Rossby Wave Propagation Research Articles

A (2+1) -dimensional evolution model of Rossby waves and its resonance Y-type soliton and interaction solutions

In this paper, we derive a Kadomtsev-Petviashvili equation by using the multi-scale expansion and perturbation method, which is a model from the potential vorticity equation in the traditional approximation and describes the Rossby wave propagation properties. The N-soliton solutions, resonance Y-type soliton solutions, resonance X-type soliton solutions and interaction solutions of the equation are obtained with the help of dependent-variable transformation. In addition, the composite graphs are given to view the resonance phenomenon of Rossby waves. The results better enrich the research of Rossby waves in ocean dynamics and atmospheric dynamics.

The synergistic effect of the preceding winter mid-latitude North Atlantic and summer tropical eastern Indian Ocean SST on summer extreme heat events in northern China

Summer extreme heat events happen frequently in northern China during recent decades, which have serious impacts on the society and ecosystem. The present study reveals that there is a synergistic effect of the preceding winter positive mid-latitude North Atlantic SST anomaly (pMNA SSTA) and summer negative tropical eastern Indian Ocean SST anomaly (nTEI SSTA) on strengthening the summer extreme heat events in northern China. The extreme heat events are stronger and more frequent when the two factors cooccur, and the probability of a strengthened extreme heat events is higher, which indicates a synergistic effect of the two factors. The preceding winter pMNA SSTA and summer nTEI SSTA exert their synergistic effect through a series of coupled oceanic-land-atmospheric bridges. The preceding winter pMNA SSTA could lead to an anomalous anticyclone over central Asia via the eastward propagating Rossby wave, which decreases snowfall and the subsequent snow cover there. The negative snow cover anomaly may persist into spring and induce a local anomalous anticyclone in spring via the snow-hydrological effect, which decreases the precipitation over the southern flank of the anomalous anticyclone. The decreased soil moisture persists into summer and induces the eastward propagating Rossby wave, and favors the increase of atmosphere thickness over northern China. The summer nTEI SSTA can also induce the anomalous anticyclone over northern China via the northeastward Rossby wave propagation. Thus, the two factors exhibit evident synergistic effect on the atmospheric circulation anomaly over northern China. The anomalous anticyclone corresponds to the increased atmosphere thickness, which favors the increase of air temperature in northern China and strengthening of extreme heat events. Therefore, the preceding winter pMNA SSTA and summer nTEI SSTA have significant synergistic effect on strengthening the summer extreme heat events in northern China.

Eurasian mid-latitude jet stream bridges an Atlantic to Asia summer teleconnection in heat extremes

Northern Europe and Northeast Asia, which are climatologically mild and temperate in early summer, both witnessed record-breaking heat extremes in June 2023. With a focus on the subseasonal time scale, this study highlights a teleconnected relationship in historical early-summer heat extremes between Northern Europe and Northeast Asia. By diagnosing the thermal budget and wave activity flux, we show that over one third of historical Northeast Asian heat extremes are initialized by Eurasian mid-latitude Rossby wave packets. The wave packets manifest as strong anticyclonic anomalies over subpolar North Atlantic, cyclonic anomalies over Ural region and anticyclonic anomalies over Northeast Asia, which directly cause coexisting heat extremes in Northern Europe and Northeast Asia. Such wave packets are strongly modulated by the Eurasian jet stream. By carrying out numerical experiments, we further explain why the wave packet is significant in early summer. We find that the quasi-stationary response of the Eurasian midlatitude Rossby wave packets to a specified heating in subpolar North Atlantic is dependent on the background jet structure. In early summer, the double-jet state over Eurasia favors Rossby wave propagation. This is of great practical importance for accurate prediction of Northeast Asian heat extreme amplitude and location.

Nonstationarity in the NAO–Gulf Stream SST Front Interaction

Abstract The interaction between the North Atlantic Oscillation (NAO) and the latitudinal shifts of the Gulf Stream sea surface temperature front (GSF) has been the subject of extensive investigations. There are indications of nonstationarity in this interaction, but differences in the methodologies used in previous studies make it difficult to draw consistent conclusions. Furthermore, there is a lack of consensus on the key mechanisms underlying the response of the GSF to the NAO. This study assesses the possible nonstationarity in the NAO–GSF interaction and the mechanisms underlying this interaction during 1950–2020, using reanalysis data. Results show that the NAO and GSF indices covary on the decadal time scale but only during 1972–2018. A secondary peak in the NAO–GSF covariability emerges on multiannual time scales but only during 2005–15. The nonstationarity in the decadal NAO–GSF covariability is also manifested in variations in their lead–lag relationship. Indeed, the NAO tends to lead the GSF shifts by 3 years during 1972–90 and by 2 years during 1990–2018. The response of the GSF to the NAO at the decadal time scale can be interpreted as the joint effect of the fast response of wind-driven oceanic circulation, the response of deep oceanic circulation, and the propagation of Rossby waves. However, there is evidence of Rossby wave propagation only during 1972–90. Here it is suggested that the nonstationarity of Rossby wave propagation caused the time lag between the NAO and the GSF shifts on the decadal time scale to differ between the two time periods.

Why Is the Westward Rossby Wave Propagation from the California Coast “Too Fast”?

Abstract Past work has shown that interannual California coastal sea level variability is mostly of equatorial origin, and decades of satellite sea surface height (SSH) and in situ dynamic height observations indicate that this interannual signal propagates westward from the California coast as nondispersive Rossby waves (RWs). These observations agree with standard linear vertical mode theory except that even when mean flow and bottom topography are considered, the fastest baroclinic vertical mode RW in each case is always much slower (1.6–2.3 cm s−1) than the observed 4.2 cm s−1. This order-1 disagreement is only resolved if the standard bottom boundary condition that the vertical velocity w′ = 0 is replaced by perturbation pressure p′ = 0. Zero p′ is an appropriate bottom boundary condition because south of San Francisco the northeastern Pacific Ocean boundary acts approximately like an impermeable vertical wall to the interannual equatorial wave signal, and therefore equatorial quasigeostrophic p′ is horizontally constant along the boundary. Thus, if equatorial p′ = 0 at the bottom, then this condition also applies off California. The large-scale equatorial ocean boundary signal is due to wind-forced eastward group velocity equatorial Kelvin waves, which at interannual and lower frequencies propagate at such a shallow angle to the horizontal that none of the baroclinic equatorial Kelvin wave signal reaches the ocean floor before striking the eastern Pacific boundary. Off California this signal can thus be approximated by a first baroclinic mode with p′ = 0 at the bottom, and hence the long RW speed there agrees with that observed (both approximately 4.2 cm s−1). Significance Statement The California Current System is one of the most biologically rich and best-documented coastal regions in the world. In this region coastal sea level propagates westward from the coast at about 110 km month−1, slow enough to enable us to make large-scale ocean climate forecasts of the California Current ecosystem using coastal sea level. Although the westward speed seems slow, theoretically it is about double what we would expect. Offered here is an explanation of why this speed is “too fast” by linking the California wave signal to the equator, El Niño, and the shallow equatorial ocean response to the wind.

Contrasting latitudinal evolution of East Asian monsoonal precipitation during the Last Interglacial (130–120 ka)

Evolution of East Asian monsoonal precipitation across the Last Interglacial (LIG) remains controversial, owing to the discrepancies between various proxies and their low temporal resolution. Through a transient high-resolution global climate simulation covering the interval of 130–120 ka, we illustrate a long-term increasing (decreasing) trend in summer precipitation over south China (northeast Asia) during the LIG (i.e. 130–120 ka). The out-of-phase precipitation evolution across latitudes were coherently regulated by the weakened monsoonal circulation, southward moved western North Pacific high, and southward displaced East Asian westerly jet from the early to late LIG. These atmospheric circulation variations were in turn determined by sea surface temperature anomalies over the Pacific and the propagation of extratropical Rossby waves originating from North Africa. Our results may provide important insights for reconciling discrepancies between precipitation proxies during the LIG and for precipitation behavior in a warmer-than-present world.

Linkage between the boreal spring Antarctic oscillation and the temperature dipole mode over the Tibetan Plateau in summer

As the extratropical Southern Hemispheric most prominent atmospheric mode, Antarctic oscillation (AAO) can impact the climate system via multi-spherical interactions, whereas its potential influence on the temperature anomaly over the Tibetan Plateau (TP) remains unclear. By employing ERA5 reanalysis during 1979–2018, we have identified a significant in-phase relationship between April–May AAO and the boreal summer (June–August) temperature anomaly over the TP. The AAO significantly modulates the meridional circulation in the Atlantic sector where the climatic subtropical jet is weaker than other regions in the subtropics as well as the equatorward extension of midlatitude jet. The divergence of transient eddy momentum related to the positive AAO strengthens the northward movement of the South American meridional circulation and cross-equatorial airflow in the low troposphere. Due to the Coriolis force, the cross-equatorial airflow turns southwesterly above the tropical North Atlantic, weakening the climatic northeasterly and the feedback of wind-evaporation-sea surface temperature (SST). Consequently, an anomalous warm SST persists into the boreal summer and triggers a Rossby wave train, which propagates along the North Atlantic-TP pathway. As a result, the positive phase of TP's temperature dipole mode with an eastern warm anomaly and a western cold anomaly appears. The numerical experiments conducted by the linear baroclinic model provide further evidence for the Rossby wave origination and propagation. The AAO, therefore, may act as a valuable predictor for the climate predictions around the TP.

Assessment of thermocline depth bias in the Seychelles-Chagos Thermocline Ridge of the Southwestern Indian Ocean simulated by the CMIP6 models

The Seychelles-Chagos Thermocline Ridge (SCTR, 5°S-10°S, 50°E-80°E) is a unique open-ocean upwelling region in the southwestern Indian Ocean. Due to the negative wind stress curl between the equatorial westerlies and southeasterly trade winds, SCTR is known as a strong upwelling region with high biological productivity, providing a primary fishing zone for the surrounding countries. Given its importance in shaping the variability of the Indian Ocean climate by understanding the sea-air interaction and its dynamics, the simulation of SCTR is evaluated using outputs from the Coupled Model Intercomparison Project Phase Sixth (CMIP6). Compared to observations, 23 out of 27 CMIP6 models tend to simulate considerably deeper SCTR thermocline depth (defined as the 20°C isotherm depth (D20))– a common bias in climate models. The deep bias is related to the easterly wind bias in the equatorial to southern Indian Ocean, which is prominent in boreal summer and fall. This easterly wind bias produces a weak annual mean Ekman pumping, especially in the boreal fall. Throughout the year, the observed Ekman pumping is positive and is driven by two components: the curl term, is associated with the wind stress curl, leads to upwelling during boreal summer to fall; the beta term, is linked to planetary beta and zonal wind stress, contributes to downwelling during boreal spring to fall. However, the easterly wind bias in the CMIP6 increases both the positive curl and negative beta terms. The beta term bias offsets the curl term bias and reduces the upwelling velocity. Furthermore, the easterly wind bias is likely caused by the reduced east-west sea surface temperature (SST) difference associated with a pronounced warm bias in the western equatorial Indian Ocean, accompanied by the east-west mean sea level pressure gradient over the Indian Ocean. Furthermore, this study finds local wind-induced Ekman pumping to be a more dominant factor in thermocline depth bias than Rossby waves, despite CMIP6 models replicating Rossby wave propagation. This study highlights the importance of the beta term in the Ekman pumping simulation. Thus, reducing the boreal summer-to-fall easterly wind bias over the Indian Ocean region may improve the thermocline depth simulation over the SCTR region.

The impact of freshening over the Antarctic Ocean on Pacific Decadal Oscillation

<p>The profound influence of the Antarctic Ocean freshening on the Pacific Decadal Oscillation (PDO) is investigated in this study by utilizing a series of fully coupled ocean-atmosphere 400-year-modeling experiments. The simulated results derived from the Fast Ocean-Atmosphere Model (FOAM) can reasonably identify the spatial pattern and time period (10–20 years and 20–50 years) of the observed PDO with slightly weak amplitudes. In the sensitivity experiment (Southern Ocean Water Hosing), 1.0 Sv (Sverdrup, 1Sv = 1.0 × 106 m<sup>3</sup>/s) freshwater flux is uniformly imposed over the Antarctic Ocean for 400 years. As a response to this Antarctic Ocean freshening, the Tropical Pacific Ocean displays a normal “La Niña pattern”, while the low-frequency variability within the North Pacific Ocean is much weakened. Preserving the PDO’s spatial pattern, the multidecadal (20–50 years) magnitude becomes weak and shifts toward higher frequency. In contrast, the decadal magnitude of the PDO (10–20 years) is slightly reinforced and also shifts towards higher frequency. Dynamical analysis indicates that the shortening of the PDO multidecadal variability is mainly caused by the acceleration of the first-baroclinic-mode Rossby waves. The spreading of the fresh anomalies and associated increasing stratification in the North Pacific Ocean result in the shortening of the long Rossby wave propagation to cross the subtropical North Pacific basin. A heat budget analysis further shows that the upper-ocean thermodynamic variability in relationship to the stratification oscillation in the North Pacific Ocean is mainly associated with the anomalous behaviors of the meridional advection, heat flux and ocean mixing.</p>

The Influence of Southeastern South American Rainfall on Weather Patterns Over the Tropical Atlantic, Northwestern Africa and Western Europe

AbstractDuring boreal winter (December–February), the South American monsoon system (SAMS) reaches its annual maximum when upper‐tropospheric westerly winds prevail over the equatorial Atlantic. Atmospheric dynamic model simulations suggest that Rossby waves generated over South America can propagate to and potentially influence weather patterns in the Northern Hemisphere (NH). However, observational evidence has been absent previously. Here we focus on southeastern South American (SESA) precipitation anomalies, which can characterize intraseasonal rainfall variability of the SAMS. Since tropical “westerly duct” and convective heating are important factors for cross‐equatorial propagation of Rossby wave (CEPRW), we identify two groups of events based on the two factors. By comparing the events associated with both SESA rainfall and tropical westerlies to the events associated with tropical westerlies only, we find that an anomalous Rossby wave train is triggered by precipitation anomalies over SESA, propagates in the southwest–northeast direction, and subsequently crosses the equator. Over a period of 4 days, near‐surface temperature over northwestern Africa and western Europe becomes warmer, accompanied by increased surface downward longwave radiation and precipitable water. The equatorward propagating Eliassen–Palm flux anomalies originated from SESA support the evidence of CEPRW. Simulations using a time‐dependent linear barotropic model forced by prescribed divergence anomalies over SESA further confirm that SESA rainfall can influence the NH weather patterns through CEPRW. Knowledge of this study will help us better understand and model interhemispheric teleconnections over the American–Atlantic–African/European sector.

Large-Scale Eddy-Mean Flow Interaction in the Earth's Extratropical Atmosphere

Large-scale circulation of the atmosphere in the Earth's extratropics is dominated by eddies, eastward (westerly) zonal winds, and their interaction. Eddies not only bring about weather variabilities but also help maintain the average state of climate. In recent years, our understanding of how large-scale eddies and mean flows interact in the extratropical atmosphere has advanced significantly due to new dynamical constraints on finite-amplitude eddies and the related eddy-free reference state. This article reviews the theoretical foundations for finite-amplitude Rossby wave activity and related concepts. Theory is then applied to atmospheric data to elucidate how angular momentum is redistributed by the generation, transmission, and dissipation of Rossby waves and to reveal how an anomalously large wave event such as atmospheric blocking may arise from regional eddy-mean flow interaction.

Two-dimensional study of Rossby waves generated by an initial disturbance

Rossby waves are the fundamental time dependent large-scale motion of the ocean and atmosphere. In this article, we are interested to find analytical solution of Rossby waves produced by an initial disturbance. These waves, as we know, are archetypical low-frequency waves, which play a central role in defining meteorological and oceanographic conditions. Our purpose here is to analyze the properties of such waves in two dimension as a guide to the construction of the dynamical framework appropriate for low-frequency motion. As a precursor to our aim, we revisited the problem of Rossby wave generation in an ocean bounded by two imaginary latitudes for a particular type of external forcing. The characteristics of this motion are explored with the help of group velocity and phase velocity. Subsequently, the two-dimensional case of generation of Rossby wave by a general form of disturbance in an unbounded ocean is solved where the concept of stationary phase method is extensively used. The rules of governing the propagation of wave packet are discussed, and we found for a wave-like disturbance, Rossby wave is not a single wave train but rather several, which moves as a wave front with a definite direction-deviation. It is natural to question what role, if any, the source plays in explicit determination of these radiated waves and their asymptotic behavior. To address these and other concerns, we have developed an analytical model of generation of Rossby waves produced by an initial disturbance in two-dimensional open ocean with a considerable generality.

Physical mechanisms for the dominant summertime high-latitude atmospheric teleconnection pattern and the related Northern Eurasian climates

Summertime atmospheric teleconnection patterns over Eurasia have a significant influence on regional weather and climate. Despite extensive studies on the subtropical patterns, the high-latitude counterpart has received relatively less attention. This study proposes physical mechanisms for the formation and maintenance of the dominant high-latitude teleconnection pattern. The formation of the pattern is associated with variability in synoptic-scale eddy activity due to the meridional gradient of sea surface temperature anomalies in the vicinity of the Gulf Stream, causing a meridional shift of the central axis of storm track at the exit of Atlantic jet. The resultant convergence of transient vorticity fluxes to the west of the British Isles induces low-frequency cyclonic circulation anomalies and continued propagation of Rossby waves downstream along northern Eurasia. Once these circulation anomalies are formed, the subsequent latent heat-related diabatic anomalies over the northern Eurasian landmass act as another source of Rossby waves to maintain the teleconnection pattern. Regional temperature and precipitation variability is closely linked to the wave pattern along a route through northern Eurasia, and even precipitation over the East Asian summer monsoon region is influenced by the teleconnection pattern.

East Asian summer rainfall stimulated by subseasonal Indian monsoonal heating

The responses of the East Asian summer monsoon (EASM) to the Indian summer monsoon (ISM) have been the subject of extensive investigation. Nevertheless, it remains uncertain whether the ISM can serve as a predictor for the EASM. Here, on the basis of both observations and a large-ensemble climate model experiment, we show that the subseasonal variability of abnormal diabatic heating over India enhances precipitation over central East China, the Korean Peninsula, and southern Japan in June. ISM heating triggers Rossby wave propagation along the subtropical jet, promoting southerly winds over East Asia. The southerly winds helps steer anomalous mid-tropospheric warm advection and lower-tropospheric moisture advection toward East Asia, providing conditions preferential for rainband formation. Cluster analysis shows that, depending on jet structures, ISM heating can serve as a trigger as well as a reinforcer of the rainband.

Diagnosing the relationship between the Madden–Julian Oscillation and United States severe convective weather outbreaks

AbstractThe Madden–Julian Oscillation (MJO) is the leading intraseasonal mode of tropical variability and has been connected to influences in extratropical weather. Recently, the MJO has been gaining traction as a useful teleconnection for extending severe weather outbreak forecast lead time. However, the MJO's intraseasonal, planetary‐scale is far from the meso‐ or micro‐scale severe weather phenomena, making identifying links between the two more difficult. In our study, we use severe weather outbreaks comprising all three main hazards (wind, hail and tornadoes), which are typically linked to a synoptic‐scale system. Outbreaks are devised using kernel density estimation and were used to determine spatial shifts for each of the eight phases of the MJO in each month. A total of 15 months/phases indicated a significant spatial shift. In the spring and summer months (March–August), southerly shifts are more likely to occur in MJO phases 7–8 and 1–3, while northerly shifts are more likely in phases 4–6. In the cool season (September–February), only 3 month/phase groups indicated significant shifts, two shifting west and one north. Composite analysis showed the likely link between the MJO and these outbreak shifts was rooted in Rossby wave breaking and generation of a Rossby wave source induced by the barotropic vorticity advection from the MJO convection. These resulting wave breaks and wave trains led to changes in the storm track (amplitude and location) that resulted in a more favourable synoptic‐scale pattern for severe weather over the United States that deviated from the climatologically favourable region.

Seasonal prediction of UK mean and extreme winds

AbstractRecent studies have shown that seasonal forecasting systems have significant skill in predicting northern European winds and storms in winter, but other seasons have not been so extensively analysed. Given this fact, and coupled with requests from users of the Met Office 3‐month outlook for the United Kingdom (UK), we have investigated the skill in predicting seasonal (3‐month) mean UK wind speed and storms (extreme winds) with a one‐month lead‐time, throughout the year, using a large ensemble from the Met Office's seasonal prediction system, GloSea. We find that seasonal UK storms and mean wind speeds are well correlated, and therefore a single prediction of UK mean wind speed will give an indication of predicted storm counts. Skill for these predictions is highest in winter (December–February), related to predictability of the North Atlantic Oscillation. In contrast, summer (June–August) UK wind skill is not significant and furthermore appears to be negative. We find evidence, in both observations and model members, for a Rossby wave from the tropics influencing UK summer winds and forming a significant predictable component in the model ensemble mean. However, the model predictable signal appears to be out of phase with that observed leading to the negative correlation. Further investigation into summer Rossby wave generation and propagation is necessary to understand whether summer predictions could be improved.

Nonlinear Effects of the Stratospheric Quasi‐Biennial Oscillation and ENSO on the North Atlantic Winter Atmospheric Circulation

AbstractPrevious studies have shown that both the stratospheric Quasi‐Biennial Oscillation (QBO) and the El Niño‐South Oscillation (ENSO) can influence the North Atlantic winter circulation. Here we use reanalysis and model output data to show that the QBO and ENSO interact to produce a nonlinear effect on the North Atlantic winter circulation. Specifically, during El Niño winters, the QBO teleconnection mainly takes a subtropical pathway with changes in the North Pacific–Atlantic subtropical jet (STJ); during La Niña winters, the QBO connects with the troposphere predominantly through a polar pathway, that is, stratospheric polar vortex (SPV) changes. Further, the QBO‐induced STJ changes in El Niño lead to anomalous Rossby wave propagation toward the North Atlantic, and the QBO‐induced SPV during La Niña anomaly exerts a downward effect on the North Atlantic. Hence, the various interactions between ENSO and QBO teleconnections result in nonlinear, and even synergistic, impacts on the North Atlantic circulation.

Exceptional multi-year prediction skill of the Kuroshio Extension in the CESM high-resolution decadal prediction system

The Kuroshio Extension (KE) has far-reaching influences on climate as well as on local marine ecosystems. Thus, skillful multi-year to decadal prediction of the KE state and understanding sources of skill are valuable. Retrospective forecasts using the high-resolution Community Earth System Model (CESM) show exceptional skill in predicting KE variability up to lead year 4, substantially higher than the skill found in a similarly configured low-resolution CESM. The higher skill is attained because the high-resolution system can more realistically simulate the westward Rossby wave propagation of initialized ocean anomalies in the central North Pacific and their expression within the sharp KE front, and does not suffer from spurious variability near Japan present in the low-resolution CESM that interferes with the incoming wave propagation. These results argue for the use of high-resolution models for future studies that aim to predict changes in western boundary current systems and associated biological fields.

Impact of the Eurasian Wave Train on the Interannual Variability of Autumn Precipitation in the Central Region of China

AbstractAutumn precipitation in the central region of China (APCC) can considerably influence on agricultural production and human livelihood. With reanalysis data from 1979 to 2020, it revealed that APCC is significantly affected by the second mode of the meridional wind field at 200 hPa over the Eurasian continent on the interannual scale, which featured a quasi‐barotropic “+‐+” wave‐like pattern from Europe to China (EC pattern) in extratropical zone. When the EC pattern is in a positive phase, central China lags behind the downstream positive geopotential height (anticyclone) anomaly. In this context, the transport of moisture from the south and east is enhanced and the advancement of the southerly monsoon is blocked, providing sufficient water vapor to support the occurrence of APCC. Further diagnosis revealed that local ascending motion is strengthened owing to the warm advection induced by anomalous southerly and positive vorticity advection associated with the anomalous anticyclone over East Asia. Additionally, wave flux analysis and model simulation suggest that the EC pattern results from the propagation of stationary Rossby wave, which could be excited by dipole sea surface temperature anomalies in the North Atlantic. These discoveries will contribute to improving the predictability of APCC.

Intermodel Spread of Historical Indian Monsoon Rainfall Change in CMIP6: The Role of the Tropical Pacific Mean State

Abstract Robust projections of the Indian summer monsoon rainfall (ISMR) are critical as it provides 80% of the annual precipitation to more than 1 billion people who are very vulnerable to climate change. However, even over the historical period, state-of-the-art climate models have difficulties in reproducing the observed ISMR trends and are affected by a large intermodel spread, which questions the reliability of ISMR projections. Such uncertainty could come from internal variability or model biases. Here, we study the impact of the latter on the historical forced change of ISMR in 34 models from CMIP6. First, we show that models’ biases over India do not significantly impact how they simulate the historical change of ISMR. However, we do find statistically significant relationships between ISMR historical forced changes and remote rainfall and temperature biases within the tropics by using a maximum covariance analysis (MCA). Our results highlight the key role of tropical Pacific sea surface temperature (SST) mean state biases as an important source of intermodel spread in the ISMR change. The physical mechanisms underlying these statistical relationships between ISMR change and the intermodel spread of Pacific SST biases are finally explored. We found that models having El Niño/La Niña–like mean SST bias in the Pacific tend to exhibit El Niño/La Niña–like changes over the historical period, impacting ISMR through a shift in the Walker circulation and Rossby wave propagation across the Pacific.