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Stephen G. Yeager and William G. Large

Abstract

Temperature and salinity (TS) profiles from the global array of Argo floats support the existence of spice-formation regions in the subtropics of each ocean basin where large, destabilizing vertical salinity gradients coincide with weak stratification in winter. In these characteristic regions, convective boundary layer mixing generates a strongly density-compensated (SDC) layer at the base of the well-mixed layer. The degree of density compensation of the TS gradients of an upper-ocean water column is quantified using a bulk vertical Turner angle (Tub) between the surface and upper pycnocline. The winter generation of the SDC layer in spice-formation zones is clearly seen in Argo data as a large-amplitude seasonal cycle of Tub in regions of the subtropical oceans characterized by high mean Tub. In formation regions, Argo floats provide ample evidence of large, abrupt spice injection (TS increase on subducted isopycnals due to vertical mixing) associated with the winter increase in Tub. A simple conceptual model of the spice-injection mechanism is presented that is based on known behavior of convective boundary layers and supported by numerical model results. It suggests that penetrative convective mixing of a partially density-compensated water column will enhance the Turner angle within a transition layer between the mixed layer and the upper pycnocline, generating seasonal TS increases on density surfaces below the mixed layer. Observations are consistent with this hypothesis. In OGCMs, regions showing high Tub mean and seasonal amplitude are also the sources of significant interannual spice variability in the permanent pycnocline. Decadal changes in the North Pacific of a model hindcast simulation show qualitative resemblance to the observed multiyear time series from the Hawaii Ocean Time series (HOT) station ALOHA. Modeled pycnocline variations near Hawaii can be linked to high Tub seasonality and winter spice injection within a formation region upstream of ALOHA, suggesting that spice injection may explain the origins of observed large, interannual variations on isopycnals in the ocean interior.

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Stephen G. Yeager and William G. Large

Abstract

The origins of density-compensating anomalies of temperature and salinity (spice) are investigated using a model forced with the most realistic surface products available over the 40 years 1958–97. In this hindcast, the largest interannual spiciness anomalies are found in the Pacific Ocean near the isopycnal σ 0 = 25.5, where deviations as great as 1.2°C and 0.6 psu are generated equatorward of winter outcropping in the eastern subtropics in both hemispheres. These source regions are characterized by very unstable salinity gradients and low mean density stratification in winter. Two related signatures of winter mixing in the southeast Pacific (SEP) are density that is well mixed deeper than either temperature or salinity and subsurface density ratios that approach 1. All ocean basins in the model are shown to have regions with these characteristics and signatures; however, the resultant spiciness signals are focused on different isopycnals ranging from σ 0 = 25.0 in the northeast Pacific to σ 0 = 26.5 in the south Indian Ocean. A detailed examination of the SEP finds that large positive anomalies are generated by diapycnal mixing across subducted isopycnals (e.g., σ 0 = 25.5), whereas negative anomalies are the result of a steady isopycnal advection, moderated by vertical advection and heave. There is considerable interannual variability in the strength of anomalies and in the density on which they occur. Historical observations are consistent with the model results but are insufficient to verify all aspects of the hindcast, including the processes of anomaly generation in the SEP. It was not possible to relate isopycnal anomaly genesis to local surface forcing of any kind. A complex scenario involving basinwide circulation of both the ocean and atmosphere, especially of surface water through the subtropical evaporation zones, is put forward to explain the decadal time scale evident in SEP salinity anomalies on σ 0 = 25.5. Pacific anomalies generated on σ 0 = 25.5 can be traced along mean geostrophic streamlines to the western boundary, where decadal salinity variations at ≈7°S are about 2 times as large (order ±0.1 psu) as at ≈12°N, although there may be more variance on shallower isopycnals in the north. At least portions of the σ 0 = 25.5 signals appear to continue along the boundary to a convergence at the equator, suggesting that the most robust sources of Pacific spiciness variance coincide with equatorial exchange pathways.

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William G. Large and Peter R. Gent

Abstract

A nonlocal K-profile parameterization (KPP) of the upper-ocean boundary layer is tested for the equatorial regions. First, the short-term performance of a one-dimensional model with KPP is found to compare favorably to large eddy simulations (LES), including nonlocal countergradient heat flux. The comparison is clean because both the surface forcing and the large-scale flow are identical in the two models. The comparison is direct because the parameterized turbulent flux profiles are explicitly computed in LES. A similar comparison is less favorable when KPP is replaced by purely downgradient diffusion with Richardson-number-dependent viscosity and diffusivity because of the absence of intense convection after sunset. Sensitivity experiments are used to establish parameter values in the interior mixing of KPP.

Second, the impact of the parameterization on annual means and the seasonal cycle in a general circulation model of the upper, equatorial Pacific Ocean is described. The results of GCM runs with and without KPP are compared to annual mean profiles of zonal velocity and temperature from the TOGA-TAO array. The two GCM solutions are closer to each other than to the observations, with biases in zonal velocity in the western Pacific and in subsurface temperature in the eastern Pacific. Such comparisons are never clean because neither the wind stress and the surface heat flux nor the forcing by the large-scale flow are known to sufficient accuracy.

Finally, comparisons are made of the equatorial Pacific Ocean GCM results when different heat flux formulations are used. These include bulk forcing where prescribed air temperature and humidity are used, SST forcing where the use of such ocean-controlled parameters is avoided, and a fully coupled atmospheric general circulation model where there is no prescribed control over any surface fluxes. It is concluded, especially in the eastern Pacific, that the use of specified air temperature and humidity does not overly constrain the model sea surface temperature.

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William G. Large, Edward G. Patton, and Peter P. Sullivan

Abstract

Observations from the Southern Ocean Flux Station provide a wide range of wind, buoyancy, and wave (Stokes) forcing for large-eddy simulation (LES) of deep Southern Ocean boundary layers. Almost everywhere there is a nonzero angle Ω between the shear and the stress vectors. Also, with unstable forcing there is usually a depth where there is stable stratification, but zero buoyancy flux and often a number of depths above where there is positive flux, but neutral stratification. These features allow nonlocal transports of buoyancy and of momentum to be diagnosed, using either the Eulerian or Lagrangian shear. The resulting profiles of nonlocal diffusivity and viscosity are quite similar when scaled according to Monin–Obukhov similarity theory in the surface layer, provided the Eulerian shear is used. Therefore, a composite shape function is constructed that may be generally applicable. In contrast, the deeper boundary layer appears to be too decoupled from the Stokes component of the Lagrangian shear. The nonlocal transports can be dominant. The diagnosed across-shear momentum flux is entirely nonlocal and is highly negatively correlated with the across-shear component of the wind stress, just as nonlocal and surface buoyancy fluxes are related. Furthermore, in the convective limit the scaling coefficients become essentially identical, with some consistency with atmospheric experience. The nonlocal contribution to the along-shear momentum flux is proportional to (1 − cosΩ) and is always countergradient, but is unrelated to the aligned wind stress component.

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Kevin E. Trenberth, William G. Large, and Jerry G. Olson

Abstract

The mean annual cycle in surface wind stress over the global oceans from surface wind analyses from the European Centre for Medium Range Weather Forecasts (ECMWF) for seven years (1980–86) is presented. The drag coefficient is a function of wind speed and atmospheric stability, and the density is computed for each observation. Annual and seasonal mean climatologies of wind stress, wind stress and Sverdrup transport and the first two annual harmonies of the wind stress are presented. The Northern and Southern hemispheres are contrasted as an the Pacific and Atlantic basins. The representativeness of the climatology is also assessed. The main shortcomings with the current results are in the topics.

The wind stress statistics over the southern ocean are believed to be the moon reliable because of the paucity of direct wind observations. Annual mean values exceed 2 dyn cm−2 over the eastern hemisphere near 50°S and locally exceed 3 dyn cm−2 in the southern Indian Ocean; values much larger than in previous climatologies. The 12 month variations dominate the annual cycle over most of the globe and are strongest in the Arabian Sea, North Pacific and North Atlantic. But strong semiannual components occur especially over the Southern Ocean and in the North Pacific. The former are associated with semiannual increases in the strength of the southern westerlies whereas in the North Pacific, the semiannual cycle occurs locally largely because of the annual variations in intensity and meridional movement of the Aleutian low and subtropical high.

The wind stress considerably from year to year. Over most of the world's ocean the mean annual cycle explains less then 45% of the monthly variance in each of the wind stress components and the curl of wind stress. In addition, mean values for the climatology differ significantly from those of previous periods. There is good reason to believe that these differences in the Northern Hemisphere an mostly real and represent climate variations on interannual and decadal time scales that have major implications for the circulation of the oceans. A related factor is that this period included two Pacific Warm Events (El Niños), but no Cold (La Niña) Events.

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Dailin Wang, James C. McWilliams, and William G. Large

Abstract

The deep diurnal cycle of turbulence at the equator is studied using the technique of large-eddy simulation (LES). Based on a scale-separation hypothesis, the LES model includes the following large-scale flow terms: the equatorial undercurrent (EUC), zonal pressure gradient, upwelling, horizontal divergence, zonal temperature gradient, and mesoscale eddy forcing terms for the zonal momentum and the heat equations. The importance of these terms in obtaining a quasi-equilibrium boundary layer solution is discussed. The model is forced with a constant easterly wind stress and diurnal cooling and heating. It is found that boundary-layer turbulence penetrates as deep as 50 m below the mixed layer during nighttime cooling. The diurnal variation of turbulence dissipation and mixed layer depth are within the range of observations. The gradient Richardson number (Ri) of the mean flow shows a diurnal cycle but the amplitudes decrease with depth. Within the mixed layer and just below the layer, Ri can be lower than the critical value of 0.25 at night. During the day, Ri > 0.25 below the mixed layer. Well below the mixed layer (below about 40 m), Ri is always greater than 0.25 because of the initial vertical profiles of EUC and temperature chosen. However, the flow is still highly nonlinear, or turbulent, as indicated by the order one ratio of fluctuating temperature gradient (root-mean-square) to the mean gradient. The authors find that this deep turbulence cycle from the model is closely related to local shear (or Kelvin–Helmholtz) instability. Distribution of local (pointwise) gradient Richardson number shows a diurnal cycle, which is the cause of the diurnal cycle of turbulence well below the mixed layer as evidenced by high levels of turbulent kinetic energy at local Richardson numbers in the range of [0, 0.25]. Eddy viscosity and diffusivity are computed from the LES solutions and are compared with observationally derived values.

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Ralph F. Milliff, William G. Large, William R. Holland, and James C. McWilliams

Abstract

High-resolution (1/5°×1/6°) quasigeostrophic models of the North Atlantic Ocean are forced by daily wind stress curl fields of controlled wavenumber content. In the low-wavenumber case, the wind stress curl is derived from a low-pass filtering of ECMWF wind fields such that the retained wavenumber band is observed to obey a k −2 power law in the spectrum for each day (where k is the wavenumber vector). In a second case, the wavenumber content of the wind stress curl fields is comparable to that derivable from an ideal scatterometer-wind dataset. Decadal-average streamfunction fields are compared with a climatology of dynamic topography and compared between the model calculations driven by these synthetic wind stress curl datasets with the goal of testing the sensitivity of the general circulation to high-wavenumber forcing. The largest signal in decadal-average streamfunction response to high-wavenumber forcing occurs in the eastern basin of the North Atlantic. Fields of mean kinetic and eddy kinetic energies are enhanced in the eastern basin of the North Atlantic by 0.5 and 1.0 orders of magnitude, respectively, in calculations forced by the scatterometer-like wind stress curl. Model solutions are compared with the implied Sverdrup streamfunctions for each forcing dataset, and parallel experiments are performed with a linearized quasigeostrophic model. Conclusions are drawn regarding the operative dynamics and the wavenumber band of importance in improving the model general circulation. It is noted that present day and planned scatterometer missions are capable of resolving the requisite spatial scales.

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William G. Large, Edward G. Patton, Alice K. DuVivier, Peter P. Sullivan, and Leonel Romero

Abstract

Monin–Obukhov similarity theory is applied to the surface layer of large-eddy simulations (LES) of deep Southern Ocean boundary layers. Observations from the Southern Ocean Flux Station provide a wide range of wind, buoyancy, and wave (Stokes drift) forcing. Two No-Stokes LES are used to determine the extent of the ocean surface layer and to adapt the nondimensional momentum and buoyancy gradients, as functions of the stability parameter. Stokes-forced LES are used to modify this parameter for wave effects, then to formulate dependencies of Stokes similarity functions on a Stokes parameter ξ. To account for wind-wave misalignment, the dimensional analysis is extended with two independent variables, namely, the production of turbulent kinetic energy in the surface layer due to Stokes shear and the total production, so that their ratio gives ξ. Stokes forcing is shown to reduce vertical shear more than stratification, and to enhance viscosity and diffusivity by factors up to 5.8 and 4.0, respectively, such that the Prandtl number can exceed unity. A practical parameterization is developed for ξ in terms of the meteorological forcing plus a Stokes drift profile, so that the Stokes and stability similarity functions can be combined to give turbulent velocity scales. These scales for both viscosity and diffusivity are evaluated against the LES, and the correlations are nearly 0.97. The benefit of calculating Stokes drift profiles from directional wave spectra is demonstrated by similarly evaluating three alternatives.

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William G. Large, Gokhan Danabasoglu, Scott C. Doney, and James C. McWilliams

Abstract

The effects of more realistic bulk forcing boundary conditions, a more physical subgrid-scale vertical mixing parameterization, and more accurate bottom topography are investigated in a coarse-resolution, global oceanic general circulation model. In contrast to forcing with prescribed fluxes, the bulk forcing utilizes the evolving model sea surface temperatures and monthly atmospheric fields based on reanalyses by the National Centers for Environmental Prediction and on satellite data products. The vertical mixing in the oceanic boundary layer is governed by a nonlocal K-profile parameterization (KPP) and is matched to parameterizations of mixing in the interior. The KPP scheme is designed to represent well both convective and wind-driven entrainment. The near- equilibrium solutions are compared to a baseline experiment in which the surface tracers are strongly restored everywhere to climatology and the vertical mixing is conventional with constant coefficients, except where there is either convective or near-surface enhancement.

The most profound effects are due to the bulk forcing boundary conditions, while KPP mixing has little effect on the annual-mean state of the ocean model below the upper few hundred meters. Compared to restoring boundary conditions, bulk forcing produces poleward heat and salt transports in better agreement with most oceanographic estimates and maintains the abyssal salinity and temperature closer to observations. The KPP scheme produces mixed layers and boundary layers with realistically large temporal and spatial variability. In addition, it allows for more near-surface vertical shear, particularly in the equatorial regions, and results in enhanced large-scale surface divergence and convergence. Generally, topographic effects are confined locally, with some important consequences. For example, realistic ocean bottom topography between Greenland and Europe locks the position of the sinking branch of the Atlantic thermohaline circulation to the Icelandic Ridge. The model solutions are especially sensitive to the under-ice boundary conditions where model tracers are strongly restored to climatology in all cases. In particular, a factor of 4 reduction in the strength of under-ice restoring diminishes the abyssal salinity improvements by about 30%.

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Scott C. Doney, Steve Yeager, Gokhan Danabasoglu, William G. Large, and James C. McWilliams

Abstract

The interannual variability in upper-ocean (0–400 m) temperature and governing mechanisms for the period 1968–97 are quantified from a global ocean hindcast simulation driven by atmospheric reanalysis and satellite data products. The unconstrained simulation exhibits considerable skill in replicating the observed interannual variability in vertically integrated heat content estimated from hydrographic data and monthly satellite sea surface temperature and sea surface height data. Globally, the most significant interannual variability modes arise from El Niño–Southern Oscillation and the Indian Ocean zonal mode, with substantial extension beyond the Tropics into the midlatitudes. In the well-stratified Tropics and subtropics, net annual heat storage variability is driven predominately by the convergence of the advective heat transport, mostly reflecting velocity anomalies times the mean temperature field. Vertical velocity variability is caused by remote wind forcing, and subsurface temperature anomalies are governed mostly by isopycnal displacements (heave). The dynamics at mid- to high latitudes are qualitatively different and vary regionally. Interannual temperature variability is more coherent with depth because of deep winter mixing and variations in western boundary currents and the Antarctic Circumpolar Current that span the upper thermocline. Net annual heat storage variability is forced by a mixture of local air–sea heat fluxes and the convergence of the advective heat transport, the latter resulting from both velocity and temperature anomalies. Also, density-compensated temperature changes on isopycnal surfaces (spice) are quantitatively significant.

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