptional Siberian warmth. Our observational and model analyses also suggest that anomalous tropical atmospheric heating contributed to the unusual warmth in Siberia through a teleconnection involving upper-troposphere dynamics and the mean meridional circulation. This tropical–extratropical teleconnection offers a possible physical mechanism by which anthropogenic climate change influenced the extreme Siberian warmth.

The impact of tropical SST variability on the northern hemisphere circumglobal teleconnection pattern

Tropical-extratropical teleconnection during boreal summer is characterized by a circumglobal teleconnection (CGT) pattern and is often considered as one of the major predictability sources of extratropical atmospheric circulation. The extent to which the summer circumglobal teleconnection pattern is remotely forced by tropical sea surface temperature (SST) variability remains to be elucidated. Here, we use a suite of pacemaker simulations from the Community Earth System Model (CESM1) to investigate the contribution of tropical sea surface temperature variability in the Eastern Tropical Pacific, Northern Atlantic, and Indo-Western Pacific to the summer circumglobal teleconnection circulation. We find that the variability of circumglobal teleconnection in the summer is dominated by the sea surface temperature variability in the Indo-Western Pacific and its remotely forced ENSO-like anomaly. As a result, the Indo-Western Pacific pacemaker experiment exhibits precipitation and surface temperature teleconnection patterns closest to the observed patterns in the Northern Hemisphere. A better understanding of such tropical-extratropical interactions can inform better seasonal forecasts and model development.

Seasonal Predictability of the East Atlantic Pattern in Late Autumn and Early Winter

AbstractThe North Atlantic Oscillation (“NAO”) and the East Atlantic Pattern (“EAP”) dominate winter atmospheric variability over the North Atlantic. Unlike the NAO, seasonal predictability of the EAP has remained elusive. A multi‐model ensemble of seasonal predictions yields skillful forecasts of the EAP in late autumn and early winter, complementing NAO prediction skill in winter. The shift in prediction skill from EAP to NAO reflects the ability of the ensemble to forecast the evolving influence of the El Niño South Oscillation on the North Atlantic region. In early winter, the ensemble correctly forecasts the key tropical–extratropical teleconnections, resulting in skillful predictions of the EAP and western European temperatures and rainfall. However, the modeled tropical–extratropical teleconnections are weak compared to observations, contributing to a signal to noise error in predictions of the EAP. Improving the strength of such teleconnections would improve predictions of the EAP and associated surface climate.

The Distinct Influence of Two Madden–Julian Trajectory Classes on the South American Dipole

Abstract The South American dipole (SAD) is the main mode of rainfall variability at the intraseasonal time scale in South America. Its phases, determined by wet/dry conditions over the South Atlantic convergence zone (SACZ) and southeastern South America (SESA) regions, have been related to extreme precipitation and droughts across South America, but are yet to be fully understood and predicted. In this work, we study the intraseasonal predictability of the SAD in relation with the Madden–Julian oscillation (MJO) in the December–March (DJFM) season. We show that significant predictability windows for the SAD’s extreme states emerge up to 10–20 days after the MJO is found in phase 2. In particular, we introduce a classification of MJO trajectories based on the RMM index, defining long and intense (LIT) and not long and intense (NLIT) sets of trajectories. These sets bear resemblances to MJO events that are able or unable to propagate beyond the Maritime Continent, respectively. Moreover, we show that they influence differently over the SAD’s predictability. While both sets present similar tropical–extratropical teleconnections, the LIT set presents a more intense and persistence tropical–tropical teleconnection. Because of this, there is a set-independent predictability for the SESA region, influenced by both tropical and extratropical teleconnections, and a LIT dependency for the SACZ predictability, influenced mainly by the presence of the tropical–tropical teleconnection. Overall, our work contributes to the intraseasonal predictability of SAD phases and understanding its relationship with MJO events.

Modes and Mechanisms of Pacific Decadal-Scale Variability.

The modes of Pacific decadal-scale variability (PDV), traditionally defined as statistical patterns of variance, reflect to first order the ocean's integration (i.e., reddening) of atmospheric forcing that arises from both a shift and a change in strength of the climatological (time-mean) atmospheric circulation. While these patterns concisely describe PDV, they do not distinguish among the key dynamical processes driving the evolution of PDV anomalies, including atmospheric and ocean teleconnections and coupled feedbacks with similar spatial structures that operate on different timescales. In this review, we synthesize past analysis using an empirical dynamical model constructed from monthly ocean surface anomalies drawn from several reanalysis products, showing that the PDV modes of variance result from two fundamental low-frequency dynamical eigenmodes: the North Pacific-central Pacific (NP-CP) and Kuroshio-Oyashio Extension (KOE) modes. Both eigenmodes highlight how two-way tropical-extratropical teleconnection dynamics are the primary mechanisms energizing and synchronizing the basin-scale footprint of PDV. While the NP-CP mode captures interannual- to decadal-scale variability, the KOE mode is linked to the basin-scale expression of PDV on decadal to multidecadal timescales, including contributions from the South Pacific.

The Boreal Summer Zonal Wavenumber-3 Trend Pattern and Its Connection with Surface Enhanced Warming

Abstract The Northern Hemisphere warms faster under global warming and suffers from more frequent heatwaves, causing considerable social and economic damage. The Northern Hemisphere surface warming exhibits strong regionality, with multiple “hotspots” (areas of enhanced warming), but the relations among them remain unclear. This study finds a dominating zonal wavenumber-3 (ZW3) trend pattern in the upper-level geopotential heights during the boreal summer. The summer geopotential heights show significant increasing trends along the latitudinal circle around 60°N, with three centers located over northeastern America, western Eurasia, and eastern Siberia. The regionally enhanced surface warming trends are closely linked to the increased geopotential through the reduced cloud cover, exhibiting a consistent ZW3 pattern. The model simulations forced by sea surface temperature (SST) and Arctic sea ice cover (SIC) indicate that the SST forcing plays an important role in generating the ZW3 pattern, while the contribution of the SIC is minimal. A theoretical barotropic model can fairly well reproduce the observed ZW3 structure forced by a heating source located over the subtropical North Atlantic, where the SSTs show prominent warming trends and a close relationship with the ZW3 pattern. Our results indicate that the hotspots may be interconnected and are related to a Rossby wave train with a ZW3 structure. It highlights a vital role of tropical/subtropical SSTs on the atmospheric circulation and the associated surface enhanced warming over the mid- to high latitudes, which may have great implications for the coordinated heatwave events and tropical–extratropical teleconnections.

Representation of Rossby wave propagation and its effect on the teleconnection between the Indian summer monsoon and extratropical rainfall in the Met Office Unified Model

Compared with Global Atmosphere 6 (GA6) of the UK Met Office Unified Model (UM), the dry bias over the Indian monsoon region in Global Atmosphere 7 (GA7) is significantly reduced. However, the physical processes controlling how this reduced dry bias in India influences rainfall teleconnections in the extratropics remain unclear. Thus, in this study, we use Rossby wave tracing in a horizontally nonuniform background flow to investigate how the improved simulation of monsoon rainfall in GA7 compared with GA6 affects extratropical rainfall teleconnections. We find that wave rays emanating from the upper troposphere in the Indian monsoon region first propagate westward, then divide into the Northern Hemisphere (NH) subtropical westerlies over Asia and the Southern Hemisphere (SH) subtropical westerlies. The wave ray trajectories in GA7 in years of strong Indian summer monsoon rainfall (ISMR) are closer to observations than those in GA6. We also find that the upper tropospheric meridional winds over the South Asian monsoon region and western Tibetan Plateau are much better simulated in GA7 than in GA6 owning to the improvement of ISMR and South Asian High (SAH), which leads to a more realistic simulation of the wave rays in GA7. The better simulated circulation teleconnections in GA7 then modulate the vertical motion and moisture transport, and hence affect extratropical rainfall anomalies in the NH and SH. This paper provides new insights for the assessment of tropical–extratropical teleconnections in models.

Effect of the North Pacific Tropospheric Waveguide on the Fidelity of Model El Niño Teleconnections

AbstractIn a free-running climate model, DJF tropical–extratropical teleconnections are assessed and compared to observed teleconnections in reanalysis data. From reanalysis, the leading mode of covariability between tropical outgoing longwave radiation (OLR) and Northern Hemisphere extratropical geopotential height (Z500) is identified using maximum covariance analysis (MCA). This mode relates closely to the El Niño pattern. The GCM captures the tropical OLR well but the associated extratropical Z500 less well. The GCM climatology has an equatorward shifted North Pacific jet bias. We examine whether the difference in the teleconnection pattern is related to the GCM’s jet bias. In both a ray-tracing analysis and a barotropic model, this jet bias is shown to affect the Rossby wave propagation from the tropical Pacific into the North Pacific. These idealized model results suggest qualitatively that the MCA difference is largely consistent with linear Rossby wave dynamics. While the basic state has a larger effect on the North Pacific MCA, a Rossby wave source (RWS) bias in the Caribbean has a larger effect on the North Atlantic MCA. The North Pacific jet bias is also proposed to affect the downstream propagation of waves from North America into the Caribbean, where it affects tropical RWS and the triggering of secondary waves into the North Atlantic. We propose that climatological biases in the tropics are one underlying cause of the jet bias. Our study may also help understand the results of other climate models with similar jet biases.

Subseasonal midlatitude prediction skill following Quasi-Biennial Oscillation and Madden–Julian Oscillation activity

Abstract. The Madden–Julian Oscillation (MJO) is known to force extratropical weather days to weeks following an MJO event through excitation of stationary Rossby waves, also referred to as tropical–extratropical teleconnections. Prior research has demonstrated that this tropically forced midlatitude response leads to increased prediction skill on subseasonal to seasonal (S2S) timescales. Furthermore, the Quasi-Biennial Oscillation (QBO) has been shown to possibly alter these teleconnections through modulation of the MJO itself and the atmospheric basic state upon which the Rossby waves propagate. This implies that the MJO–QBO relationship may affect midlatitude circulation prediction skill on S2S timescales. In this study, we quantify midlatitude circulation sensitivity and prediction skill following active MJOs and QBOs across the Northern Hemisphere on S2S timescales through an examination of the 500 hPa geopotential height field. First, a comparison of the spatial distribution of Northern Hemisphere sensitivity to the MJO during different QBO phases is performed for European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim reanalysis and ECMWF and the National Centers for Environmental Prediction (NCEP) hindcasts. Secondly, differences in prediction skill in ECMWF and NCEP hindcasts are quantified following MJO–QBO activity. In both hindcast systems, we find that regions across the Pacific, North America, and the Atlantic demonstrate an enhanced MJO impact on prediction skill during strong QBO periods with lead times of 1–4 weeks compared to MJO events during neutral QBO periods.

The Consistency of MJO Teleconnection Patterns on Interannual Time Scales

AbstractThe Madden–Julian oscillation (MJO) excites strong variations in extratropical geopotential heights that modulate extratropical weather, making the MJO an important predictability source on subseasonal to seasonal time scales (S2S). Previous research demonstrates a strong similarity of teleconnection patterns across MJO events for certain MJO phases (i.e., pattern consistency) and increased model ensemble agreement during these phases that is beneficial for extended numerical weather forecasts. However, the MJO’s ability to modulate extratropical weather varies greatly on interannual time scales, which brings extra uncertainty in leveraging the MJO for S2S prediction. Few studies have investigated the mechanisms responsible for variations in the consistency of MJO tropical–extratropical teleconnections on interannual time scales. This study uses reanalysis data, ensemble simulations of a linear baroclinic model, and a Rossby wave ray tracing algorithm to demonstrate that two mechanisms largely determine the interannual variability of MJO teleconnection consistency. First, the meridional shift of stationary Rossby wave ray paths indicates increases (decreases) in the MJO’s extratropical modulation during La Niña (El Niño) years. Second, a previous study proposed that the constructive interference of Rossby wave signals caused by a dipole Rossby wave source pattern across the subtropical jet during certain MJO phases produces a consistent MJO teleconnection. However, this dipole feature is less clear in both El Niño and La Niña years due to the extension and contraction of MJO convection, respectively, which would decrease the MJO’s influence in the extratropics. Hence, considering the joint influence of the basic state and MJO forcing, this study suggests a diminished potential to leverage the MJO for S2S prediction in El Niño years.

Tropical atmospheric drivers of wintertime European precipitation events

AbstractFrom observations, we identify a wave‐like pattern associated with northwestern European seasonal precipitation events. These events are associated with tropical precipitation anomalies, prompting us to investigate if there are any tropical–extratropical teleconnections, in particular the role of tropical anomalies in driving extratropical dynamics through Rossby wave propagation. Using a hierarchy of models from ray tracing to barotropic and baroclinic models, we investigate the Rossby wave mechanism and test potential tropical drivers and yield qualitative results. Using a barotropic model, we identify potential Rossby wave source regions which are consistent between the observations and the model. These regions include the tropical western and eastern Atlantic, the subtropical eastern Atlantic and, to a smaller degree, the subtropical eastern Pacific. Zonal wavenumber 2 and 3 components of the barotropic model responses match well with the observations and ray tracing supports the importance of these components. We use a baroclinic model to investigate the link between the observed Rossby wave source anomalies and the observed tropical precipitation anomalies. The reduced precipitation observed in the tropical Atlantic just north of the Equator can generate some of the observed Rossby wave source anomalies in the tropical Atlantic, while the increased precipitation observed in the tropical eastern Pacific can generate some of the observed Rossby wave source anomalies in the subtropical eastern Pacific. Our results can also be applied to European drought events because of the qualitative linearity in the observations and in our linear methods.

Australia-Asian monsoon in two versions of the UK Met Office Unified Model and their impacts on tropical–extratropical teleconnections

Many climate models have dry biases in tropical monsoon regions, but it is less clear how these errors can affect these model-simulated tropical–extratropical interactions and rainfall teleconnections. In this study, we evaluate the Australia and Asian (A–A) monsoon rainfall in two versions [Global Atmosphere version 6 (GA6), and version 7 (GA7)] of the UK Met Office Unified Model (UM) with uncoupled atmosphere-only simulations using two horizontal resolutions (N96 ~ 135 km and N216 ~ 60 km). Although UM can capture broad features of rainfall seasonal variations in the monsoon region, there are significant model errors in the locations of monsoon rainfall, its magnitudes and time evolutions, especially a climatological rainfall dry bias over the Indian subcontinent. This dry bias is progressively reduced by increased model resolutions from N96 to N216 and improved model physics from GA6 to GA7. Area-averaged peak monsoon rainfall over the Indian subcontinent (70°–90°E, 5°–25°N) increases by ~ 70% from 3 mm day−1 in GA6N216 to 5 mm day−1 in GA7N216, and rainfall dry bias averaged over a large tropical Asian monsoon domain is reduced by 2/3. In the uncoupled atmosphere-only experiment, the model rainfall is excessively sensitive to its underlying SST conditions, with positive local SST-rainfall correlations occurring over whole tropical oceans, while observations show negative correlations in regions dominated by atmospheric influence although such model performance is improved in its fully coupled runs. Most significant improvement of monsoon rainfall and ENSO (El Nino Southern Oscillations) relationship in GA7 occurred in the Australian continent, with GA7N216 better capturing the observed relationship than GA6N216. Although GA7N216 still suffers a dry bias in the Indian monsoon region, the increased monsoon rainfall substantially increases atmospheric diabatic heating over the region in the middle-upper troposphere, leading to more realistic extratropical circulation Gill-type responses to the heating. This results in improved monsoon-desert rainfall teleconnections, more realistic linkage between tropical Asian monsoon with its subtropical East Asian component, and its interhemispheric influence in Southern Hemisphere. Such results are further confirmed by the model nudging experiments in which the atmospheric wind and temperature are nudged towards observations over the Indian monsoon domain. Our study clearly demonstrates that uncertainty associated with monsoon simulations needs to be considered in future climate projections even outside the monsoon domain.