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- Author or Editor: Rong Zhang x
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Abstract
High-resolution space-based observations reveal significant two-way air–sea interactions associated with tropical instability waves (TIWs); their roles in budgets of heat, salt, momentum, and biogeochemical fields in the tropical oceans have been recently demonstrated. However, dynamical model-based simulations of the atmospheric response to TIW-induced sea surface temperature (SSTTIW) perturbations remain a great challenge because of the limitation in spatial resolution and realistic representations of the related processes in the atmospheric planetary boundary layer (PBL) and their interactions with the overlying free troposphere. Using microwave remote sensing data, an empirical model is derived to depict wind stress perturbations induced by TIW-related SST forcing in the eastern tropical Pacific Ocean. Wind data are based on space–time blending of Quick Scatterometer (QuikSCAT) Direction Interval Retrieval with Thresholded Nudging (DIRTH) satellite observations and NCEP analysis fields; SST data are from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). These daily data are first subject to a spatial filter of 12° moving average in the zonal direction to extract TIW-related wind stress (τ TIW) and SSTTIW perturbations. A combined singular value decomposition (SVD) analysis is then applied to these zonal high-pass-filtered τ TIW and SSTTIW fields. It is demonstrated that the SVD-based analysis technique can effectively extract TIW-induced covariability patterns in the atmosphere and ocean, acting as a filter by passing wind signals that are directly related with the SSTTIW forcing over the TIW active regions. As a result, the empirical model can well represent TIW-induced wind stress responses as revealed directly from satellite measurements (e.g., the structure and phase), but the amplitude can be underestimated significantly. Validation and sensitivity experiments are performed to illustrate the robustness of the empirical τ TIW model. Further applications are discussed for taking into account the TIW-induced wind responses and feedback effects that are missing in large-scale climate models and atmospheric reanalysis data, as well as for uncoupled ocean and coupled mesoscale and large-scale air–sea modeling studies.
Abstract
High-resolution space-based observations reveal significant two-way air–sea interactions associated with tropical instability waves (TIWs); their roles in budgets of heat, salt, momentum, and biogeochemical fields in the tropical oceans have been recently demonstrated. However, dynamical model-based simulations of the atmospheric response to TIW-induced sea surface temperature (SSTTIW) perturbations remain a great challenge because of the limitation in spatial resolution and realistic representations of the related processes in the atmospheric planetary boundary layer (PBL) and their interactions with the overlying free troposphere. Using microwave remote sensing data, an empirical model is derived to depict wind stress perturbations induced by TIW-related SST forcing in the eastern tropical Pacific Ocean. Wind data are based on space–time blending of Quick Scatterometer (QuikSCAT) Direction Interval Retrieval with Thresholded Nudging (DIRTH) satellite observations and NCEP analysis fields; SST data are from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). These daily data are first subject to a spatial filter of 12° moving average in the zonal direction to extract TIW-related wind stress (τ TIW) and SSTTIW perturbations. A combined singular value decomposition (SVD) analysis is then applied to these zonal high-pass-filtered τ TIW and SSTTIW fields. It is demonstrated that the SVD-based analysis technique can effectively extract TIW-induced covariability patterns in the atmosphere and ocean, acting as a filter by passing wind signals that are directly related with the SSTTIW forcing over the TIW active regions. As a result, the empirical model can well represent TIW-induced wind stress responses as revealed directly from satellite measurements (e.g., the structure and phase), but the amplitude can be underestimated significantly. Validation and sensitivity experiments are performed to illustrate the robustness of the empirical τ TIW model. Further applications are discussed for taking into account the TIW-induced wind responses and feedback effects that are missing in large-scale climate models and atmospheric reanalysis data, as well as for uncoupled ocean and coupled mesoscale and large-scale air–sea modeling studies.
Abstract
The role of subsurface temperature variability in modulating El Niño–Southern Oscillation (ENSO) properties is examined using an intermediate coupled model (ICM), consisting of an intermediate dynamic ocean model and a sea surface temperature (SST) anomaly model. An empirical procedure is used to parameterize the temperature of subsurface water entrained into the mixed layer (Te ) from sea level (SL) anomalies via a singular value decomposition (SVD) analysis for use in simulating sea surface temperature anomalies (SSTAs). The ocean model is coupled to a statistical atmospheric model that estimates wind stress anomalies also from an SVD analysis. Using the empirical Te models constructed from two subperiods, 1963–79 (T 63–79 e ) and 1980–96 (T 80–96 e ), the coupled system exhibits strikingly different properties of interannual variability (the oscillation period, spatial structure, and temporal evolution). For the T 63–79 e model, the system features a 2-yr oscillation and westward propagation of SSTAs on the equator, while for the T 80–96 e model, it is characterized by a 5-yr oscillation and eastward propagation. These changes in ENSO properties are consistent with the behavior shift of El Niño observed in the late 1970s. Heat budget analyses further demonstrate a controlling role played by the vertical advection of subsurface temperature anomalies in determining the ENSO properties.
Abstract
The role of subsurface temperature variability in modulating El Niño–Southern Oscillation (ENSO) properties is examined using an intermediate coupled model (ICM), consisting of an intermediate dynamic ocean model and a sea surface temperature (SST) anomaly model. An empirical procedure is used to parameterize the temperature of subsurface water entrained into the mixed layer (Te ) from sea level (SL) anomalies via a singular value decomposition (SVD) analysis for use in simulating sea surface temperature anomalies (SSTAs). The ocean model is coupled to a statistical atmospheric model that estimates wind stress anomalies also from an SVD analysis. Using the empirical Te models constructed from two subperiods, 1963–79 (T 63–79 e ) and 1980–96 (T 80–96 e ), the coupled system exhibits strikingly different properties of interannual variability (the oscillation period, spatial structure, and temporal evolution). For the T 63–79 e model, the system features a 2-yr oscillation and westward propagation of SSTAs on the equator, while for the T 80–96 e model, it is characterized by a 5-yr oscillation and eastward propagation. These changes in ENSO properties are consistent with the behavior shift of El Niño observed in the late 1970s. Heat budget analyses further demonstrate a controlling role played by the vertical advection of subsurface temperature anomalies in determining the ENSO properties.
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.
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.
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.
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.
Abstract
In this study the mechanisms for low-frequency variability of summer Arctic sea ice are analyzed using long control simulations from three coupled models (GFDL CM2.1, GFDL CM3, and NCAR CESM). Despite different Arctic sea ice mean states, there are many robust features in the response of low-frequency summer Arctic sea ice variability to the three key predictors (Atlantic and Pacific oceanic heat transport into the Arctic and the Arctic dipole) across all three models. In all three models, an enhanced Atlantic (Pacific) heat transport into the Arctic induces summer Arctic sea ice decline and surface warming, especially over the Atlantic (Pacific) sector of the Arctic. A positive phase of the Arctic dipole induces summer Arctic sea ice decline and surface warming on the Pacific side, and opposite changes on the Atlantic side. There is robust Bjerknes compensation at low frequency, so the northward atmospheric heat transport provides a negative feedback to summer Arctic sea ice variations. The influence of the Arctic dipole on summer Arctic sea ice extent is more (less) effective in simulations with less (excessive) climatological summer sea ice in the Atlantic sector. The response of Arctic sea ice thickness to the three key predictors is stronger in models that have thicker climatological Arctic sea ice.
Abstract
In this study the mechanisms for low-frequency variability of summer Arctic sea ice are analyzed using long control simulations from three coupled models (GFDL CM2.1, GFDL CM3, and NCAR CESM). Despite different Arctic sea ice mean states, there are many robust features in the response of low-frequency summer Arctic sea ice variability to the three key predictors (Atlantic and Pacific oceanic heat transport into the Arctic and the Arctic dipole) across all three models. In all three models, an enhanced Atlantic (Pacific) heat transport into the Arctic induces summer Arctic sea ice decline and surface warming, especially over the Atlantic (Pacific) sector of the Arctic. A positive phase of the Arctic dipole induces summer Arctic sea ice decline and surface warming on the Pacific side, and opposite changes on the Atlantic side. There is robust Bjerknes compensation at low frequency, so the northward atmospheric heat transport provides a negative feedback to summer Arctic sea ice variations. The influence of the Arctic dipole on summer Arctic sea ice extent is more (less) effective in simulations with less (excessive) climatological summer sea ice in the Atlantic sector. The response of Arctic sea ice thickness to the three key predictors is stronger in models that have thicker climatological Arctic sea ice.
Abstract
The impacts of freshwater flux (FWF) forcing on interannual variability in the tropical Pacific climate system are investigated using a hybrid coupled model (HCM), constructed from an oceanic general circulation model (OGCM) and a simplified atmospheric model, whose forcing fields to the ocean consist of three components. Interannual anomalies of wind stress and precipitation minus evaporation, (P − E), are calculated respectively by their statistical feedback models that are constructed from a singular value decomposition (SVD) analysis of their historical data. Heat flux is calculated using an advective atmospheric mixed layer (AML) model. The constructed HCM can well reproduce interannual variability associated with ENSO in the tropical Pacific. HCM experiments are performed with varying strengths of anomalous FWF forcing. It is demonstrated that FWF can have a significant modulating impact on interannual variability. The buoyancy flux (QB ) field, an important parameter determining the mixing and entrainment in the equatorial Pacific, is analyzed to illustrate the compensating role played by its two contributing parts: one is related to heat flux (QT ) and the other to freshwater flux (QS ). A positive feedback is identified between FWF and SST as follows: SST anomalies, generated by El Niño, nonlocally induce large anomalous FWF variability over the western and central regions, which directly influences sea surface salinity (SSS) and QB , leading to changes in the mixed layer depth (MLD), the upper-ocean stability, and the mixing and the entrainment of subsurface waters. These oceanic processes act to enhance the SST anomalies, which in turn feedback to the atmosphere in a coupled ocean–atmosphere system. As a result, taking into account anomalous FWF forcing in the HCM leads to an enhanced interannual variability and ENSO cycles. It is further shown that FWF forcing is playing a different role from heat flux forcing, with the former acting to drive a change in SST while the latter represents a passive response to the SST change. This HCM-based modeling study presents clear evidence for the role of FWF forcing in modulating interannual variability in the tropical Pacific. The significance and implications of these results are further discussed for physical understanding and model improvements of interannual variability in the tropical Pacific ocean–atmosphere system.
Abstract
The impacts of freshwater flux (FWF) forcing on interannual variability in the tropical Pacific climate system are investigated using a hybrid coupled model (HCM), constructed from an oceanic general circulation model (OGCM) and a simplified atmospheric model, whose forcing fields to the ocean consist of three components. Interannual anomalies of wind stress and precipitation minus evaporation, (P − E), are calculated respectively by their statistical feedback models that are constructed from a singular value decomposition (SVD) analysis of their historical data. Heat flux is calculated using an advective atmospheric mixed layer (AML) model. The constructed HCM can well reproduce interannual variability associated with ENSO in the tropical Pacific. HCM experiments are performed with varying strengths of anomalous FWF forcing. It is demonstrated that FWF can have a significant modulating impact on interannual variability. The buoyancy flux (QB ) field, an important parameter determining the mixing and entrainment in the equatorial Pacific, is analyzed to illustrate the compensating role played by its two contributing parts: one is related to heat flux (QT ) and the other to freshwater flux (QS ). A positive feedback is identified between FWF and SST as follows: SST anomalies, generated by El Niño, nonlocally induce large anomalous FWF variability over the western and central regions, which directly influences sea surface salinity (SSS) and QB , leading to changes in the mixed layer depth (MLD), the upper-ocean stability, and the mixing and the entrainment of subsurface waters. These oceanic processes act to enhance the SST anomalies, which in turn feedback to the atmosphere in a coupled ocean–atmosphere system. As a result, taking into account anomalous FWF forcing in the HCM leads to an enhanced interannual variability and ENSO cycles. It is further shown that FWF forcing is playing a different role from heat flux forcing, with the former acting to drive a change in SST while the latter represents a passive response to the SST change. This HCM-based modeling study presents clear evidence for the role of FWF forcing in modulating interannual variability in the tropical Pacific. The significance and implications of these results are further discussed for physical understanding and model improvements of interannual variability in the tropical Pacific ocean–atmosphere system.
Abstract
A three-box model of haline and thermal mode overturning is developed to study thermohaline oscillations found in a number of ocean general circulation models and that might have occurred in warm equable paleoclimates. By including convective adjustment modified to represent the localized nature of deep convection, the box model shows that a steady haline mode circulation is unstable. For certain ranges of freshwater forcing/vertical diffusivity, a self-sustained oscillatory circulation is found in which haline–thermal mode switching occurs with a period of centuries to millennia. It is found that mode switching is most likely to occur in warm periods of earth's history with, relative to the present climate, a reduced Pole&ndash=uator temperature gradient, an enhanced hydrological cycle, and somewhat smaller values of oceanic diffusivities.
Abstract
A three-box model of haline and thermal mode overturning is developed to study thermohaline oscillations found in a number of ocean general circulation models and that might have occurred in warm equable paleoclimates. By including convective adjustment modified to represent the localized nature of deep convection, the box model shows that a steady haline mode circulation is unstable. For certain ranges of freshwater forcing/vertical diffusivity, a self-sustained oscillatory circulation is found in which haline–thermal mode switching occurs with a period of centuries to millennia. It is found that mode switching is most likely to occur in warm periods of earth's history with, relative to the present climate, a reduced Pole&ndash=uator temperature gradient, an enhanced hydrological cycle, and somewhat smaller values of oceanic diffusivities.
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.
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.
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.
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.
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 Latif and Barnett. 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.
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 Latif and Barnett. 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.