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Zhengyu Liu

A coupled theory is proposed to account for the magnitude of the Walker circulation in the tropical Pacific. It is suggested that the Pacific Walker circulation is at a saturation state, at which the zonal sea surface temperature difference is bounded by about a quarter of the latitudinal difference of the radiative–convective equilibrium sea surface temperature.

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Zhengyu Liu

Abstract

Thermocline variability forced by zonally uniform Ekman pumping with annual to decadal periods is investigated. Both analytical and numerical solutions are obtained by the method of characteristics. As found in Part I, there is little thermocline variability in the ventilated zone or pool zone. In contrast, strong variability may exist in the shadow zone.

For annual forcings, nonlinearity is negligible. However, the linear solution is influenced substantially by the basic-state thermocline structure. As a result, local responses dominate for a shallow interface, while remote Rossby waves dominate for a deep interface.

Under a strong decadal forcing, nonlinearity may become important. The time-mean thermocline in the shadow zone is shallower than the steady thermocline under the mean Ekman pumping, particularly in the western part of a shadow zone where the mean deviation may reach the order often meters. This shallower mean thermocline is caused by the nonlinear Rossby wave.

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Zhengyu Liu

Abstract

The emerging interest in decadal climate prediction highlights the importance of understanding the mechanisms of decadal to interdecadal climate variability. The purpose of this paper is to provide a review of our understanding of interdecadal climate variability in the Pacific and Atlantic Oceans. In particular, the dynamics of interdecadal variability in both oceans will be discussed in a unified framework and in light of historical development. General mechanisms responsible for interdecadal variability, including the role of ocean dynamics, are reviewed first. A hierarchy of increasingly complex paradigms is used to explain variability. This hierarchy ranges from a simple red noise model to a complex stochastically driven coupled ocean–atmosphere mode. The review suggests that stochastic forcing is the major driving mechanism for almost all interdecadal variability, while ocean–atmosphere feedback plays a relatively minor role. Interdecadal variability can be generated independently in the tropics or extratropics, and in the Pacific or Atlantic. In the Pacific, decadal–interdecadal variability is associated with changes in the wind-driven upper-ocean circulation. In the North Atlantic, some of the multidecadal variability is associated with changes in the Atlantic meridional overturning circulation (AMOC). In both the Pacific and Atlantic, the time scale of interdecadal variability seems to be determined mainly by Rossby wave propagation in the extratropics; in the Atlantic, the time scale could also be determined by the advection of the returning branch of AMOC in the Atlantic. One significant advancement of the last two decades is the recognition of the stochastic forcing as the dominant generation mechanism for almost all interdecadal variability. Finally, outstanding issues regarding the cause of interdecadal climate variability are discussed. The mechanism that determines the time scale of each interdecadal mode remains one of the key issues not understood. It is suggested that much further understanding can be gained in the future by performing specifically designed sensitivity experiments in coupled ocean–atmosphere general circulation models, by further analysis of observations and cross-model comparisons, and by combining mechanistic studies with decadal prediction studies.

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Zhengyu Liu

Abstract

The effect of annual wind migration on the inertial recirculation is investigated using quasigeostrophic models. It is found that recirculation cells can he suppressed significantly by the wind migration. The two key dynamic conditions for the suppression are 1) the mismatch of formation timescales between the western boundary current and recirculation, and 2) the interaction between the two neighboring recirculation cells, which is related to chaotic intercell transport. The first condition tends to disconnect the potential vorticity anomaly source on the western boundary from the recirculation cell, while the second condition can generate strong eddy enstrophy flux and therefore the mixing of potential vorticity anomalies. Both conditions tend to destroy the potential vorticity anomaly, and in turn the recirculation.

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Zhengyu Liu

Abstract

A two-layer quasigeostrophic model is used to investigate the influence of stratification on the inertial recirculation in a full basin model. It is found that the barotropic transport of the inertial recirculation is intensified significantly through barotropic–baroclinic interactions in the presence of a shallow thermocline or a strong stratification. Weakly nonlinear theories and numerical experiments show that a strong baroclinic–barotropic interaction intensifies the advection of potential vorticity anomaly toward the inertial recirculation and therefore forces a stronger recirculation. Furthermore, from the potential vorticity point of view, our model recirculations belong to the generalized “modonlike” recirculation (with dQ/d ψ < 0). The increased zonal penetration of recirculation cells with stratification is not caused by the internal dynamics of the recirculation cells. Instead, it is caused by the increased advection of potential vorticity anomaly—an external forcing to the recirculation cells.

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Zhengyu Liu

Abstract

The probabilistic modal response of vegetation to stochastic precipitation variability is studied in a conceptual climate–ecosystem model. It is found that vegetation can exhibit bimodality in a monostable climate–ecosystem under strong rainfall variability and with soil moisture memory comparable with that of the vegetation. The bimodality of vegetation is generated by a convolution of a nonlinear vegetation response and a colored stochastic noise. The nonlinear vegetation response is such that vegetation becomes insensitive to precipitation variability near either end state (green or desert), providing the potential for two preferred modes. The long memory of soil moisture allows the vegetation to respond to a slow stochastic forcing such that the vegetation tends to grow toward its equilibrium states. The implication of the noise-induced bimodality to abrupt changes in the climate–ecosystem is also discussed.

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Zhengyu Liu

Abstract

A two-layer planetary geostrophic model is used to investigate the thermocline variability under a suddenly changing Ekman pumping. The effect of ventilation and the associated advection is particularly emphasized in the ventilated zone. The governing equation is a quasi-linear equation, which is solved analytically by the method of characteristics.

It is found that the dynamics differs substantially between a shadow zone and a ventilated zone. In the shadow zone, the Rossby wave is the dominant mechanism to balance the Ekman pumping. After a sudden change in the wind field, the Ekman pumping changes rapidly, but the baroclinic Rossby wave evolves at a much slower time scale (years to decades). This mismatch of response time scale produces an imbalance in forcings and in turn results in a strong thermocline variability. However, in the ventilated zone, the cold advection replaces the Rossby wave to become the major opposing mechanism to the Ekman pumping. After a sudden wind change, both the Ekman pumping and the cold advection vary rapidly at the time scale of barotropic Rossby waves (about one week) to achieve a new steady balance, leaving little thermocline variability.

The evolution of thermocline structure and circulation differs dramatically between a spinup and a spindown. For instance, with a change in the Ekman pumping field, the lower-layer fluid in the shadow zone is no longer motionless. After a spinup, the lower-layer water moves southward because of the compression on planetary vortex tubes by the downward anomalous Ekman pumping. The associated circulation is an anticyclonic gyre. In contrast, during a spindown, the water moves northward because of the stretching of planetary vortex tubes by the upward anomalous Ekman pumping. The lower-layer circulation now consists of two counterrotating gyres: an anticyclonic gyre to the north and a cyclonic gyre to the south.

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Zhengyu Liu

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A simple linear coupled ocean–atmosphere model is used to study the equatorial annual cycle. The ocean is a stab mixed-layer model and the atmosphere is the Lindzen–Nigam model. The model is shown to capture most features of the observed equatorial annual cycle. A significant part of the tropical annual cycle is found to be generated by the extratropical annual variability that propagates toward the equator through a coupled ocean–atmosphere wave. The back-pressure effect in the atmosphere model can contribute to several important aspects of the variability, especially in the vicinity of the equator. Comparison with other mechanisms for the equatorial annual cycle is also discussed.

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Zhengyu Liu

Abstract

A simple theoretical analysis identified three possible interannual positive feedbacks in the extratropics: the upwelling mode, the SST-Sverdrup mode, and the SST-evaporation mode. The upwelling mode becomes unstable when the atmosphere responses to a warm SST anomaly predominantly with a high surface pressure. In contrast, the SST-Sverdrup mode is destabilized when the atmosphere responses to a warm SST with a low pressure. In the region of mean westerly wind, the SST-evaporation mode is unstable when the atmospheric response to a warm SST is a qurater-wavelength to the south. The upwelling mode seems to favor low-latitude regions, while the two SST modes seem to favor midhigh latitudes. It is suggested that the relative position of the stationary atmospheric response to anomalous SST is of crucial importance for the extratropical ocean-atmosphere interaction.

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Zhengyu Liu

Abstract

A theory of tropical climatology is used to study the role of ocean in the response of tropical climatology to global warming. Special emphasis is given to the response of the west–east SST contrast along the equator. The transient response of tropical sea surface temperature to a global warming is shown to have two distinctive stages: a fast surface adjustment stage of years and a slow thermocline adjustment stage of decades.

Under a global warming heat flux that does not vary much in space, the initial response is always an enhanced west–east SST contrast. The final equilibrium response, however, depends on the effective latitudinal differential heating. The west–east SST contrast increases for an enhanced latitudinal differential heating, and vise versa.

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