Asian Monsoon Forcing of Subtropical Easterlies in the Community Atmosphere Model: Summer Climate Implications for the Western Atlantic

Kelly, Patrick; Mapes, Brian E.

doi:10.1175/jcli-d-12-00339.1

Cited by 16 publications

(16 citation statements)

References 30 publications

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“…only in the zonally asymmetric part of the circulation. Strong zonal overturnings do exist in the northern subtropics during boreal summer, and meridional advection of momentum and vorticity by the upper-tropospheric Asian anticyclone is large even when compared to transports by the zonal-mean flow and transient eddies (e.g., Kelly and Mapes 2013;Shaw 2014). Thus, the overly large strength of the summer cell in our model would not seem to make our results irrelevant to Earth's observed summer circulation.…”

Section: Summary and Discussionmentioning

confidence: 77%

“…This change in sign of the absolute vorticity occurred as part of a southward migration of negative relative vorticity from the Northern Hemisphere as the stationary wave amplitude was increased (T. A. Shaw 2015, personal communication) and is distinct from the regime transitions marked by absolute vorticity reaching zero in the region of the off-equatorial heating (e.g., Plumb and Hou 1992;Hsu and Plumb 2000). Nevertheless, the results of Shaw (2014), as well as those of Kelly andMapes (2011, 2013), illustrate the important role played by stationary eddy momentum transports in the boreal summer general circulation. This study is complementary to that work in that we also force an idealized model with a zonally asymmetric thermal forcing, but here we focus on the scaling of the total vertical mass flux, integrated over all longitudes, instead of just the zonal mean.…”

Section: Introductionmentioning

confidence: 97%

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Regime Transitions of Cross-Equatorial Hadley Circulations with Zonally Asymmetric Thermal Forcings

Zhai

Boos

2015

Journal of the Atmospheric Sciences

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Observed nonlinearities in the seasonal evolution of monsoons have been previously explained using theories for Hadley circulations driven by zonally symmetric thermal forcings, even though monsoonal forcings deviate strongly from the assumption of zonal symmetry. Here, an idealized model of a dry, three-dimensional atmosphere is used to compare the response to zonally symmetric and asymmetric off-equatorial thermal forcings. For symmetric forcings, the zonal-mean, cross-equatorial mass flux increases more rapidly with the amplitude of the forcing once the forcing becomes strong enough to reduce the upper-tropospheric absolute vorticity to near zero, consistent with previous studies of the transition to angular momentum–conserving flow. For zonally asymmetric forcings, the zonal-mean cross-equatorial flow exhibits a similar dependence on forcing strength and a similar reduction of the zonal-mean upper-level vorticity, but asymmetric forcings also produce strong zonal overturnings with subsidence west of the heating, as in the well-known linear response to off-equatorial heatings. The mass flux in these zonal overturnings increases linearly with forcing strength until its rate of increase tapers off for the strongest forcings; the total upward mass flux (i.e., the zonal-mean plus zonally asymmetric components) increases linearly with the strength of zonally asymmetric forcings and exhibits no abrupt or nonlinear dependence on forcing amplitude. These results indicate the importance of considering the zonally asymmetric part of the divergent response to off-equatorial forcings and suggest that theories based on zonally symmetric forcings need further examination before they can be assumed to describe observed monsoons.

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Section: Summary and Discussionmentioning

confidence: 77%

Section: Introductionmentioning

confidence: 97%

Regime Transitions of Cross-Equatorial Hadley Circulations with Zonally Asymmetric Thermal Forcings

Zhai

Boos

2015

Journal of the Atmospheric Sciences

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“…Dima et al (2005) showed that the structure of the planetary-scale wave transport is consistent with the response of the nonlinear shallow-water equations to a zonally asymmetric off-equatorial heating suggesting that the tropical wave transport is connected to heating in the summer hemisphere. Kelly andMapes (2011, 2013) showed that subtropical stationary wave momentum transport associated with the upper-tropospheric Tibetan anticyclone modulates North American drought through an impact on the North Atlantic subtropical anticyclone. Finally, Shaw and Pauluis (2012) showed that latent heat transport in the vicinity of subtropical anticyclones and monsoons dominates the mass transport by the NH summer circulation in isentropic coordinates.…”

Section: Introductionmentioning

confidence: 99%

On the Role of Planetary-Scale Waves in the Abrupt Seasonal Transition of the Northern Hemisphere General Circulation

Shaw

2014

Journal of the Atmospheric Sciences

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The role of planetary-scale waves in the abrupt seasonal transition of the Northern Hemisphere (NH) general circulation is studied. In reanalysis data, the winter-to-summer transition involves the growth of planetary-scale wave latent heat and momentum transports in the region of monsoons and anticyclones that dominate over the zonal-mean transport beginning in midspring. The wave-dominated regime coincides with an abrupt northward expansion of the cross-equatorial circulation and reversal of the trade winds. In the upper troposphere, the transition coincides with the growth of cross-equatorial planetary-scale wave momentum transport and a poleward shift of subplanetary-scale wave transport and jet stream.The dynamics of the seasonal transition are captured by idealized aquaplanet model simulations with a prescribed subtropical planetary-scale wave sea surface temperature (SST) perturbation. The SST perturbation generates subtropical planetary-scale wave streamfunction variance and transport in the lower and upper troposphere consistent with quasigeostrophic theory. Beyond a threshold SST, a transition of the zonalmean circulation occurs, which coincides with a localized reversal of absolute vorticity in the NH tropical upper troposphere. The transition is abrupt in the lower troposphere because of the quadratic dependence of the wave transport on the SST perturbation and involves seasonal-time-scale feedbacks between the wave and zonal-mean flow in the upper troposphere, including cross-equatorial wave propagation. The zonal-mean vertical and meridional flows associated with the circulation response are in balance with the planetary-scale wave momentum and latent heat meridional flux divergences. The results highlight the leading-order role of monsoonanticyclone transport in the seasonal transition, including its impact on the meridional extent of the Hadley and Ferrel cells. They can also be used to explain why the transition is less abrupt in the Southern Hemisphere.

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“…Previous studies have demonstrated that regional thermal forcing was associated with global teleconnection patterns; for example, anomalous heating rate in association with the variability of the Indian summer monsoon excited the summertime circumglobal pattern although thermal forcing was regionally prescribed [ Ding and Wang , ; Lin , ]. Kelly and Mapes [] also found that land surface heating over South Asia led to an enhanced Rossby wave source region out of the forcing area. Thus, we will examine the effect of ULSF on the upper tropospheric Rossby wave next.…”

Section: Resultsmentioning

confidence: 99%

Atmospheric responses to idealized urban land surface forcing in eastern China during the boreal spring

Deng

2015

JGR Atmospheres

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In contrast to the impacts of anthropogenic aerosols and greenhouse gases, little is known about the impact of urban land surface forcing (ULSF) on large‐scale atmospheric circulation. This study explores atmospheric responses to idealized ULSF in eastern China during the boreal spring using the Community Atmosphere Model version 5.1 coupled with the Community Land Model version 4. Results show that the ULSF leads to an increased air temperature in northern China both near the surface and in the lower troposphere. Related to a strong thermal feedback loop, a middle‐upper tropospheric cooling is found in eastern China while a relatively strong warming occurs in the middle‐high latitudes, which acts to enhance the meridional temperature gradient to the north of the source region and then shifts the East Asian subtropical jet stream (EASJ) southward. A weakened southwesterly in the lower troposphere in southern China slows down moisture transportation to northern China, and the southward shifted EASJ induces strong anomalous sinking motion to the north of the Yangtze River Valley (YRV). The associated changes in moisture and vertical airflow result in moisture divergence along the YRV and convergence in southern China. Thus, the spring rain belt is shifted southward, as characterized by below‐normal rainfall extending from the Huai River Valley to South Korea and above‐normal rainfall from southern China to the south coast of Japan. In addition, analysis of the upper tropospheric wave activity signifies that large‐scale atmospheric responses due to the ULSF also exert an important influence on local climate.

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Asian Monsoon Forcing of Subtropical Easterlies in the Community Atmosphere Model: Summer Climate Implications for the Western Atlantic

Cited by 16 publications

References 30 publications

Regime Transitions of Cross-Equatorial Hadley Circulations with Zonally Asymmetric Thermal Forcings

Regime Transitions of Cross-Equatorial Hadley Circulations with Zonally Asymmetric Thermal Forcings

On the Role of Planetary-Scale Waves in the Abrupt Seasonal Transition of the Northern Hemisphere General Circulation

Atmospheric responses to idealized urban land surface forcing in eastern China during the boreal spring

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