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Haijun Yang and Zhengyu Liu

Abstract

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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Yishuai Jin and Zhengyu Liu

Abstract

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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Sang-Ik Shin and Zhengyu Liu

Abstract

The GFDL Modular Ocean Model and the Miami Isopycnal Ocean Model are used to investigate the response of the equatorial thermocline to extratropical buoyancy forcing. Passive tracers and analytical theories are also used to shed light on the dynamics of the thermocline response. The major findings are the following. (i) The midlatitude region seems to be the optimal region for surface buoyancy forcing to affect the equatorial thermocline. This occurs because, first, thermocline anomalies in the midlatitudes can penetrate into the equator very efficiently; second, buoyancy forcing generates a strong local response in the midlatitudes. (ii) Dynamic waves as well as thermocline ventilation contribute to the response in the equatorial thermocline. Consequently, equatorward penetration is substantially greater for a temperature anomaly than for a passive tracer. (iii) Midlatitude forcing generates a significant temperature response in the equatorial thermocline for forcing periods longer than decadal. (iv) For a low latitude (10°–20°) buoyancy forcing, the equatorial thermocline could be dominated by a temperature anomaly that has the opposite sign to the surface forcing because of the strong higher mode baroclinic response in the ventilated thermocline. Finally, the relevance of this work to observations and climate variability is also discussed.

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Rong-Hua Zhang and Zhengyu Liu

Abstract

Yearly in situ temperature anomaly data in the North Pacific Ocean for 1961–90 have been analyzed along constant-density surfaces (isopycnals) in order to better describe and understand decadal thermocline variability in the region. Various empirical orthogonal function analyses are performed on isopycnals to depict the dominant three-dimensional patterns. The major finding is of two preferential pathways associated with decadal temperature variability around the subtropical gyre. A subduction pathway, with a large signal in the upper thermocline, originates from the North Pacific central–eastern outcrop regions (about 40°N, 150°W) and then basically follows the mean gyre circulation southwestward along isopycnals toward the western Tropics. A subtropical pathway extends from the eastern subtropical–tropical and boundary regions and appears to continue predominantly westward across the southern part of the gyre (between 15° and 30°N) and then along the Kuroshio path toward the midlatitudes. Along these two pathways, thermal anomalies show coherent phase relationships to one another in the surface layer and in the thermocline around the gyre, with their source regions (variability centers) being out of phase on decadal timescales. Two examples of each type of anomaly pattern can be illustrated for the periods analyzed. In the 1960s, a negative temperature anomaly signal propagated predominantly westward across the subtropics, followed by a subducted warm anomaly from the outcrop region in the early 1970s that subsequently moved southwestward along isopycnals toward the western Tropics. A similar pattern was observed in the late 1970s and in the 1980s but with the opposite sign: a westward propagating positive temperature anomaly signal along the subtropics in the late 1970s through the 1980s, and a subducted cold anomaly in the early 1980s that also made its way southwestward with the expected gyre circulation to the western Tropics in the late 1980s. It is suggested that the southwestward subduction pathway provides a mechanism that connects surface anomalies in the outcrop region to thermocline variations in the western subtropics and in the Tropics, and that the westward subtropical pathway presents a possible link of tropical–subtropical variability to surface temperature anomalies around the Kuroshio and its extension regions, which may further force variations in the overlying atmospheric circulation in the midlatitudes. The results provide an observational basis for verification of theoretical studies and model simulations.

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Feiyu Lu and Zhengyu Liu

Abstract

The extratropical influence on the observed events of El Niño–Southern Oscillation (ENSO) variability from 1948 to 2015 is assessed by constraining the extratropical atmospheric variability in a coupled general circulation model (CGCM) using the regional coupled data assimilation (RCDA) method. The ensemble-mean ENSO response to extratropical atmospheric forcing, which is systematically and quantitatively studied through a series of RCDA experiments, indicates robust extratropical influence on some observed ENSO events. Furthermore, an event-by-event quantitative analysis shows significant differences of the extratropical influence among the observed ENSO events, both in its own strength and in its relation to tropical precursors such as the equatorial Pacific heat content anomaly. This study provides the first dynamic quantitative assessment of the extratropical influence on observed ENSO variability on an event-by-event basis.

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Sheng Wu and Zheng-Yu Liu

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We investigate the response of decadal variability in the North Pacific and North Atlantic under global warming and its mechanism in this study. To do so, we use four models (BCC-CSM1–1, CCSM4, IPSL-CM5A-LR, and MPI-ESM-LR) that participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5), focusing on three global warming scenarios (RCP2.6, RCP4.5, and RCP8.5). Our analysis shows that the intensified global warming leads to a decrease in amplitude of both the Pacific decadal oscillation (PDO) and Atlantic multidecadal variability (AMV), resulting in reduced decadal variability of sea surface temperature (SST) in both the North Pacific and North Atlantic. In comparison, interannual variability is less impacted by global warming and has a tendency to increase, which leads to a shift of spectral power from decadal toward interannual variability. We then show the weakening decadal variability is caused partly by the weakened forcing of atmospheric heat flux variability, and partly by the increased SST damping rate. In addition, an enhanced upper-ocean stratification under global warming also contributes to the acceleration of Rossby waves, and a shift of decadal variability spectral power toward a shorter period.

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Jianling Yang, Qinyu Liu, and Zhengyu Liu

Abstract

The authors investigate the relationship between sea surface temperature (SST) in the tropical Indian Ocean (TIO) and the seasonal atmosphere circulation in the Asian monsoon region (AMR) using the maximum covariance analyses (MCAs). The results show that the Asian monsoon circulation is significantly correlated with two dominant SST anomaly (SSTA) modes: the Indian Ocean Basin mode (IOB) and the Indian Ocean dipole mode (IOD). The peak SSTA of the IOB appears in spring and has a much stronger relationship with the Asian summer monsoon than the peak of the IOD does, whereas the peak SSTA for the IOD appears in fall and shows a stronger link to the Asian winter monsoon than to the Asian summer monsoon. In addition, the IOB in spring has a relatively stronger link with the atmospheric circulation in summer than in other seasons.

The large-scale atmospheric circulation and SSTA patterns of the covariability of the first two dominant MCA modes are described. For the first MCA mode, a warm IOB, persists from spring to summer, and the atmospheric circulation is enhanced by the establishment of the climatological summer monsoon. The increased evaporative moisture associated with the warm IOB is transported to South Asia by the climatological summer monsoon, which increases the moisture convergence toward this region, leading to a significant increase in summer monsoon precipitation. For the second MCA mode, a positive IOD possibly corresponds to a weaker Indian winter monsoon and more precipitation over the southwestern and eastern equatorial TIO.

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Dong Eun Lee, Zhengyu Liu, and Yun Liu

Abstract

Prescribing sea surface temperature (SST) for the atmospheric general circulation models (GCM) may not lead to underestimation of the coupled variability. In this study, a set of SST-driven atmospheric GCM experiments, starting from slightly different multiple initial conditions, is performed. The SST used here is prepared by a coupled GCM, which has the same atmospheric GCM component as the AGCM used in the experiment with the prescribed SST. The results indicate that prescribing SST leads to underestimation of the coupled air temperature variance only in subtropics. Meanwhile, in midlatitudes, prescribing SST may result in the overestimation of the coupled air temperature variance, where the major role of ocean–atmosphere contrast is to provide damping for SST.

The simple stochastically driven coupled model is revisited with an extension to the direct wind-driven forcing for SST. In addition to the previous setup relying exclusively on the stochastic perturbation for air temperature, the ocean temperature is also forced by the pure random wind. By this extended model, it is speculated that the coupled air temperature variance can be overestimated by prescribing SST, depending on the sensitivity of SST to the wind-driven heat flux. The midlatitude is the most probable place for the overestimation since the wind-driven ocean dynamics can enhance the wind-driven surface heat flux due to the dominant zonal wind anomaly.

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Wei Liu, Zhengyu Liu, and Esther C. Brady

Abstract

This paper is concerned with the question: why do coupled general circulation models (CGCM) seem to be biased toward a monostable Atlantic meridional overturning circulation (AMOC)? In particular, the authors investigate whether the monostable behavior of the CGCMs is caused by a bias of model surface climatology. First observational literature is reviewed, and it is suggested that the AMOC is likely to be bistable in the real world in the past and present. Then the stability of the AMOC in the NCAR Community Climate System Model, version 3 (CCSM3) is studied by comparing the present-day control simulation (without flux adjustment) with a sensitivity experiment with flux adjustment. It is found that the monostable AMOC in the control simulation is altered to a bistable AMOC in the flux-adjustment experiment because a reduction of the surface salinity biases in the tropical and northern North Atlantic leads to a reduction of the bias of freshwater transport in the Atlantic. In particular, the tropical bias associated with the double ITCZ reduces salinity in the upper South Atlantic Ocean and, in turn, the AMOC freshwater export, which tends to overstabilize the AMOC and therefore biases the AMOC from bistable toward monostable state. This conclusion is consistent with a further analysis of the stability indicator of two groups of IPCC Fourth Assessment Report (AR4) CGCMs: one without and the other with flux adjustment. Because the tropical bias is a common feature among all CGCMs without flux adjustment, the authors propose that the surface climate bias, notably the tropical bias in the Atlantic, may contribute significantly to the monostability of AMOC behavior in current CGCMs.

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Zhengyu Liu, Haijun Yang, and Qinyu Liu

Abstract

Dynamics of the seasonal cycle of sea surface height (SSH) in the South China Sea (SCS) are studied using observations as well as numerical and theoretical models. Seasonal variability of the SCS is interpreted in light of large-scale dynamics and Rossby waves. It is found that the seasonal cycle over most of the SCS basin is determined predominantly by the regional ocean dynamics within the SCS. The SSH variability is shown to be forced mainly by surface wind curl on baroclinic Rossby waves. Annual baroclinic Rossby waves cross the basin in less than a few months, leaving the upper ocean in a quasi-steady Sverdrup balance. An anomalous cyclonic (anticyclonic) gyre is generated in winter (summer) by the anomalous cyclonic (anticyclonic) wind curl that is associated with the northeasterly (southwesterly) monsoon. In addition, surface heat flux acts to enhance the wind-generated variability. The winter surface cooling (warming) cools (warms) the mixed layer especially in the central SCS, reducing (increasing) the SSH.

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