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Arthur Coquereau
,
Florian Sévellec
,
Thierry Huck
,
Joël J.-M. Hirschi
, and
Antoine Hochet

Abstract

As well as having an impact on the background state of the climate, global warming due to human activities could affect its natural oscillations and internal variability. In this study, we use four initial-condition ensembles from the CMIP6 framework to investigate the potential evolution of internal climate variability under different warming pathways for the 21st century. Our results suggest significant changes in natural climate variability, and point to two distinct regimes driving these changes. First, a decrease of internal variability of surface air temperature at high latitudes and all frequencies, associated with a poleward shift and the gradual disappearance of sea-ice edges, which we show to be an important component of internal variability. Second, an intensification of the interannual variability of surface air temperature and precipitation at low latitudes, which appears to be associated with the El Niño–Southern Oscillation (ENSO). This second regime is particularly alarming because it may contribute to making the climate more unstable and less predictable, with a significant impact on human societies and ecosystems.

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Verónica Martín-Gómez
,
Belén Rodríguez-Fonseca
,
Irene Polo
, and
Marta Martín-Rey

Abstract

In the last decades, many efforts have been made to understand how different tropical oceanic basins are able to impact El Niño Southern Oscillation (ENSO). However, the collective connectivity among the tropical oceans and their associated influence on ENSO is less understood. Using a complex network methodology, the degree of collective connectivity among the tropical oceans is analyzed focusing on the detection of periods when the tropical basins collectively interact and Atlantic and Indian basins influence the equatorial Pacific sea surface temperatures (SST). The background state for the periods of strong collective connectivity is also investigated.

Our results show a marked multidecadal variability in the tropical interbasin connection, with periods of stronger and weaker collective connectivity. These changes seem to be modulated by changes in the North Atlantic ocean mean state a decade in advance. In particular, strong connectivity occurs in periods with colder than average tropical north Atlantic surface ocean. Associated with this cooling an anomalous convergence of the vertical integral of total energy flux (VIEF) takes place over the tropical north-west Atlantic, associated with anomalous divergence of VIEF over the equatorial eastern Pacific. In turn, an anomalous zonal surface pressure gradient over the tropical Pacific weakens the trades over the western equatorial Pacific. Consequently, a shallower thermocline emerges over the western equatorial Pacific, which can enhance thermocline feedbacks, the triggering of ENSO events, and therefore, ENSO variability. By construction, our results put forward opposite conditions for periods of weak tropical basins connectivity. These results have important implications for seasonal to decadal predictions.

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Tingting Zhu
and
Jin-Yi Yu

Abstract

Utilizing a 2200-year CESM1 pre-industrial simulation, this study examines the influence of property distinctions between single-year (SY) and multi-year (MY) La Niñas on their respective impacts on winter surface air temperatures across mid-to-high latitude continents in the model, focusing on specific teleconnection mechanisms. Distinct impacts were identified in four continent sectors: North America, Europe, Western Siberia (W-Siberia), and Eastern Siberia (E-Siberia). The typical impacts of simulated SY La Niña events are featured with anomalous warming over Europe and W&E-Siberia and anomalous cooling over North America. Simulated MY La Niña events reduce the typical anomalous cooling over North America and the typical anomalous warming over W&E-Siberia but intensify the typical anomalous warming over Europe. The distinct impacts of simulated MY La Niñas are more prominent during their first winter than during the second winter, except over W-Siberia, where the distinct impact is more pronounced during the second winter. These overall distinct impacts in the CESM1 simulation can be attributed to the varying sensitivities of these continent sectors to the differences between MY and SY La Niñas in their intensity, location, and induced sea surface temperature anomalies in the Atlantic Ocean. These property differences were linked to the distinct climate impacts through the Pacific North America, North Atlantic Oscillation, Indian Ocean-induced wave train, and Tropical North Atlantic-induced wave train mechanisms. The modeling results are then validated against observations from 1900 to 2022 to identify disparities in the CESM1 simulation.

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Fang Zhou
,
Siseho Christonette Siseho
,
Minghong Liu
,
Dapeng Zhang
, and
Haoxin Zhang

Abstract

Focusing on summer precipitation over the Tibetan Plateau (TP), this study mainly investigates the joint impacts of the North African and the Western Pacific subtropical highs (i.e., NASH and WPSH) through examining circulation and moisture anomalies. Results show that there are several boundary combination types of the two subtropical highs. The anomalous vertical motion with sufficient moisture transport under different boundary types plays the dominant role in TP precipitation anomaly. When the WPSH strengthens westward approaching to the TP, it can transport water vapor northward from Northwest Pacific and North Indian Oceans to the south edge of the TP and induce ascending motion over the southeastern TP, contributing to more precipitation there. When the NASH enhances and extends eastward, it can transport water vapor eastward from North Atlantic Ocean to the southwest eastern TP and give rise to ascending motion there, inducing positive precipitation anomaly over the southwest eastern TP. When the two subtropical highs simultaneously intensify and extend to the TP, water vapor can be transported to the TP widely from the North Atlantic Ocean, the North Indian Ocean and the northwest Pacific Ocean with the strengthening of the westerly, resulting in the location of the ascending motion and rain belt shifting obviously northward. Further analyses indicate that the pre-winter ENSO and summer North Atlantic air–sea interaction are two indispensable possible modulation factors for the joint impact of the two subtropical highs on TP precipitation.

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F. Guo
,
S. C. Clemens
,
X. Du
,
X. Liu
,
Y. Liu
,
J. Sun
,
H. Fan
,
T. Wang
, and
Y. Sun

Abstract

Millennial-scale climate change is thought to be synchronous throughout the northern hemisphere and has been demonstrated to be strongly modulated by longer-term glacial-interglacial and orbital scale processes. However, processes that modulate the magnitude of millennial-scale variability (MMV) at the glacial-interglacial timescale remain unclear. We present multi-proxy evidence showing out-of-phase relationships between the MMV of East Asian and North Atlantic climate proxies at the eccentricity band. During most late Pleistocene glacial intervals, the MMV in North Atlantic SST and East Asian Monsoon proxies show a gradual weakening trend from glacial inceptions into glacial maxima, inversely proportional to that of North Atlantic ice rafted detritus record. The inverse glacial-age trends apply to both summer- and winter-monsoon proxies across the loess, speleothem, and marine archives, indicating fundamental linkages between MMV records of the North Atlantic and East Asia. We infer that intensified glacial-age iceberg discharge is accompanied by weakened Atlantic meridional overturning circulation via changes in freshwater input and water-column stability, leading to reduction in North Atlantic SST and wind anomalies, subsequently propagating dampened millennial-scale variability into the mid-latitude East Asian Monsoon region via the westerlies. Our results indicate that the impact of North Atlantic iceberg discharge and the associated variability in water-column stability at the millennial-scale is a primary influence on hydroclimate instability in East Asia.

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Ying Dai
,
Peter Hitchcock
, and
Isla R. Simpson

Abstract

This study evaluates the representation of the composite-mean surface response to Sudden Stratospheric Warmings (SSWs) in 28 CMIP6 models. Most models can reproduce the magnitude of the SLP response over the Arctic, although the simulated Arctic SLP response varies from model to model. Regarding the structure of the SLP response, most models exhibit a basin-symmetric negative NAM-like response with a cyclonic Pacific SLP response, whereas the reanalysis shows a highly basin-asymmetric negative NAO-like response without a robust Pacific center.

We then explore the drivers of these model biases and spread by applying a multiple linear regression. The results show that the polar-cap temperature anomalies at 100 hPa (ΔT 100) modulate both the magnitude of the Arctic SLP response and the cyclonic Pacific SLP response. Apart from ΔT 100, the intensity and latitudinal location of the climatological eddy-driven jet in the troposphere also affect the magnitude of the Arctic SLP response. The compensation of model biases in these two tropospheric metrics and the good model representation of ΔT 100 explains the good agreement between the ensemble mean and the reanalysis on the magnitude of the Arctic SLP response, as indicated by the fact that the ensemble mean lies well within the reanalysis uncertainty range and that the reanalysis mean sits well within the model distribution. The Niño-3.4 SST anomalies and North Pacific SST dipole anomalies together with ΔT 100 modulate the cyclonic Pacific SLP response. In this case, biases in both oceanic drivers work in the same direction and lead to the cyclonic Pacific SLP response in models that is not present in the reanalysis.

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P. J. Tuckman
,
Jane Smyth
,
Nicholas J. Lutsko
, and
John Marshall

Abstract

The Intertropical Convergence Zone (ITCZ) is associated with a zonal band of strong precipitation that migrates meridionally over the seasonal cycle. Tropical precipitation also migrates zonally; such as from the South Asian monsoon in Northern Hemisphere summer (JJA) to the precipitation maximum of the West Pacific in Northern Hemisphere winter (DJF). To explore this zonal movement in the Indo-Pacific sector, we analyze the seasonal cycle of tropical precipitation using a 2D energetic framework and study idealized atmosphere-ocean simulations with and without ocean dynamics. In the observed seasonal cycle, an atmospheric energy and precipitation anomaly forms over South Asia in northern spring and summer due to heating over land. It is then advected eastward into the West Pacific in northern autumn and remains there due to interactions with the Pacific cold tongue and equatorial easterlies. We interpret this phenomenon as a “monsoonal mode,” a zonally propagating moist energy anomaly of continental and seasonal scale. To understand the behavior of the monsoonal mode, we develop and explore an analytical model in which the monsoonal mode is advected by low-level winds, is sustained by interaction with the ocean, and decays due to free tropospheric mixing of energy.

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Xianghui Fang
,
Henk Dijkstra
,
Claudia Wieners
, and
Francesco Guardamagna

Abstract

As the strongest year-to-year fluctuation of the global climate system, El Niño-Southern Oscillation (ENSO) exhibits spatial-temporal diversity, which challenges the classical ENSO theories that mainly focus on the canonical eastern Pacific (EP) type. Besides, the complicated interplay between the interannual anomaly fields and the decadally varying mean state is another difficulty in current ENSO theory. To better account for these issues, the nonlinear two-region recharge paradigm model is extended to a three-region full-field conceptual model to capture the physics in the western Pacific (WP), central Pacific (CP) and EP regions. The results show that the extended conceptual model displays a rich dynamical behavior as parameters setting the efficiencies of upwelling and zonal advection are varied. The model can not only generate El Niño bursting behavior, but also simulate the statistical asymmetries between the two types of El Niño and the warm and cold phases of ENSO. Finally, since both the anomaly fields and mean states are simulated by the model, it provides a simple tool to investigate their interactions. The strengthening of the upwelling efficiency, which can be seen as an analogy to a cooling thermocline associated with the oceanic tunnel to the mid-latitudes, will increase the zonal gradient of the mean state temperature between the WP and EP, i.e., resembling a negative Pacific Decadal Oscillation (PDO) pattern along the equatorial Pacific. The influence of the zonal advection efficiency is quite the opposite, i.e., its strengthening will reduce the zonal gradient of the mean state temperature along the equatorial Pacific.

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Valentina Ortiz-Guzmán
,
Martin Jucker
, and
Steven Sherwood

Abstract

The Southern Hemisphere climate and weather are affected by several modes of variability and climate phenomena across different time and spatial scales. An additional key component of the atmosphere dynamics that greatly influences weather is quasi-stationary Rossby waves, which attract particular interest as they are often associated with synoptic scale extreme events. In the Southern Hemisphere extratropical circulation, the most prominent quasi-stationary Rossby wave pattern is the zonal wavenumber 3 (ZW3), which has been shown to have impacts on meridional heat and momentum transport in mid to high-latitudes, and on Antarctic sea-ice extent. However, little is known about its impacts outside of polar regions. In this work, we use ERA5 reanalysis data on monthly time scales to explore the influence of phase and amplitude of ZW3 on temperature and precipitation across the Southern Hemisphere midlatitudes. Our results show significant impact in various regions for all seasons. One of the most substantial effects is observed in precipitation over southeastern Brazil during austral summer, where different phases of the ZW3 force opposite anomalies. When using ZW3 phase and amplitude as prior information, the probability of occurrence of precipitation extremes in this region increases up to three times. Additionally, we find that this ZW3 weather signature is largely independent of the zonally symmetric Southern Annular Mode (SAM); neither does it seem to be linked to El Niño Southern Oscillation (ENSO) or Indian Ocean Dipole (IOD) signal.

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Shaohua Chen
,
Haikun Zhao
,
Philip J. Klotzbach
,
Jian Cao
,
Jia Liang
,
Weican Zhou
, and
Liguang Wu

Abstract

On inter-annual time scales, there is significant meridional migration of the boreal summer (May–October) synoptic-scale wave train (SSW) relative to the summer monsoon trough line over the western North Pacific (WNP) during 1979–2021. The associated plausible physical reasons for the SSW meridional migration are investigated by comparing analyses between two distinct groups: atypical SSW years where SSWs tend to prevail northward of the summer monsoon trough line and typical SSW years where SSWs largely occur along the summer monsoon trough line. During typical SSW years, SSWs originate primarily from equatorial mixed Rossby-gravity (MRG) waves and then develop into off-equatorial tropical depression (TD) waves in the lower troposphere of the monsoon region. During atypical SSW years, SSWs appear to be sourced from upper-level easterlies, propagating downward to the lower troposphere in the monsoon region, with a prevailing TD wave structure.

A budget analysis of barotropic eddy kinetic energy suggests that interannual meridional SSW migration is closely related to changes in the vorticity distribution along the summer monsoon trough over the WNP, especially the western part of the summer monsoon trough. These changes cause low-frequency zonal convergence and shear differences, changing barotropic conversion around the monsoon trough and modulating interannual SSW meridional movement. In response to these changes, there are corresponding differences in SSW sources: a predominate MRG-TD wave pattern in typical SSW years and a predominate TD wave pattern in atypical SSW years. These results improve our understanding of the interannual variability of the large-scale circulation and tropical cyclones.

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