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Terrence M. Joyce and Rong Zhang

Abstract

The Atlantic meridional overturning circulation (AMOC) simulated in various ocean-only and coupled atmosphere–ocean numerical models often varies in time because of either forced or internal variability. The path of the Gulf Stream (GS) is one diagnostic variable that seems to be sensitive to the amplitude of the AMOC, yet previous modeling studies show a diametrically opposed relationship between the two variables. In this note this issue is revisited, bringing together ocean observations and comparisons with the GFDL Climate Model version 2.1 (CM2.1), both of which suggest a more southerly (northerly) GS path when the AMOC is relatively strong (weak). Also shown are some examples of possible diagnostics to compare various models and observations on the relationship between shifts in GS path and changes in AMOC strength in future studies.

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Rong Zhang and Thomas L. Delworth

Abstract

In this study, a mechanism is demonstrated whereby a large reduction in the Atlantic thermohaline circulation (THC) can induce global-scale changes in the Tropics that are consistent with paleoevidence of the global synchronization of millennial-scale abrupt climate change. Using GFDL’s newly developed global coupled ocean–atmosphere model (CM2.0), the global response to a sustained addition of freshwater to the model’s North Atlantic is simulated. This freshwater forcing substantially weakens the Atlantic THC, resulting in a southward shift of the intertropical convergence zone over the Atlantic and Pacific, an El Niño–like pattern in the southeastern tropical Pacific, and weakened Indian and Asian summer monsoons through air–sea interactions.

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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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Yuchao Zhu and Rong-Hua Zhang

Abstract

Climate models suffer from significant biases over the tropical Pacific Ocean, including a too-cold cold tongue and too-warm temperature at the depth of the thermocline. The emergence of model biases can be partly attributed to vertical mixing parameterizations, in which there are great uncertainties in selections of functional forms and empirical parameters. In this paper, the impacts of two different vertical mixing schemes on the tropical Pacific temperature simulations are investigated using version 5 of the Modular Ocean Model (MOM5). One vertical mixing scheme is the widely used K-profile parameterization (KPP) scheme, and the other is a hybrid mixing scheme (the Chen scheme) by combining a Kraus–Turner-type bulk mixed layer (ML) model with Peters et al.’s shear instability mixing model (PGT model). It is shown that the Chen scheme works better than the KPP scheme for SST simulation but produces an exaggerated subsurface warm bias simultaneously. The better SST simulation can be attributed to the employment of the PGT model, which produces lower levels of shear instability mixing than its counterpart in the KPP scheme. Furthermore, a modified KPP scheme is presented in which its shear instability mixing model and constant background diffusivity are replaced by the PGT model and the Argo-derived background diffusivity, respectively. This new scheme is then employed into MOM5-based ocean-only and coupled simulations, demonstrating substantial improvements in temperature simulations over the tropical Pacific. The modified KPP scheme can be easily employed into other ocean models, offering an effective way to improve ocean simulations.

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Rong-Hua Zhang and Sydney Levitus

Abstract

Yearly upper-ocean in situ temperature anomaly data for the period 1961–90 are analyzed to reveal spatial structure and evolution of decadal variability in the North Pacific Ocean. An EOF analysis has been performed on individual temperature anomaly fields at upper-ocean standard levels, as well as simultaneously on the entire upper-ocean data to depict the combined three-dimensional structure in a coherent manner. Time evolution of anomaly fields is depicted by using a regression analysis.

The analyses detect the principal basin-scale structure of decadal warm period (DWP) and decadal cold period (DCP). There is a well-defined subsurface thermal anomaly pattern, characterized by a prominent seesaw structure with opposite anomaly polarity between the midlatitude North Pacific and the subtropical regions. During a DWP, a positive temperature anomaly is found in the central midlatitude upper ocean, with the maximum at about 100-m depth. This is accompanied by a corresponding negative anomaly in the American coastal region and in the subtropics. A reverse pattern of these anomalies is observed during the DCP. Evolution between the DWP and the DCP involves significant zonal and meridional propagation of anomaly phase around the North Pacific, showing consistent and coherent variations from subsurface to sea surface, from central midlatitudes to the American coastal regions, and to the subtropical Pacific Ocean. This phase propagation is much more well-organized at subsurface depths than that at the sea surface, suggesting an anomaly decadal-scale cycle circulating clockwise around the subtropical gyre, which supports earlier findings by . There is a systematic and coherent westward transpacific phase propagation in the subtropical region.

These analyses present evidence of the manner in which upper-ocean temperature anomalies evolved in the North Pacific, thus providing an observational basis for evaluating theoretical studies and model simulations. The dynamical implication for physical understanding and prediction of decadal climate variability are discussed.

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Rong-Hua Zhang and Sydney Levitus

Abstract

Upper-ocean temperature and surface marine meteorological observations are used to examine interannual variability of the coupled tropical Pacific climate system. The basinwide structure and evolution of meteorological and oceanographic fields associated with ENSO events are described using composites, empirical orthogonal functions, and a lagged correlation analysis.

The analyses reveal well-defined spatial structures and coherent phase relations among various anomaly fields. There are prominent seesaw patterns and orderly movement of subsurface ocean thermal anomalies. During an El Niño year, positive temperature anomalies occur in the eastern and central tropical Pacific upper ocean. Westerly wind anomalies, displaced well to the west of SST anomalies, occur over the western and central equatorial region. These patterns are accompanied by subsurface negative temperature anomalies in the west, with maxima located at thermocline depths off the equator. A reverse pattern is observed during La Niña.

The ENSO evolution is characterized by a very slow propagation of subsurface thermal anomalies around the tropical Pacific basin, showing consistent and coherent oceanic variations in the west and in the east, at subsurface depths and at the sea surface, and on the equator and off the equator of the tropical North Pacific. A common feature associated with the onset of El Niño is an appearance of subsurface thermal anomalies in the western Pacific Ocean, which propagate systematically eastward along the equator. Their arrival to the east results in a reversal of SST anomaly polarity, which then correspondingly produces surface wind anomalies in the west, which in turn produce and intensify the subsurface anomalies off the equator, thus terminating one phase of the Southern Oscillation. At the same time, the continual anomaly movement at depth from east to west off the equator provides a phase transition mechanism back to the west. In due course, opposite anomalies are located in the subsurface equatorial western Pacific, introducing an opposite SO phase and beginning a new cycle. Therefore, the phase transitions at the sea surface in the east and at depth in the west are both caused by these preferential, slowly propagating subsurface temperature anomalies, which are essential to the ENSO evolution. Their cycling time around the tropical Pacific basin may determine the period of the El Niño occurrence.

The authors’ data analyses show an important role of the thermocline displacement in producing and phasing SST anomalies in the eastern and central equatorial Pacific. The coherent subsurface anomaly movement and its phase relation with SST and surface winds determine the nature of interannual variability and provide an oscillation mechanism for the tropical Pacific climate system. It appears that interannual variability represents a slowly evolving air–sea coupled mode, rather than individual free oceanic Rossby and Kelvin wave modes. These results provide an observational basis for verifying theoretical studies and model simulations.

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Rong Zhang and Geoffrey K. Vallis

Abstract

The mechanisms affecting the path of the depth-integrated North Atlantic western boundary current and the formation of the northern recirculation gyre are investigated using a hierarchy of models, namely, a robust diagnostic model, a prognostic model using a global 1° ocean general circulation model coupled to a two-dimensional atmospheric energy balance model with a hydrological cycle, a simple numerical barotropic model, and an analytic model. The results herein suggest that the path of this boundary current and the formation of the northern recirculation gyre are sensitive to both the magnitude of lateral viscosity and the strength of the deep western boundary current (DWBC). In particular, it is shown that bottom vortex stretching induced by a downslope DWBC near the south of the Grand Banks leads to the formation of a cyclonic northern recirculation gyre and keeps the path of the depth-integrated western boundary current downstream of Cape Hatteras separated from the North American coast. Both south of the Grand Banks and at the crossover region of the DWBC and Gulf Stream, the downslope DWBC induces strong bottom downwelling over the steep continental slope, and the magnitude of the bottom downwelling is locally stronger than surface Ekman pumping velocity, providing strong positive vorticity through bottom vortex-stretching effects. The bottom vortex-stretching effect is also present in an extensive area in the North Atlantic, and the contribution to the North Atlantic subpolar and subtropical gyres is on the same order as the local surface wind stress curl. Analytic solutions show that the bottom vortex stretching is important near the western boundary only when the continental slope is wider than the Munk frictional layer scale.

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Rong Zhang and Geoffrey K. Vallis

Abstract

In this paper, it is shown that coherent large-scale low-frequency variabilities in the North Atlantic Ocean—that is, the variations of thermohaline circulation, deep western boundary current, northern recirculation gyre, and Gulf Stream path—are associated with high-latitude oceanic Great Salinity Anomaly events. In particular, a dipolar sea surface temperature anomaly (warming off the U.S. east coast and cooling south of Greenland) can be triggered by the Great Salinity Anomaly events several years in advance, thus providing a degree of long-term predictability to the system. Diagnosed phase relationships among an observed proxy for Great Salinity Anomaly events, the Labrador Sea sea surface temperature anomaly, and the North Atlantic Oscillation are also discussed.

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Qiushi Zhang, Yuchao Zhu, and Rong-Hua Zhang

Abstract

Realistic ocean subsurface simulations of thermal structure and variation are critically important to success in climate prediction and projection; currently, substantial systematic subsurface biases still exist in the state-of-the-art ocean and climate models. In this paper, subsurface biases in the tropical Atlantic Ocean (TA) are investigated by analyzing simulations from the Ocean Model Intercomparison Project (OMIP) and conducting ocean-only experiments that are based on the Parallel Ocean Program, version 2 (POP2). The subsurface biases are prominent in almost all OMIP simulations, characterized by two warm-bias patches off the equator. By conducting two groups of POP2-based ocean-only experiments, two potential origins of the biases are explored, including uncertainties in wind forcing and vertical mixing parameterization, respectively. It is illustrated that the warm bias near 10°N can be slightly reduced by modulating the prescribed wind field, and the warm biases over the entire basin are significantly reduced by reducing background diffusivity in the ocean interior in ways to match observations. By conducting a heat-budget analysis, it is found that the improved subsurface simulations are attributed to the enhanced cooling effect by constraining the vertical mixing diffusivity in terms of the observational estimate, implying that overestimation of vertical mixing is primarily responsible for the subsurface warm biases in the TA. Since the climate simulation is very sensitive to the vertical mixing parameterization, more accurate representations of ocean vertical mixing are clearly needed in ocean and climate models.

Significance Statement

The purpose of our study is to analyze the characteristics of subsurface temperature biases in the tropical Atlantic Ocean and to investigate the causes for the biases. This is important because subsurface biases greatly reduce the reliability of models in climate prediction and projection. It is found that significant subsurface warm biases arise in 100–150 m over the entire tropical Atlantic basin and the biases are mainly attributed to overestimated ocean vertical mixing. Our work highlights that subsurface ocean simulations are highly sensitive to vertical mixing parameterization, and further research is necessary for more accurate representations of ocean vertical mixing in ocean and climate modeling.

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Rong-Hua Zhang and Stephen E. Zebiak

Abstract

An embedding approach is developed and tested to improve El Niño–Southern Oscillation (ENSO) simulations in a hybrid coupled model (HCM), focusing on the ocean thermocline effects on sea surface temperature (SST) in the eastern equatorial Pacific. The NOAA/GFDL Modular Ocean Model (MOM 3) is coupled to a statistical atmospheric model that estimates wind stress anomalies based on a singular value decomposition (SVD) of the covariance between observed wind stress and SST anomalies. Analogous to the Cane–Zebiak (CZ) coupled model, a separate SST anomaly model is explicitly embedded into the z-coordinate ocean general circulation model (OGCM). The three components exchange predicted anomalies within the coupled system: The OGCM provides anomalies of ocean currents in the surface mixed layer and the thermocline depth, which are used to calculate SST anomalies from the embedded SST model; wind anomalies are then determined according to the statistical atmospheric model, which in turn force the OGCM. Results from uncoupled and coupled runs with and without the embedding are compared. With the standard coupling, the system exhibits similar behavior to previous HCMs, including interannual variability with a dominant quasi-biennial oscillation and a westward propagation of SST anomalies on the equator. These characteristics suggest that the horizontal advection is playing a more important role than the vertical advection in determining SST changes over the eastern equatorial Pacific. Incorporating the embedded SST anomaly model, with which the thermocline effects on SST can be enhanced in the eastern equatorial Pacific, has a significant impact on performance of the HCM. The embedded HCM exhibits more realistic SST variability and coupled behavior, characterized by 3–4-yr oscillations and a more standing SST pattern along the equator.

The results support the hypothesis that current physical parameterizations in the OGCM provide insufficient thermal linkage between the thermocline and the sea surface in the eastern equatorial Pacific. It is demonstrated that the long-known deficiency of some OGCMs in their depiction of the thermocline and its interactions with SST may contribute to unrealistic coupled variability in HCMs of ENSO. The embedding approach not only provides a diagnosis for parameterization deficiencies in current OGCMs but, pending progress on this difficult problem, provides a straightforward means to bypass it and improve coupled model performance.

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