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

Abstract

Empirical rules for both entrainment and detrainment are developed from LES of the Southern Ocean boundary layer when the turbulence, stratification and shear cannot be assumed to be in equilibrium with diurnal variability in surface flux and wave (Stokes drift) forcing. A major consequence is the failure of down-gradient eddy viscosity, which becomes more serious with Stokes drift and is overcome by relating the angle between the stress and shear vectors to the orientations of Lagrangian shear to the surface and of local Eulerian shear over five meters. Thus, the momentum flux can be parameterized as a stress magnitude and this empirical direction. In addition, the response of a deep boundary layer to sufficiently strong diurnal heating includes boundary layer collapse and the subsequent growth of a morning boundary layer, whose depth is empirically related to the time history of the forcing, as are both morning detrainment and afternoon entrainment into weak diurnal stratification. Below the boundary layer, detrainment rules give the maximum buoyancy flux and its depth, as well a specific stress direction. Another rule relates both afternoon and night-time entrainment depth and buoyancy flux to surface layer turbulent kinetic energy production integrals. These empirical relationships are combined with rules for boundary layer transport to formulate two parameterizations; one based on eddy diffusivity and viscosity profiles and another on flux profiles of buoyancy and of stress magnitude. Evaluations against LES fluxes show the flux profiles to be more representative of the diurnal cycle, especially with Stokes drift.

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

Abstract

Computations of the surface wind stress and pseudostress over the global oceans have been made using surface winds from the European Centre for Medium Range Weather Forecasts for 7 years. The drag coefficient is a function of wind speed and atmospheric stability, and the air density is computed for each observation. Assuming a constant density, the effective drag coefficient required to convert the pseudostress into a stress has been computed for each month of the year using several methods. Because the drag coefficient varies from day-to-day and with the seasons, the effective drag coefficient cannot be uniquely defined and is a useful concept if only the very gross characteristics of the field are of interest and errors of the order of 10% are tolerable. Even then, the spatial and seasonal variations in CD must be taken into amount, and occasionally the wind stress may be greatly in error.

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

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The approach to equilibrium of a coarse-resolution, seasonally forced, global oceanic general circulation model is investigated, considering the affects of a widely used acceleration technique that distorts the dynamics by using unequal time steps in the governing equations. A measure of the equilibration time for any solution property is defined as the time it takes to go 90% of the way from its present-value to its equilibrium value. This measure becomes approximately time invariant only after sufficiently long integration. It indicates that the total kinetic energy and most mass transport rates attain equilibrium within about 90 and 40 calender years, respectively. The upper-ocean potential temperature and salinity equilibrium times are about 480 and 380 calender years, following 150- and 20-year initial adjustments, respectively. In the abyssal ocean, potential temperature and salinity equilibration take about 4500 and 3900 calender years, respectively. These longer equilibration times are due to the slow diffusion of tracers both along and across the isopycnal surfaces in stably stratified regions, and these times vary with the associated diffusivities. An analysis of synchronous (i.e., not accelerated) integrations shows that there is a complex interplay between convective, advective, and diffusive timescales. Because of the distortion by acceleration of the seasonal cycle, the solutions display some significant adjustments upon switching to synchronous integration. However, the proper seasonal cycle is recovered within five years. Provided that a sufficient equilibrium state has been achieved with acceleration, the model must be integrated synchronously for only about 15 years thereafter to closely approach synchronous equilibrium.

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

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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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Svein Vagle, William G. Large, and David M. Farmer

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The potential of the WOTAN technique to estimate oceanic winds from underwater ambient sound is thoroughly evaluated. Anemometer winds and sound spectrum levels at 11 frequencies in the range 3–25 kHz from the FASINEX Experiment are used to establish both the frequency and wind speed dependencies of ambient sound. These relationships are then tested using independent data from four other deployments, and found to hold in the deep ocean in the OCEAN STORMS but not in shallow coastal waters. The OCEAN STORMS ambient-sound wind speed estimates are within ±0.5 m s−1 of anemometer values for wind speeds between 4 and 15 m s−1. Causes of differences, including disequilibrium of the surface wave field, are discussed and it is argued that they are no larger than expected.

The procedure for processing ambient-sound data is developed. It includes temperature dependent calibration detection of shipping and precipitation contamination, and standardization of measurements to 1 m depth. The latter procedure allows data from different depths and sound speed profiles to be compared. The potential for using the technique on remote platforms is assessed. On-board processing and subsequent despiking and interpolation would result in a continuous wind record. For time scales of 12 hours or longer the results would be very similar to those obtained with an anemometer. Over shorter time scales there may be some important differences.

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