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Sarah M. Kang
and
Shang-Ping Xie

Abstract

This study shows that the magnitude of global surface warming greatly depends on the meridional distribution of surface thermal forcing. An atmospheric model coupled to an aquaplanet slab mixed layer ocean is perturbed by prescribing heating to the ocean mixed layer. The heating is distributed uniformly globally or confined to narrow tropical or polar bands, and the amplitude is adjusted to ensure that the global mean remains the same for all cases. Since the tropical temperature is close to a moist adiabat, the prescribed heating leads to a maximized warming near the tropopause, whereas the polar warming is trapped near the surface because of strong atmospheric stability. Hence, the surface warming is more effectively damped by radiation in the tropics than in the polar region. As a result, the global surface temperature increase is weak (strong) when the given amount of heating is confined to the tropical (polar) band. The degree of this contrast is shown to depend on water vapor– and cloud–radiative feedbacks that alter the effective strength of prescribed thermal forcing.

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Tomomichi Ogata
and
Shang-Ping Xie

Abstract

The semiannual cycle in zonal wind over the equatorial Indian Ocean is investigated by use of ocean–atmospheric reanalyses, and linear ocean–atmospheric models. In observations, the semiannual cycle in zonal wind is dominant on the equator and confined in the planetary boundary layer (PBL). Results from a momentum budget analysis show that momentum advection generated by the cross-equatorial monsoon circulation is important for the semiannual zonal-wind cycle in the equatorial Indian Ocean. In experiments with a linearized primitive model of the atmosphere, semiannual momentum forcing due to the meridional advection over the central equatorial Indian Ocean is important to simulate the observed maxima of the semiannual cycle in equatorial zonal wind. Off Somalia, diabatic heating and surface friction over land weaken the semiannual response to large momentum forcing there. Results from a linear ocean model suggest that the semiannual zonal wind stress over the central equatorial Indian Ocean generates large semiannual variability in zonal current through a basin-mode resonance.

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Takeaki Sampe
and
Shang-Ping Xie

High winds at sea are feared by sailors, but their distribution is poorly known because ships have avoided them as much as possible. The accumulation of spaceborne scatterometer measurements now allows a global mapping of high winds over the ocean. Seven years of Quick Scatterometer (QuikSCAT) data gathered since July 1999 show that high-wind events, defined as wind speeds greater than 20 m s−1 (“strong gale” and higher on the Beaufort scale), mostly happen in winter. Over coastal regions, land orography is the major cause of high winds, forcing wind jets of various types. Over the open ocean, high winds tend to be collocated with the extratropical storm tracks, along which migratory low and high pressure systems travel eastward. In comparison, tropical cyclones do not leave a strong signature in the climatology of high-wind occurrence except in the western Pacific east of Taiwan. In the extratropics, sea surface temperature (SST) fronts and their meanders significantly change the frequency of high-wind events. For example, high winds occur twice as often (or more) over the warmer than the colder flank of the Gulf Stream, and over the poleward than equatorward meanders of the Antarctic Circumpolar Current. The collocation of frequent high winds and SST frontal zones is not a mere coincidence because SST gradients anchor storm tracks, which in turn sustain the surface westerlies against friction with lateral heat and momentum flux. Both the high mean speed and large variance of wind increase the probability of high winds. Implications for navigation safety and oceanographic and climate research are discussed.

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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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Masami Nonaka
and
Shang-Ping Xie

Abstract

Satellite microwave measurements are analyzed, revealing robust covariability in sea surface temperature (SST) and wind speed over the Kuroshio Extension (KE) east of Japan. Ocean hydrodynamic instabilities cause the KE to meander and result in large SST variations. Increased (reduced) wind speeds are found to be associated with warm (cold) SST anomalies. This positive SST–wind correlation in KE is confirmed by in situ buoy measurements and is consistent with a vertical shear adjustment mechanism. Namely, an increase in SST reduces the static stability of the near-surface atmosphere, intensifying the vertical turbulence mixing and bringing fast-moving air from aloft to the sea surface.

South of Japan, the Kuroshio is known to vary between nearshore and offshore paths. These paths are very persistent and can last for months to years. As the Kuroshio shifts its path, coherent wind changes are detected from satellite data. In particular, winds are high south of Tokyo when the Kuroshio takes the nearshore path while they are greatly reduced when this warm current leaves the coast in the offshore path.

The positive SST–wind correlation over the strong Kuroshio Current and its extension is opposite to the negative one often observed in regions of weak currents such as south of the Aleutian low. The latter correlation is considered to be indicative of atmosphere-to-ocean forcing.

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Renguang Wu
and
Shang-Ping Xie

Abstract

This note compares equatorial Pacific surface wind changes around 1977 in the NCEP–NCAR reanalysis and the Comprehensive Ocean–Atmosphere Data Set (COADS) observations. Significant discrepancies are found in wind changes over the equatorial central and eastern Pacific. In the NCEP–NCAR reanalysis, the easterlies weakened over the eastern equatorial Pacific, while the southerlies strengthened over the north equatorial central Pacific. As a result, the low-level convergence and precipitation decreased over the equatorial central Pacific. These wind and precipitation anomalies are opposite to those derived from the COADS observations. Independent observations of ocean heat content are used to validate the changes in equatorial zonal wind, and it is found that the zonal slope of the thermocline in an ocean model forced by the COADS wind is more consistent with ocean observations than forced by the reanalysis wind. The equatorial wind biases are also identified in the NCEP–NCAR reanalysis climatology, reaching a maximum in the cold season from August to October. This seasonality of wind biases calls for improved representation of atmospheric boundary layer processes in climate models.

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Yuko Okumura
and
Shang-Ping Xie

Abstract

The seasonal cycle of equatorial Atlantic sea surface temperature (SST) is characterized by a rapid cooling from April to July, coinciding with the onset of the West African summer monsoon and followed by a slow warming that lasts 3 times longer. Two ensemble simulations are carried out with an atmospheric general circulation model to investigate the mechanisms for the wind changes that cause this rapid oceanic cooling and its feedback onto the African monsoon. In the control simulation, SST is globally prescribed in its full climatological seasonal cycle, while in the second simulation, equatorial Atlantic SST is held constant in time from 15 April onward.

Comparison of these simulations indicates that the equatorial cooling exerts a significant influence on the African monsoon, intensifying the southerly winds in the Gulf of Guinea and pushing the continental rainband inland away from the Guinean coast. The intensification of the cross-equatorial southerlies associated with the onset of the African monsoon, in turn, triggers the oceanic cooling in the east. The equatorial easterlies are also important for the seasonal cooling by inducing local upwelling and raising the thermocline in the east.

Three mechanisms are identified for the easterly wind acceleration in the equatorial Atlantic in boreal summer. First, the monsoon rainfall distribution is such that it induces zonal sea level pressure gradients and easterly anomalies in the eastern Atlantic. Second, the strong cross-equatorial southerlies advect the easterly momentum from the south into the equator. Finally, zonal pressure gradients associated with the equatorial ocean cooling accelerate surface easterly winds in the middle and western Atlantic. This interaction of equatorial SST and zonal wind causes their westward copropagation, analogous to that in the equatorial Pacific.

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Shang-Ping Xie
and
Kaori Saito

Abstract

Despite the equatorial symmetry of the annual-mean insolation, the intertropical convergence zone (ITCZ) and the collocated band of high sea surface temperature (SST) assume perennial northern latitudes over the eastern Pacific and Atlantic. An atmospheric general circulation model is coupled with an intermediate ocean model to study continental forcing and oceanic–atmospheric interaction that act to break the equatorial symmetry. The model reaches a statistically symmetric mean state under perfectly symmetric conditions with the continental coasts running along meridians. When a bulge of landmass is added to the eastern continent north of the equator, it initiates a coupled ocean–atmosphere wave front that propagates westward across the ocean basin, cooling the ocean surface and suppressing deep convection on and south of the equator. As a result, the ITCZ shifts into the Northern Hemisphere. In contrast to this basinwide response, little latitudinal asymmetry develops in the coupled model when the same land bulge is moved to the western continent, with the ITCZ staying on the equator. These model experiments demonstrate that the meridional structure of the oceanic ITCZ is largely determined by the continental geometry in the east, lending support to a westward control hypothesis based on simple linear wave dynamics.

Under steady solar forcing, the model ITCZ displays substantial interannual variability in both intensity and latitude. A coherent interhemispheric SST pattern with opposing polarities is found to be associated with this ITCZ variability. Various oceanic–atmospheric feedback mechanisms involved in the formation and variability of the ITCZ are examined.

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H. Annamalai
,
Ping Liu
, and
Shang-Ping Xie

Abstract

An atmospheric general circulation model (AGCM) is used to examine the role of Indian Ocean sea surface temperature (SST) anomalies in regional climate variability. In particular, the authors focus on the effect of the basinwide warming that occurs during December through May after the mature phase of El Niño. To elucidate the relative importance of local and remote forcing, model solutions were sought for experiments where SST anomalies are inserted in the (i) tropical Indo-Pacific Oceans, (ii) tropical Pacific Ocean, and (iii) tropical Indian Ocean. A 10-member ensemble simulation is carried out for each of the three forcing scenarios.

The model solutions demonstrate that precipitation variations over the southwest Indian Ocean are tied to local SST anomalies and are highly reproducible. Changes in the Indian Ocean–Walker circulation suppress precipitation over the tropical west Pacific–Maritime Continent, contributing to the development of a low-level anticyclone over the Philippine and South China Seas. Our model results indicate that more than 50% of the total precipitation anomalies over the tropical west Pacific–Maritime Continent is forced by remote Indian Ocean SST anomalies, offering an additional mechanism for the Philippine Sea anticyclone apart from Pacific SST. This anticyclone increases precipitation along the East Asian winter monsoon front from December to May. The anomalous subsidence over the Maritime Continent in conjunction with persistent anomalies of SST and precipitation over the Indian Ocean in spring prevent the northwestward migration of the ITCZ and the associated deep moist layer, causing a significant delay in the Indian summer monsoon onset in June by 6–7 days. At time scales of 5 days, however, the reproducibility of the northward progression of the ITCZ during the onset is low.

Results indicate that Indian Ocean SST anomalies during December through May that develop in response to both atmospheric and oceanic processes to El Niño need to be considered for a complete understanding of regional climate variability, particularly around the Indian Ocean rim.

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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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