Search Results
You are looking at 1 - 10 of 82 items for
- Author or Editor: Fei-Fei Jin x
- Refine by Access: All Content x
Abstract
A linear coupled model for the atmosphere–upper-ocean system is proposed to highlight the mechanisms of decadal to interdecadal climate variability in the North Pacific. In this model, wind stress anomalies over the North Pacific are related to anomalies in the meridional temperature gradient of the upper ocean. The latter depends upon air–sea thermodynamical feedbacks and meridional heat transport by upper-ocean currents. Slow adjustment of the oceanic gyre circulation to the change in wind stress is accomplished by the forced baroclinic oceanic Rossby waves, which carry out the meridional heat transport. Uncoupled ocean dynamic adjustment can produce a weak decadal to interdecadal peak in the power spectrum of the meridional transport under temporal white noise wind stress forcing with organized spatial structure. Coupled dynamics produce a basin-scale interdecadal oscillatory mode. This mode arises from the dynamic coupling and the memory of the system, residing in the slow gyre circulation adjustment. Its stability is heavily controlled by the ocean thermal damping, and its period is about one and one-half to three times the decadal ocean dynamic adjustment time. In the relevant parameter regime, this coupled mode produces a robust and pronounced interdecadal spectral peak in the upper-ocean temperature and the Sverdrup transport of the gyre circulation. The interdecadal oscillations reproduced in the simple model provide insights into main physical mechanisms of the North Pacific decadal–interdecadal variability observed in nature and simulated in coupled general circulation models.
Abstract
A linear coupled model for the atmosphere–upper-ocean system is proposed to highlight the mechanisms of decadal to interdecadal climate variability in the North Pacific. In this model, wind stress anomalies over the North Pacific are related to anomalies in the meridional temperature gradient of the upper ocean. The latter depends upon air–sea thermodynamical feedbacks and meridional heat transport by upper-ocean currents. Slow adjustment of the oceanic gyre circulation to the change in wind stress is accomplished by the forced baroclinic oceanic Rossby waves, which carry out the meridional heat transport. Uncoupled ocean dynamic adjustment can produce a weak decadal to interdecadal peak in the power spectrum of the meridional transport under temporal white noise wind stress forcing with organized spatial structure. Coupled dynamics produce a basin-scale interdecadal oscillatory mode. This mode arises from the dynamic coupling and the memory of the system, residing in the slow gyre circulation adjustment. Its stability is heavily controlled by the ocean thermal damping, and its period is about one and one-half to three times the decadal ocean dynamic adjustment time. In the relevant parameter regime, this coupled mode produces a robust and pronounced interdecadal spectral peak in the upper-ocean temperature and the Sverdrup transport of the gyre circulation. The interdecadal oscillations reproduced in the simple model provide insights into main physical mechanisms of the North Pacific decadal–interdecadal variability observed in nature and simulated in coupled general circulation models.
Abstract
Basinwide very low-frequency (VLF) modes with zonal uniformly deepening and shoaling of the equatorial thermocline are found as solutions of linear shallow-water equations with the meridional basin boundaries under an equatorial β plane. They can be understood as the free heat-content recharge oscillations. The interannual VLF modes had been recognized as the essential part of the coupled recharge oscillator theory for the El Niño–Southern Oscillation. It is suggested that the decadal VLF modes may also be transformed into coupled modes relevant to the Pacific decadal climate variability through the ocean–atmosphere interaction in the Tropics.
Abstract
Basinwide very low-frequency (VLF) modes with zonal uniformly deepening and shoaling of the equatorial thermocline are found as solutions of linear shallow-water equations with the meridional basin boundaries under an equatorial β plane. They can be understood as the free heat-content recharge oscillations. The interannual VLF modes had been recognized as the essential part of the coupled recharge oscillator theory for the El Niño–Southern Oscillation. It is suggested that the decadal VLF modes may also be transformed into coupled modes relevant to the Pacific decadal climate variability through the ocean–atmosphere interaction in the Tropics.
Abstract
A new conceptual model for ENSO has been constructed based upon the positive feedback of tropical ocean–atmosphere interaction proposed by Bjerknes as the growth mechanism and the recharge–discharge of the equatorial heat content as the phase-transition mechanism suggested by Cane and Zebiak and by Wyrtki. This model combines SST dynamics and ocean adjustment dynamics into a coupled basinwide recharge oscillator that relies on the nonequilibrium between the zonal mean equatorial thermocline depth and wind stress. Over a wide range of the relative coupling coefficient, this recharge oscillator can be either self-excited or stochastically sustained. Its period is robust in the range of 3–5 years. This recharge oscillator model clearly depicts the slow physics of ENSO and also embodies the delayed oscillator (Schopf and Suarez; Battisti and Hirst) without requiring an explicit wave delay. It can also be viewed as a mixed SST–ocean dynamics oscillator due to the fact that it arises from the merging of two uncoupled modes, a decaying SST mode and a basinwide ocean adjustment mode, through the tropical ocean–atmosphere coupling. The basic characteristics of this recharge oscillator, including the relationship between the equatorial western Pacific thermocline depth and the eastern Pacific SST anomalies, are in agreement with those of ENSO variability in the observations and simulations with the Zebiak–Cane model.
Abstract
A new conceptual model for ENSO has been constructed based upon the positive feedback of tropical ocean–atmosphere interaction proposed by Bjerknes as the growth mechanism and the recharge–discharge of the equatorial heat content as the phase-transition mechanism suggested by Cane and Zebiak and by Wyrtki. This model combines SST dynamics and ocean adjustment dynamics into a coupled basinwide recharge oscillator that relies on the nonequilibrium between the zonal mean equatorial thermocline depth and wind stress. Over a wide range of the relative coupling coefficient, this recharge oscillator can be either self-excited or stochastically sustained. Its period is robust in the range of 3–5 years. This recharge oscillator model clearly depicts the slow physics of ENSO and also embodies the delayed oscillator (Schopf and Suarez; Battisti and Hirst) without requiring an explicit wave delay. It can also be viewed as a mixed SST–ocean dynamics oscillator due to the fact that it arises from the merging of two uncoupled modes, a decaying SST mode and a basinwide ocean adjustment mode, through the tropical ocean–atmosphere coupling. The basic characteristics of this recharge oscillator, including the relationship between the equatorial western Pacific thermocline depth and the eastern Pacific SST anomalies, are in agreement with those of ENSO variability in the observations and simulations with the Zebiak–Cane model.
Abstract
The conceptual recharge oscillator model intuitively established in Part I is derived from a dynamical framework of a Cane–Zebiak type model for tropical ocean–atmosphere interaction. A two-strip approximation to the equatorial ocean dynamics and one-strip approximation to the SST dynamics are employed to obtain a stripped-down coupled model that captures the main physics of the Cane–Zebiak type model. It is shown that the conceptual recharge oscillator model can be obtained from the stripped-down coupled model with a two-box approximation in the zonal direction or a low-frequency approximation to filter out high-frequency modes. Linear solutions of the stripped-down model are analytically solved and the dependence of coupled modes on various model parameters is delineated. In different parameter regimes, the stripped-down coupled model describes a coupled-wave mode and a mixed SST–ocean-dynamics mode that results from the merger of a nonoscillatory ocean adjustment mode with an SST mode. These two coupled oscillatory modes undergo a further merger. In the neighborhood of this merger, the leading mode of the system becomes a generalized mixed mode. It is suggested that the slow ENSO regime can be best characterized by this generalized mixed mode whose essential physics are described by the conceptual recharge oscillator model proposed in Part I.
Abstract
The conceptual recharge oscillator model intuitively established in Part I is derived from a dynamical framework of a Cane–Zebiak type model for tropical ocean–atmosphere interaction. A two-strip approximation to the equatorial ocean dynamics and one-strip approximation to the SST dynamics are employed to obtain a stripped-down coupled model that captures the main physics of the Cane–Zebiak type model. It is shown that the conceptual recharge oscillator model can be obtained from the stripped-down coupled model with a two-box approximation in the zonal direction or a low-frequency approximation to filter out high-frequency modes. Linear solutions of the stripped-down model are analytically solved and the dependence of coupled modes on various model parameters is delineated. In different parameter regimes, the stripped-down coupled model describes a coupled-wave mode and a mixed SST–ocean-dynamics mode that results from the merger of a nonoscillatory ocean adjustment mode with an SST mode. These two coupled oscillatory modes undergo a further merger. In the neighborhood of this merger, the leading mode of the system becomes a generalized mixed mode. It is suggested that the slow ENSO regime can be best characterized by this generalized mixed mode whose essential physics are described by the conceptual recharge oscillator model proposed in Part I.
Abstract
The structure of the Equatorial Undercurrent in a two-hemisphere ocean is studied, using a simple ideal-fluid model in which both potential vorticity and the Bernoulli function are conserved along streamlines. For the case of a symmetric forcing, the solution is reduced to the case discussed in previous studies. For the case of asymmetric forcing, the western boundary current from the hemisphere with stronger forcing overshoots the equator where the two western boundary currents merge and form an undercurrent that is asymmetric with respect to the equator. Layer thicknesses are continuous across the matching streamline, but zonal velocity can be discontinuous. Both the wind stress pattern and the Indonesian Throughflow are the most important factors dictating the asymmetric nature of the undercurrent.
Abstract
The structure of the Equatorial Undercurrent in a two-hemisphere ocean is studied, using a simple ideal-fluid model in which both potential vorticity and the Bernoulli function are conserved along streamlines. For the case of a symmetric forcing, the solution is reduced to the case discussed in previous studies. For the case of asymmetric forcing, the western boundary current from the hemisphere with stronger forcing overshoots the equator where the two western boundary currents merge and form an undercurrent that is asymmetric with respect to the equator. Layer thicknesses are continuous across the matching streamline, but zonal velocity can be discontinuous. Both the wind stress pattern and the Indonesian Throughflow are the most important factors dictating the asymmetric nature of the undercurrent.
Abstract
A meridional-plane, hemispherical ocean model is developed to study interdecadal variability of the thermohaline circulation (THC). The model differs from previous formulations of zonally averaged ocean models by using a prognostic equation to calculate the meridional velocity. This allows the incorporation of an adjustment timescale comparable to the advective timescale of the meridional overturning. An interdecadal oscillation is documented for an idealized ocean of homogeneous salinity forced by a time-independent surface heat flux.
The governing equations are linearized about a basic state in order to isolate the effect of parameter changes on the oscillation. An eigenvalue analysis reveals that the frequency of the oscillation is independent of the advective timescale. Instead, the interdecadal timescale of the oscillation is set by the slow adjustment of the overturning intensity to a meridional pressure gradient anomaly. The oscillatory instability, on the other hand, is dependent on both advective and adjustment processes. Advection by the basic state acts locally to reinforce the meridional pressure gradient anomaly, whereas the delayed adjustment of the overturning intensity to this pressure gradient anomaly modifies the poleward transport of heat, thereby initiating the phase reversal of the pressure gradient anomaly. Thus, the advection of temperature anomalies by the basic state provides a positive feedback while the adjustment of the overturning intensity serves as the phase-switching mechanism.
The relevance of this “adjustment oscillator” to the interdecadal variability simulated in idealized ocean general circulation models is discussed. The results strongly suggest that internal, interdecadal variability of the THC is not an inherently three-dimensional or nonlinear phenomenon and that this type of variability cannot be conceptualized as a loop oscillator.
Abstract
A meridional-plane, hemispherical ocean model is developed to study interdecadal variability of the thermohaline circulation (THC). The model differs from previous formulations of zonally averaged ocean models by using a prognostic equation to calculate the meridional velocity. This allows the incorporation of an adjustment timescale comparable to the advective timescale of the meridional overturning. An interdecadal oscillation is documented for an idealized ocean of homogeneous salinity forced by a time-independent surface heat flux.
The governing equations are linearized about a basic state in order to isolate the effect of parameter changes on the oscillation. An eigenvalue analysis reveals that the frequency of the oscillation is independent of the advective timescale. Instead, the interdecadal timescale of the oscillation is set by the slow adjustment of the overturning intensity to a meridional pressure gradient anomaly. The oscillatory instability, on the other hand, is dependent on both advective and adjustment processes. Advection by the basic state acts locally to reinforce the meridional pressure gradient anomaly, whereas the delayed adjustment of the overturning intensity to this pressure gradient anomaly modifies the poleward transport of heat, thereby initiating the phase reversal of the pressure gradient anomaly. Thus, the advection of temperature anomalies by the basic state provides a positive feedback while the adjustment of the overturning intensity serves as the phase-switching mechanism.
The relevance of this “adjustment oscillator” to the interdecadal variability simulated in idealized ocean general circulation models is discussed. The results strongly suggest that internal, interdecadal variability of the THC is not an inherently three-dimensional or nonlinear phenomenon and that this type of variability cannot be conceptualized as a loop oscillator.
Abstract
Origin of the Arctic Oscillation (AO) was explored from a dynamical perspective, using a primitive equation model linearized about the observed winter climatology. In order to obtain a leading low-frequency mode of the model atmosphere, singular vector analysis is performed on the linear dynamical operator. The singular mode (v vector) with the smallest singular value, referred to as the neutral mode, has a considerable similarity to the observed AO in many aspects, suggesting that, at least in a linear dynamical framework, the AO is a dynamically consistent mode of variability that arises from the zonal asymmetry of the time-mean state.
Diagnosis of the neutral mode shows that a zonal flow–stationary wave interaction and an interaction between anomalous and climatological stationary waves are both of importance for maintenance of the spatial structure. Besides, vortex stretching–shrinking is significant in the upper levels, which indicates that a baroclinic process has a certain role as well. It is found that the formation of a hemispheric scale of the neutral mode is accomplished within several days via propagation of Rossby wave packets along jet streams that act as a waveguide. The neutral mode structure is robust for a wide range of model damping parameters, and especially preferred when the damping is weakened in the free troposphere. The corresponding optimal forcing (u vector) indicates that, though its robustness is relatively ambiguous, the neutral mode is most effectively excited by a certain pattern of the extratropical thermal forcing, in addition to a modest sensitivity to the deep heating anomaly over the tropical Indian Ocean.
Abstract
Origin of the Arctic Oscillation (AO) was explored from a dynamical perspective, using a primitive equation model linearized about the observed winter climatology. In order to obtain a leading low-frequency mode of the model atmosphere, singular vector analysis is performed on the linear dynamical operator. The singular mode (v vector) with the smallest singular value, referred to as the neutral mode, has a considerable similarity to the observed AO in many aspects, suggesting that, at least in a linear dynamical framework, the AO is a dynamically consistent mode of variability that arises from the zonal asymmetry of the time-mean state.
Diagnosis of the neutral mode shows that a zonal flow–stationary wave interaction and an interaction between anomalous and climatological stationary waves are both of importance for maintenance of the spatial structure. Besides, vortex stretching–shrinking is significant in the upper levels, which indicates that a baroclinic process has a certain role as well. It is found that the formation of a hemispheric scale of the neutral mode is accomplished within several days via propagation of Rossby wave packets along jet streams that act as a waveguide. The neutral mode structure is robust for a wide range of model damping parameters, and especially preferred when the damping is weakened in the free troposphere. The corresponding optimal forcing (u vector) indicates that, though its robustness is relatively ambiguous, the neutral mode is most effectively excited by a certain pattern of the extratropical thermal forcing, in addition to a modest sensitivity to the deep heating anomaly over the tropical Indian Ocean.
Abstract
A newly developed, linear baroclinic model (LBM) and its application to the tropical ENSO teleconnection is described. The model, based on primitive equations linearized about the observed, zonally varying basic state in northern winter, involves linear schemes for the cumulus convection, and surface sensible and latent heat fluxes, referred to as the moist LBM. This enables us to solve a steady-state response of the coupled dynamical–convective system to a given SST anomaly, which is fairly different from the conventional dry LBM. Linear representation of the convection is acceptable for a realistic range of SST anomalies, reproducing well the Rossby wave train computed in the conventional LBM forced by a tropical heating.
The moist LBM is used to examine the formation mechanisms of an anomalous low-level anticyclone near the Philippines that links El Niño with the Asian winter monsoon. Given that the conventional LBM simulates the Philippine Sea anticyclone as a Rossby response to the anomalous diabatic cooling associated with the weakened convection over the Maritime Continent, causes of the convective suppression are examined. Moist LBM experiments forced by observed El Niño SST anomalies indicate that a basinwide warming of the Indian Ocean, in addition to SST anomalies in the Pacific, has a considerable impact in weakening the convection over the Maritime Continent through a modulation of the Walker circulation. Observational analysis supports this idea and further suggests that the lagged Indian Ocean response to El Niño contributes to determining when the Philippine Sea anticyclone is developed. The moist LBM identified a positive wind–evaporation feedback at work between the Philippine anticyclone and the western Pacific SST anomaly, which might also contribute.
Abstract
A newly developed, linear baroclinic model (LBM) and its application to the tropical ENSO teleconnection is described. The model, based on primitive equations linearized about the observed, zonally varying basic state in northern winter, involves linear schemes for the cumulus convection, and surface sensible and latent heat fluxes, referred to as the moist LBM. This enables us to solve a steady-state response of the coupled dynamical–convective system to a given SST anomaly, which is fairly different from the conventional dry LBM. Linear representation of the convection is acceptable for a realistic range of SST anomalies, reproducing well the Rossby wave train computed in the conventional LBM forced by a tropical heating.
The moist LBM is used to examine the formation mechanisms of an anomalous low-level anticyclone near the Philippines that links El Niño with the Asian winter monsoon. Given that the conventional LBM simulates the Philippine Sea anticyclone as a Rossby response to the anomalous diabatic cooling associated with the weakened convection over the Maritime Continent, causes of the convective suppression are examined. Moist LBM experiments forced by observed El Niño SST anomalies indicate that a basinwide warming of the Indian Ocean, in addition to SST anomalies in the Pacific, has a considerable impact in weakening the convection over the Maritime Continent through a modulation of the Walker circulation. Observational analysis supports this idea and further suggests that the lagged Indian Ocean response to El Niño contributes to determining when the Philippine Sea anticyclone is developed. The moist LBM identified a positive wind–evaporation feedback at work between the Philippine anticyclone and the western Pacific SST anomaly, which might also contribute.
Abstract
El Niño events (warm) are often stronger than La Niña events (cold). This asymmetry is an intrinsic nonlinear characteristic of the El Niño–Southern Oscillation (ENSO) phenomenon. In order to measure the nonlinearity of ENSO, the maximum potential intensity (MPI) index and the nonlinear dynamic heating (NDH) of ENSO are proposed as qualitative and quantitative measures. The 1997/98 El Niño that was recorded as the strongest event in the past century and another strong El Niño event in 1982/83 nearly reached the MPI. During these superwarming events, the normal climatological conditions of the ocean and atmosphere were collapsed completely. The huge bursts of ENSO activity manifested in these events are attributable to the nonlinear dynamic processes.
Through a heat budget analysis of the ocean mixed layer it is found that throughout much of the ENSO episodes of 1982/83 and 1997/98, the NDH strengthened these warm events and weakened subsequent La Niña events. This led to the warm–cold asymmetry. It is also found that the eastward-propagating feature in these two El Niño events provided a favorable phase relationship between temperature and current that resulted in the strong nonlinear dynamical warming. For the westward-propagating El Niño events prior to the late 1970s (e.g., 1957/58 and 1972/73 ENSOs) the phase relationships between zonal temperature gradient and current and between the surface and subsurface temperature anomalies are unfavorable for nonlinear dynamic heating, and thereby the ENSO events are not strong.
Abstract
El Niño events (warm) are often stronger than La Niña events (cold). This asymmetry is an intrinsic nonlinear characteristic of the El Niño–Southern Oscillation (ENSO) phenomenon. In order to measure the nonlinearity of ENSO, the maximum potential intensity (MPI) index and the nonlinear dynamic heating (NDH) of ENSO are proposed as qualitative and quantitative measures. The 1997/98 El Niño that was recorded as the strongest event in the past century and another strong El Niño event in 1982/83 nearly reached the MPI. During these superwarming events, the normal climatological conditions of the ocean and atmosphere were collapsed completely. The huge bursts of ENSO activity manifested in these events are attributable to the nonlinear dynamic processes.
Through a heat budget analysis of the ocean mixed layer it is found that throughout much of the ENSO episodes of 1982/83 and 1997/98, the NDH strengthened these warm events and weakened subsequent La Niña events. This led to the warm–cold asymmetry. It is also found that the eastward-propagating feature in these two El Niño events provided a favorable phase relationship between temperature and current that resulted in the strong nonlinear dynamical warming. For the westward-propagating El Niño events prior to the late 1970s (e.g., 1957/58 and 1972/73 ENSOs) the phase relationships between zonal temperature gradient and current and between the surface and subsurface temperature anomalies are unfavorable for nonlinear dynamic heating, and thereby the ENSO events are not strong.
Abstract
The vertical advection of anomalous subsurface temperature by the mean upwelling and the zonal advection of mean sea surface temperature (SST) by anomalous current are known to be essential for the equatorial SST anomaly associated with the El Niño–Southern Oscillation (ENSO). In the coupled model, these two processes are referred to as the thermocline feedback and the zonal advective feedback, respectively. Using a version of a recharge oscillator model for ENSO obtained from the stripped-down approximation of the Cane–Zebiak-type model, it is demonstrated that these two feedbacks, which are linked dynamically through the geostrophic approximation, tend constructively to contribute to the growth and phase transition of ENSO. However, these two feedbacks control the leading coupled mode in different ways. The thermocline feedback leads to a coupled mode through the merging of the damped SST mode and ocean adjustment mode, whereas the zonal advective feedback tends to destabilize the gravest ocean basin mode. With both of these feedbacks, the leading modes of the coupled model still can be traced back to these different origins under moderate changes in the model setup. The main consequence of these sensitivities is that the growth rate and frequency of the ENSO mode may be sensitive to slight changes in basic-state parameters, which control the strength of these feedbacks.
Abstract
The vertical advection of anomalous subsurface temperature by the mean upwelling and the zonal advection of mean sea surface temperature (SST) by anomalous current are known to be essential for the equatorial SST anomaly associated with the El Niño–Southern Oscillation (ENSO). In the coupled model, these two processes are referred to as the thermocline feedback and the zonal advective feedback, respectively. Using a version of a recharge oscillator model for ENSO obtained from the stripped-down approximation of the Cane–Zebiak-type model, it is demonstrated that these two feedbacks, which are linked dynamically through the geostrophic approximation, tend constructively to contribute to the growth and phase transition of ENSO. However, these two feedbacks control the leading coupled mode in different ways. The thermocline feedback leads to a coupled mode through the merging of the damped SST mode and ocean adjustment mode, whereas the zonal advective feedback tends to destabilize the gravest ocean basin mode. With both of these feedbacks, the leading modes of the coupled model still can be traced back to these different origins under moderate changes in the model setup. The main consequence of these sensitivities is that the growth rate and frequency of the ENSO mode may be sensitive to slight changes in basic-state parameters, which control the strength of these feedbacks.
