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Tingting Zhu
and
Jin-Yi Yu

Abstract

Utilizing a 2200-yr CESM1 preindustrial simulation, this study examines the influence of property distinctions between single-year (SY) and multiyear (MY) La Niñas on their respective impacts on winter surface air temperatures across mid–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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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 time scale remain unclear. We present multiproxy 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 (EAM) proxies shows a gradual weakening trend from glacial inceptions into glacial maxima, inversely proportional to that of the 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 a reduction in North Atlantic SST and wind anomalies, subsequently propagating dampened millennial-scale variability into the midlatitude 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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Lingyu Zhou
,
Yan Xia
,
Fei Xie
,
Chen Zhou
, and
Chuanfeng Zhao

Abstract

The variability of stratospheric water vapor (SWV) plays a crucial role in stratospheric chemistry and Earth’s energy budget, strongly influenced by sea surface temperature (SST). In this study, we systematically investigate the response of lower-SWV (LSWV) to regional sea surface temperature changes using idealized SST patch experiments within a climate model. The results indicate that LSWV is most sensitive to tropical sea surface temperature, with the strongest response occurring in late autumn and early winter. Warming of the tropical Indian Ocean and western Pacific (WP) leads to stratospheric drying, while warming of the tropical Atlantic (TA) and eastern Pacific results in stratospheric moistening. The drying impact on LSWV due to warming in the western Pacific Ocean exceeds the wet effect in the eastern Pacific Ocean by approximately 60%. The variations in tropical SST influence LSWV by modulating the temperature at the tropical tropopause layer, especially over the Indo-Pacific warm pool through Matsuno–Gill responses. Furthermore, the response of LSWV to tropical SST changes exhibits nonnegligible nonlinearity, which indicates the importance of nonlinearity in determining the LSWV response to global surface warming.

Significance Statement

In this study, we explore how changes in the temperature of the ocean’s surface can affect the amount of water vapor in the stratosphere, a layer of Earth’s atmosphere. Understanding this relationship is important because water vapor in the stratosphere can influence both our climate and the chemistry of the atmosphere. Using a climate model, we found that water vapor in the lower stratosphere is especially responsive to temperature changes in tropical ocean regions. Specifically, when the Indian Ocean and the western Pacific get warmer, the stratosphere tends to get drier. On the other hand, warming in the Atlantic and eastern Pacific leads to more moisture in the stratosphere. The way these changes add up is complex and not simply a sum of individual parts, especially in tropical warm pool regions. Our findings have implications for how we understand and predict the impacts of climate change on stratospheric water vapor.

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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 Northern Annular Mode (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 (MLR). The results show that the polar cap temperature anomalies at 100 hPa (ΔT 100) modulate the magnitude of both 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 explain 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 are not present in the reanalysis.

Significance Statement

Sudden stratospheric warmings (SSWs) represent an important source of skill for forecasting winter weather on subseasonal-to-seasonal time scales. To what extent SSWs could be used to improve the prediction of surface weather depends on how well stratosphere–troposphere coupling associated with SSWs is represented in climate models. Therefore, we evaluate the representation of stratosphere–troposphere coupling associated with SSWs in 28 state-of-the-art climate models. The representation is found to diverge widely among climate models, and some are biased noticeably from the reanalysis. The models’ spread and bias are largely driven by five major factors and can be reduced substantially by making bias corrections to these factors.

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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 subtropical high (NASH) and the western Pacific subtropical high (WPSH) by 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 a dominant role in TP precipitation anomaly. When the WPSH strengthens westward approaching the TP, it can transport water vapor northward from the 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 the 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 prewinter El Niño–Southern Oscillation (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.

Significance Statement

Under the thermal and topographic effects of the Tibetan Plateau (TP), the subtropical high belt breaks over the TP, forming the western Pacific subtropical high (WPSH) on its east and the North African subtropical high (NASH) on its west. As a permanent anticyclonic circulation over the northwest Pacific, the formation, impact, and prediction of WPSH have been fully revealed. However, few studies have been focused on the NASH. In this study, we examine the joint impacts of the two subtropical highs on TP summer precipitation based on various reanalysis datasets and phase 6 of Coupled Model Intercomparison Project (CMIP6) historical simulations. The results show distinct precipitation features with several boundary combination types of the two subtropical highs. Besides, controlling water vapor transport and vertical motion is an important way for the two subtropical highs to affect TP precipitation in summer. Further study shows that the effect of the two subtropical highs on TP summer precipitation is generally modulated by the prewinter El Niño–Southern Oscillation (ENSO) and summer North Atlantic air–sea interaction. The results from CMIP6 support the conclusions drawn from reanalysis. These findings could help deepen our understanding of the joint influencing factors on TP summer precipitation.

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Arshdeep Singh
,
Sanjiv Kumar
,
Liang Chen
,
Montasir Maruf
,
Peter Lawrence
, and
Min-Hui Lo

Abstract

This study examines the effects of land-use (LU) change on regional climate, comparing historical and future scenarios using seven climate models from phase 6 of Coupled Model Intercomparison Project–Land Use Model Intercomparison Project experiments. LU changes are evaluated relative to land-use conditions during the preindustrial climate. Using the Community Earth System Model, version 2–Large Ensemble (CESM2-LE) experiment, we distinguish LU impacts from natural climate variability. We assess LU impact locally by comparing the impacts of climate change in neighboring areas with and without LU changes. Further, we conduct CESM2 experiments with and without LU changes to investigate LU-related climate processes. A multimodel analysis reveals a shift in LU-induced climate impacts, from cooling in the past to warming in the future climate across midlatitude regions. For instance, in North America, LU’s effect on air temperature changes from −0.24° ± 0.18°C historically to 0.62° ± 0.27°C in the future during the boreal summer. The CESM2-LE shows a decrease in LU-driven cooling from −0.92° ± 0.09°C in the past to −0.09° ± 0.09°C in future boreal summers in North America. A hydroclimatic perspective linking LU and climate feedback indicates LU changes causing soil moisture drying in the midlatitude regions. This contrasts with hydrology-only views showing wetter soil conditions due to LU changes. Furthermore, global warming causes widespread drying of soil moisture across various regions. Midlatitude regions shift from a historically wet regime to a water-limited transitional regime in the future climate. This results in reduced evapotranspiration, weakening LU-driven cooling in future climate projections. A strong linear relationship exists between soil moisture and evaporative fraction in midlatitudes.

Significance Statement

Land–atmosphere feedback involving soil moisture can increase local temperature and affect how land-use (LU) change impacts manifest in a warming climate. Conversely, an increased surface reflectance due to LU change can decrease local temperature in the midlatitude regions. Further, the LU change signal is often mixed with the internal climate variability, making it harder to separate. This study uses a novel technique to separate LU change impact from other climate forcing in the latest generation of climate and Earth system models. In the future climate, soil moisture drying lessens the cooling impact. A large-ensemble climate experiment analysis confirms a significant weakening of the LU-driven cooling impact in the midlatitudes. Both LU and climate changes exacerbate soil moisture drying, leading to a shift toward a water-limited system where hydrological feedback becomes more influential than radiative feedback.

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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 the free tropospheric mixing of energy.

Significance Statement

Regional concentrations of tropical precipitation, such as the South Asian monsoon, provide water to billions of people. These features have strong seasonal cycles that have typically been framed in terms of meridional shifts of precipitation following the sun’s movement. Here, we study zonal shifts of tropical precipitation over the seasonal cycle in observations and idealized simulations. We find that land–ocean contrasts trigger a monsoon with concentrated precipitation over Asia in northern summer and near-surface eastward winds carry this precipitation into the west Pacific during northern autumn in what we call a “monsoonal mode.” This concentrated precipitation remains over the west Pacific during northern winter, as further migration is impeded by the cold sea surface temperatures (SSTs) and easterly winds of the east Pacific.

Open access
Yinxing Liu
,
Zhiwei Zhang
,
Qingguo Yuan
, and
Wei Zhao

Abstract

Meridional heat transport induced by oceanic mesoscale eddies (EHT) plays a significant role in the heat budget of the Southern Ocean (SO) but the decadal trends in EHT and its associated mechanisms are still obscure. Here, this scientific issue is investigated by combining concurrent satellite observations and Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) reanalysis data over the 24 years between 1993 and 2016. The results reveal that the surface EHTs from both satellite and ECCO2 data consistently show decadal poleward increasing trends in the SO, particularly in the latitude band of the Antarctic Circumpolar Current (ACC). In terms of average in the ACC band, the ECCO2-derived EHT over the upper 1000 m has a linear trend of 1.1 × 10−2 PW decade−1 or 16% per decade compared with its time-mean value of 0.07 PW. Diagnostic analysis based on “mixing length” theory suggests that the decadal strengthening of eddy kinetic energy (EKE) is the dominant mechanism for the increase in EHT in the SO. By performing an energy budget analysis, we further find that the decadal increase in EKE is mainly caused by the strengthened baroclinic instability of large-scale circulation that converts more available potential energy to EKE. For the strengthened baroclinic instability in the SO, it is attributed to the increasing large-scale wind stress work on the large-scale circulation corresponding to the positive phase of the Southern Annular Mode between 1993 and 2016. The decadal trends in EHT identified here may help understand decadal variations of heat storage and sea ice extent in the SO.

Significance Statement

Oceanic mesoscale-eddy-induced meridional heat transport (EHT) is a key process of heat redistribution in the Southern Ocean (SO), but the decadal variations of EHT and the associated mechanisms remain obscure. Here, by analyzing satellite and reanalysis data between 1993 and 2016, we find that the poleward EHT has significant decadal increasing trends in the SO, particularly in the Antarctic Circumpolar Current latitude band. Further analysis suggests that the increasing EHT is mainly caused by enhanced eddy kinetic energy converted by the strengthened baroclinic instability of large-scale circulation, which is attributed to the strengthening winds modulated by the Southern Annular Mode. The above findings may improve our understanding of the decadal variations of heat storage and sea ice extent in the SO.

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Zihan Song
,
Shang-Ping Xie
,
Lixiao Xu
,
Xiao-Tong Zheng
,
Xiaopei Lin
, and
Yu-Fan Geng

Abstract

A deep winter mixed layer forms north of the Antarctic Circumpolar Current (ACC) in the Indo-Pacific sectors, while the mixed layer depth (MLD) is shallow in the Atlantic. Using observations and a global atmospheric model, this study investigates the contribution of surface buoyancy flux and background stratification to interbasin MLD variations. The surface heat flux is decomposed into broad-scale and frontal-scale variations. At the broad scale, the meandering ACC path is accompanied by a zonal wavenumber-1 structure of sea surface temperature (SST) with a warmer Pacific than the Atlantic; under the prevailing westerly winds, this temperature contrast results in larger surface heat loss facilitating deeper MLD in the Indo-Pacific sectors than in the Atlantic. In the Indian sector, the intense ACC fronts strengthen surface heat loss compared to the Pacific. The surface freshwater flux pattern largely follows that of evaporation and reinforces the heat flux pattern, especially in the southeast Pacific. A diagnostic relationship is introduced to highlight the role of ACC’s sloping isopycnals in setting a weak submixed layer stratification north of ACC. This weak stratification varies in magnitude across basins. In the Atlantic and western Indian Oceans where the ACC is at a low latitude (∼45°S), solar heating, intrusions of subtropical gyres, and energetic mesoscale eddies together maintain relatively strong stratification. In the southeast Pacific, in comparison, the ACC reaches the southernmost latitude (56°S), far away from the subtropical front. This creates weaker stratification, allowing deep mixed layers to form, aided by surface buoyancy loss.

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Matthew Patterson
,
Christopher O’Reilly
,
Jon Robson
, and
Tim Woollings

Abstract

The coupled nature of the ocean–atmosphere system frequently makes understanding the direction of causality difficult in ocean–atmosphere interactions. This study presents a method to decompose turbulent surface heat fluxes into a component which is directly forced by atmospheric circulation and a residual which is assumed to be primarily “ocean-forced.” This method is applied to the North Atlantic in a 500-yr preindustrial control run using the Met Office’s HadGEM3-GC3.1-MM model. The method shows that atmospheric circulation dominates interannual to decadal heat flux variability in the Labrador Sea, in contrast to the Gulf Stream where the ocean primarily drives the variability. An empirical orthogonal function analysis identifies several residual heat flux modes associated with variations in ocean circulation. The first of these modes is characterized by the ocean warming the atmosphere along the Gulf Stream and North Atlantic Current and the second by a dipole of cooling in the western subtropical North Atlantic and warming in the subpolar North Atlantic. Lead–lag regression analysis suggests that atmospheric circulation anomalies in prior years partly drive the ocean heat flux modes; however, there is no significant atmospheric circulation response in years following the peaks of the modes. Overall, the heat flux dynamical decomposition method provides a useful way to separate the effects of the ocean and atmosphere on heat flux and could be applied to other ocean basins and to either models or reanalysis datasets.

Significance Statement

Variability of the ocean affects atmospheric circulation and provides a source of long-term predictability for surface weather. However, the atmosphere also affects the ocean. This makes the separation of cause and effect in such atmosphere–ocean interactions difficult. This paper introduces a method to separate “turbulent heat fluxes,” the primary means by which the atmosphere and ocean influence one another, into a component driven by atmospheric variability and a component which is primarily related to ocean variability. The method is tested by applying it to a climate model simulation and is able to identify regions in which the exchange of heat between the ocean and atmosphere is dominated by atmospheric variability and regions which are dominated by the ocean.

Open access