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Zhengyu Liu and Na Wen

Abstract

The equilibrium feedback assessment (EFA) is combined with the singular value decomposition (SVD) to assess the large-scale feedback modes from a lower boundary variability field onto an atmospheric field. The leading EFA-SVD modes are the optimal feedback modes, with the lower boundary forcing patterns corresponding to those that generate the largest atmospheric responses, and therefore provide upper bounds of the feedback response. The application of EFA-SVD to an idealized coupled ocean–atmosphere model demonstrates that EFA-SVD is able to extract the leading feedback modes successfully. Furthermore, these large-scale modes are the least sensitive to sampling errors among all the feedback processes and therefore are the most robust for statistical estimation. The EFA-SVD is then applied to the observed North Atlantic ocean–atmosphere system for the assessment of the sea surface temperature (SST) feedback on the surface heat flux and the geopotential height, respectively. The dominant local negative feedback of SST on heat flux is confirmed, with an upper bound of about 40 W m−2 K−1 for basin-scale anomalies. The SST also seems to exert a strong feedback on the atmospheric geopotential height: the optimal SST forcing has a dipole pattern that generates an optimal response of a North Atlantic Oscillation (NAO) pattern, with an upper bound of about 70 m K−1 at 500 hPa. Further issues on the EFA-SVD analysis are also discussed.

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Zhengyu Liu and Boyin Huang

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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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Haijun Yang and Zhengyu Liu

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The full spectrum of basin modes in a tropical–extratropical basin is studied comprehensively in a linear shallow-water system. Two sets of least-damped basin modes are identified. At the low-frequency end is the planetary wave basin mode, whose period is determined by the cross-basin time of the planetary wave on the poleward boundary of the basin, consistent with recent theories. At the higher-frequency end is the Kelvin wave basin mode, whose period is determined by the around-basin traveling time of the Kelvin wave. Sensitivity experiments are also performed on the eigenvalue problem to study the dynamics of these basin modes. It is found that the period of the planetary wave basin mode is determined by an effective basin boundary that is always at a latitude no higher than the geometric basin boundary. The effective poleward boundary is located at the most poleward latitude where the planetary wave can cross the entire basin. It is also found that the Kelvin wave basin modes are vulnerable to boundary perturbations. If the coastal Kelvin wave propagation is suppressed along the basin boundary, the Kelvin wave basin mode would degenerate to the equatorial basin mode that has been obtained theoretically from the long-wave approximation.

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Zhengyu Liu and Boyin Huang

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Based on results from analytic and general circulation models, the authors propose a theory for the coupled warm pool, cold tongue, and Walker circulation system. The intensity of the coupled system is determined by the coupling strength, the local equilibrium time, and latitudinal differential heating. Most importantly, this intensity is strongly regulated in the coupled system, with a saturation level that can be reached at a modest coupling strength. The saturation west–east sea surface temperature difference (and the associated Walker circulation) corresponds to about one-quarter of the latitudinal differential equilibrium temperature. This regulation is caused primarily by the decoupling of the SST gradient from a strong ocean current. The author’s estimate suggests that the present Pacific is near the saturation state. Furthermore, the much weaker Walker circulation system in the Atlantic Ocean is interpreted as being the result of the influence of the adjacent land, which is able to extend into the entire Atlantic to change the zonal distribution of the trade wind. The theory is also applied to understand the tropical climatology in coupled GCM simulations, in the Last Glacial Maximum climate, and in the global warming climate, as well as in the regulation of the tropical sea surface temperature.

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Lixin Wu and Zhengyu Liu

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Decadal variability in the North Pacific is studied in a series of coupled global ocean–atmosphere simulations using coupled modeling surgery—a set of modeling approaches that can be used to identify the origins and causes of a specific variability mode in the coupled climate system. Both modeling and observational studies suggest two distinctive internal modes in the North Pacific: the North Pacific mode (NPM) and the eastern North Pacific mode (ENPM). The ENPM originates from atmospheric stochastic forcing through spatial resonance. Both local ocean–atmosphere coupling and remote tropical teleconnective forcing can enhance the ENPM, but none of them is a necessary precondition. The influence of the tropical forcing in the midlatitudes is dominated by atmospheric teleconnection, while the oceanic teleconnection is negligible. The upper-ocean heat budget reveals that SST anomalies in the central North Pacific and the eastern North Pacific are generated by anomalous Ekman advection and surface heat flux, respectively. In contrast to the ENPM, the NPM critically depends on local ocean–atmosphere coupled feedback, although the atmospheric stochastic forcing can generate a NPM-like mode with much reduced amplitudes and no preferred timescale.

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Lixin Wu and Zhengyu Liu

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In this paper, the causes and mechanisms of North Atlantic decadal variability are explored in a series of coupled ocean–atmosphere simulations. The model captures the major features of the observed North Atlantic decadal variability. The North Atlantic SST anomalies in the model control simulation exhibit a prominent decadal cycle of 12–16 yr, and a coherent propagation from the western subtropical Atlantic to the subpolar region. A series of additional modeling experiments are conducted in which the air–sea coupling is systematically modified in order to evaluate the importance of air–sea coupling for the North Atlantic decadal variability being studied. This shall be referred to as “modeling surgery.” The results suggest the critical role of ocean–atmosphere coupling in sustaining the North Atlantic decadal oscillation at selected time scales. The coupling in the North Atlantic is characterized by a robust North Atlantic Oscillation (NAO)-like atmospheric response to the SST tripole anomaly, which tends to intensify the SST anomaly and, meanwhile, also provide a delayed negative feedback. This delayed negative feedback is predominantly associated with the adjustment of the subtropical gyre in response to the anomalous wind stress curl in the subtropical Atlantic. Atmospheric stochastic forcing can drive SST patterns similar to those in the fully coupled ocean–atmosphere system, but fails to generate any preferred decadal time scales. The simulated North Atlantic decadal variability, therefore, can be viewed as a coupled ocean–atmosphere mode under the influence of stochastic forcing.

This modeling study also suggests some potential resonance between the Pacific and the North Atlantic decadal fluctuations mediated by the atmosphere. The modeling surgery indicates that the Pacific climate, although not a necessary precondition, can impact the North Atlantic climate variability substantially.

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Zhengyu Liu and Huijun Yang

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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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Huijun Yang and Zhengyu Liu

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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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Yishuai Jin and Zhengyu Liu

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In this paper, we investigate the role of the period of El Niño–Southern Oscillation (ENSO) in the spring persistence barrier (SPB), mainly using the neutral recharge oscillator (NRO) model both analytically and numerically. It is suggested that a shorter ENSO period strengthens the SPB. Moreover, in contrast to the strict phase locking of the SPB in the Langevin equation, the phase of the SPB is no longer locked exactly to a particular time of the calendar year in the NRO model. Instead, the phases of the SPB for different initial months shift earlier with lag months of maximum persistence decline. In particular, the phase of the SPB will be shifted from the early summer to early spring, corresponding to the initial months of the early half year and later half year. This feature demonstrates that for the later half year, ENSO predictability decreases as the presence of ENSO period. For realistic parameters, the range of the phase change is modest, smaller than 2–3 months. A similar phase shift is also identified for the SPB in the damped ENSO regime, unstable ENSO regime, and observations. Our theory provides a null hypothesis for the role of ENSO period with regard to the SPB.

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Zhengyu Liu and Lixin Wu

Abstract

Atmospheric response to a midlatitude winter SST anomaly is studied in a coupled ocean–atmosphere general circulation model. The role of ocean–atmosphere coupling is examined with ensemble experiments of different coupling configurations. The atmospheric response is found to depend critically on ocean–atmosphere coupling. The full coupling experiment produces the strongest warm-ridge response and agrees the best with a statistical estimation of the atmospheric response. The fixed SST experiment and the thermodynamic coupling experiment also generate a warm-ridge response, but with a substantially weaker magnitude. This weaker warm-ridge response is associated with an excessive heat flux into the atmosphere, which tends to force an anomalous warm- low response and, therefore, weakens the warm-ridge response of the full coupling experiment.

This study suggests that the atmospheric response is associated with both the SST and heat flux. The SST forcing favors a warm-ridge response, while the heat flux forcing tends to be associated with a warm-low response. The correct atmospheric response is generated in the fully coupled model that produces the correct combination of SST and heat flux naturally.

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