Mechanisms of Fast Walker Circulation Responses to CO2 Forcing

Abstract There is a lack of consensus on the physical mechanisms that drive the anthropogenic weakening of tropical circulation. This study investigates the relative roles of direct CO2 forcing, mean SST warming, and the pattern of SST change on the weakening of the tropical circulation using an ensemble of AMIP and aquaplanet simulations. In terms of the mean weakening of the tropical circulation, the SST warming dominates over the direct CO2 forcing through its control over the tropical mean hydrological cycle and tropospheric stratification. In terms of the spatial pattern of circulation weakening, however, the three forcing agents are all important contributors, especially over the ocean. The increasing CO2 weakens convection over ocean directly by stabilizing the lower troposphere and indirectly via the land–sea warming contrast. The mean SST warming drives strong weakening over the centers and edges of convective zones. The pattern of SST warming plays a crucial role on the spatial pattern of circulation weakening over the tropical Pacific. The anthropogenic weakening of the Walker circulation is mostly driven by the mean SST warming. Increasing CO2 strengthens the Walker circulation through its indirect effect on land–sea warming contrast. Changes in the upper-level velocity potential indicate that the pattern of SST warming does not weaken the Walker circulation despite being “El Niño–like.” A weakening caused by the mean SST warming also dominates changes in the Hadley circulation in the AMIP simulations. However, this weakening is not simulated in the Southern Hemisphere in coupled simulations.

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Joint insolation and ice sheet/CO2 forcing on northern China precipitation during Pliocene warmth

Science Bulletin ◽

10.1016/j.scib.2020.10.025 ◽

2020 ◽

Author(s):

Zeng Luo ◽

Junsheng Nie ◽

Annelisa Ehret Moe ◽

Richard V. Heermance ◽

Carmala Garzione ◽

...

Keyword(s):

Ice Sheet ◽

Northern China ◽

Co2 Forcing

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An Analytical Model for the Meridional Gradient in CO2 Forcing

10.1002/essoar.10500379.1 ◽

2019 ◽

Author(s):

Nadir Jeevanjee ◽

Jacob Seeley ◽

David Paynter ◽

Stephan Fueglistaler

Keyword(s):

Analytical Model ◽

Meridional Gradient ◽

Co2 Forcing

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Quantifying global climate feedbacks, responses and forcing under abrupt and gradual CO2 forcing

Climate Dynamics ◽

10.1007/s00382-013-1677-0 ◽

2013 ◽

Vol 41 (9-10) ◽

pp. 2471-2479 ◽

Cited By ~ 10

Author(s):

David J. Long ◽

Matthew Collins

Keyword(s):

Global Climate ◽

Climate Feedbacks ◽

Co2 Forcing

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Associated atmospheric mechanisms for the increased cold season precipitation over the Three-River Headwaters region from the late 1980s

Journal of Climate ◽

10.1175/jcli-d-21-0077.1 ◽

2021 ◽

pp. 1

Author(s):

Shasha Shang ◽

Gaofeng Zhu ◽

Jianhui Wei ◽

Yan Li ◽

Kun Zhang ◽

...

Keyword(s):

Water Vapor ◽

Hadley Circulation ◽

Walker Circulation ◽

Cold Season ◽

Precipitation Variability ◽

Vapor Transport ◽

Water Vapor Transport ◽

The Tibetan Plateau ◽

Westerly Jet ◽

Tropical Oceans

AbstractPrecipitation in the Three-River Headwater (TRH) region has undergone significant changes as a result of global warming, which can affect water resources in downstream regions of Asia. However, the underlying mechanisms of the precipitation variability during the cold season (October to April), are still not fully understood. In this study, the daily China gridded precipitation product of CN05.1 as well as the NCEP-NCAR reanalysis are used to investigate the characteristics of the cold season precipitation variability over the TRH region and associated atmospheric mechanisms. The cold season precipitation shows an increasing trend (5.5 mm decade-1) from 1961 to 2014, with a dry-to-wet shift in around the late 1980s. The results indicate that the increased precipitation is associated with the enhanced easterly anomalies over the Tibetan Plateau (TP) and enhanced southeasterly water vapor transport. The enhanced Walker circulations, caused by the gradients of sea surface temperature between the equatorial central-eastern Pacific and Indo-western Pacific in tropical oceans, resulted in strengthened easterly anomalies over the TP and the westward expansion of the anticyclone in the western North Pacific. Meanwhile, the changed Walker circulation is accompanied by a strengthened local Hadley circulation which leads to enhanced meridional water vapor transport from tropical oceans and the South China Sea toward the TRH region. Furthermore, the strengthened East Asia Subtropical Westerly jet may contribute to the enhanced divergence at upper level and anomalous ascending motion above the TRH region leading to more precipitation.

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Impact of Resolution on the Tropical Pacific Circulation in a Matrix of Coupled Models

Journal of Climate ◽

10.1175/2008jcli2537.1 ◽

2009 ◽

Vol 22 (10) ◽

pp. 2541-2556 ◽

Cited By ~ 69

Author(s):

Malcolm J. Roberts ◽

A. Clayton ◽

M.-E. Demory ◽

J. Donners ◽

P. L. Vidale ◽

...

Keyword(s):

Coupled Model ◽

Walker Circulation ◽

Ocean Model ◽

Tropical Pacific ◽

Coupled Models ◽

Model Stability ◽

The Mean ◽

The Impact ◽

Mean State ◽

The Tropical Pacific

Abstract Results are presented from a matrix of coupled model integrations, using atmosphere resolutions of 135 and 90 km, and ocean resolutions of 1° and 1/3°, to study the impact of resolution on simulated climate. The mean state of the tropical Pacific is found to be improved in the models with a higher ocean resolution. Such an improved mean state arises from the development of tropical instability waves, which are poorly resolved at low resolution; these waves reduce the equatorial cold tongue bias. The improved ocean state also allows for a better simulation of the atmospheric Walker circulation. Several sensitivity studies have been performed to further understand the processes involved in the different component models. Significantly decreasing the horizontal momentum dissipation in the coupled model with the lower-resolution ocean has benefits for the mean tropical Pacific climate, but decreases model stability. Increasing the momentum dissipation in the coupled model with the higher-resolution ocean degrades the simulation toward that of the lower-resolution ocean. These results suggest that enhanced ocean model resolution can have important benefits for the climatology of both the atmosphere and ocean components of the coupled model, and that some of these benefits may be achievable at lower ocean resolution, if the model formulation allows.

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Isotopic evidence for twentieth-century weakening of the Pacific Walker circulation

Earth and Planetary Science Letters ◽

10.1016/j.epsl.2018.12.002 ◽

2019 ◽

Vol 507 ◽

pp. 85-93

Author(s):

Zhongfang Liu ◽

Zhimin Jian ◽

Christopher J. Poulsen ◽

Liang Zhao

Keyword(s):

Twentieth Century ◽

Walker Circulation ◽

Isotopic Evidence ◽

The Pacific

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Walker circulation and tropical upper tropospheric water vapor

Journal of Geophysical Research Atmospheres ◽

10.1029/95jd02275 ◽

1996 ◽

Vol 101 (D1) ◽

pp. 1961-1974 ◽

Cited By ~ 47

Author(s):

Reginald E. Newell ◽

Yong Zhu ◽

Edward V. Browell ◽

William G. Read ◽

Joe W. Waters

Keyword(s):

Water Vapor ◽

Walker Circulation

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Indo-Pacific Variability on Seasonal to Multidecadal Time Scales. Part II: Multiscale Atmosphere–Ocean Linkages

Journal of Climate ◽

10.1175/jcli-d-17-0031.1 ◽

2018 ◽

Vol 31 (2) ◽

pp. 693-725 ◽

Cited By ~ 7

Author(s):

Dimitrios Giannakis ◽

Joanna Slawinska

Keyword(s):

Time Scales ◽

El Niño ◽

El Nino ◽

Walker Circulation ◽

La Niña ◽

Central Pacific ◽

Australian Monsoon ◽

La Nina ◽

Combination Modes ◽

Sst Anomalies

The coupled atmosphere–ocean variability of the Indo-Pacific domain on seasonal to multidecadal time scales is investigated in CCSM4 and in observations through nonlinear Laplacian spectral analysis (NLSA). It is found that ENSO modes and combination modes of ENSO with the annual cycle exhibit a seasonally synchronized southward shift of equatorial surface zonal winds and thermocline adjustment consistent with terminating El Niño and La Niña events. The surface winds associated with these modes also generate teleconnections between the Pacific and Indian Oceans, leading to SST anomalies characteristic of the Indian Ocean dipole. The family of NLSA ENSO modes is used to study El Niño–La Niña asymmetries, and it is found that a group of secondary ENSO modes with more rapidly decorrelating temporal patterns contributes significantly to positively skewed SST and zonal wind statistics. Besides ENSO, fundamental and combination modes representing the tropospheric biennial oscillation (TBO) are found to be consistent with mechanisms for seasonally synchronized biennial variability of the Asian–Australian monsoon and Walker circulation. On longer time scales, a multidecadal pattern referred to as the west Pacific multidecadal mode (WPMM) is established to significantly modulate ENSO and TBO activity, with periods of negative SST anomalies in the western tropical Pacific favoring stronger ENSO and TBO variability. This behavior is attributed to the fact that cold WPMM phases feature anomalous decadal westerlies in the tropical central Pacific, as well as an anomalously flat zonal thermocline profile in the equatorial Pacific. Moreover, the WPMM is found to correlate significantly with decadal precipitation over Australia.

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