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- Author or Editor: Rong Zhang x
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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.
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.
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.
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.
Abstract
A simple scheme is proposed for penetrating atmospheric momentum flux over the ocean surface boundary layer or mixed layer (BL/ML) and is tested in the z-coordinate NOAA/Geophysical Fluid Dynamics Laboratory Modular Ocean Model (MOM 3) for improving its performance. Analogous to the treatment in layered ocean models, wind stress is applied, as a body force, to the entire BL/ML whose depth is calculated from a nonlocal K-profile parameterization scheme. The penetrating scheme presents an explicit and effective way to distribute a priori momentum flux throughout the BL/ML that has varying depth in space and time, instead of just over the uppermost model level with fixed thickness. This additional procedure introduces an explicit mechanism that directly relates wind stress to the BL/ML formulation, which in turn controls current and thermal structure in the upper ocean and the interaction with the underlying thermocline. Two penetrating runs, one over the BL and the other over the ML, have similar results that differ systematically from those with the penetration over fixed depths (control run). It is demonstrated that, with coherent and systematic improvements, this penetrating scheme can have significant effects on simulated equatorial ocean currents and thermal structure not only in the surface layer, but also in the thermocline. Besides more reasonable ML depth simulation in the equatorial central basin, there is substantial reduction in the mean offset of simulated isotherm depths and warm bias in the thermocline, due to downward shift of the maximum upwelling zone in the equatorial central Pacific. Consistent with observations, the penetrating scheme realistically reproduces the springtime reversal of the South Equatorial Current and the corresponding surface warming in the central equatorial Pacific, with accompanying surfacing of the Equatorial Undercurrent Current in March–May.
Abstract
A simple scheme is proposed for penetrating atmospheric momentum flux over the ocean surface boundary layer or mixed layer (BL/ML) and is tested in the z-coordinate NOAA/Geophysical Fluid Dynamics Laboratory Modular Ocean Model (MOM 3) for improving its performance. Analogous to the treatment in layered ocean models, wind stress is applied, as a body force, to the entire BL/ML whose depth is calculated from a nonlocal K-profile parameterization scheme. The penetrating scheme presents an explicit and effective way to distribute a priori momentum flux throughout the BL/ML that has varying depth in space and time, instead of just over the uppermost model level with fixed thickness. This additional procedure introduces an explicit mechanism that directly relates wind stress to the BL/ML formulation, which in turn controls current and thermal structure in the upper ocean and the interaction with the underlying thermocline. Two penetrating runs, one over the BL and the other over the ML, have similar results that differ systematically from those with the penetration over fixed depths (control run). It is demonstrated that, with coherent and systematic improvements, this penetrating scheme can have significant effects on simulated equatorial ocean currents and thermal structure not only in the surface layer, but also in the thermocline. Besides more reasonable ML depth simulation in the equatorial central basin, there is substantial reduction in the mean offset of simulated isotherm depths and warm bias in the thermocline, due to downward shift of the maximum upwelling zone in the equatorial central Pacific. Consistent with observations, the penetrating scheme realistically reproduces the springtime reversal of the South Equatorial Current and the corresponding surface warming in the central equatorial Pacific, with accompanying surfacing of the Equatorial Undercurrent Current in March–May.
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.
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.
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.
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.
Abstract
Thermohaline structure and time evolution in the subsurface ocean play a critical role in climate variability and predictability. They are still poorly represented in ocean and climate models. Here, the characteristics of subsurface thermohaline biases in the southern tropical Pacific and their causes are investigated through CMIP-based analyses and model-based experiments. There exists a pronounced subsurface cold bias at 200-m depth over the southern tropical Pacific in CMIP6 simulations with an ensemble mean of about −4°C and an extreme close to −10°C. This cold bias is accompanied by a fresh subsurface bias of about −0.9 psu in the ensemble mean (−1.9 psu minimum). Similar subsurface thermohaline biases also exist in CMIP5 outputs, indicating that reduction of these biases remains a long-standing challenge for model developments. To understand the causes of these biases, attribution analyses and POP2-based sensitivity experiments are performed. It is found that the subsurface thermohaline biases are attributed to the model deficiencies in simulating wind stress curl and precipitation in the southern tropical Pacific. By conducting CESM2-based coupled experiments, a warm SST bias in the southeastern tropical Pacific is found to be responsible for the poor simulations in wind stress curl and precipitation. The consequences of these biases are also analyzed. The subsurface thermohaline biases cause the density field to increase substantially along 10°S, flattening the zonal isopycnal surface and reducing equatorward interior transport. In addition, the anomalously cold and fresh subsurface signals in the southern tropical Pacific are seen to propagate to the equator, leading to an overall spurious cooling in the equatorial subsurface.
Significance Statement
Subsurface biases severely degrade the credibility of climate models in their predictions and projections; hence, it is important to understand the causes of these subsurface biases. Our study analyzes the characteristics of subsurface thermohaline biases in the southern tropical Pacific and investigates their causes. A pronounced subsurface cold bias is found over the southern tropical Pacific, accompanied by an obvious subsurface fresh bias. By performing attribution analyses and numerical experiments, it is found that the subsurface thermohaline biases are attributed to the model deficiencies in simulating wind stress and precipitation, which are further attributed to the warm SST bias in the southeastern tropical Pacific. These results provide a guide for improving climate model performances.
Abstract
Thermohaline structure and time evolution in the subsurface ocean play a critical role in climate variability and predictability. They are still poorly represented in ocean and climate models. Here, the characteristics of subsurface thermohaline biases in the southern tropical Pacific and their causes are investigated through CMIP-based analyses and model-based experiments. There exists a pronounced subsurface cold bias at 200-m depth over the southern tropical Pacific in CMIP6 simulations with an ensemble mean of about −4°C and an extreme close to −10°C. This cold bias is accompanied by a fresh subsurface bias of about −0.9 psu in the ensemble mean (−1.9 psu minimum). Similar subsurface thermohaline biases also exist in CMIP5 outputs, indicating that reduction of these biases remains a long-standing challenge for model developments. To understand the causes of these biases, attribution analyses and POP2-based sensitivity experiments are performed. It is found that the subsurface thermohaline biases are attributed to the model deficiencies in simulating wind stress curl and precipitation in the southern tropical Pacific. By conducting CESM2-based coupled experiments, a warm SST bias in the southeastern tropical Pacific is found to be responsible for the poor simulations in wind stress curl and precipitation. The consequences of these biases are also analyzed. The subsurface thermohaline biases cause the density field to increase substantially along 10°S, flattening the zonal isopycnal surface and reducing equatorward interior transport. In addition, the anomalously cold and fresh subsurface signals in the southern tropical Pacific are seen to propagate to the equator, leading to an overall spurious cooling in the equatorial subsurface.
Significance Statement
Subsurface biases severely degrade the credibility of climate models in their predictions and projections; hence, it is important to understand the causes of these subsurface biases. Our study analyzes the characteristics of subsurface thermohaline biases in the southern tropical Pacific and investigates their causes. A pronounced subsurface cold bias is found over the southern tropical Pacific, accompanied by an obvious subsurface fresh bias. By performing attribution analyses and numerical experiments, it is found that the subsurface thermohaline biases are attributed to the model deficiencies in simulating wind stress and precipitation, which are further attributed to the warm SST bias in the southeastern tropical Pacific. These results provide a guide for improving climate model performances.
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.
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.
Abstract
El Niño and La Niña exhibit asymmetric evolution characteristics during their decay phases. The decay speed of El Niño is significantly greater than that of La Niña. This study systematically and quantitatively investigates the relative contributions of the equatorial western Pacific (WP) and central-eastern Pacific (CEP) wind stress anomalies to ENSO decay and its asymmetry through data analysis, numerical experiments, and dynamic and thermodynamic diagnoses. It is demonstrated that the sea surface temperature anomalies (SSTAs) forced by the wind stress anomalies in the equatorial CEP play a dominant role in ENSO decay and contribute to ENSO decay asymmetry, while the forcing by the equatorial WP wind stress anomalies has a small contribution. Diagnoses of the oceanic mixed layer heat budget indicate that anomalous zonal advection term and vertical advection term forced by the wind stress anomalies in the equatorial CEP are the most important dynamic terms contributed to ENSO decay. Both terms in El Niño decay phase are much larger than in La Niña decay phase, resulting in a larger decay speed in El Niño than in La Niña. The contributions of these two terms do not depend on the equatorial WP wind field, confirming that the equatorial WP wind stress anomalies do not act as a pivotal part in ENSO asymmetric decay. Moreover, it is demonstrated that within the equatorial CEP, dominant contribution comes from the wind stress anomalies in the equatorial central Pacific, in which those in the equatorial southern central Pacific play a major role.
Significance Statement
Previous studies proposed why wind fields in the equatorial western Pacific (WP) or central-eastern Pacific (CEP) are asymmetric and how the asymmetric wind fields affect ENSO decay and decay asymmetry. By using an oceanic general circulation model, we quantitatively estimate the relative contributions of the wind stress anomalies over the equatorial WP and CEP. It is demonstrated that the wind stress anomalies over the equatorial CEP and the associated ocean response play a dominant role in the asymmetric decay. Additionally, it is further illustrated the predominant role comes from the wind stress anomalies in the equatorial southern central Pacific within the equatorial CEP. Our study provides a physical explanation on the ENSO decay and its asymmetry.
Abstract
El Niño and La Niña exhibit asymmetric evolution characteristics during their decay phases. The decay speed of El Niño is significantly greater than that of La Niña. This study systematically and quantitatively investigates the relative contributions of the equatorial western Pacific (WP) and central-eastern Pacific (CEP) wind stress anomalies to ENSO decay and its asymmetry through data analysis, numerical experiments, and dynamic and thermodynamic diagnoses. It is demonstrated that the sea surface temperature anomalies (SSTAs) forced by the wind stress anomalies in the equatorial CEP play a dominant role in ENSO decay and contribute to ENSO decay asymmetry, while the forcing by the equatorial WP wind stress anomalies has a small contribution. Diagnoses of the oceanic mixed layer heat budget indicate that anomalous zonal advection term and vertical advection term forced by the wind stress anomalies in the equatorial CEP are the most important dynamic terms contributed to ENSO decay. Both terms in El Niño decay phase are much larger than in La Niña decay phase, resulting in a larger decay speed in El Niño than in La Niña. The contributions of these two terms do not depend on the equatorial WP wind field, confirming that the equatorial WP wind stress anomalies do not act as a pivotal part in ENSO asymmetric decay. Moreover, it is demonstrated that within the equatorial CEP, dominant contribution comes from the wind stress anomalies in the equatorial central Pacific, in which those in the equatorial southern central Pacific play a major role.
Significance Statement
Previous studies proposed why wind fields in the equatorial western Pacific (WP) or central-eastern Pacific (CEP) are asymmetric and how the asymmetric wind fields affect ENSO decay and decay asymmetry. By using an oceanic general circulation model, we quantitatively estimate the relative contributions of the wind stress anomalies over the equatorial WP and CEP. It is demonstrated that the wind stress anomalies over the equatorial CEP and the associated ocean response play a dominant role in the asymmetric decay. Additionally, it is further illustrated the predominant role comes from the wind stress anomalies in the equatorial southern central Pacific within the equatorial CEP. Our study provides a physical explanation on the ENSO decay and its asymmetry.
Abstract
Substantial model biases are still prominent even in the latest CMIP6 simulations; attributing their causes is defined as one of the three main scientific questions addressed in CMIP6. In this paper, cold temperature biases in the North Pacific subtropics are investigated using simulations from the newly released CMIP6 models, together with other related modeling products. In addition, ocean-only sensitivity experiments are performed to characterize the biases, with a focus on the role of oceanic vertical mixing schemes. Based on the Argo-derived diffusivity, idealized vertical diffusivity fields are designed to mimic the seasonality of vertical mixing in this region, and are employed in ocean-only simulations to test the sensitivity of this cold bias to oceanic vertical mixing. It is demonstrated that the cold temperature biases can be reduced when the mixing strength is enhanced within and beneath the surface boundary layer. Additionally, the temperature simulations are rather sensitive to the parameterization of static instability, and the cold biases can be reduced when the vertical diffusivity for convection is increased. These indicate that the cold temperature biases in the North Pacific can be largely attributed to biases in oceanic vertical mixing within ocean-only simulations, which likely contribute to the even larger biases seen in coupled simulations. This study therefore highlights the need for improved oceanic vertical mixing in order to reduce these persistent cold temperature biases seen across several CMIP models.
Abstract
Substantial model biases are still prominent even in the latest CMIP6 simulations; attributing their causes is defined as one of the three main scientific questions addressed in CMIP6. In this paper, cold temperature biases in the North Pacific subtropics are investigated using simulations from the newly released CMIP6 models, together with other related modeling products. In addition, ocean-only sensitivity experiments are performed to characterize the biases, with a focus on the role of oceanic vertical mixing schemes. Based on the Argo-derived diffusivity, idealized vertical diffusivity fields are designed to mimic the seasonality of vertical mixing in this region, and are employed in ocean-only simulations to test the sensitivity of this cold bias to oceanic vertical mixing. It is demonstrated that the cold temperature biases can be reduced when the mixing strength is enhanced within and beneath the surface boundary layer. Additionally, the temperature simulations are rather sensitive to the parameterization of static instability, and the cold biases can be reduced when the vertical diffusivity for convection is increased. These indicate that the cold temperature biases in the North Pacific can be largely attributed to biases in oceanic vertical mixing within ocean-only simulations, which likely contribute to the even larger biases seen in coupled simulations. This study therefore highlights the need for improved oceanic vertical mixing in order to reduce these persistent cold temperature biases seen across several CMIP models.
Abstract
Ocean biology components affect the vertical redistribution of incoming solar radiation in the upper ocean of the tropical Pacific and can significantly modulate El Niño–Southern Oscillation (ENSO). The biophysical interactions in the region were represented by coupling an ocean biology model with an ocean general circulation model (OGCM); the coupled ocean physics–biology model is then forced by prescribed wind anomalies during 1980–2007. Two ocean-only experiments were performed with different representations of chlorophyll (Chl). In an interannual Chl run (referred to as Chlinter), Chl was interannually varying, which was interactively calculated from the ocean biology model to explicitly represent its heating feedback on ocean thermodynamics. The structure and relationship of the related heating terms were examined to understand the Chl-induced feedback effects and the processes involved. The portion of solar radiation penetrating the bottom of the mixed layer (Q pen) was significantly affected by interannual Chl anomalies in the western-central equatorial Pacific. In a climatological run (Chlclim), the Chl concentration was prescribed to be its seasonally varying climatology derived from the Chlinter run. Compared with the Chlclim run, interannual variability in the Chlinter run tended to be reduced. The sea surface temperature (SST) differences between the two runs exhibited an asymmetric bioeffect: they were stronger during La Niña events but relatively weaker during El Niño events. The signs of the SST differences between the two runs indicated a close relationship with Chl: a cooling effect was associated with a low Chl concentration during El Niño events, and a strong warming effect was associated with a high Chl concentration during La Niña events.
Abstract
Ocean biology components affect the vertical redistribution of incoming solar radiation in the upper ocean of the tropical Pacific and can significantly modulate El Niño–Southern Oscillation (ENSO). The biophysical interactions in the region were represented by coupling an ocean biology model with an ocean general circulation model (OGCM); the coupled ocean physics–biology model is then forced by prescribed wind anomalies during 1980–2007. Two ocean-only experiments were performed with different representations of chlorophyll (Chl). In an interannual Chl run (referred to as Chlinter), Chl was interannually varying, which was interactively calculated from the ocean biology model to explicitly represent its heating feedback on ocean thermodynamics. The structure and relationship of the related heating terms were examined to understand the Chl-induced feedback effects and the processes involved. The portion of solar radiation penetrating the bottom of the mixed layer (Q pen) was significantly affected by interannual Chl anomalies in the western-central equatorial Pacific. In a climatological run (Chlclim), the Chl concentration was prescribed to be its seasonally varying climatology derived from the Chlinter run. Compared with the Chlclim run, interannual variability in the Chlinter run tended to be reduced. The sea surface temperature (SST) differences between the two runs exhibited an asymmetric bioeffect: they were stronger during La Niña events but relatively weaker during El Niño events. The signs of the SST differences between the two runs indicated a close relationship with Chl: a cooling effect was associated with a low Chl concentration during El Niño events, and a strong warming effect was associated with a high Chl concentration during La Niña events.
