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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 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 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 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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Zhengyu Liu

Abstract

The study of a forced delayed oscillator ENSO model suggests that the intensity of ENSO can be suppressed significantly by an external periodic forcing due to the nonlinear mechanism of frequency entrainment. This suppression of ENSO is most effective for ENSOs in the regime of unstable self-exciting oscillation and for forcing of frequencies close to that of ENSO. In particular, an annual cycle forcing can suppress ENSO substantially. This ENSO suppression effect by an external annual cycle is in contrast to the effect of the seasonal change of the coupled instability: the latter predominantly generates the seasonal phase locking of ENSO but has little effect on the amplitude of ENSO. Potential implications are also discussed for the evolution of ENSO in the Holocene and the observed monsoon–ENSO relationship.

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Wei Liu and Zhengyu Liu

Abstract

A diagnostic indicator ΔM ov is proposed in this paper to monitor the stability of the Atlantic meridional overturning circulation (AMOC). The ΔM ov is a diagnostic for a basinwide salt-advection feedback and defined as the difference between the freshwater transport induced by the AMOC across the southern border of the Atlantic Ocean and the overturning liquid freshwater transport from the Arctic Ocean to the North Atlantic. As validated in the Community Climate System Model, version 3 (CCSM3), for an AMOC in the conveyor state, a positive ΔM ov (freshwater convergence) in the Atlantic basin indicates a monostable AMOC and a negative ΔM ov (freshwater divergence) indicates a bistable AMOC. Based on ΔM ov, the authors investigate the AMOC stability in the Last Glacial Maximum (LGM) and analyze the modulation of the AMOC stability by an open/closed Bering Strait. Moreover, the authors estimate that the real AMOC is likely to be bistable in the present day, since some observations suggest a negative ΔM ov (freshwater divergence) is currently in the Atlantic basin. However, this estimation is very sensitive to the choice of the observational data.

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Wei Liu and Zhengyu Liu

Abstract

This study examines the validity of the net freshwater transport ΔM ov as a stability indicator of the Atlantic meridional overturning circulation (AMOC) in a low-resolution version of the NCAR Community Climate System Model, version 3 (CCSM3). It is shown that the sign of ΔM ov indicates the monostability or bistability of the AMOC, which is based on a hypothesis that a collapsed AMOC induces a zero net freshwater transport. In CCSM3, this hypothesis is satisfied in that the collapsed AMOC, with a nonzero strength, induces a zero net freshwater transport ΔM ov across the Atlantic basin by generating equivalent freshwater export M ovS and freshwater import M ovN at the southern and northern boundaries, respectively. Because of the satisfaction of the hypothesis, ΔM ov is consistent with a generalized indicator L for a slowly evolving AMOC, both of which correctly monitor the AMOC stability.

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Xiaojie Zhu and Zhengyu Liu

Abstract

The trend of sea surface temperature (SST) in the twentieth century is examined in observations and the Intergovernmental Panel on Climate Change (IPCC) twentieth-century simulations. The observed SST neither shows a clear signal of the enhanced equatorial response (EER) warming nor exhibits a clear trend of the El Niño–like warming in the last century. Similarly, the IPCC simulations show neither a clear EER warming nor an El Niño–like warming in the last century. Furthermore, the comparison of heat fluxes in model simulations of the global warming scenario and the twentieth century indicates that the aerosol cooling effect, opposite to the greenhouse gases warming effect, plays an important role in the twentieth century and explains the EER-like signal in the twentieth-century simulations. Therefore, a conclusion that the IPCC model simulations of the twentieth century are consistent with observations within the error bars as well as the future projection of the EER warming pattern in the global warming scenario are validated.

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Michael Notaro and Zhengyu Liu

Abstract

The authors demonstrate that variability in vegetation cover can potentially influence oceanic variability through the atmospheric bridge. Experiments aimed at isolating the impact of variability in forest cover along the poleward side of the Asian boreal forest on North Pacific SSTs are performed using the fully coupled model, Fast Ocean Atmosphere Model–Lund Potsdam Jena (FOAM-LPJ), with dynamic atmosphere, ocean, and vegetation. The northern edge of the simulated Asian boreal forest is characterized by substantial variability in annual forest cover, with an east–west dipole pattern marking its first EOF mode. Simulations in which vegetation cover is allowed to vary over north/central Russia exhibit statistically significant greater SST variance over the Kuroshio Extension. Anomalously high forest cover over North Asia supports a lower surface albedo with higher temperatures and lower sea level pressure, leading to a reduction in cold advection into northern China and in turn a decrease in cold air transport into the Kuroshio Extension region. Variability in the large-scale circulation pattern is indirectly impacted by the aforementioned vegetation feedback, including the enhancement in upper-level jet wind variability along the north–south flanks of the East Asian jet stream.

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Yafang Zhong and Zhengyu Liu

Abstract

Atmospheric response to North Pacific oceanic variability is assessed in Community Climate System Model, version 3 (CCSM3) using two statistical methods and one dynamical method. All methods identify an equivalent barotropic low response to a warmer sea surface temperature (SST) anomaly in the Kuroshio Extension region (KOE) during early–midwinter. While all three methods capture the major features of the response, the generalized equilibrium feedback assessment method (GEFA) isolates the impact of KOE SST from a complex context, and thus makes itself an excellent choice for similar practice.

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