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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 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

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

Abstract

A simple ventilated thermocline model is used to study the subtropical-tropical mass exchange. It is found that the water subducted in the western subtropical gyre (recirculating window) tends to recirculate within the subtropical gyre. while the water subducted in the eastern part (exchange window) tends to penetrate equatorward. The exchange window expands with an increased easterly wind or basin width on the southern boundary of the subtropical gyre, but shrinks with an increased wind curl within the subtropical gyre.

Furthermore, the total exchange transport increases with the easterly wind or the width of the basin on the southern boundary of the subtropical gyre, but it is independent of subtropical wind. The ventilation mechanism is important in supporting the exchange transport. For wind with realistic strength at the southern boundary, the reduction of the exchange transport is about 15%–30% of the Ekman transport.

Finally, relative to the exchange transport in the interior of the ocean, the exchange transport through the low-latitude western boundary current decreases with increased total exchange transport.

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Zhengyu Liu

Abstract

The response of a thermocline gyre to anomalies in surface wind stress forcing and surface buoyancy forcing is investigated in light of planetary wave dynamics, both analytically and numerically. The author’s theory suggests that anomalous Ekman pumping most efficiently generates the non-Doppler-shift mode, which resembles the first baroclinic mode and has the clearest signal in the sea surface height field and the lower thermocline temperature field. The non-Doppler-shift mode propagates westward rapidly regardless of the mean circulation. In contrast, anomalous surface buoyancy forcing, which can be simulated by an entrainment velocity, produces the strongest response in the advective mode, which resembles the second baroclinic mode and has the largest signature in the upper thermocline temperature field. The advective mode tends to propagate in the direction of the subsurface flow, but its propagation speed may differ substantially from that of the mean flow. The theory is further substantiated by numerical experiments in three ocean models: a 3-layer eddy-resolving quasigeostrophic model, a 2.5-layer primitive equation model, and an oceanic general circulation model. Finally, relevance of the theory to recent observations of decadal variability in the upper ocean and the climate system is also discussed.

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Zhengyu Liu and Boyin Huang

Abstract

It is suggested that the tritium maximum in the central Pacific is caused by two water pathways across the North Equatorial Countercurrent (NECC), one from the central Pacific and the other from the Mindanao Current. It is argued that an interior pathway exists, by which tritium-rich thermocline waters from the subtropical North Pacific cross the NECC in the central Pacific. The transport in this pathway, however, is small compared with that from the Mindanao Current.

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Huijun Yang and Zhengyu Liu

Abstract

The aim of this paper is to renew interest in the Lagrangian view of global and basin ocean circulations and its implications in physical and biogeochemical ocean processes. The paper examines the Lagrangian transport, mixing, and chaos in a simple, laminar, three-dimensional, steady, basin-scale oceanic flow consisting of the gyre and the thermohaline circulation mode. The Lagrangian structure of this flow could not be chaotic if the steady oceanic flow consists of only either one of the two modes nor if the flow is zonally symmetric, such as the Antarctic Circumpolar Current. However, when both the modes are present, the Lagrangian structure of the flow is chaotic, resulting in chaotic trajectories and providing the enhanced transport and mixing and microstructure of a tracer field. The Lagrangian trajectory and tracer experiments show the great complexity of the Lagrangian geometric structure of the flow field and demonstrate the complicated transport and mixing processes in the World Ocean. The finite-time Lyapunov exponent analysis has successfully characterized the Lagrangian nature. One of the most important findings is the distinct large-scale barrier—which the authors term the great ocean barrier—within the ocean interior with upper and lower branches, as remnants of the Kolmogorov–Arnold–Moser (KAM) invariant surfaces. The most fundamental reasons for such Lagrangian structure are the intrinsic nature of the long time mean, global and basin-scale oceanic flow: the three-dimensionality and incompressibility giving rise to chaos and to the great ocean barrier, respectively. Implications of these results are discussed, from the great ocean conveyor hypothesis to the predictability of the (quasi) Lagrangian drifters and floats in the climate observing system.

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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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Zhengyu Liu and Huijun Yang

Abstract

The effect of the annual migration of the wind field on the intergyre transport is investigated in a double-gyre circulation. It is found that the trajectories of the water columns advected by the gyre-scale circulation exhibit a strongly chaotic behavior. The resulted cross-gyre chaotic transport amounts to about one-third of the Sverdrup transport.

The chaotic intergyre transport causes strong mixing between the two gyres. The study with a passive tracer shown that the equivalent diffusivity of the chaotic mixing is at the order of 107 cm2, s−1, comparable to that estimated for strong synoptical eddies in the region of the Gulf Stream. It is suggested that the chaotic transport may contribute significantly to the intergyre exchange.

Further parameter sensitivity studies show that the chaotic transport is the strongest under the migration with frequencies from interannual to decadal, and with the migration distance about 1000 km. Some possible applications of the chaotic transport to the general oceanic circulation are also discussed.

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