Abstract

This article continues the study of the radiation of energy from nonzonal ocean currents in the nonlinear regime. The effects of the interactions between different waves in the initial spectrum on the radiating properties in the model are studied. The process is modeled by three numerical experiments, each initialized with a pair of waves. One wave is typically less radiative than the other.

The development of radiation by an initially nonradiating set of modes is observed. The radiative response in the far field is dominated by the waves with a spatial scale longer than the jet width. A brief comparison of the results to the data in the North Atlantic is presented at the end.

Abstract

The energy radiation from oceanic boundary currents is assumed to be one of the main mechanisms responsible for the production of the highly energetic eddy field in the interior of the ocean. The efficiency of the process is demonstrated in an example of a simple model of a nonzonal flow. The nonzonal orientation of the current proves to be a key dynamical factor setting the radiation in the model.

The effects of the nonlinear interactions on the radiating properties of the solution are studied in detail numerically. The efficient numerical algorithm with open boundary conditions is used. The solutions of the linear problem reported previously by Kamenkovich and Pedlosky are used as initial conditions.

The results show that even rapidly growing linear solutions, which are trapped during the initial stage of development, can radiate energy in the nonlinear regime if the basic current is nonzonal. The radiation starts as soon as the initially fast exponential growth significantly slows. The initial apparent trapping of those solutions is caused by their fast temporal growth. The new mechanism for radiation is related to the nonzonality of a current.

Abstract

No abstract available

Abstract

The linear stability of nonzonal flow, uniform in the alongjet direction on a beta plane, is studied. The flow is balanced by a forcing term in the potential vorticity equation. The problem is analyzed in both barotropic and two-layer models. The stability computations are performed for piecewise constant and continuous velocity profiles. New stability properties of nonzonal jets are discussed. The destabilizing effect of the meridional tilt of the jet is demonstrated. The study focuses on the ability of the current to support radiating instabilities capable of significant penetration into the far field. The radiating properties of nonzonal currents are found to be very different from those of zonal currents. In particular, purely zonal flows do not support radiating instabilities, whereas flows with a meridional component are capable of radiating long waves. The new mechanism for radiation is related to the nonzonality of a current.

Abstract

The zonally localized instability of a zonal flow that has an added meridional component in the zonal interval |x| < a is examined. The flow occurs on the infinite β plane and is analyzed in a two-layer model. The basic flow in the lower layer is zero. The zonal flow velocity is small enough so that it is subcritical with regard to baroclinic instability. The flow is rendered unstable only by its horizontal change of direction, which introduces the meridional flow. The stability problem is solved by matching simple plane wave solutions in the regions upstream, downstream, and within the region of meridional flow.

Localized instability is found for all values of the unstable interval a. The growth rate diminishes as a shrinks, but no critical value of a ≠ 0 needs to be exceeded for instability.

For small values of a, the disturbance extends well beyond a as a wake of slowly damped downstream radiating waves. However, the heat fluxes driving the instability are sharply confined to |x| < a.

Although the instability is possible only because the meridional component of the flow is different from zero, the baroclinic energy conversion of the available potential energy associated with the zonal flow is at least as large as, and usually larger than, that associated with the meridional flow.

The authors suggest that the process described in this simple model may be present whenever zonally accelerated flows are locally unstable since the region of zonal acceleration of the flow must be accompanied by meridional flow.

Abstract

The authors identify and describe the important dynamical mechanisms that explain the significant sensitivity of the Atlantic thermohaline circulation to the parameterization of heat and salt transports by mesoscale eddies in numerical models. In particular, the effects of the Gent–McWilliams (GM) scheme, which has a strong flattening effect on isopycnals, and a simple horizontal diffusion scheme are considered and compared. Two control runs, one with each scheme, exhibit very different circulations and density structures. To analyze the dynamical reasons for the differences between the control runs, a number of numerical experiments with regionally varying diffusion coefficients are carried out, emphasizing the effects of different schemes in key regions. The main effect of eddies in the Southern Ocean in nature is to shoal the subsurface isopycnal surfaces, thus increasing the density of the northward inflow of relatively dense intermediate waters into the Atlantic—as will be seen, this is more effectively done by the GM parameterization of the eddies. The resulting increase in the subsurface density at low latitudes decreases the meridional density contrast with the high latitudes of the North Atlantic, shoals the pycnocline, and consequently weakens the meridional overturning. By contrast, the effect of the eddy transports in the western boundary current in the Northern Hemisphere on the strength of the North Atlantic Deep Water (NADW) formation is shown to be smaller. The Northern Hemisphere upwelling and horizontal flow structure is strongly affected by local eddy transports, and the outflow of the NADW is very sensitive to the Northern Hemisphere eddy transports as a result. The original scaling of Gnanadesikan is modified to include the effects of horizontal mixing in low latitudes. The results confirm the leading role of the Southern Ocean eddies in affecting the strength of NADW formation, while the Northern Hemisphere horizontal mixing mostly affects local upwelling. The eddy transports in the Southern Ocean also affect the properties of Antarctic Bottom Water, which influences the vertical penetration of the NADW overturning cell as well as the density of the deep ocean.

Abstract

Restoring boundary conditions, wherein the temperature and salinity are restored to surface target fields of temperature and salinity, are traditionally used for studies of the ocean circulation in ocean general circulation models. The canonical problem with these boundary conditions is that, when the target fields are chosen as the observed fields, accurate simulation of the surface fields of temperature and salinity would imply that the surface fluxes and therefore the ocean heat transports approach zero, a clearly unrealistic situation. It is clear that the target fields cannot be chosen as the observed fields. A simple but effective method of modifying conventional restoring boundary conditions is introduced, designed to keep the calculated values of surface temperature and salinity as close to observations as possible. The technique involves calculating the optimal target fields in the restoring boundary conditions by an iterative procedure. The method accounts for oceanic processes, such as advection and eddy mixing in the derivation of the new boundary conditions. A reduced version of this method is introduced that produces comparable results but offers greater simplicity in implementation. The simplicity of the method is particularly attractive in idealized studies, which often employ restoring surface boundary conditions. The success of the new method is, however, limited by several factors that cannot be easily compensated by the adjustment of the target profiles. These factors include inaccurate model dynamics, errors in the observations, and the too-simplified form of restoring surface boundary conditions themselves. The application of the method in this study with a coarse-resolution model leads to considerable improvements of the simulation of sea surface temperature (SST) and sea surface salinity (SSS). Both amplitude and phase of the annual cycle in SST greatly improve. The resulting magnitudes of surface heat and freshwater fluxes increase on average, and the meridional heat transport gets stronger. However, the fluxes in some regions remain unrealistic, notably the too-strong freshwater forcing of the western boundary currents in the Northern Hemisphere. Southern Ocean cooling and freshening are also likely to be too strong. The subsurface values of temperature improve greatly, proving that a large part of errors in the subsurface temperature distribution in our model can be corrected by reducing errors at the surface. In contrast, the reduction of errors in surface salinity fails to improve uniformly the simulated subsurface salinity values.

Abstract

The Southern Ocean’s Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) are two globally significant upper-ocean water masses that circulate in all Southern Hemisphere subtropical gyres and cross the equator to enter the North Pacific and North Atlantic Oceans. Simulations of SAMW and AAIW for the twentieth century in eight climate models [GFDL-CM2.1, CCSM3, CNRM-CM3, MIROC3.2(medres), MIROC3.2(hires), MRI-CGCM2.3.2, CSIRO-Mk3.0, and UKMO-HadCM3] that provided their output in support of the Intergovernmental Panel on Climate Change’s Fourth Assessment Report (IPCC AR4) have been compared to the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Atlas of Regional Seas. The climate models, except for UKMO-HadCM3, CSIRO-Mk3.0, and MRI-CGCM2.3.2, provide a reasonable simulation of SAMW and AAIW isopycnal temperature and salinity in the Southern Ocean. Many models simulate the potential vorticity minimum layer and salinity minimum layer of SAMW and AAIW, respectively. However, the simulated SAMW layer is generally thinner and at lighter densities than observed. All climate models display a limited equatorward extension of SAMW and AAIW north of the Antarctic Circumpolar Current. Errors in the simulation of SAMW and AAIW property characteristics are likely to be due to a combination of many errors in the climate models, including simulation of wind and buoyancy forcing, inadequate representation of subgrid-scale mixing processes in the Southern Ocean, and midlatitude diapycnal mixing parameterizations.

Abstract

The ocean heat uptake (OHU) is studied using the Massachusetts Institute of Technology (MIT) ocean general circulation model (OGCM) with idealized ocean geometry. The OGCM is coupled with a statistical–dynamic atmospheric model. The simulation of OHU in the coupled model is consistent with other coupled ocean–atmosphere GCMs in a transient climate change when CO₂ concentration increases by 1% yr^–1. The global average surface air temperature increases by 1.7°C at the time of CO₂ concentration doubling (year 70). The ocean temperature increases by about 1.0°C near the surface, 0.1°C at 1000 m in the Pacific, and 0.3°C in the Atlantic. The maximum overturning circulation (MOTC) in the Atlantic at 1350 m decreases by about 4.5 Sv (1 Sv ≡ 10⁶ m³ s^–1). The center of MOTC drifts upward about 300 m, and therefore a large OTC anomaly (14 Sv) is found at 2700 m. The MOTC recovers gradually, but the OTC anomaly at 2700 m does not seem to recover after CO₂ concentration is kept constant during 400-yr simulation period.

The diagnosis of heat flux convergence anomaly indicates that the warming in the lower latitudes of the Atlantic is associated with large-scale advection. But, the warming in the higher latitudes is associated with the heat brought down from the surface by convection and eddy mixing. In global average, the treatments of convection and eddy mixing are the two main factors affecting the OHU.

The uncertainty of OHU due to subgrid-scale eddy mixing is studied. In the MIT OGCM this mixing is a combination of Gent–McWilliams bolus advection and Redi isopycnal diffusion (GMR), with a single diffusivity being used to calculate the isopycnal and thickness diffusion. Experiments are carried out with values of the diffusivity of 500, 1000, and 2000 m² s^–1. The total OHU is insensitive to these changes. The insensitivity is mainly due to the changes in the vertical heat flux by GMR mixing being compensated by changes in the other vertical heat flux components.

In the Atlantic when the diffusivity is reduced from 1000 to 500 m² s^–1, the surface warming can penetrate deeper. Therefore, the warming decreases by about 0.15°C above 2000 m but increases by about 0.15°C below 2500 m. Similarly, when the diffusivity is increased from 1000 to 2000 m² s^–1, the surface warming becomes shallower; the warming increases by about 0.2°C above 1000 m but decreases by about 0.2°C below 1000 m. These changes in the vertical distribution of the OHU also contribute to the insensitivity of the total OHU to changes in the GMR mixing. The analysis of heat flux convergence indicates that the difference of OHU seems to be associated with the MOTC circulation.

Abstract

The diapycnal diffusivity in the ocean is one of the least known parameters in current climate models. Measurements of this diffusivity are sparse and insufficient for compiling a global map. Inferences from inverse methods and energy budget calculations suggest as much as a factor of 5 difference in the global mean value of the diapycnal diffusivity. Yet, the climate is extremely sensitive to the diapycnal diffusivity. In this paper the sensitivity of the current climate to the diapycnal diffusivity is studied, focusing on the changes occurring in the ocean circulation. To this end, a coupled model with a three-dimensional ocean with idealized geometry is used.

The results show that, at equilibrium, the strength of the thermohaline circulation in the North Atlantic scales with the 0.44 power of the diapycnal diffusivity, in contrast to the theoretical value based on scaling arguments for uncoupled models of 2/3. On the other hand, the strength of the circulation in the South Pacific scales with the 0.63 power of the diapycnal diffusivity in closer accordance with the theoretical value.

The vertical heat balance in the global ocean is controlled by, in the downward direction, (i) advection and (ii) diapycnal diffusion; in the upward direction, (iii) isopycnal diffusion and (iv) parameterized mesoscale eddy [Gent–McWilliams (GM)] advection. The size of the latter three fluxes increases with diapycnal diffusivity, because the thickness of the thermocline also increases with diapycnal diffusivity leading to greater isopycnal slopes at high latitudes, and hence, enhanced isopycnal diffusion and GM advection. Thus larger diapycnal diffusion is compensated for by changes in isopycnal diffusion and GM advection. Little change is found for the advective flux because of compensation between downward and upward advection.

Sensitivity results are presented for the hysteresis curve of the thermohaline circulation. The stability of the climate system to slow freshwater perturbations is reduced as a consequence of a smaller diapycnal diffusivity. This result is consistent with the findings of two-dimensional climate models. However, contrary to the results of these studies, a common threshold for the shutdown of the thermohaline circulation is not found in this model.