Search Results

You are looking at 51 - 60 of 94 items for

  • Author or Editor: Zhengyu Liu x
  • All content x
Clear All Modify Search
Fuyao Wang, Zhengyu Liu, and Michael Notaro

Abstract

The seasonal impacts of the dominant sea surface temperature (SST) modes to North American climate are assessed comprehensively in observations using the multivariate generalized equilibrium feedback assessment (GEFA) method. The GEFA method is first validated before applying it to observations. Impacts of each individual SST mode are quantified and the associated mechanisms are discussed. Four critical SST modes for North American climate are found: the ENSO mode, Indian Ocean Basin (IOB) mode, North Pacific first empirical orthogonal function (EOF) mode, and tropical Atlantic second EOF mode. The impacts of the ENSO mode are consistent with previous studies qualitatively, while the impact strength is further quantified here. The IOB mode has a strong influence on surface air temperature across North America, and it is demonstrated for the first time that its impact strength might even exceed that of ENSO during both winter and summer. The IOB mode also affects the year-round precipitation. A deeper understanding of the impact of North Pacific SSTs on wintertime surface air temperature is achieved: namely, positive SST anomalies in the Kuroshio Extension region correspond to colder (warmer) air in western (eastern) North America. The tropical Atlantic has a more significant influence on North American precipitation than does the extratropical Atlantic, with colder than normal tropical North Atlantic SSTs supporting wetter conditions across much of the United States, especially during autumn. Because of the linearity of GEFA, the total impacts of multiple SST modes can be obtained by the linear combination of each individual mode's impact. The GEFA method is a potentially powerful tool for seasonal climate prediction.

Full access
Haijun Yang, Zhengyu Liu, and Qiong Zhang

Abstract

Oceanic response to stochastic wind forcing is studied in a tropical–extratropical basin using two shallow water models: a periodically forced model and a time-forward model. Consistent with theory, extratropical stochastic wind forces a decadal spectral peak in the tropical and eastern boundary ocean as a resonant response of the planetary wave basin mode. This resonant response is characterized by a rather uniform amplitude and phase in the equatorial and eastern boundary region. In comparison, away from the eastern boundary, the extratropical ocean is affected significantly by the local Ekman pumping forcing, with spectral peaks varying with location. A complex EOF (CEOF) analysis of the time-forward model simulation further suggests that these resonant responses are robust, and can be extracted as the leading CEOF modes. Thus, the resonance of the planetary wave basin mode gives a physically based guideline for the interpretation of decadal oceanic variability in the tropical–extratropical ocean.

Full access
Yingying Zhao, Haijun Yang, and Zhengyu Liu

Abstract

The Bjerknes compensation (BJC) refers to the tendency for changes in the atmosphere heat transport (AHT) and ocean heat transport (OHT) to compensate each other. However, the nature of this compensation varies with the time scale of changes. In this study, a new approach was developed to diagnose BJC for climate variability by considering the correlation between AHT and OHT and their relative magnitudes. The correlation is equivalent to the cosine of phase difference between AHT and OHT. For high-frequency climate variability, AHT lags or leads OHT by π/2, the correlation is zero, and BJC does not occur concurrently. For low-frequency climate variability, AHT lags or leads OHT by π, the correlation is −1, and BJC is concurrent. With increasing time scale, the phase difference between AHT and OHT changes from π/2 to π, and the BJC reaches equilibrium. A coupled box model is used to justify the approach and to understand the temporal change of BJC from a theoretical perspective. The correlation and BJC rate derived from theory and from the box model exhibit similar transient behaviors, approaching equilibrium monotonically with increasing time scale. The equilibrium BJC is established at decadal time scale. Since the BJC is closely related to climate feedback, a proper identification of BJC processes in climate variability can reveal the nature of dominant climate feedback processes at different time scales.

Full access
Zhengyu Liu, Lixin Wu, and Eric Bayler

Abstract

The interaction of open and coastal oceans in a midlatitude ocean basin is investigated in light of Rossby and coastal Kelvin waves. The basinwide pressure adjustment to an initial Rossby wave packet is studied both analytically and numerically, with the focus on the low-frequency modulation of the resulting coastal Kelvin wave. It is shown that the incoming mass is redistributed by coastal Kelvin waves as well as eastern boundary planetary waves, while the incoming energy is lost mostly to short Rossby waves at the western boundary. The amplitude of the Kelvin wave is determined by two mass redistribution processes: a fast process due to the coastal Kelvin wave along the ocean boundary and a slow process due to the eastern boundary planetary wave in the interior ocean. The amplitude of the Kelvin wave is smaller than that of the incident planetary wave because the initial mass of the Rossby wave is spread to the entire basin. In a midlatitude ocean basin, the slow eastern boundary planetary wave is the dominant mass sink. The resulting coastal Kelvin wave peaks when the peak of the incident planetary wave arrives at the western boundary.

The theory is also extended to an extratropical–tropical basin to shed light on the modulation effect of extratropical oceanic variability on the equatorial thermocline. In contrast to a midlatitude basin, the fast mass redistribution becomes the dominant process, which is now accomplished mainly by equatorial Rossby and Kelvin waves, rather than the coastal Kelvin wave. The coastal Kelvin wave and the modulation of the equatorial thermocline peak close to the time when the wave trail of the incident Rossby wave arrives at the western boundary. Finally, the theory is also applied to the wave interaction around an extratropical island.

Full access
Michael Notaro, Steve Vavrus, and Zhengyu Liu

Abstract

Transient simulations are presented of future climate and vegetation associated with continued rising levels of CO2. The model is a fully coupled atmosphere–ocean–land–ice model with dynamic vegetation. The impacts of the radiative and physiological forcing of CO2 are diagnosed, along with the role of vegetation feedbacks. While the radiative effect of rising CO2 produces most of the warming, the physiological effect contributes additional warming by weakening the hydrologic cycle through reduced evapotranspiration. Both effects cause drying over tropical rain forests, while the radiative effect enhances Arctic and Indonesian precipitation.

A global greening trend is simulated primarily due to the physiological effect, with an increase in photosynthesis and total tree cover associated with enhanced water-use efficiency. In particular, tree cover is enhanced by the physiological effect over moisture-limited regions. Over Amazonia, South Africa, and Australia, the radiative forcing produces soil drying and reduced forest cover. A poleward shift of the boreal forest is simulated as both the radiative and physiological effects enhance vegetation growth in the northern tundra and the radiative effect induces drying and summertime heat stress on the central and southern boreal forest. Vegetation feedbacks substantially impact local temperature trends through changes in albedo and evapotranspiration. The physiological effect increases net biomass across most land areas, while the radiative effect results in an increase over the tundra and decrease over tropical forests and portions of the boreal forest.

Full access
Zhengyu Liu and S. G. H. Philander

Abstract

An oceanic GCM is used to investigate the response of the tropical and subtropical thermocline circulation and structure to different wind stress patterns. Although the subtropical winds do not affect the transport or the speed of the Equatorial Undercurrent significantly, they do change the equatorial temperature field in the lower part of the equatorial thermocline significantly. A weaker subtropical wind curl causes a cooling of the subsurface equatorial region and, hence, an intensification of the equatorial thermocline. A weakening of the subtropical wind curl by a factor of 2 cools the equatorial lower thermocline water by 2°C.

Full access
Haijun Yang, Yingying Zhao, and Zhengyu Liu

Abstract

A coupled box model is used to study the compensation between atmosphere and ocean heat transports. An analytical solution to the Bjerknes compensation (BJC) rate, defined as the ratio of anomalous atmosphere heat transport (AHT) to anomalous ocean heat transport (OHT), is obtained. The BJC rate is determined by local feedback between surface temperature and net heat flux at the top of atmosphere (TOA) and the AHT efficiency. In a stable climate that ensures global energy conservation, the changes between AHT and OHT tend to be always out of phase, and the BJC is always valid. This can be demonstrated when the climate is perturbed by freshwater flux. The BJC in this case exhibits three different behaviors: the anomalous AHT can undercompensate, overcompensate, or perfectly compensate the anomalous OHT, depending on the local feedback. Stronger negative local feedback will result in a lower BJC rate. Stronger positive local feedback will result in a larger overcompensation. If zero climate feedback occurs in the system, the AHT will compensate the OHT perfectly. However, the BJC will fail if the climate system is perturbed by heat flux. In this case, the changes in AHT and OHT will be in phase, and their ratio will be closely related to the mean AHT and OHT. In a more realistic situation when the climate is perturbed by both heat and freshwater fluxes, whether the BJC will occur depends largely on the interplay among meridional temperature and salinity gradients and the thermohaline circulation strength. This work explicitly shows that the energy conservation is the intrinsic mechanism of BJC and establishes a specific link between radiative feedback and the degree of compensation. It also implies a close relationship between the energy balance at the TOA and the ocean thermohaline dynamics.

Full access
Zhengyu Liu, Na Wen, and Lei Fan

Abstract

Three statistic methods [generalized equilibrium feedback analysis (GEFA), linear inverse modeling (LIM), and fluctuation–dissipation theorem (FDT)] are compared for their assessment of the atmospheric response to sea surface temperature variability in the coupled climate system with a sample length comparable with the observations (decades). The comparison is made first in an idealized coupled model and then in the observations. For daily to pentad data, for a linear stochastic system, the simple model study demonstrates that all three methods are able to provide a consistent assessment of the atmospheric response. For monthly data, GEFA is able to produce an assessment comparable with the daily or pentad assessments using the three methods. The consistence of the three methods is further confirmed in the observations for the responses of the atmospheric geopotential height (at 200 hPa) to the tropical ENSO mode and the North Pacific mode. It is found that the three methods produce a consistent response with the overall pattern correlation over 0.95 and the amplitude difference within 10%–20%. The consistent results in both the simple model and the observations suggest that the three statistical methods can be used as a cross validation on the robustness of the assessment of the atmospheric response to surface forcing in the observations.

Full access
Michael Notaro, Guangshan Chen, and Zhengyu Liu

Abstract

Vegetation feedbacks on climate, on the subannual time scale, are examined across six monsoon regions with a fully coupled atmosphere–ocean–ice–land model with dynamic vegetation. Initial value ensemble experiments are run in which the total vegetation cover fraction across the six monsoon regions is reduced and the climatic response assessed. Consistent responses among the regions include reductions in leaf area index, turbulent fluxes, and atmospheric moisture; enhanced subsidence; and increases in ground and surface air temperature. The most distinct changes in vertical motion, precipitable water, and precipitation occur along the flanks of the monsoon season, with small changes in midmonsoon rainfall. Unique responses to reduced vegetation cover are noted among the monsoon regions. While the monsoon is delayed and weaker over north Australia owing to diminished leaf area, it occurs earlier over China and the southwest United States. The subtropical monsoon regions are characterized by a larger decrease in sensible heat than latent heat flux, while the opposite is true for tropical monsoon regions. North Australia experiences the most substantial decline in both moisture flux convergence and precipitation.

Full access
Lixin Wu, Zhengyu Liu, and Harley E. Hurlburt

Abstract

The effect of continental slope on buoyancy-driven circulation has been studied using a two-layer quasigeostrophic model. In the model, buoyancy flux is incorporated as interfacial mass flux, which consists of narrow intense detrainment in the north and broad entrainment in the south. The model explicitly shows that, in the presence of the continental slope, a small amount of buoyancy flux can drive a strong barotropic flow. This flow develops because the beta effect of bottom topography either reduces or deflects the buoyancy-driven deep flow so that it cannot compensate its overlying counterflow, thus generating a net transport. As a result, in a double gyre circulation with a western continental slope, a small amount of detrainment/entrainment water mass can substantially enhance the transport of the western boundary current through southwestern deflection of the deep subpolar circulation. For example, with a reasonable western continental slope, a 10 Sv (Sv ≡ 106 m3 s−1) detrainment mass flux can increase the transport of the western boundary current from 40 Sv of the wind-driven transport to 148 Sv. Relevance to the North Atlantic is then discussed.

Full access