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Abstract
Oceanic interdecadal thermohaline oscillations are studied in the context of a conceptual two box model. In this mechanistic model, two essential processes associated with convection, subsurface advective heating and surface freshening, are modeled in terms of horizontal relaxation processes, with the surface relaxation timescale shorter than the subsurface one.
The model can oscillate with interdecadal periods within a certain parameter regime, and captures the fundamental characteristics of interdecadal oscillations: a clockwise trajectory in the T-S plane. The dependence of the amplitude and period of the oscillation on different model parameters is explored. It is shown that there are two important parameters in the model: 1) p, the ratio of the timescale of the surface freshening process to that of the subsurface advective heating process, and 2) q, the ratio of the saline forcing to the thermal forcing in maintaining the vertical halocline and inverted thermocline structure. Oscillatory solutions exist when p < q < 1. The period of the oscillation increases with q and decreases with p. When p is small enough, the period of the oscillation is mostly determined by q. The amplitude of the oscillation increases with the period.
Abstract
Oceanic interdecadal thermohaline oscillations are studied in the context of a conceptual two box model. In this mechanistic model, two essential processes associated with convection, subsurface advective heating and surface freshening, are modeled in terms of horizontal relaxation processes, with the surface relaxation timescale shorter than the subsurface one.
The model can oscillate with interdecadal periods within a certain parameter regime, and captures the fundamental characteristics of interdecadal oscillations: a clockwise trajectory in the T-S plane. The dependence of the amplitude and period of the oscillation on different model parameters is explored. It is shown that there are two important parameters in the model: 1) p, the ratio of the timescale of the surface freshening process to that of the subsurface advective heating process, and 2) q, the ratio of the saline forcing to the thermal forcing in maintaining the vertical halocline and inverted thermocline structure. Oscillatory solutions exist when p < q < 1. The period of the oscillation increases with q and decreases with p. When p is small enough, the period of the oscillation is mostly determined by q. The amplitude of the oscillation increases with the period.
Abstract
A two-dimensional (in a vertical and meridional plane) model for steady equatorial undercurrents is described. Compared to the primitive equation model, the zonal pressure gradient and associated zonal temperature gradients (both vary vertically) are prescribed in this model, and all other terms involving zonal variations are ignored. With zonal pressure gradients resembling actual ocean gradients, model undercurrents agree well with observations as far as the main features are concerned. In particular, the model simulates a stronger undercurrent in the Pacific than in the Atlantic, suggesting that a weaker zonal wind stress, a shallower thermocline, a more surface-confined zonal pressure gradient, and an associated larger magnitude of near-surface zonal temperature gradient around 30°W in the Atlantic than around 150°W in the Pacific, which is related to the longitudinal structure of the zonal wind stress and longitudinal basin extent, are the cause of this difference. An argument based on geostrophy and heat balance is also given.
The model is used to examine the dynamic nature and heat balance of steady equatorial undercurrents for a symmetric circulation about the equator. With a full, nonlinear heat balance, an undercurrent is generated in both linear and nonlinear dynamic balances, but the dynamical features are different in the two cases. In the nonlinear dynamic case, vertical-momentum transports play a key role; in the linear dynamic case, though the eastward zonal pressure gradient provides a necessary forcing, the existence of the undercurrent also relies on the meridional diffusive momentum transport near the surface, which is positive instead of negative. For a doubling of zonal wind stress and a fixed vertical profile of zonal pressure gradient, the speed of the undercurrent core increases by about 25% in the nonlinear case but remains unchanged in the linear case; surface temperature increases by about 1.3 K in the nonlinear case and decreases by 3 K in the linear case.
Within the undercurrent core, the dominant momentum balance is between the zonal pressure gradient and meridional diffusive friction, and the heat balance is between zonal and vertical advections. It is proposed that the position of the undercurrent core relative to the thermocline reflects different advective heat balances: the undercurrent core is above (or below) the thermocline if the net heat advection balance tends to heat (or cool). The fact that the undercurrent core is more or less in the thermocline suggests that three-dimensional advective heat transports almost cancel each other.
Abstract
A two-dimensional (in a vertical and meridional plane) model for steady equatorial undercurrents is described. Compared to the primitive equation model, the zonal pressure gradient and associated zonal temperature gradients (both vary vertically) are prescribed in this model, and all other terms involving zonal variations are ignored. With zonal pressure gradients resembling actual ocean gradients, model undercurrents agree well with observations as far as the main features are concerned. In particular, the model simulates a stronger undercurrent in the Pacific than in the Atlantic, suggesting that a weaker zonal wind stress, a shallower thermocline, a more surface-confined zonal pressure gradient, and an associated larger magnitude of near-surface zonal temperature gradient around 30°W in the Atlantic than around 150°W in the Pacific, which is related to the longitudinal structure of the zonal wind stress and longitudinal basin extent, are the cause of this difference. An argument based on geostrophy and heat balance is also given.
The model is used to examine the dynamic nature and heat balance of steady equatorial undercurrents for a symmetric circulation about the equator. With a full, nonlinear heat balance, an undercurrent is generated in both linear and nonlinear dynamic balances, but the dynamical features are different in the two cases. In the nonlinear dynamic case, vertical-momentum transports play a key role; in the linear dynamic case, though the eastward zonal pressure gradient provides a necessary forcing, the existence of the undercurrent also relies on the meridional diffusive momentum transport near the surface, which is positive instead of negative. For a doubling of zonal wind stress and a fixed vertical profile of zonal pressure gradient, the speed of the undercurrent core increases by about 25% in the nonlinear case but remains unchanged in the linear case; surface temperature increases by about 1.3 K in the nonlinear case and decreases by 3 K in the linear case.
Within the undercurrent core, the dominant momentum balance is between the zonal pressure gradient and meridional diffusive friction, and the heat balance is between zonal and vertical advections. It is proposed that the position of the undercurrent core relative to the thermocline reflects different advective heat balances: the undercurrent core is above (or below) the thermocline if the net heat advection balance tends to heat (or cool). The fact that the undercurrent core is more or less in the thermocline suggests that three-dimensional advective heat transports almost cancel each other.
Abstract
The implicit vertical diffusion (IVD) convective adjustment scheme in common use in ocean general circulation models (OGCMs) could have large residual static gravitational instability at each time step. An iterative and explicit scheme is devised, based on similar physical considerations as the ones for the IVD scheme. It guarantees a complete removal of static instability in a vertical water column and is more efficient than the IVD scheme in overall spinup of the model.
The two convective schemes are compared in an ocean model that is in a state of interdecadal limit cycles. While the model solution with either of these two schemes is characterized by interdecadal oscillations, the variability is different in each scheme. The primary oscillation has a period of about 11 years, but the basin mean kinetic energy shows large differences. The 11-year cycle is modulated by a 33-year oscillation with the IVD scheme, while it is modulated by a 22-year cycle with the complete scheme. The amplitude of the variation of kinetic energy with the IVD scheme is also about twice as large as that with a complete adjustment scheme. It is therefore suggested that complete and incomplete convective schemes can lead to different model variability when convective changes in temperature and salinity have large variations over a short period of time.
Abstract
The implicit vertical diffusion (IVD) convective adjustment scheme in common use in ocean general circulation models (OGCMs) could have large residual static gravitational instability at each time step. An iterative and explicit scheme is devised, based on similar physical considerations as the ones for the IVD scheme. It guarantees a complete removal of static instability in a vertical water column and is more efficient than the IVD scheme in overall spinup of the model.
The two convective schemes are compared in an ocean model that is in a state of interdecadal limit cycles. While the model solution with either of these two schemes is characterized by interdecadal oscillations, the variability is different in each scheme. The primary oscillation has a period of about 11 years, but the basin mean kinetic energy shows large differences. The 11-year cycle is modulated by a 33-year oscillation with the IVD scheme, while it is modulated by a 22-year cycle with the complete scheme. The amplitude of the variation of kinetic energy with the IVD scheme is also about twice as large as that with a complete adjustment scheme. It is therefore suggested that complete and incomplete convective schemes can lead to different model variability when convective changes in temperature and salinity have large variations over a short period of time.
Abstract
Oceanic interdecadal thermohaline oscillations are investigated with a coarse-resolution version of the Geophysical Fluid Dynamics Laboratory Modular Ocean Model. The geometry of the model is a box with a depth of 5000 m and a longitudinal width of 60°, spanning latitudes from 14.5° to 66.5°N. The model ocean is forced by a zonal wind stress, a heat flux parameterized by restoring the surface temperature toward a reference value, and a specified surface freshwater flux. Zonal wind stress, reference temperature, and freshwater flux are all longitudinally uniform, time-independent, and vary meridionally.
It is shown that the ocean model can be in a state of interdecadal oscillations, and a physical mechanism is explained. For these oscillatory solutions, both surface mean heat flux and basin mean kinetic energy vary with interdecadal periods. Temperature and salinity budget analyses reveal that these oscillations depend primarily on advective and convective processes. Horizontal advective heat transports from the subtropical region warm the subsurface water in the subpolar region, destablize the water column, and thereby enhance convection. Convection, in turn, induces surface cyclonic and equatorward flows, which, together with horizontal diffusion and surface freshwater input, transport subpolar fresh water into convecting regions, subsequently weakening or suppressing convection. During an oscillation, convection vertically homogenizes the water column, increases the surface salinity, creates a larger meridional gradient of surface salinity, and increases the efficiency of surface advective freshening in the convective region. The periodic strengthening and weakening of convection caused by subsurface advective warming and surface freshening in the subpolar region results in model interdecadal oscillations.
These advective and convective interdecadal oscillations are not sensitive to either the detailed distribution of subpolar freshwater flux or the horizontal diffusivity. They are mainly a result of halocline and inverted thermocline structure in the subpolar region, maintained by horizontal advective subsurface heating and surface freshening processes.
Abstract
Oceanic interdecadal thermohaline oscillations are investigated with a coarse-resolution version of the Geophysical Fluid Dynamics Laboratory Modular Ocean Model. The geometry of the model is a box with a depth of 5000 m and a longitudinal width of 60°, spanning latitudes from 14.5° to 66.5°N. The model ocean is forced by a zonal wind stress, a heat flux parameterized by restoring the surface temperature toward a reference value, and a specified surface freshwater flux. Zonal wind stress, reference temperature, and freshwater flux are all longitudinally uniform, time-independent, and vary meridionally.
It is shown that the ocean model can be in a state of interdecadal oscillations, and a physical mechanism is explained. For these oscillatory solutions, both surface mean heat flux and basin mean kinetic energy vary with interdecadal periods. Temperature and salinity budget analyses reveal that these oscillations depend primarily on advective and convective processes. Horizontal advective heat transports from the subtropical region warm the subsurface water in the subpolar region, destablize the water column, and thereby enhance convection. Convection, in turn, induces surface cyclonic and equatorward flows, which, together with horizontal diffusion and surface freshwater input, transport subpolar fresh water into convecting regions, subsequently weakening or suppressing convection. During an oscillation, convection vertically homogenizes the water column, increases the surface salinity, creates a larger meridional gradient of surface salinity, and increases the efficiency of surface advective freshening in the convective region. The periodic strengthening and weakening of convection caused by subsurface advective warming and surface freshening in the subpolar region results in model interdecadal oscillations.
These advective and convective interdecadal oscillations are not sensitive to either the detailed distribution of subpolar freshwater flux or the horizontal diffusivity. They are mainly a result of halocline and inverted thermocline structure in the subpolar region, maintained by horizontal advective subsurface heating and surface freshening processes.
Abstract
A multilayer ocean model that is physically simple and computationally efficient is developed for studies of competition and interaction among deep-water sources in determining ocean circulation. The model is essentially geostrophic and hydrostatic in the ocean interior with Rayleigh friction added in boundary-layer and equatorial regions. A stably stratified density structure is specified at static equilibrium, and cross-isopycnal mixing is parameterized as a diffusive flux. The model is forced by latitudinally varying Ekman pumping velocities at the base of the ocean surface Ekman layer and localized deep-water sources.
A four-layer version of the model has been run in a rectangular basin with 5000-m depth, extending from 65°S to 65°N latitude and covering 70 degrees of longitude. The four layers mimic the major water masses observed in the Atlantic Ocean: thermocline water, intermediate water, North Atlantic Deep Water (NADW), and Antarctic Bottom Water (AABW). For forcing corresponding to the current climate, warm water and cold water circulation routes produced in the model agree with those inferred from observations, for example, southward-flowing NADW overriding northward-flowing AABW in the western boundary.
The model shows that subtropical gyres intensify, and thermocline depths become shallow, when deep-water formation rates increase, or when vertical diffusivity kv decreases, or when more NADW is formed from the thermocline layer than that from the intermediate layer. Consistent with the advective thermocline depth scaling, distributions of the Ekman pumping contribute little to deep-water circulations.
The interaction between NADW and AABW sources is demonstrated. Changes in the formation rate of a deep-water source alter cross-isopycnal flows, especially along the related circulation route, thus altering the extent that the other sources can travel before they detrain significantly. These changes feed back onto the thermocline circulation and cross-equatorial transports.
The model suggests that reduction in deep-water formation rate may increase the transient response time of the atmosphere to perturbations, because the thermocline depth becomes deeper. Also, poleward heat transport may decrease, thus acting to self-regulate the temperatures in polar regions.
Abstract
A multilayer ocean model that is physically simple and computationally efficient is developed for studies of competition and interaction among deep-water sources in determining ocean circulation. The model is essentially geostrophic and hydrostatic in the ocean interior with Rayleigh friction added in boundary-layer and equatorial regions. A stably stratified density structure is specified at static equilibrium, and cross-isopycnal mixing is parameterized as a diffusive flux. The model is forced by latitudinally varying Ekman pumping velocities at the base of the ocean surface Ekman layer and localized deep-water sources.
A four-layer version of the model has been run in a rectangular basin with 5000-m depth, extending from 65°S to 65°N latitude and covering 70 degrees of longitude. The four layers mimic the major water masses observed in the Atlantic Ocean: thermocline water, intermediate water, North Atlantic Deep Water (NADW), and Antarctic Bottom Water (AABW). For forcing corresponding to the current climate, warm water and cold water circulation routes produced in the model agree with those inferred from observations, for example, southward-flowing NADW overriding northward-flowing AABW in the western boundary.
The model shows that subtropical gyres intensify, and thermocline depths become shallow, when deep-water formation rates increase, or when vertical diffusivity kv decreases, or when more NADW is formed from the thermocline layer than that from the intermediate layer. Consistent with the advective thermocline depth scaling, distributions of the Ekman pumping contribute little to deep-water circulations.
The interaction between NADW and AABW sources is demonstrated. Changes in the formation rate of a deep-water source alter cross-isopycnal flows, especially along the related circulation route, thus altering the extent that the other sources can travel before they detrain significantly. These changes feed back onto the thermocline circulation and cross-equatorial transports.
The model suggests that reduction in deep-water formation rate may increase the transient response time of the atmosphere to perturbations, because the thermocline depth becomes deeper. Also, poleward heat transport may decrease, thus acting to self-regulate the temperatures in polar regions.
Abstract
The performance of a new historical reanalysis, the NOAA–CIRES–DOE Twentieth Century Reanalysis version 3 (20CRv3), is evaluated via comparisons with other reanalyses and independent observations. This dataset provides global, 3-hourly estimates of the atmosphere from 1806 to 2015 by assimilating only surface pressure observations and prescribing sea surface temperature, sea ice concentration, and radiative forcings. Comparisons with independent observations, other reanalyses, and satellite products suggest that 20CRv3 can reliably produce atmospheric estimates on scales ranging from weather events to long-term climatic trends. Not only does 20CRv3 recreate a “best estimate” of the weather, including extreme events, it also provides an estimate of its confidence through the use of an ensemble. Surface pressure statistics suggest that these confidence estimates are reliable. Comparisons with independent upper-air observations in the Northern Hemisphere demonstrate that 20CRv3 has skill throughout the twentieth century. Upper-air fields from 20CRv3 in the late twentieth century and early twenty-first century correlate well with full-input reanalyses, and the correlation is predicted by the confidence fields from 20CRv3. The skill of analyzed 500-hPa geopotential heights from 20CRv3 for 1979–2015 is comparable to that of modern operational 3–4-day forecasts. Finally, 20CRv3 performs well on climate time scales. Long time series and multidecadal averages of mass, circulation, and precipitation fields agree well with modern reanalyses and station- and satellite-based products. 20CRv3 is also able to capture trends in tropospheric-layer temperatures that correlate well with independent products in the twentieth century, placing recent trends in a longer historical context.
Abstract
The performance of a new historical reanalysis, the NOAA–CIRES–DOE Twentieth Century Reanalysis version 3 (20CRv3), is evaluated via comparisons with other reanalyses and independent observations. This dataset provides global, 3-hourly estimates of the atmosphere from 1806 to 2015 by assimilating only surface pressure observations and prescribing sea surface temperature, sea ice concentration, and radiative forcings. Comparisons with independent observations, other reanalyses, and satellite products suggest that 20CRv3 can reliably produce atmospheric estimates on scales ranging from weather events to long-term climatic trends. Not only does 20CRv3 recreate a “best estimate” of the weather, including extreme events, it also provides an estimate of its confidence through the use of an ensemble. Surface pressure statistics suggest that these confidence estimates are reliable. Comparisons with independent upper-air observations in the Northern Hemisphere demonstrate that 20CRv3 has skill throughout the twentieth century. Upper-air fields from 20CRv3 in the late twentieth century and early twenty-first century correlate well with full-input reanalyses, and the correlation is predicted by the confidence fields from 20CRv3. The skill of analyzed 500-hPa geopotential heights from 20CRv3 for 1979–2015 is comparable to that of modern operational 3–4-day forecasts. Finally, 20CRv3 performs well on climate time scales. Long time series and multidecadal averages of mass, circulation, and precipitation fields agree well with modern reanalyses and station- and satellite-based products. 20CRv3 is also able to capture trends in tropospheric-layer temperatures that correlate well with independent products in the twentieth century, placing recent trends in a longer historical context.
