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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

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

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

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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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Wei Liu and Zhengyu Liu

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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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Zhengyu Liu and Shangping Xie

Abstract

A simple coupled ocean–atmospheric boundary layer model is used to study the annual variability in the eastern tropical Pacific. The air–sea coupling, particularly the feedback of the total wind speed effect on evaporation and wind mixing entrainment, produces a rapid equatorward and westward propagation of annual disturbances. For reasonable parameters, both amplitude and phase of the annual disturbance can be reproduced fairly well over most of the Tropics. It is then suggested that a substantial part of equatorial annual variability may come from the extratropics (say beyond 15°) due to the propagation of coupled waves.

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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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Tomoko Inui and Zhengyu Liu

Abstract

An OGCM (MOM1) is used to examine the oceanic response to localized anomalous surface wind and buoyancy forcings. Wind stress and surface cooling anomalies are imposed at several different locations with respect to the positions of the mixed layer front and the LPVP (low potential vorticity path). Surface cooling locally creates sea surface temperature anomalies, which are subducted to the thermocline in remote places. The way in which wind anomalies affect the thermocline structure can be observed by using the following indicator. The LPVP is defined as a line that consists of water with minimum potential vorticity at each latitude. It is defined at each isopycnal surface and is affected through changes in the mixed layer depth or the position of the outcrop lines.

Sea surface height (SSH) anomalies created by localized anomalous wind stress forcing propagate westward at the same speed as the lower-thermocline depth anomalies, corresponding to the first baroclinic mode. When the forcing region is east of the LPVP, the depth of various isopycnal surfaces induces large variability in the region of the LPVP, caused either by propagation of the first baroclinic mode wave or variations in the mixed layer front position. These results imply that the subsurface temperature anomalies, associated with the change of isopycnal depths, are large in the vicinity of the LPVP, even if the wind stress anomaly is remote.

Previous studies suggest that propagation of subsurface temperature anomalies is forced primarily by surface cooling. In this work, the authors observe that temperature anomalies created by surface cooling primarily follow the subtropical circulation. However, it is shown that the subducted temperature anomalies may also be generated by remote wind-forcing effects, through their impact on the position of the LPVP.

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Boyin Huang and Zhengyu Liu

Abstract

The linear temperature trend of the last 40 yr (1955–94) in the upper Pacific Ocean above 400 m is studied using an objectively analyzed dataset and simulations of an ocean general circulation model. Both the data and simulations suggest a warming trend in the western tropical Pacific (10°S–10°N) near the surface and in the eastern tropical Pacific above 400 m but a cooling trend in the thermocline of the western tropical Pacific. In the midlatitude North Pacific (30°–50°N), the temperature trend is positive east of 150°W but negative to the west.

Simulated heat budget indicates that the temperature trend in the tropical Pacific may result from oceanic advection. In the central and western Pacific, the surface warming is associated with the reduction of cold advection from the off-equatorial divergent flow and the South Equatorial Current, while the cooling in the thermocline is related to the reduction of equatorward warm advection. In the eastern Pacific, the warming is associated with the reduction of upwelling. The reduction of these ocean currents, in turn, may result largely from the weakening of the trade winds.

In the midlatitude North Pacific, the ocean temperature trends similarly may result from the oceanic advection associated with the reduction of the westerlies. The effect of net surface heat flux into the ocean is a damping factor to the sea surface temperature. These studies highlight the importance of oceanic advection in producing long-term temperature trends.

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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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