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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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Yafang Zhong and Zhengyu Liu

Abstract

Previous analyses of the Community Climate System Model, version 3 (CCSM3) standard integration have revealed pronounced multidecadal variability in the Pacific climate system. The purpose of the present work is to investigate physical mechanism underlying this Pacific multidecadal variability (PMV). To better isolate the mechanism that selects the long multidecadal time scale for the PMV, a few specifically designed sensitivity experiments are carried out. When the propagating Rossby waves are blocked in the subtropics from the midbasin, the PMV remains outstanding. In contrast, when the Rossby waves are blocked beyond the subtropics across the entire North Pacific, the PMV is virtually suppressed. It suggests that the PMV relies on propagating Rossby waves in the subpolar Pacific, whereas those in the subtropics are not critical.

A novel mechanism of PMV is advanced based on a more comprehensive analysis, which is characterized by a crucial role of the subpolar North Pacific Ocean. The multidecadal ocean temperature and salinity anomalies may originate from the subsurface of the subpolar North Pacific because of the wave adjustment to the preceding basin-scale wind curl forcing. The anomalies then ascend to the surface and are amplified through local temperature–salinity convective feedback. Along the southward Oyashio, these anomalies travel to the Kuroshio Extension (KOE) region and are further intensified through a similar convective feedback. The oceanic temperature anomaly in the KOE is able to feed back to the large-scale atmospheric circulation, inducing a wind curl anomaly over the subpolar North Pacific, which in turn generates anomalous oceanic circulation and causes temperature and salinity variability in the subpolar subsurface. Thereby, a closed loop of PMV is established in the form of an extratropical delayed oscillator. The phase transition of PMV is driven by the delayed negative feedback that resides in the wave adjustment of the subpolar North Pacific via propagating Rossby waves, whereas the convective positive feedback provides the growth mechanism. A significant role of salinity variability is unveiled in both the delayed negative feedback and convective positive feedback.

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Yishuai Jin and Zhengyu Liu

Abstract

In this paper, we investigate the potential factors that control the relationship between the El Niño-Southern Oscillation (ENSO) persistence barriers (PB) in sea surface temperature (SST) and ocean heat content (OHC) and apply it to explain observational ENSO PBs. With the addition of seasonal growth rate in SST in the neutral recharge oscillator (NRO) model, approximate analytical solutions of autocorrelation functions for SST and OHC suggest strictly that the timing of PB for OHC leads that of SST by half a year and the strength of the two PBs are the same. The numerical solutions of the NRO model also show a similar relationship. The role of ENSO growth rate to PBs in SST and OHC is then identified in the damped and unstable ENSO regime. Therefore, it is suggested that for the observational ENSO, the seasonally varying ENSO growth rate in SST controls PBs in SST and OHC simultaneously.

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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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Sheng Wu and Zheng-Yu Liu

Abstract

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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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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Na Wen, Zhengyu Liu, and Qinyu Liu

Abstract

Most previous studies have proven the local negative heat flux feedback (the surface heat flux response to SST anomalies) in the midlatitude areas. However, it is uncertain whether a nonlocal heat flux feedback can be observed. In this paper, the generalized equilibrium feedback assessment (GEFA) method is employed to examine the full surface turbulent heat flux response to SST in the North Atlantic Ocean using NCEP–NCAR reanalysis data. The results not only confirm the dominant local negative feedback, but also indicate a robust nonlocal positive feedback of the Gulf Stream Extension (GSE) SST to the downstream heat flux in the subpolar region. This nonlocal feedback presents a strong seasonality, with response magnitudes of in winter and in summer. Further study indicates that the nonlocal effect is initiated by the adjustments of the downstream surface wind to the GSE SST anomalies.

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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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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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