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Shang-Ping Xie

Abstract

An ocean general circulation model is coupled with a simple atmosphere model to investigate the formation mechanism of the intertropical convergence zone in the eastern Pacific, which is observed in the Northern Hemisphere. The coupled model develops an asymmetric state under conditions symmetric about the equator. The zonal variation in equatorial upwelling leads to pronounced differences between the western and other parts of the ocean. In the western warm water pool region, where the cooling effect of the equatorial upwelling is suppressed, both atmospheric and oceanic surface conditions are symmetric about the equator. On the other hand. in the central region where the upwelling cools the equatorial ocean, a single ITCZ forms off the equator in the Northern or Southern Hemisphere, depending on the initial condition. A strong contrast exists in the sea surface temperature SST between the hemispheres; SST is much higher at the latitude of the ITCZ than that on the other side of the equator. This high SST is crucial for the development of deep convection in the ITCZ. An air-sea interaction mechanism. where the wind speed-dependent surface evaporation plays a crucial role, maintains the asymmetric state. confirming the results from a previous two-dimensional model study.

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Shang-Ping Xie

Abstract

Hemispheric asymmetries of continental geometry have long been speculated to be the cause of the Northern Hemisphere position of the intertropical convergence zone over the central and eastern Pacific. It is unknown, however, how the effects of continental asymmetries are transmitted to and felt by the central Pacific thousands of kilometers away. This paper proposes a transmitter mechanism by investigating the response of a coupled ocean–atmospheric model to a symmetry-breaking force by the American continents. The model treats land forcing implicitly as an eastern boundary condition. In the absence of oceanic feedback, the model response to the eastern boundary forcing is tightly trapped and confined to a small longitudinal extent off the coast, whereas the climate over the interior ocean is symmetric about the equator. Ocean–atmosphere coupling greatly enhances the transmissibility of the effects of the land forcing, establishing large latitudinal asymmetry over a great zonal extent. A westward propagating coupled instability is found to be responsible, which is antisymmetric about the equator and is caused by a wind–evaporation–SST feedback proposed previously by Xie and Philander. The solution to an initial value problem shows that a coupled ocean–atmosphere wave front generated by the land forcing amplifies as it moves westward, leaving behind a latitudinally asymmetric steady state.

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Jan Hafner
and
Shang-Ping Xie

Abstract

Recent satellite observations reveal far-reaching effects of the Hawaiian Islands on surface wind, cloud, ocean current, and sea surface temperature (SST) that extend leeward over an unusually long distance (>1000 km). A three-dimensional regional atmospheric model with full physics is used to investigate the cause of this long wake. While previous wind–wake studies tend to focus on regions near the islands, the emphasis here is the far-field effects of SST and orography well away from the Hawaiian Islands. In response to an island-induced SST pattern, the model produces surface wind and cloud anomaly patterns that resemble those observed by satellites. In particular, anomalous surface winds are found to converge onto a zonal band of warmer water, with cloud liquid water content enhanced over it but reduced on the northern and southern sides. In the vertical, a two-cell meridional circulation develops of a baroclinic structure with the rising motion and thicker clouds over the warm water band. The model response in the wind and cloud fields supports the hypothesis that ocean–atmosphere interaction is crucial for sustaining the island effects over a few thousand kilometers.

Near Hawaii, mountains generate separate wind wakes in the model lee of individual islands as observed by satellites. Under orographic forcing, the model simulates the windward cloud line and the southwest-tilted cloud band leeward of the Big Island. In the far field, orographically induced wind perturbations are found to be in geostrophic balance with pressure anomalies, indicative of quasigeostrophic Rossby wave propagation. A shallow-water model is developed for disturbances trapped in the inversion-capped planetary boundary layer. The westward propagation of Rossby waves is found to increase the wake length significantly, consistent with the three-dimensional simulation.

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Wenyu Zhou
and
Shang-Ping Xie

Abstract

The tropical tropospheric temperature is close to but typically cooler than that of the moist adiabat. The negative temperature deviation from the moist adiabat manifests a C-shape profile and is projected to increase and stretch upward under warming in both comprehensive climate models and idealized radiative–convective equilibrium (RCE) simulations. The increased temperature deviation corresponds to a larger convective available potential energy (CAPE) under warming. The extreme convective updraft velocity in RCE increases correspondingly but at a smaller fractional rate than that of CAPE. A conceptual model for the tropical temperature deviation and convective updraft velocities is formulated to understand these features. The model builds on the previous zero-buoyancy model but replaces the bulk zero-buoyancy plume by a spectrum of entraining plumes that have distinct entrainment rates and are positively buoyant until their levels of neutral buoyancy. Besides the negative temperature deviation and its increasing magnitude with warming, this allows the spectral plume model to further predict the C-shape profile as well as its upward stretch with warming. By representing extreme convective updrafts as weakly entraining plumes, the model is able to reproduce the smaller fractional increase in convective velocities with warming as compared to that of CAPE. The smaller fractional increase is mainly caused by the upward stretch in the temperature deviation profile with warming, which reduces the ratio between the integrated plume buoyancy and CAPE. The model thus provides a useful tool for understanding the tropical temperature profile and convective updraft velocities.

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Shang-Ping Xie
,
Atsushi Kubokawa
, and
Kimio Hanawa

Abstract

The nonlinear evolution of the quasi-linear (QL) evaporation-wind feedback (EWFB) instability obtained in Part I of this study is investigated in a two-level model of the aqua-planet atmosphere. In this model, the QL-EWFB instability causes tropical convection to organize on the planetary scale and a wavenumber one Kelvin wave-like structure dominates the east-west circulation in the tropics. An increase of the static stability that is in phase with the surface evaporation stabilizes the EWFB mode. For large surface humidity, a hierarchy of convective structures appear as a result of the nonlinear adjustment of the QL mode at large amplitudes. Isolated grid-size individual convective zones move randomly, while a wavenumber one envelope of this convection propagates eastward at a constant speed.

In the conditionally unstable parameter regime, the model atmosphere is found to be stable on the planetary scale, but it can be conditionally unstable on the scale of individual convection events. The EWFB and conditional instabilities are not mutually exclusive as in the QL model but cooperate in organizing convection. The development of the fast-growing conditional instability acts to stabilize the large-scale atmosphere, allowing the EWFB mechanism to organize convection into a wavenumber one structure.

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Masaru Inatsu
,
Hitoshi Mukougawa
, and
Shang-Ping Xie

Abstract

A set of atmospheric general circulation model (AGCM) experiments under idealized conditions is performed to investigate atmospheric response to surface boundary forcing by extratropical land–sea contrast, large-scale orography, and tropical sea surface temperature (SST) distribution. Stationary eddies forced by the extratropical land–sea distribution are strongest in high latitudes, but their amplitudes are modest and comparable to internal chaotic variability. By contrast, the stationary eddy response to zonal variations in tropical SST is strong and robust in both the subtropics and midlatitudes. While these SST-forced stationary waves are trapped within the troposphere, those induced by orography show a strong vertical propagation into the stratosphere. Analysis of transient eddies indicates that orography is effective in generating a zonally localized storm track while extratropical land–sea contrast has little effect on the zonal variation of upper-level storm activity.

A vorticity budget analysis is carried out to understand tropical SST forcing mechanism to set up extratropical stationary eddies. In the subtropics, the dominant balance is reached between the vortex stretching and zonal advection. North of the tropical warm water pool, a subtropical anticyclone forms in the upper troposphere in response to the divergence of the locally enhanced Hadley circulation. The authors further show that this subtropical response to tropical SST variations has nonlinear characteristics in both its amplitude and zonal phase.

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Hyo-Seok Park
,
Shang-Ping Xie
, and
Seok-Woo Son

Abstract

The orographic effect of the Tibetan Plateau on atmospheric poleward heat transport is investigated using an atmospheric general circulation model. The linear interference between the Tibetan Plateau–induced winds and the eddy temperature field associated with the land–sea thermal contrast is a key factor for enhancing the poleward stationary eddy heat transport. Specifically, Tibetan Plateau–induced stationary waves produce northerlies over the cold eastern Eurasian continent, leading to a poleward heat transport. In another hot spot of stationary eddy heat transport over the eastern North Pacific, Tibetan Plateau–induced stationary waves transport relatively warm marine air northward.

In an experiment where the Tibetan Plateau is removed, the poleward heat transport is mostly accomplished by transient eddies, similar to the Southern Hemisphere. In the presence of the Tibetan Plateau, the enhanced stationary eddy heat transport is offset by a comparable reduction in transient eddy heat transport. This compensation between stationary and transient eddy heat transport is seen in observed interannual variability. Both the model and observations indicate that an enhanced poleward heat transport by stationary waves weakens transient eddies by decreasing the meridional temperature gradient and the associated westerlies in midlatitudes.

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R. Justin Small
,
Shang-Ping Xie
,
Yuqing Wang
,
Steven K. Esbensen
, and
Dean Vickers

Abstract

Recent observations from spaceborne microwave sensors have revealed detailed structure of the surface flow over the equatorial eastern Pacific in the boreal fall season. A marked acceleration of surface wind across the northern sea surface temperature (SST) front of the cold tongue is a prominent feature of the regional climate. Previous studies have attributed the acceleration to the effect of enhanced momentum mixing over the warmer waters. A high-resolution numerical model is used to examine the cross-frontal flow adjustment. In a comprehensive comparison, the model agrees well with many observed features of cross-equatorial flow and boundary layer structure from satellite, Tropical Atmosphere Ocean (TAO) moorings, and the recent Eastern Pacific Investigation of Climate Processes (EPIC) campaign. In particular, the model simulates the acceleration across the SST front, and the change from a stable to unstable boundary layer. Analysis of the model momentum budget indicates that the hydrostatic pressure gradient, set up in response to the SST gradient, drives the surface northward acceleration. Because of thermal advection by the mean southerly flow, the pressure gradient is located downstream of the SST gradient and consequently, divergence occurs over the SST front, as observed by satellite. Pressure gradients also act to change the vertical shear of the wind as the front is crossed. However, the model underpredicts the changes in vertical wind shear across the front, relative to the EPIC observations. It is suggested that the vertical transfer of momentum by mixing, a mechanism described by Wallace et al. may also act to enhance the change in shear in the observations, but the model does not simulate this effect. Reasons for this are discussed.

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