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Martin Jucker
,
Chris Lucas
, and
Deepashree Dutta

Abstract

The amount of water vapor injected into the stratosphere after the eruption of Hunga Tonga-Hunga Ha’apai (HTHH) was unprecedented, and it is therefore unclear what it might mean for surface climate. We use chemistry climate model simulations to assess the long-term surface impacts of stratospheric water vapor (SWV) anomalies similar to those caused by HTHH, but neglect the relatively minor aerosol loading from the eruption. The simulations show that the SWV anomalies lead to strong and persistent warming of Northern Hemisphere landmasses in boreal winter, and austral winter cooling over Australia, years after eruption, demonstrating that large SWV forcing can have surface impacts on a decadal timescale. We also emphasize that the surface response to SWV anomalies is more complex than simple warming due to greenhouse forcing and is influenced by factors such as regional circulation patterns and cloud feedbacks. Further research is needed to fully understand the multi-year effects of SWV anomalies and their relationship with climate phenomena like El Nino Southern Oscillation.

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Zhen Liu
,
Changlin Chen
,
Guihua Wang
,
Shouwei Li
, and
Shouhua Liu

Abstract

Using a range of Detection and Attribution Model Intercomparison Project (DAMIP) simulations from phase 6 of Coupled Model Intercomparison Project (CMIP6), we study the response of dynamic sea level (DSL) to external anthropogenic climate forcing [greenhouse gases (GHGs), aerosols, and stratospheric ozone] with a focus on the differences over the twentieth and twenty-first century. In the second half of the twentieth century, the DSL nonuniformity in the Northern Hemisphere (NH) was relatively small due to a cancellation between the effects of increasing GHGs and aerosols. In contrast, the DSL signal in the Southern Hemisphere (SH) over this period was large because stratospheric ozone depletion reinforced the effects of increasing GHGs. In the twenty-first century, the DSL response has been intensified in the NH because the warming effects of diminishing aerosols have acted to reinforce the effects of increasing GHGs. Meanwhile, the distribution of SH DSL has also become uneven although stratospheric ozone recovery has partially offset the effects of rising GHGs. Using a global ocean circulation model, we decompose the changes in the twenty-first century DSL into distinct responses to surface forcings including sea surface temperature, salinity, and wind stress. Our results show that the dipole-like pattern of DSL in the North Pacific can be attributed largely to sea surface warming, while the dipole-like pattern in the North Atlantic is attributed to subpolar surface salinity freshening. The belted pattern of DSL changes in the Southern Ocean is induced by both surface warming and intensifying/poleward-shifting westerly winds.

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Yuqiong Zheng
,
Shangfeng Chen
,
Wen Chen
,
Renguang Wu
,
Zhibiao Wang
,
Bin Yu
,
Peng Hu
, and
Jinling Piao

Abstract

The spring Pacific meridional mode (PMM) is an important precursor of El Niño–Southern Oscillation (ENSO). However, recent studies reported that only about half of the spring PMM events were followed by ENSO events. This study examines the role of internal climate variability in modulating the impact of PMM on ENSO using 100-member ensemble simulations of the Max Planck Institute Earth System Model (MPI-ESM). The relationship between spring PMM and following winter ENSO shows a large spread among the 100 members. The variation of spring Aleutian low (AL) intensity is identified to be an important factor modulating the PMM–ENSO relation. The spring AL affects the PMM–ENSO relationship by modifying PMM-generated low-level zonal wind anomalies over the tropical western Pacific. The strengthening of the spring AL is accompanied by westerly wind anomalies over the midlatitude northwestern Pacific, leading to sea surface temperature (SST) cooling there via an enhancement of upward surface heat flux. This results in increased meridional SST gradient and leads to northerly wind anomalies over the subtropical northwestern Pacific, which turn to surface westerly wind anomalies after reaching the equatorial western Pacific due to the conservation of potential vorticity. Thus, the low-level westerly (easterly) wind anomalies over the tropical western Pacific associated with the positive (negative) spring PMM were strengthened (weakened), which further contributes to an enhanced (a weakened) PMM–ENSO relation. The mechanism for the modulation of the AL on the spring PMM–ENSO relationship is verified by a set of AGCM simulations. This study suggests that the condition of the spring AL should be considered when predicting ENSO on the basis of the PMM.

Significance Statement

Spring Pacific meridional mode (PMM) is a predictor of ENSO, but not all spring PMM events are accompanied by the occurrence of ENSO events. This study aims to explore the influence of internal climate variability on the relationship between spring PMM and following ENSO. It is revealed that the Aleutian low exerts a crucial modulation on the spring PMM–ENSO relationship. The underlying physical mechanisms for the impact of the Aleutian low on the relationship between spring PMM and ENSO are further examined. The results of this study have important implications for improving the prediction of ENSO.

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Shihua Liu
,
Sihua Huang
,
Yanke Tan
,
Zhiping Wen
,
Xiaodan Chen
, and
Yuanyuan Guo

Abstract

Previous studies have pointed out that the tropical easterly jet (TEJ) core varies longitudinally or latitudinally. Whether there is a linkage between longitudinal and latitudinal variations of the TEJ core remains unclear. We found that, on the interannual time scale, the northward (southward) movement of the TEJ core is typically accompanied by a westward (eastward) shift, characterized by a noticeable northwest–southeast (NW–SE) displacement. This NW–SE shift is most evident in July. A locational index is defined to capture this shift by the difference of area-averaged 200-hPa zonal winds between the western Arabian Sea (AS) and the southern tip of the Indian Peninsula. Observations and numerical simulations demonstrated that the northwestward-shifted (southeastward-shifted) TEJ core is caused by the joint and individual influences from the enhanced (suppressed) convective activities over the eastern AS and suppressed (enhanced) convective activities over the northern Bay of Bengal–South China Sea (BOB–SCS). Enhanced (suppressed) convective activities over the eastern AS can induce upper-tropospheric divergence (convergence) and anticyclonic (cyclonic) circulations to the northwest of the convection, leading to anomalous easterly (westerly) over the western AS. The suppressed (enhanced) convective activities over the northern BOB–SCS can further facilitate the northwestward (southeastward) shift through inducing anomalous cyclonic (anticyclonic) circulation centering at the BOB and the associated anomalous westerly (easterly) over the southern tip of the Indian Peninsula. The NW–SE shift of the TEJ core may have an implication for the change in the area of the intense rainfall in South Asia.

Significance Statement

The purpose of this study is to explore the linkage between the zonal and meridional variations of the core of the tropical easterly jet (TEJ) and its underlying mechanisms. We found that the TEJ core features a pronounced northwest–southeast shift and this phenomenon only occurs in July. Thus, we defined a locational index to depict this unique characteristic and reveal its relationship with the anomalous convective activities over the eastern Arabian Sea and the northern Bay of Bengal–South China Sea. These results may help improve our understanding of the characteristics and mechanisms of the variations of the TEJ core.

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Alain T. Tamoffo
,
Torsten Weber
,
William Cabos
,
Paul-Arthur Monerie
,
Kerry H. Cook
,
Dmitry V. Sein
,
Alessandro Dosio
,
Nana A. B. Klutse
,
Akintomide A. Akinsanola
, and
Daniela Jacob

Abstract

This study explores the added value (AV) of a regional earth system model (ESM) compared to an atmosphere-only regional climate model (RCM) in simulating West African Monsoon (WAM) rainfall. The primary goals are to foster discussions on the suitability of coupled RCMs for WAM projections and deepen our understanding of ocean-atmosphere coupling’s influence on the WAM system. The study employs results from dynamical downscaling of the ERA-Interim reanalysis and Max Plank Institute ESM (MPI-ESM-LR) by two RCMs, REMO (atmosphere-only) and ROM (REMO coupled with Max Planck Institute Ocean Model; MPIOM), at ∼25-km horizontal resolution. Results show that in regions distant from coupling domain boundaries such as West Africa (WA), constraint conditions from ERA-Interim are more beneficial than coupling effects. REMO, reliant on oceanic sea surface temperatures (SSTs) from observations and influenced by ERA-Interim, is biased under coupling conditions, although coupling offers potential advantages in representing heat and mass fluxes. Contrastingly, as intended, coupling improves SSTs-monsoon fluxes’ relationships under ESM-forced conditions. In this latter case, coupling features a dipole-like spatial structure of AV, improving precipitation over the Guinean coast but degrading precipitation over half of the Sahel. Our extensive examination of physical processes and mechanisms underpinning the WAM system supports the plausibility of AV. Additionally, we found that the monsoonal dynamics over the ocean respond to convective activity, with the Sahara-Sahel surface temperature gradient serving as the maintenance mechanism. While further efforts are needed to enhance the coupled RCM, we advocate for its use in the context of WAM rainfall forecasts and projections.

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Chen Liu
,
Lei Chen
, and
Stefan Liess

Abstract

The features of large-scale atmospheric circulations, storm tracks, and the mean flow-eddy interaction during winter Pacific-North American (PNA) events are investigated using National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data at subseasonal timescale from 1979 to 2022. The day-to-day variations of storm-track activity and stream function reveal that storm-track activity varies along the evolution of mean flow. To better understand storm track variability with the mean flow-eddy interaction, further exploration is made by analyzing local energy energetics. The changes in horizontal and vertical baroclinic energy conversions from background flow correspond to the storm track anomalies over the North Pacific, indicating that the anomalies in storm tracks are due to the anomalous mean flow associated with PNA patterns impacting energy conversion through mean flow-eddy interaction. Eddy feedback driven by vorticity and heat fluxes is analyzed. This provides a concrete illustration of how eddy feedback serves as a positive factor for the upper-tropospheric circulation anomalies associated with the PNA pattern.

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Suqiong Hu
,
Wenjun Zhang
,
Masahiro Watanabe
,
Feng Jiang
,
Fei-Fei Jin
, and
Han-Ching Chen

Abstract

El Niño–Southern Oscillation (ENSO), the dominant mode of interannual variability in the tropical Pacific, is well known to affect the extratropical climate via atmospheric teleconnections. Extratropical atmospheric variability may in turn influence the occurrence of ENSO events. The winter North Pacific Oscillation (NPO), as the secondary dominant mode of atmospheric variability over the North Pacific, has been recognized as a potential precursor for ENSO development. This study demonstrates that the preexisting winter NPO signal is primarily excited by sea surface temperature (SST) anomalies in the equatorial western–central Pacific. During ENSO years with a preceding winter NPO signal, which accounts for approximately 60% of ENSO events observed in 1979–2021, significant SST anomalies emerge in the equatorial western–central Pacific in the preceding autumn and winter. The concurrent presence of local convection anomalies can act as a catalyst for NPO-like atmospheric circulation anomalies. In contrast, during other ENSO years, significant SST anomalies are not observed in the equatorial western–central Pacific during the preceding winter, and correspondingly, the NPO signal is absent. Ensemble simulations using an atmospheric general circulation model driven by observed SST anomalies in the tropical western–central Pacific can well reproduce the interannual variability of observed NPO. Therefore, an alternative explanation for the observed NPO–ENSO relationship is that the preceding winter NPO is a companion to ENSO development, driven by the precursory SST signal in the equatorial western–central Pacific. Our results suggest that the lagged relationship between ENSO and the NPO involves a tropical–extratropical two-way coupling rather than a purely stochastic forcing of the extratropical atmosphere on ENSO.

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Zifan Su
,
Yongkun Xie
,
Jianping Huang
,
Guoxiong Wu
,
Yuzhi Liu
, and
Xiaodan Guan

Abstract

The Tibetan Plateau’s (TP) topography has long been recognized for its impact on climate. However, recognition of the influence of the TP on global weather variability remains insufficient. Therefore, this study used numerical simulations to demonstrate the influences of the TP and its mechanical and thermal forcing on global high-frequency temperature variability and eddy kinetic energy (EKE). Despite local influences, the TP influenced the high-frequency temperature variability in far-flung regions like North America. In summer, the TP’s influence on high-frequency temperature variability showed dipole patterns in Eurasia and tripole patterns in North America, which were mainly induced by TP thermal forcing. In winter, the TP’s influence on high-frequency temperature variability was dominated by mechanical forcing and was less significant for remote regions than in summer. Mechanical forcing dominated EKE in both summer and winter. Furthermore, the horizontal temperature advection dominated the TP’s influence on high-frequency temperature variability for both its thermal effect in summer and its mechanical effect in winter, wherein EKE, as the dynamical factor, determined the horizontal temperature advection rather than the thermodynamical factor, the temperature gradient. Our findings suggest that the TP, via its mechanical and thermal forcing, may have an impact on temperature-related weather extremes around the world.

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Hairu Ding
,
Li Dong
,
Kaijun Liu
,
Ting Lin
,
Zhiang Xie
,
Bo Zhang
, and
Xiaoxue Wang

Abstract

As the only remaining ice sheet in the Northern Hemisphere, the Greenland ice sheet (GrIS) plays a crucial role in influencing atmospheric circulations, particularly with its rapid melting under global warming. In this paper, the influences of GrIS topography and surface thermal conditions are investigated by a series of aquaplanet experiments. The results show that the GrIS topography induces stationary waves and favors more blocking events through the generation of negative potential vorticity (PV) anomalies, while it tends to suppress local storm activities through the induced stationary waves. The surface cooling center of the GrIS is found to strengthen the jet streams by enhancing the meridional temperature gradient and thermal wind, while it causes the PV and static stability to increase during near-Greenland blocking days, thereby disfavoring blocking onset. Altogether, the topography and surface thermal effects of GrIS appear to compete with each other so that the net effect would determine the final response. Nevertheless, nonlinearity is found in both GrIS-topography alone and GrIS-surface temperature alone experiments, where nonlinear responses of atmospheric circulation are detected when the GrIS topography height or surface temperature exceeds their critical values, respectively. Hence, through this study, the response of the blocking in the vicinity of Greenland to the combined effects of topography and surface thermal conditions may shed light on comprehending the underlying mechanism of blocking aleration in a changing climate.

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Qinlu Gu
,
Renguang Wu
, and
Sang-Wook Yeh

Abstract

El Niño–Southern Oscillation (ENSO) exhibits nonlinearity in its amplitude and impacts. This study investigates the dependence of summertime synoptic-scale disturbance (SSD) intensity over the tropical western North Pacific (TWNP) on the ENSO amplitude. A tendency of nonlinearity exists in the observed response of the TWNP SSD intensity to the amplitude of tropical central-eastern Pacific (CEP) sea surface temperature (SST) anomalies in boreal summer. Numerical experiments are conducted with an atmospheric general circulation model with linearly varying tropical CEP SST anomalies imposed to illustrate the nonlinearity exclusively induced by changes in the ENSO amplitude. A linear increase in the amplitude of El Niño–like SST anomalies results in a nonlinear enhancement of SSD intensity over the TWNP, manifested as the increase in SSD intensity at a rate larger than expected by linear response with an eastward shift. This is attributed to the nonlinear intensification of anomalous ascent over the TWNP induced by tropical convection response to positive tropical CEP SST anomalies and the nonlinear effect of anomalous convection on the synoptic-scale activity. In contrast, as La Niña–like SST anomalies increase linearly, the SSD intensity over the TWNP decreases at a rate slower than expected from a linear response and even reaches saturation with little longitudinal shift. Due to the thermodynamic control on the occurrence of deep convection in the tropics, enhanced negative SST anomalies do not induce additional changes in anomalous descent over the tropical CEP. Thus, the TWNP SSD intensity no longer decreases with further increase in tropical CEP cold SST anomalies.

Significance Statement

Synoptic-scale disturbances (SSDs) over the tropical western North Pacific (TWNP) play an important role in weather and climate over East and Southeast Asia. Impacts of those SSDs are contingent on their intensity. El Niño–Southern Oscillation (ENSO) has substantial impacts on weather and climate worldwide, including the SSDs over the TWNP. ENSO displays a diversity in amplitude, spatial pattern, and temporal evolution. The present study investigates the response of the TWNP SSD intensity to varying ENSO amplitude during boreal summer and reveals a distinctive nonlinear response of the TWNP SSD intensity to the amplitude of El Niño and La Niña, which has important implication for understanding the impacts of ENSO on climate over the TWNP and Asia.

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