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- Author or Editor: Eli Tziperman x
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Abstract
The relation between the mean state of the thermohaline circulation (THC) and its stability is examined using a realistic-geometry primitive equation coupled ocean–atmosphere–ice global general circulation model. The main finding is that a thermohaline circulation that is 25% weaker and less dominated by thermal forcing than that of today’s ocean is unstable within this coupled GCM. Unstable initial ocean climates lead in the coupled model to an increase of the THC, to strong oscillations, or to a THC collapse.
The existence of an unstable range of weak states of the THC provides a natural explanation for large-amplitude THC variability seen in the paleo record prior to the past 10 000 years: A weakening of the THC due to an external forcing (e.g., ice melting and freshening of the North Atlantic) may push it into the unstable regime. Once in this regime, the THC strongly oscillates due to the inherent instability of a weak THC. Hence the strong THC variability in this scenario does not result from switches between two or more quasi-stable steady states.
Abstract
The relation between the mean state of the thermohaline circulation (THC) and its stability is examined using a realistic-geometry primitive equation coupled ocean–atmosphere–ice global general circulation model. The main finding is that a thermohaline circulation that is 25% weaker and less dominated by thermal forcing than that of today’s ocean is unstable within this coupled GCM. Unstable initial ocean climates lead in the coupled model to an increase of the THC, to strong oscillations, or to a THC collapse.
The existence of an unstable range of weak states of the THC provides a natural explanation for large-amplitude THC variability seen in the paleo record prior to the past 10 000 years: A weakening of the THC due to an external forcing (e.g., ice melting and freshening of the North Atlantic) may push it into the unstable regime. Once in this regime, the THC strongly oscillates due to the inherent instability of a weak THC. Hence the strong THC variability in this scenario does not result from switches between two or more quasi-stable steady states.
Abstract
The relation between the circulation calculated from averaged hydrographic data (such as the Levitus data), and the actual time average circulation is examined using a CTD dataset which provides both time and space coverage of a region of the Mediterranean Sea. The connection between eddy mixing coefficients calculated from hydrographic data and the eddy fluxes (
An inverse model is used to calculate circulation and mixing coefficients from the time average data. Then, the actual time averaged circulation is estimated by averaging six realizations of the instantaneous velocity field, and mixing coefficients are calculated by directly parameterizing the eddy fluxes of heat and salt.
Comparing the results obtained by the different procedures, it is concluded that the horizontal time average circulation can be reliably estimated from averaged and smoothed climatological data, but that it is nearly impossible to obtain physically meaningful mixing coefficient from such data.
Abstract
The relation between the circulation calculated from averaged hydrographic data (such as the Levitus data), and the actual time average circulation is examined using a CTD dataset which provides both time and space coverage of a region of the Mediterranean Sea. The connection between eddy mixing coefficients calculated from hydrographic data and the eddy fluxes (
An inverse model is used to calculate circulation and mixing coefficients from the time average data. Then, the actual time averaged circulation is estimated by averaging six realizations of the instantaneous velocity field, and mixing coefficients are calculated by directly parameterizing the eddy fluxes of heat and salt.
Comparing the results obtained by the different procedures, it is concluded that the horizontal time average circulation can be reliably estimated from averaged and smoothed climatological data, but that it is nearly impossible to obtain physically meaningful mixing coefficient from such data.
Abstract
The problem of determining the (eastern boundary) basic stratification and the buoyancy-driven circulation of the oceans is addressed. A global integral constraint relating the interior stratification and the air-sea heat fluxes is derived, based on the condition that the total mass of water of given density is constant in a steady state ocean. This constraint is then applied to two simple analytic models: The first is a continuous nonlinear diffusive model of the lower mid-depth and bottom circulation below the influence of the wind-driven circulation. It shows the tendency of the vertical density profile to look like an exponential profile in the presence of mixing. The integral constraint is used to relate the stratification and circulation of the bottom and mid-depth waters to the air-sea heat fluxes at the surface, where the deep densities outcrop. The second model is a layered one of the wind-driven circulation, mid-depth, and bottom water. The eastern boundary stratification of the model is determined from the air-sea heat fluxes, using the integral constraint and a parameterization of the mixing processes in layer models. A two gyre mid-depth circulation is found, driven by the cross-isopycnal diffusive velocities and affected by the variations in the depth of the main thermocline above it. The bottom circulation in both models is similar to that of the Stommel-Arons model.
Air-sea heat fluxes affect the deep buoyancy-driven flows not by direct cooling or heating, but through the formation of water masses that sink and spreak in the deep ocean. Thermal boundary conditions for the interior thermocline problem seem to require specification of the air-sea heat fluxes in addition to the specification of the density distribution at the base of the Ekman layer. Cross-isopycnal mixing processes are a crucial part of the dynamics, although numerically small. Together with the air-sea heat fluxes, they determine the basic vertical stratification of the wind-driven and deep circulation.
Abstract
The problem of determining the (eastern boundary) basic stratification and the buoyancy-driven circulation of the oceans is addressed. A global integral constraint relating the interior stratification and the air-sea heat fluxes is derived, based on the condition that the total mass of water of given density is constant in a steady state ocean. This constraint is then applied to two simple analytic models: The first is a continuous nonlinear diffusive model of the lower mid-depth and bottom circulation below the influence of the wind-driven circulation. It shows the tendency of the vertical density profile to look like an exponential profile in the presence of mixing. The integral constraint is used to relate the stratification and circulation of the bottom and mid-depth waters to the air-sea heat fluxes at the surface, where the deep densities outcrop. The second model is a layered one of the wind-driven circulation, mid-depth, and bottom water. The eastern boundary stratification of the model is determined from the air-sea heat fluxes, using the integral constraint and a parameterization of the mixing processes in layer models. A two gyre mid-depth circulation is found, driven by the cross-isopycnal diffusive velocities and affected by the variations in the depth of the main thermocline above it. The bottom circulation in both models is similar to that of the Stommel-Arons model.
Air-sea heat fluxes affect the deep buoyancy-driven flows not by direct cooling or heating, but through the formation of water masses that sink and spreak in the deep ocean. Thermal boundary conditions for the interior thermocline problem seem to require specification of the air-sea heat fluxes in addition to the specification of the density distribution at the base of the Ekman layer. Cross-isopycnal mixing processes are a crucial part of the dynamics, although numerically small. Together with the air-sea heat fluxes, they determine the basic vertical stratification of the wind-driven and deep circulation.
Abstract
The physical mechanism underlying ENSO’s phase locking to the seasonal cycle is examined in three parameter regimes: the fast-SST limit, the fast-wave limit, and the mixed SST–wave dynamics regime. The seasonal cycle is imposed on simple ordinary differential equation models for each physical regime either as a seasonal ocean–atmosphere coupling strength obtained from the model of Zebiak and Cane or as a climatological seasonal upwelling. In all three parameter regimes, the seasonal variations in the ocean–atmosphere coupling strength force the events to peak toward the end of the calendar year, whereas the effect of upwelling is shown to be less important. The phase locking mechanism in the mixed-mode and fast-SST regimes relies on the seasonal excitation of the Kelvin and the Rossby waves by wind stress anomalies in the central Pacific basin. The peak time of the events is set by the dynamics to allow a balance between the warming and cooling trends due to downwelling Kelvin and upwelling Rossby waves. This balance is obtained because the warming trend due to the large-amplitude Kelvin waves, amplified by a weak Northern Hemisphere wintertime ocean–atmosphere coupling strength, balances the cooling trend due to weak Rossby waves, amplified by a strong summertime coupling strength. The difference between the locking mechanisms in the mixed-mode regime and in the fast-SST regime is used to understand the effect of the SST adjustment time on the timing of the phase locking. Finally, in the less realistic fast-wave regime, a different physical mechanism for ENSO’s phase locking is revealed through the SST adjustment time and the interaction between the east Pacific region and the central Pacific region.
Abstract
The physical mechanism underlying ENSO’s phase locking to the seasonal cycle is examined in three parameter regimes: the fast-SST limit, the fast-wave limit, and the mixed SST–wave dynamics regime. The seasonal cycle is imposed on simple ordinary differential equation models for each physical regime either as a seasonal ocean–atmosphere coupling strength obtained from the model of Zebiak and Cane or as a climatological seasonal upwelling. In all three parameter regimes, the seasonal variations in the ocean–atmosphere coupling strength force the events to peak toward the end of the calendar year, whereas the effect of upwelling is shown to be less important. The phase locking mechanism in the mixed-mode and fast-SST regimes relies on the seasonal excitation of the Kelvin and the Rossby waves by wind stress anomalies in the central Pacific basin. The peak time of the events is set by the dynamics to allow a balance between the warming and cooling trends due to downwelling Kelvin and upwelling Rossby waves. This balance is obtained because the warming trend due to the large-amplitude Kelvin waves, amplified by a weak Northern Hemisphere wintertime ocean–atmosphere coupling strength, balances the cooling trend due to weak Rossby waves, amplified by a strong summertime coupling strength. The difference between the locking mechanisms in the mixed-mode regime and in the fast-SST regime is used to understand the effect of the SST adjustment time on the timing of the phase locking. Finally, in the less realistic fast-wave regime, a different physical mechanism for ENSO’s phase locking is revealed through the SST adjustment time and the interaction between the east Pacific region and the central Pacific region.
Abstract
The possibility of generating decadal ENSO variability via an ocean teleconnection to the midlatitude Pacific is studied. This is done by analyzing the sensitivity of the equatorial stratification to midlatitude processes using an ocean general circulation model, the adjoint method, and a quasigeostrophic normal-mode stability analysis. It is found that, on timescales of 2–15 yr, the equatorial Pacific is most sensitive to midlatitude planetary Rossby waves traveling from the midlatitudes toward the western boundary and then to the equator. Those waves that propagate through baroclinically unstable parts of the subtropical gyre are amplified by the baroclinic instability and therefore dominate the midlatitude signal arriving at the equator. This result implies that decadal variability in the midlatitude Pacific would be efficiently transmitted to the equatorial Pacific from specific areas of the midlatitude Pacific that are baroclinically unstable, such as the near-equatorial edges of the subtropical gyres (15°N and 12°S). The Rossby waves that propagate via the baroclinically unstable areas are of the advective mode type, which follow the gyre circulation to some degree and arrive from as far as 25°N and 30°S in the east Pacific. It is shown that the baroclinic instability amplifying these waves involves critical layers due to the vertical shear of the subtropical gyre circulation, at depths of 150–200 m.
Abstract
The possibility of generating decadal ENSO variability via an ocean teleconnection to the midlatitude Pacific is studied. This is done by analyzing the sensitivity of the equatorial stratification to midlatitude processes using an ocean general circulation model, the adjoint method, and a quasigeostrophic normal-mode stability analysis. It is found that, on timescales of 2–15 yr, the equatorial Pacific is most sensitive to midlatitude planetary Rossby waves traveling from the midlatitudes toward the western boundary and then to the equator. Those waves that propagate through baroclinically unstable parts of the subtropical gyre are amplified by the baroclinic instability and therefore dominate the midlatitude signal arriving at the equator. This result implies that decadal variability in the midlatitude Pacific would be efficiently transmitted to the equatorial Pacific from specific areas of the midlatitude Pacific that are baroclinically unstable, such as the near-equatorial edges of the subtropical gyres (15°N and 12°S). The Rossby waves that propagate via the baroclinically unstable areas are of the advective mode type, which follow the gyre circulation to some degree and arrive from as far as 25°N and 30°S in the east Pacific. It is shown that the baroclinic instability amplifying these waves involves critical layers due to the vertical shear of the subtropical gyre circulation, at depths of 150–200 m.
Abstract
The dependence of ENSO's period on its amplitude is examined using a simple delayed oscillator model. This dependence is first calculated in the strongly nonlinear regime by extracting and analyzing unstable periodic orbits from the chaotic attractor of the model. In this regime, the period is found to decrease with increasing amplitude. Next, the dependence of the period on the amplitude is also calculated analytically and numerically in the weakly nonlinear regime by varying the ocean–atmosphere coupling coefficient. In this case, the period increases with the amplitude. The weakly nonlinear result reflects the dependence of the period on the ocean–atmosphere coupling strength rather than the dependence on the amplitude, while the strongly nonlinear result is the robust take-home message here: the period reduces with increasing amplitude.
Abstract
The dependence of ENSO's period on its amplitude is examined using a simple delayed oscillator model. This dependence is first calculated in the strongly nonlinear regime by extracting and analyzing unstable periodic orbits from the chaotic attractor of the model. In this regime, the period is found to decrease with increasing amplitude. Next, the dependence of the period on the amplitude is also calculated analytically and numerically in the weakly nonlinear regime by varying the ocean–atmosphere coupling coefficient. In this case, the period increases with the amplitude. The weakly nonlinear result reflects the dependence of the period on the ocean–atmosphere coupling strength rather than the dependence on the amplitude, while the strongly nonlinear result is the robust take-home message here: the period reduces with increasing amplitude.
Abstract
Numerous studies have demonstrated statistical associations between the El Niño–Southern Oscillation (ENSO) and precipitation in the Mediterranean basin. The dynamical bases for these teleconnections have yet to be fully identified. Here, observational analyses and model simulations are used to show how ENSO variability affects rainfall over southwestern Europe (Iberia, Southern France, and Italy). A precipitation index for the region, named southwestern European Precipitation (SWEP), is used. The observational analyses show that ENSO modulates SWEP during the September–December wet season. These precipitation anomalies are associated with changes in large-scale atmospheric fields to the west of Iberia that alter low-level westerly winds and onshore moisture advection from the Atlantic.
The vorticity anomalies associated with SWEP variability are linked to ENSO through a stationary barotropic Rossby wave train that emanates from the eastern equatorial Pacific and propagates eastward to the Atlantic and Mediterranean. Solutions of the linearized barotropic vorticity equation produce such eastward-propagating Rossby waves with trajectories that traverse the region of observed ENSO-related anomalies. In addition, these linearized barotropic vorticity equation solutions produce a dipole of positive and negative vorticity anomalies to the west of Iberia that matches observations and is consistent with the onshore advection of moisture. Thus, interannual variability of fall and early winter precipitation over southwestern Europe is linked to ENSO variability in the eastern Pacific via an eastward-propagating atmospheric stationary barotropic Rossby wave train.
Abstract
Numerous studies have demonstrated statistical associations between the El Niño–Southern Oscillation (ENSO) and precipitation in the Mediterranean basin. The dynamical bases for these teleconnections have yet to be fully identified. Here, observational analyses and model simulations are used to show how ENSO variability affects rainfall over southwestern Europe (Iberia, Southern France, and Italy). A precipitation index for the region, named southwestern European Precipitation (SWEP), is used. The observational analyses show that ENSO modulates SWEP during the September–December wet season. These precipitation anomalies are associated with changes in large-scale atmospheric fields to the west of Iberia that alter low-level westerly winds and onshore moisture advection from the Atlantic.
The vorticity anomalies associated with SWEP variability are linked to ENSO through a stationary barotropic Rossby wave train that emanates from the eastern equatorial Pacific and propagates eastward to the Atlantic and Mediterranean. Solutions of the linearized barotropic vorticity equation produce such eastward-propagating Rossby waves with trajectories that traverse the region of observed ENSO-related anomalies. In addition, these linearized barotropic vorticity equation solutions produce a dipole of positive and negative vorticity anomalies to the west of Iberia that matches observations and is consistent with the onshore advection of moisture. Thus, interannual variability of fall and early winter precipitation over southwestern Europe is linked to ENSO variability in the eastern Pacific via an eastward-propagating atmospheric stationary barotropic Rossby wave train.
Abstract
Westerly wind bursts (WWBs), a significant player in ENSO dynamics, are modeled using an observationally motivated statistical approach that relates the characteristics of WWBs to the large-scale sea surface temperature. Although the WWB wind stress at a given location may be a nonlinear function of SST, the characteristics of WWBs are well described as a linear function of SST. Over 50% of the interannual variance in the WWB likelihood, zonal location, duration, and fetch is explained by changes in SST. The model captures what is seen in a 17-yr record of satellite-derived winds: the eastward migration and increased occurrence of wind bursts as the western Pacific warm pool extends. The WWB model shows significant skill in predicting the interannual variability of the characteristics of WWBs, while the prediction skill of the WWB seasonal cycle is limited by the record length of available data. The novel formulation of the WWB model can be implemented in a stochastic or deterministic mode, where the deterministic mode predicts the ensemble-mean WWB characteristics. Therefore, the WWB model is especially appropriate for ensemble prediction experiments with existing ENSO models that are not capable of simulating realistic WWBs on their own. Should only the slowly varying component of WWBs be important for ENSO prediction, this WWB model allows a shortcut to directly compute the slowly varying ensemble-mean wind field without performing many realizations.
Abstract
Westerly wind bursts (WWBs), a significant player in ENSO dynamics, are modeled using an observationally motivated statistical approach that relates the characteristics of WWBs to the large-scale sea surface temperature. Although the WWB wind stress at a given location may be a nonlinear function of SST, the characteristics of WWBs are well described as a linear function of SST. Over 50% of the interannual variance in the WWB likelihood, zonal location, duration, and fetch is explained by changes in SST. The model captures what is seen in a 17-yr record of satellite-derived winds: the eastward migration and increased occurrence of wind bursts as the western Pacific warm pool extends. The WWB model shows significant skill in predicting the interannual variability of the characteristics of WWBs, while the prediction skill of the WWB seasonal cycle is limited by the record length of available data. The novel formulation of the WWB model can be implemented in a stochastic or deterministic mode, where the deterministic mode predicts the ensemble-mean WWB characteristics. Therefore, the WWB model is especially appropriate for ensemble prediction experiments with existing ENSO models that are not capable of simulating realistic WWBs on their own. Should only the slowly varying component of WWBs be important for ENSO prediction, this WWB model allows a shortcut to directly compute the slowly varying ensemble-mean wind field without performing many realizations.
Abstract
The correlation between parameters characterizing observed westerly wind bursts (WWBs) in the equatorial Pacific and the large-scale SST is analyzed using singular value decomposition. The WWB parameters include the amplitude, location, scale, and probability of occurrence for a given SST distribution rather than the wind stress itself. This approach therefore allows for a nonlinear relationship between the SST and the wind signal of the WWBs. It is found that about half of the variance of the WWB parameters is explained by only two large-scale SST modes. The first mode represents a developed El Niño event, while the second mode represents the seasonal cycle. More specifically, the central longitude of WWBs, their longitudinal extent, and their probability seem to be determined to a significant degree by the ENSO-driven signal. The amplitude of the WWBs is found to be strongly influenced by the phase of the seasonal cycle. It is concluded that the WWBs, while partially stochastic, seem an inherent part of the large-scale deterministic ENSO dynamics. Implications for ENSO predictability and prediction are discussed.
Abstract
The correlation between parameters characterizing observed westerly wind bursts (WWBs) in the equatorial Pacific and the large-scale SST is analyzed using singular value decomposition. The WWB parameters include the amplitude, location, scale, and probability of occurrence for a given SST distribution rather than the wind stress itself. This approach therefore allows for a nonlinear relationship between the SST and the wind signal of the WWBs. It is found that about half of the variance of the WWB parameters is explained by only two large-scale SST modes. The first mode represents a developed El Niño event, while the second mode represents the seasonal cycle. More specifically, the central longitude of WWBs, their longitudinal extent, and their probability seem to be determined to a significant degree by the ENSO-driven signal. The amplitude of the WWBs is found to be strongly influenced by the phase of the seasonal cycle. It is concluded that the WWBs, while partially stochastic, seem an inherent part of the large-scale deterministic ENSO dynamics. Implications for ENSO predictability and prediction are discussed.
Abstract
The sensitivity of long-term averaged air–sea fluxes calculated by a 3D atmospheric general circulation model to SST perturbations of an idealized spatial structure is investigated as a function of the SST perturbation amplitude, spatial scale, latitude, and season. This sensitivity is a dominant, yet largely unknown, parameter determining the stability and variability behavior of ocean-only model studies of decadal climate variability.
The air–sea heat-flux anomaly induced by the SST perturbation varies linearly with respect to the SST perturbation amplitude in a wide range of perturbation amplitudes (±3°C). The implied restoring time of the nonglobal SST perturbations by the air–sea fluxes is found to vary from 1.5 to 3 months (for a 50-m oceanic mixed layer and perturbation scale of less than 3.4 × 106 m) depending on the spatial scale, latitude, and season of the SST anomaly. The calculated restoring time for global perturbation is of the order of two years, and the latent heat flux is found to be the air–sea heat-flux component most sensitive to SST perturbations. Both longwave and shortwave radiative fluxes are much less sensitive than latent and sensible turbulent fluxes for nonglobal SST anomalies.
The restoring time is found to be significantly longer for large-scale anomalies than for the small-scale ones, because of the dominant effect of the heat advection by wind over small perturbations. No significant difference is found between the restoring times for SST perturbations in midlatitudes and in the Tropics. SST perturbations are dissipated faster by the air–sea fluxes during the winter than during the summer. The air–sea freshwater flux anomaly is also found to strongly depend on the SST perturbation amplitude, and to vary almost linearly with the SST perturbation in the midlatitudes, but in a nonlinear way in the Tropics. The possible model dependence of the calculated restoring times is analyzed.
Abstract
The sensitivity of long-term averaged air–sea fluxes calculated by a 3D atmospheric general circulation model to SST perturbations of an idealized spatial structure is investigated as a function of the SST perturbation amplitude, spatial scale, latitude, and season. This sensitivity is a dominant, yet largely unknown, parameter determining the stability and variability behavior of ocean-only model studies of decadal climate variability.
The air–sea heat-flux anomaly induced by the SST perturbation varies linearly with respect to the SST perturbation amplitude in a wide range of perturbation amplitudes (±3°C). The implied restoring time of the nonglobal SST perturbations by the air–sea fluxes is found to vary from 1.5 to 3 months (for a 50-m oceanic mixed layer and perturbation scale of less than 3.4 × 106 m) depending on the spatial scale, latitude, and season of the SST anomaly. The calculated restoring time for global perturbation is of the order of two years, and the latent heat flux is found to be the air–sea heat-flux component most sensitive to SST perturbations. Both longwave and shortwave radiative fluxes are much less sensitive than latent and sensible turbulent fluxes for nonglobal SST anomalies.
The restoring time is found to be significantly longer for large-scale anomalies than for the small-scale ones, because of the dominant effect of the heat advection by wind over small perturbations. No significant difference is found between the restoring times for SST perturbations in midlatitudes and in the Tropics. SST perturbations are dissipated faster by the air–sea fluxes during the winter than during the summer. The air–sea freshwater flux anomaly is also found to strongly depend on the SST perturbation amplitude, and to vary almost linearly with the SST perturbation in the midlatitudes, but in a nonlinear way in the Tropics. The possible model dependence of the calculated restoring times is analyzed.
