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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

The climate over the equatorial Pacific displays a pronounced asymmetry in the zonal direction that is characterized by the Walker circulation in the atmosphere and the cold tongue in the ocean. An intermediate coupled ocean–atmosphere model is used to investigate the driving force and the ocean–atmosphere interaction mechanism for the generation of the zonal asymmetry. In the far eastern Pacific, the upwelling at the equator is weak because zonal winds are blocked by the Andes. The off-equatorial upwelling induced by southerly cross- equatorial winds is thus crucial for cooling the eastern Pacific. A realistic cold tongue appears in the coupled model only when this southerly wind forcing is included. The southerly winds cause the sea surface temperature to fall in the east, enhancing the zonal heat contrast and hence intensifying easterly winds across the basin. These anomalous easterlies induce more equatorial upwelling and raise the thermocline in the east, amplifying the initial cooling by the southerlies. A simple analog model is presented to illustrate this coupled ocean–atmosphere feedback originally proposed by Bjerknes.

From an oceanographic point of view, the equatorial cold tongue is caused by easterly winds. In the coupled model, much of these easterlies arises as part of the Bjerknes feedback and can be attributed to the southerly wind forcing in the east. Were the earth climate symmetric about the equator, cross-equatorial wind would vanish, and the cold tongue would be much weaker and have a very different zonal structure than is observed today.

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Shang-Ping Xie

Abstract

The interaction between the annual and interannual variations is investigated by contrasting a pair of experiments with a general circulation model of the tropical Pacific Ocean. The atmospheric forcing applied to the model includes both annual and interannual components The phase of the annual forcing is shifted one-half year in the two runs, which are otherwise identical. Significant differences are found in the sea surface temperature (SST) evolution between the two runs that have the same interannual forcing function. SST anomalies tend to be phase-locked to the solar calendar and to appear in the cold season. A SST variance function in response to interannual forcings with a random phase distribution is constructed, which has an annual cycle and reaches its maximum in the cold season as is observed. It is suggested that this seasonality of an ocean origin is amplified by the interaction with the atmosphere, leading to the observed phase-locking.

The phase-locking of the interannual cycle and the interannual variation of the annual cycle in SST are two manifestations of the interaction between the annual and interannual cycles. A simple conceptual model is proposed to explain these two features, in which strong interaction between the annual and interannual cycles occurs through the nonlinearity associated with the thermocline depth change and upwelling.

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Shang-Ping Xie

Over most of the World Ocean, sea surface temperature (SST) is below 26°C and atmospheric deep convection rarely takes place. Cool ocean–atmosphere interaction is poorly understood and this lack of understanding is a stumbling block in the current effort to study non-ENSO climate variability. Using new satellite observations, the response of surface wind and low clouds to changes in SST is investigated over cool oceans, where the planetary boundary layer (PBL) is often capped by a temperature inversion. While one-way atmospheric forcing is a major mechanism for basinscale SST variability in the extratropics, clear wind response is detected in regions of strong ocean currents. In particular, SST modulation of vertical momentum mixing emerges as the dominant mechanism for SST-induced wind variability near oceanic fronts around the world, which is characterized by a positive SST–wind speed correlation. Several types of boundary layer cloud response are found, whose correlation with SST varies from positive to negative, depending on the role of surface moisture convergence. Noting that the surface moisture convergence is strongly scale dependent, it is proposed that horizontal scale is important for setting the sign of this SST–cloud correlation. Finally, the processes by which a shallow PBL response might lead to a deep, tropospheric-scale response and the implications for the study of extratropical basin-scale air–sea interaction are discussed.

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Shang-Ping Xie

Abstract

Over a large zonal extent of the central and eastern Pacific, the intertropical convergence zone (ITCZ) is located to the north of the equator. Collocated with this ITCZ is a zonal band of warm sea surface, where the highest sea surface temperatures (SST) along a meridian are found. A one-dimensional coupled ocean–atmosphere model that neglects zonal variations is used to investigate this problem of latitudinal asymmetry in the tropical climate. The equatorially symmetric model solution is found to be unstable to infinitesimal disturbances and equatorial asymmetries develop spontaneously. A linear instability that is stationary in space and antisymmetric about the equator is responsible for the unstable transition of the model from the symmetric state. The destabilizing mechanism involves a positive feedback between the scalar wind speed and SST through surface evaporation, which is illustrated with a simple low-order model that contains only two SST grid points, one in each hemisphere.

The existence of the equatorially antisymmetric instability indicates that in a zonally uniform setting, a latitudinally asymmetric climate with a single ITCZ off the equator could emerge on a symmetric planet.

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Shang-Ping Xie

Abstract

A linear model that couples an ocean mixed layer with a simple dynamic atmosphere is used to study the mechanism for decadal variability over the tropical Atlantic. An unstable mode with a dipole sea surface temperature (SST) pattern similar to observed decadal variability in the tropical Atlantic emerges in the time integration of the model. A wind–evaporation–SST feedback is responsible for the growth and oscillation of the unstable mode whereas the mean state of the Atlantic climate is essential for maintaining the spatially quasi-standing dipole structure. The oscillation period ranges from several to a few tens of years and is sensitive to coupling strength.

The oscillation is not self-sustainable as the realistic damping rate exceeds the growth rate. In response to white noise forcing, the model produces a red SST spectrum without a peak at finite frequencies. Therefore it is suggested that the tropical dipole’s preferred timescales, if any, arise from the forcing by or interaction with the extratropics. In a model run where the forcing is confined to the extratropics, a dipole SST pattern still dominates the forcing-free Tropics, in support of the proposed linkage between the Tropics and extratropics.

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Shang-Ping Xie

Abstract

A linear theory is proposed that can explain both the period and the westward propagation of the equatorial annual cycles in the SST and zonal wind. A coupled model linearized about a mean state of the air-sea system is used. This model allows the surface winds to directly change the SST through surface evaporation and vertical mixing, in contrast to the formulation of the conventional air-sea coupling models that emphasize the effects of winds on ocean dynamics. It is demonstrated that the characteristics of the equatorial seasonal cycle, including its period, phase, and amplitude, are determined to a large extent by the mean state of the air-sea coupling system. The meridional wind, which is specified here, is the most important forcing for the annual cycle in SST, suggesting that the equatorial annual cycle is the response to an annual solar forcing in the off-equatorial regions.

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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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Shang-Ping Xie

Abstract

A coupled ocean-atmosphere model is used to investigate the effects of seasonal variation in solar radiation on the configuration of the intertropical convergence zone. The model maintains a Northern Hemispheric ITCZ under annual mean insolation, with convection being suppressed in the Southern Hemisphere. In the presence of seasonal variations, a Southern Hemispheric ITCZ develops in boreal winter and spring in response to the seasonal rise in local solar radiation. As a result, the equatorial asymmetry of the annual-mean model climatology is reduced. The latitudinal asymmetry of the model climate is thus determined by a balance between the symmetry-breaking land forcing and the symmetry-restoring seasonal solar forcing.

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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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