The Role of the Dry Static Stability for the Recent Change in the Pacific Walker Circulation

Abstract There is an uncertainty in how the Pacific Walker circulation (PWC) will change in response to increased greenhouse gas (GHG) warming. On average, climate models predict that the PWC will weaken. Observational evidence is mixed, with some evidence supporting the models while others do not. In this study, insight into the PWC trend is provided by examining the tropical dry static stability, a quantity that is inversely proportional to the strength of the PWC. For the 1979–2012 period, the static stability increased markedly in all phase 5 of the Coupled Model Intercomparison Project (CMIP5) models, far more so than in the satellite and global reanalysis data, which show a strengthening of the PWC. The stabilization is greater for a subset of models that simulate a significant weakening of the PWC. With the observed sea surface temperature as the lower boundary condition, over the western tropical Pacific, atmospheric models that belong to the weakening-PWC-CMIP5 group produce greater stabilization than those that belong to the strengthening-PWC-CMIP5 group. Compared with the latter group, the former group of atmospheric models simulates weaker trade winds over the western and central tropical Pacific and, consistent with the Bjerknes mechanism, the corresponding CMIP5 models produce a weaker west–east gradient in tropical SST. Given that the models’ convective parameterizations overstabilize the atmosphere compared with an explicit convection, the findings here suggest that the models’ representations of tropical convection and stability contribute to the models’ tendency to simulate a weakening of the PWC and an El Niño–like SST.

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Surfex : une plateforme pour simuler les flux des surfaces océaniques et continentales

La Météorologie ◽

10.37053/lameteorologie-2020-0020 ◽

2020 ◽

pp. 082

Author(s):

Patrick Le Moigne ◽

Marie Minvielle

Keyword(s):

Weather Prediction ◽

Lower Boundary ◽

Atmospheric Model ◽

Horizontal Resolution ◽

Atmospheric Models ◽

Physical Processes ◽

Surface Type ◽

Lower Boundary Condition ◽

Continuous Increase ◽

The Earth

Dans les modèles atmosphériques de prévision numérique du temps et de climat, la surface constitue la condition à la limite inférieure. La grande variété des paysages présents sur l'ensemble du globe et les spécificités de chacun des types de surface rendent complexe sa description dans les modèles. Par ailleurs, l'augmentation constante de la résolution horizontale des modèles nécessite une description fine des surfaces ainsi que des processus mis en jeu lorsqu'atmosphère et surface interagissent. Cet article décrit la plateforme de modélisation Surfex, en particulier comment la grande variété des types de surface est prise en compte, quelles sont les principales paramétrisations physiques et enfin comment est réalisé le couplage à un modèle atmosphérique. Surface is the lower boundary condition of numerical weather prediction and climate atmospheric models. Its representation in models is complicated by the large diversity in landscapes over the Earth, and the specificities of each surface type. In addition, the continuous increase in model horizontal resolution requires an accurate description of surfaces as well as the processes involved when surface and atmosphere are coupled. This article describes the Surfex modelling platform, particularly how the large amount of surface types are accounted for, which are the main physical processes represented and how the coupling to an atmospheric model can be achieved.

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Understanding the Anthropogenically Forced Change of Equatorial Pacific Trade Winds in Coupled Climate Models*

Journal of Climate ◽

10.1175/jcli-d-14-00115.1 ◽

2014 ◽

Vol 27 (22) ◽

pp. 8510-8526 ◽

Cited By ~ 10

Author(s):

Baoqiang Xiang ◽

Bin Wang ◽

Juan Li ◽

Ming Zhao ◽

June-Yi Lee

Keyword(s):

Surface Temperature ◽

Sea Surface Temperature ◽

Climate Models ◽

Walker Circulation ◽

Tropical Pacific ◽

Equatorial Pacific ◽

Sea Surface ◽

Trade Wind ◽

Trade Winds ◽

The Walker Circulation

Abstract Understanding the change of equatorial Pacific trade winds is pivotal for understanding the global mean temperature change and the El Niño–Southern Oscillation (ENSO) property change. The weakening of the Walker circulation due to anthropogenic greenhouse gas (GHG) forcing was suggested as one of the most robust phenomena in current climate models by examining zonal sea level pressure gradient over the tropical Pacific. This study explores another component of the Walker circulation change focusing on equatorial Pacific trade wind change. Model sensitivity experiments demonstrate that the direct/fast response due to GHG forcing is to increase the trade winds, especially over the equatorial central-western Pacific (ECWP) (5°S–5°N, 140°E–150°W), while the indirect/slow response associated with sea surface temperature (SST) warming weakens the trade winds. Further, analysis of the results from 19 models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) and the Parallel Ocean Program (POP)–Ocean Atmosphere Sea Ice Soil (OASIS)–ECHAM model (POEM) shows that the projected weakening of the trades is robust only in the equatorial eastern Pacific (EEP) ( 5°S–5°N, 150°–80°W), but highly uncertain over the ECWP with 9 out of 19 CMIP5 models producing intensified trades. The prominent and robust weakening of EEP trades is suggested to be mainly driven by a top-down mechanism: the mean vertical advection of more upper-tropospheric warming downward to generate a cyclonic circulation anomaly in the southeast tropical Pacific. In the ECWP, the large intermodel spread is primarily linked to model diversity in simulating the relative warming of the equatorial Pacific versus the tropical mean sea surface temperature. The possible root causes of the uncertainty for the trade wind change are also discussed.

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Assessing the Climate Impacts of the Observed Atlantic Multidecadal Variability Using the GFDL CM2.1 and NCAR CESM1 Global Coupled Models

Journal of Climate ◽

10.1175/jcli-d-16-0127.1 ◽

2017 ◽

Vol 30 (8) ◽

pp. 2785-2810 ◽

Cited By ~ 78

Author(s):

Yohan Ruprich-Robert ◽

Rym Msadek ◽

Frederic Castruccio ◽

Stephen Yeager ◽

Tom Delworth ◽

...

Keyword(s):

North Atlantic ◽

Climate Models ◽

Walker Circulation ◽

Climate Impacts ◽

Boreal Summer ◽

Boreal Winter ◽

Atlantic Multidecadal Variability ◽

Coupled Models ◽

Multidecadal Variability ◽

The Pacific

The climate impacts of the observed Atlantic multidecadal variability (AMV) are investigated using the GFDL CM2.1 and the NCAR CESM1 coupled climate models. The model North Atlantic sea surface temperatures are restored to fixed anomalies corresponding to an estimate of the internally driven component of the observed AMV. Both models show that during boreal summer the AMV alters the Walker circulation and generates precipitation anomalies over the whole tropical belt. A warm phase of the AMV yields reduced precipitation over the western United States, drier conditions over the Mediterranean basin, and wetter conditions over northern Europe. During boreal winter, the AMV modulates by a factor of about 2 the frequency of occurrence of El Niño and La Niña events. This response is associated with anomalies over the Pacific that project onto the interdecadal Pacific oscillation pattern (i.e., Pacific decadal oscillation–like anomalies in the Northern Hemisphere and a symmetrical pattern in the Southern Hemisphere). This winter response is a lagged adjustment of the Pacific Ocean to the AMV forcing in summer. Most of the simulated global-scale impacts are driven by the tropical part of the AMV, except for the winter North Atlantic Oscillation–like response over the North Atlantic–European region, which is driven by both the subpolar and tropical parts of the AMV. The teleconnections between the Pacific and Atlantic basins alter the direct North Atlantic local response to the AMV, which highlights the importance of using a global coupled framework to investigate the climate impacts of the AMV. The similarity of the two model responses gives confidence that impacts described in this paper are robust.

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The representation of Northern Hemisphere blocking in current global climate models

10.5194/wcd-2019-19 ◽

2020 ◽

Cited By ~ 2

Author(s):

Reinhard Schiemann ◽

Panos Athanasiadis ◽

David Barriopedro ◽

Francisco Doblas-Reyes ◽

Katja Lohmann ◽

...

Keyword(s):

Climate Models ◽

Climate Model ◽

Global Climate ◽

Model Development ◽

Reanalysis Data ◽

Global Climate Models ◽

Cmip5 Models ◽

The Pacific ◽

Intercomparison Project ◽

Blocking Index

Abstract. Global Climate Models (GCMs) are known to suffer from biases in the simulation of atmospheric blocking, and this study provides an assessment of how blocking is represented by the latest generation of GCMs. It is evaluated (i) how historical CMIP6 (Climate Model Intercomparison Project Phase 6) simulations perform compared to CMIP5 simulations, and (ii) how horizontal model resolution affects the simulation of blocking in the CMIP6-HighResMIP (PRIMAVERA) model ensemble, which is designed to address this type of question. Two blocking indices are used to evaluate the simulated mean blocking frequency and blocking persistence for the Euro-Atlantic and Pacific regions in winter and summer against the corresponding estimates from atmospheric reanalysis data. There is robust evidence that CMIP6 models simulate blocking frequency and persistence better than CMIP5 models in the Atlantic and Pacific and in winter and summer. This improvement is sizeable so that, for example, winter blocking frequency in the median CMIP5 model in a large Euro-Atlantic domain is underestimated by 32 % using the absolute geopotential height (AGP) blocking index, whereas the same number is 19 % for the median CMIP6 model. As for the sensitivity of simulated blocking to resolution, it is found that the resolution increase, from typically 100 km to 20 km grid spacing, in the PRIMAVERA models, which are not re-tuned at the higher resolutions, benefits the mean blocking frequency in the Atlantic in winter and summer, and in the Pacific in summer. Simulated blocking persistence, however, is not seen to improve with resolution. Our results are consistent with previous studies suggesting that resolution is one of a number of interacting factors necessary for an adequate simulation of blocking in GCMs. The improvements reported in this study hold promise for further reductions in blocking biases as model development continues.

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Changes in the Eastward Movement Speed of the Madden–Julian Oscillation with Fluctuation in the Walker Circulation

Journal of Climate ◽

10.1175/jcli-d-21-0269.1 ◽

2021 ◽

pp. 1-50

Author(s):

Tamaki Suematsu ◽

Hiroaki Miura

Keyword(s):

Indian Ocean ◽

Large Scale ◽

Lower Boundary ◽

Low Frequency ◽

Walker Circulation ◽

Movement Speed ◽

Madden Julian Oscillation ◽

Lower Boundary Condition ◽

The Indian Ocean ◽

The Walker Circulation

AbstractThe eastward movement of a convectively active region is a distinguishing characteristic of the Madden–Julian oscillation (MJO). However, knowledge about the mechanisms that determine the eastward movement speed remains limited. This study investigates how the background environment modulates the speed of the boreal winter MJO and describes an intrinsic relationship between the MJO and background atmospheric circulation. We calculated the speed of the MJO events from the daily tracking of the locations of the minimum values of the outgoing longwave radiation anomaly in the time–longitude space. These speeds were then used to analyze systematic differences in the sea surface temperature (SST) distribution associated with the MJO speed. The analysis revealed a deceleration of the MJO under low-frequency (> 90 days) SST distributions that increased toward the western Pacific from both the Indian Ocean and the eastern Pacific. In contrast, the dependency on SST variability in intraseasonal frequencies (20–90 days) was small. Subsequently, the relationship between the MJO speed and background circulation, which is largely determined by the lower boundary condition set by the low-frequency SST distribution, was analyzed. The analysis counterintuitively revealed that the MJO tends to decelerate when the large-scale zonal circulation with low-level westerlies and upper-level easterlies from the Indian Ocean to the Maritime Continents is strong. The results suggest a novel view that the MJO is an integral component of the Walker circulation and that its eastward movement is modulated by the state of the large-scale flow of the Walker circulation.

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Local Energetic Constraints on Walker Circulation Strength

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-16-0219.1 ◽

2017 ◽

Vol 74 (6) ◽

pp. 1907-1922 ◽

Cited By ~ 6

Author(s):

Robert C. Wills ◽

Xavier J. Levine ◽

Tapio Schneider

Keyword(s):

Global Warming ◽

Energy Input ◽

Climate Models ◽

Walker Circulation ◽

Static Stability ◽

Net Energy ◽

Open Questions ◽

Wide Range ◽

Gross Moist Stability ◽

The Walker Circulation

Abstract The weakening of tropical overturning circulations is a robust response to global warming in climate models and observations. However, there remain open questions on the causes of this change and the extent to which this weakening affects individual circulation features such as the Walker circulation. The study presents idealized GCM simulations of a Walker circulation forced by prescribed ocean heat flux convergence in a slab ocean, where the longwave opacity of the atmosphere is varied to simulate a wide range of climates. The weakening of the Walker circulation with warming results from an increase in gross moist stability (GMS), a measure of the tropospheric moist static energy (MSE) stratification, which provides an effective static stability for tropical circulations. Baroclinic mode theory is used to determine changes in GMS in terms of the tropical-mean profiles of temperature and MSE. The GMS increases with warming, owing primarily to the rise in tropopause height, decreasing the sensitivity of the Walker circulation to zonally anomalous net energy input. In the absence of large changes in net energy input, this results in a rapid weakening of the Walker circulation with global warming.

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Robust Strengthening and Westward Shift of the Tropical Pacific Walker Circulation during 1979–2012: A Comparison of 7 Sets of Reanalysis Data and 26 CMIP5 Models

Journal of Climate ◽

10.1175/jcli-d-15-0398.1 ◽

2016 ◽

Vol 29 (9) ◽

pp. 3097-3118 ◽

Cited By ~ 37

Author(s):

Shuangmei Ma ◽

Tianjun Zhou

Keyword(s):

Vertical Section ◽

Coupled Model ◽

Walker Circulation ◽

Reanalysis Data ◽

Tropical Pacific ◽

La Niña ◽

Cmip5 Models ◽

La Nina ◽

Independent Observations ◽

The Tropical Pacific

Abstract In this study, the zonal mass streamfunction Ψ, which depicts intuitively the tropical Pacific Walker circulation (PWC) structure characterized by an enclosed and clockwise rotation cell in the zonal–vertical section over the equatorial Pacific, was used to study the changes of PWC spatial structure during 1979–2012. To examine the robustness of changes in PWC characteristics, the linear trends of PWC were evaluated and compared among the current seven sets of reanalysis data, along with a comparison to the trends of surface climate variables. The spatial pattern of Ψ trend exhibited a strengthening and westward-shifting trend of PWC in all reanalysis datasets, with the significantly positive Ψ dominating the western Pacific and negative Ψ controlling the eastern Pacific. This kind of change is physically in agreement with the changes of the sea level pressure (SLP), surface winds, and precipitation derived from both the reanalyses and independent observations. Quantitative analyses of the changes in the PWC intensity and western edge, defined based on the zonal mass streamfunction, also revealed a robust strengthening and westward-shifting trend among all reanalysis datasets, with a trend of 15.08% decade−1 and 3.70° longitude decade−1 in the ensemble mean of seven sets of reanalysis data, with the strongest (weakest) intensification of 17.53% decade−1 (7.96% decade−1) in the Twentieth Century Reanalysis (NCEP-2) and largest (smallest) westward shift of −4.68° longitude decade−1 (−2.55° longitude decade−1) in JRA-55 (JRA-25). In response to the recent observed La Niña–like anomalous SST forcing, the ensemble simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), with 26 models in the ensemble, reasonably reproduced the observed strengthening and westward-shifting trend of PWC, implying the dominant forcing of the La Niña–like SST anomalies to the recent PWC change.

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Analysis of variability of tropical Pacific sea surface temperatures

Advances in Statistical Climatology Meteorology and Oceanography ◽

10.5194/ascmo-2-155-2016 ◽

2016 ◽

Vol 2 (2) ◽

pp. 155-169

Author(s):

Georgina Davies ◽

Noel Cressie

Keyword(s):

Standard Deviation ◽

Pacific Ocean ◽

Climate Models ◽

Robust Regression ◽

Southern Oscillation ◽

Tropical Pacific ◽

Global Climate Models ◽

Sea Surface ◽

The Pacific ◽

Sst Anomalies

Abstract. Sea surface temperature (SST) in the Pacific Ocean is a key component of many global climate models and the El Niño–Southern Oscillation (ENSO) phenomenon. We shall analyse SST for the period November 1981–December 2014. To study the temporal variability of the ENSO phenomenon, we have selected a subregion of the tropical Pacific Ocean, namely the Niño 3.4 region, as it is thought to be the area where SST anomalies indicate most clearly ENSO's influence on the global atmosphere. SST anomalies, obtained by subtracting the appropriate monthly averages from the data, are the focus of the majority of previous analyses of the Pacific and other oceans' SSTs. Preliminary data analysis showed that not only Niño 3.4 spatial means but also Niño 3.4 spatial variances varied with month of the year. In this article, we conduct an analysis of the raw SST data and introduce diagnostic plots (here, plots of variability vs. central tendency). These plots show strong negative dependence between the spatial standard deviation and the spatial mean. Outliers are present, so we consider robust regression to obtain intercept and slope estimates for the 12 individual months and for all-months-combined. Based on this mean–standard deviation relationship, we define a variance-stabilizing transformation. On the transformed scale, we describe the Niño 3.4 SST time series with a statistical model that is linear, heteroskedastic, and dynamical.

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Large shift of the Pacific Walker Circulation across the Cenozoic

10.5194/egusphere-egu21-10454 ◽

2021 ◽

Author(s):

Qing Yan

Keyword(s):

Late Miocene ◽

Coupled Model ◽

Walker Circulation ◽

Tropical Pacific ◽

South American ◽

Early Eocene ◽

Large Shift ◽

The Pacific ◽

Large Spread ◽

Eastern Edge

<p>Fluctuations in the Pacific Walker circulation (PWC), a zonally-oriented overturning cell across the tropical Pacific, can cause widespread climatic and biogeochemical perturbations. It remains unknown how the PWC developed during the Cenozoic era, with its substantial changes in greenhouse gases and continental positions. Through a suite of coupled model simulations on tectonic timescales, we demonstrate that the PWC was ~38&#186; broader and ~5% more intense during the Early Eocene relative to present. As the climate cooled from the Early Eocene to the Late Miocene, the width of the PWC shrank, accompanied by an increase in intensity that was tied to the enhanced Pacific zonal temperature gradient. However, the locations of the western and eastern branches behave differently from the Early Eocene to the Late Miocene, with the western edge remained steady with time due to the relatively stable geography of the western tropical Pacific; the eastern edge migrates westward with time as the South American continent moves northwest. A transition occurs in the PWC between the Late Miocene and Late Pliocene, manifested by an eastward shift (both the western and eastern edges migrate eastward by >12&#186;) and weakening (by ~22%), which we show here is linked with the closure of the tropical seaways. Moreover, our results suggest that rising CO<sub>2</sub> favors a weaker PWC under the same land-sea configurations, a robust feature across the large spread of Cenozoic climates considered here, supporting a weakening of the PWC in a warmer future.</p>

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A very likely weakening of Pacific Walker Circulation in constrained near-future projections

Nature Communications ◽

10.1038/s41467-021-26693-y ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Mingna Wu ◽

Tianjun Zhou ◽

Chao Li ◽

Hongmei Li ◽

Xiaolong Chen ◽

...

Keyword(s):

Climate Models ◽

Quantitative Estimation ◽

Walker Circulation ◽

Internal Variability ◽

External Forcing ◽

Interdecadal Pacific Oscillation ◽

Ensemble Simulations ◽

The Pacific ◽

Near Future ◽

Earth System Models

AbstractThe observational records have shown a strengthening of the Pacific Walker circulation (PWC) since 1979. However, whether the observed change is forced by external forcing or internal variability remains inconclusive, a solid answer to more societal relevantly question of how the PWC will change in the near future is still a challenge. Here we perform a quantitative estimation on the contributions of external forcing and internal variability to the recent observed PWC strengthening using large ensemble simulations from six state-of-the-art Earth system models. We find the phase transition of the Interdecadal Pacific Oscillation (IPO), which is an internal variability mode related to the Pacific, accounts for approximately 63% (~51–72%) of the observed PWC strengthening. Models with sufficient ensemble members can reasonably capture the observed PWC and IPO changes. We further constrain the projection of PWC change by using climate models’ credit in reproducing the historical phase of IPO. The result shows a high probability of a weakened PWC in the near future.

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