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E. Joseph Metzger and Harley E. Hurlburt

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A ⅛°, 6-layer Pacific version of the Naval Research Laboratory Layered Ocean Model is used to investigate the nondeterministic nature of Kuroshio intrusion and eddy shedding into the South China Sea (SCS) on annual and interannual timescales. Four simulations, which only differ in the initial state, are forced with 1979–93 European Centre for Medium-Range Weather Forecasts reanalysis 1000 hectopascal (hPa) winds and then continued in 1994–97 with ECMWF operational 1000-hPa winds. The model shows differing amounts of Kuroshio penetration across all four simulations for the yearly means, indicating a large degree of nondeterminism at this timescale. This nondeterminism is quantified by a technique that separates the variability of a model variable into deterministic (caused by direct atmospheric forcing) and nondeterministic (caused by mesoscale flow instabilities) components. Analysis indicates substantial nondeterministic sea surface height and upper-layer velocity variability in the vicinity of Luzon Strait. A quantitative measure of Kuroshio intrusion into the SCS is presented that allows interexperiment comparisons and investigation of interannual variability, and attempts are made to positively correlate it with the oceanic and atmospheric environment. Yearly mean Kuroshio intrusion is not strongly linked to Luzon Strait transports or to changes in the North Equatorial Current bifurcation latitude (which is related to the northward Kuroshio transport east of Luzon). Likewise, no relationship could be found that linked interannual variability of yearly mean Kuroshio intrusion or monsoon season mean Luzon Strait transport with the corresponding zonal or meridional wind components, wind stress magnitude, or wind stress curl. However, there was a close relationship between the mean seasonal cycles of the Luzon Strait transport and the northeast–southwest monsoon. Eddy shedding and deep Kuroshio intrusion are rare events during the period of ECMWF reanalysis forcing, but are persistent features during the ECMWF operational time frame. While the wind stress is consistent across the reanalysis/operational time boundary, large differences exist in the wind stress curl pattern over the Luzon Strait and interior of the SCS basin. For contemporaneous years, the ECMWF operational winds produce higher curl extrema (by a factor of 2) and a much sharper north–south gradient in Luzon Strait. The net effect is to produce more Ekman pumping, a deepening of the thermocline, and a more deeply penetrating Kuroshio during the 1994–97 ECMWF operational forcing time frame. Thus, while normal interannual variations of the wind curl did not produce a deterministic response of simulated Kuroshio intrusion, the marked differences in curl between the two atmospheric products did have a substantial impact.

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Lixin Wu, Zhengyu Liu, and Harley E. Hurlburt

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The effect of continental slope on buoyancy-driven circulation has been studied using a two-layer quasigeostrophic model. In the model, buoyancy flux is incorporated as interfacial mass flux, which consists of narrow intense detrainment in the north and broad entrainment in the south. The model explicitly shows that, in the presence of the continental slope, a small amount of buoyancy flux can drive a strong barotropic flow. This flow develops because the beta effect of bottom topography either reduces or deflects the buoyancy-driven deep flow so that it cannot compensate its overlying counterflow, thus generating a net transport. As a result, in a double gyre circulation with a western continental slope, a small amount of detrainment/entrainment water mass can substantially enhance the transport of the western boundary current through southwestern deflection of the deep subpolar circulation. For example, with a reasonable western continental slope, a 10 Sv (Sv ≡ 106 m3 s−1) detrainment mass flux can increase the transport of the western boundary current from 40 Sv of the wind-driven transport to 148 Sv. Relevance to the North Atlantic is then discussed.

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Patrick J. Hogan and Harley E. Hurlburt

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A regional primitive equation ocean model is used to investigate the impact of grid resolution, baroclinic instability, bottom topography, and isopycnal outcropping on the dynamics of the wind and throughflow-forced surface circulation in the Japan/East Sea. The results demonstrate that at least 1/32° (3.5 km) horizontal grid resolution is necessary to generate sufficient baroclinic instability to produce eddy-driven cyclonic deep mean flows. These abyssal currents follow the f/h contours of the bottom topography and allow the bottom topography to strongly influence mean pathways of the upper-ocean currents in the Japan/East Sea. This upper ocean–topographical coupling via baroclinic instability (actually a mixed baroclinic–barotropic instability) requires that mesoscale variability be very well resolved to obtain sufficient coupling. For example, 1/32° resolution is required to obtain a realistic separation latitude of the East Korean Warm Current (EKWC) from the Korean coast when Hellerman–Rosenstein monthly climatological wind stress forcing is used. Separation of the EKWC is more realistic at 1/8° resolution when the model is forced with climatological winds formed from the ECMWF 10-m reanalysis due to strong positive wind stress curl north of the separation latitude, but at 1/8° the level of baroclinic instability is insufficient to initiate upper ocean–topographical coupling. Hence, this major topographical effect is largely missed at coarser resolution and leads to erroneous conclusions about the role of bottom topography and unexplained errors in the pathways of current systems. Results from a 1/64° simulation are similar to those at 1/32°, particularly where the EKWC separates from the Korean coast, suggesting statistical simulation convergence for mesoscale variability has been nearly achieved at 1/32° resolution. Isopycnal outcropping and associated vertical mixing provide an alternate mechanism to topographical control in developing and maintaining a boundary current along the west coast of Japan, but are less important than baroclinic instability in driving deep mean flows.

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Toshiaki Shinoda, Weiqing Han, E. Joseph Metzger, and Harley E. Hurlburt

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The seasonal variation of Indonesian Throughflow (ITF) transport is investigated using ocean general circulation model experiments with the Hybrid Coordinate Ocean Model (HYCOM). Twenty-eight years (1981–2008) of ⅓° Indo-Pacific basin HYCOM simulations and three years (2004–06) from a global HYCOM simulation are analyzed. Both models are able to simulate the seasonal variation of upper-ocean currents and the total transport through Makassar Strait measured by International Nusantara Stratification and Transport (INSTANT) moorings reasonably well. The annual cycle of upper-ocean currents is then calculated from the Indo-Pacific HYCOM simulation. The reduction of southward currents at Makassar Strait during April–May and October–November is evident, consistent with the INSTANT observations. Analysis of the upper-ocean currents suggests that the reduction in ITF transport during April–May and October–November results from the wind variation in the tropical Indian Ocean through the generation of a Wyrtki jet and the propagation of coastal Kelvin waves, while the subsequent recovery during January–March originates from upper-ocean variability associated with annual Rossby waves in the Pacific that are enhanced by western Pacific winds. These processes are also found in the global HYCOM simulation during the period of the INSTANT observations. The model experiments forced with annual-mean climatological wind stress in the Pacific and 3-day mean wind stress in the Indian Ocean show the reduction of southward currents at Makassar Strait during October–November but no subsequent recovery during January–March, confirming the relative importance of wind variations in the Pacific and Indian Oceans for the ITF transport in each season.

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Toshiaki Shinoda, Weiqing Han, E. Joseph Metzger, and Harley E. Hurlburt
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A. Birol Kara, Alan J. Wallcraft, and Harley E. Hurlburt

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The Naval Research Laboratory (NRL) Layered Ocean Model (NLOM) with an embedded mixed layer submodel is used to predict the climatological monthly mean sea surface temperature (SST) and surface ocean mixed layer depth (MLD) over the global ocean. The thermodynamic model simulations presented in this paper are performed using six dynamical layers plus the embedded mixed layer at 1/2° resolution in latitude and 0.703125° in longitude, globally spanning from 72°S to 65°N. These model simulations use climatological wind and thermal forcing and include no assimilation of SST or MLD data. To measure the effectiveness of the NLOM mixed layer, the annual mean and seasonal cycle of SST and MLD obtained from the model simulations are compared to those from different climatological datasets at each grid point over the global ocean. Analysis of the global error maps shows that the embedded mixed layer in NLOM gives accurate SST with atmospheric forcing even with no SST relaxation/assimilation. In this case the model gives a global root-mean-square (rms) difference of 0.37°C for the annual mean and 0.59°C over the seasonal cycle over the global ocean. The mean global correlation coefficient (R) is 0.91 for the seasonal cycle of the SST. NLOM predicts SST with an annual mean error of <0.5°C in most of the North Atlantic and North Pacific Oceans. For the MLD the model gave a global rms difference of 34 m for the annual mean and 63 m over the seasonal cycle over the global ocean in comparison to the NRL MLD climatology (NMLD). The mean global R value is 0.62 for the seasonal cycle of the MLD. Additional model–data comparisons use climatological monthly mean SST time series from 18 National Oceanic Data Center (NODC) buoys and 11 ocean weather station (OWS) hydrographic locations in the North Pacific Ocean. The median rms difference between the NLOM SSTs and SSTs at these 29 locations is 0.49°C for the seasonal cycle. Deepening and shallowing of the MLD at the all OWS locations in the northeast Pacific are captured by the model with an rms difference of <20 m and an R value of >0.85 for the seasonal cycle.

Using several statistical measures and climatologies of SST and MLD we have demonstrated that NLOM with an embedded mixed layer is able to simulate with substantial skill the climatological SST and MLD when using accurate and computationally efficient surface heat flux and solar radiation attenuation parameterizations over the global ocean. Further, this was accomplished using a model with only seven layers in the vertical, including the embedded mixed layer. Success of climatological predictions from the NLOM with an embedded mixed layer is a prerequisite for simulations using interannual atmospheric forcing with high temporal resolution. NLOM gives accurate upper-ocean quantities with atmospheric forcing even with no SST relaxation or assimilation, a strong indication that the model is a good candidate for assimilation of SST data. Finally, the techniques and datasets used here can be applied to evaluation of other ocean models in predicting the SST and MLD.

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A. Birol Kara, Alan J. Wallcraft, and Harley E. Hurlburt

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A fine-resolution (≈3.2 km) Hybrid Coordinate Ocean Model (HYCOM) is used to investigate the impact of solar radiation attenuation with depth on the predictions of monthly mean sea surface height (SSH), mixed layer depth (MLD), buoyancy and heat fluxes, and near-sea surface circulation as well. The model uses spatially and temporally varying attenuation of photosynthetically available radiation (k PAR) climatologies as processed from the remotely sensed Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) to take water turbidity into account in the Black Sea. An examination of the k PAR climatology reveals a strong seasonal cycle in the water turbidity, with a basin-averaged annual climatological mean value of 0.19 m−1 over the Black Sea. Climatologically forced HYCOM simulations demonstrate that shortwave radiation below the mixed layer can be quite different based on the water turbidity, thereby affecting prediction of upper-ocean quantities in the Black Sea. The clear water constant solar attenuation depth assumption results in relatively deeper MLD (e.g., ≈+15 m in winter) in comparison to standard simulations due to the unrealistically large amount of shortwave radiation below the mixed layer, up to 100 W m−2 during April to October. Such unrealistic sub–mixed layer heating causes weaker stratification at the base of the mixed layer. The buoyancy gain associated with high solar heating in summer effectively stabilizes the upper ocean producing shallow mixed layers and elevated SSH over the most of the Black Sea. In particular, the increased stability resulting from the water turbidity reduces vertical mixing in the upper ocean and causes changes in surface-layer currents, especially in the easternmost part of the Black Sea. Monthly mean SSH anomalies from the climatologically forced HYCOM simulations were evaluated against a monthly mean SSH anomaly climatology, which was constructed using satellite altimeter data from TOPEX/ Poseidon (T/P), Geosat Follow-On (GFO), and the Earth Remote Sensing Satellite-2 (ERS-2) over 1993–2002. Model–data comparisons show that the basin-averaged root-mean-square (rms) difference is ≈4 cm between the satellite-based SSH climatology and that obtained from HYCOM simulations using spatial and temporal k PAR fields. In contrast, when all solar radiation is absorbed at the sea surface or clear water constant solar attenuation depth values of 16.7 m are used in the model simulations, the basin-averaged SSH rms difference with respect to the climatology is ≈6 cm (≈50% more). This demonstrates positive impact from using monthly varying solar attenuation depths in simulating upper-ocean quantities in the Black Sea. The monthly mean k PAR and SSH anomaly climatologies presented in this paper can also be used for other Black Sea studies.

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A. Birol Kara, Alan J. Wallcraft, and Harley E. Hurlburt

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Ocean models need over-ocean atmospheric forcing. However, such forcing is not necessarily provided near the land–sea boundary because 1) the atmospheric model grid used for forcing is frequently much coarser than the ocean model grid, and 2) some of the atmospheric model grid over the ocean includes land values near coastal regions. This paper presents a creeping sea-fill methodology to reduce the improper representation of scalar atmospheric forcing variables near coastal regions, a problem that compromises the usefulness of the fields for ocean model simulations and other offshore applications. For demonstration, atmospheric forcing variables from archived coarse-resolution gridded products—the 1.125° × 1.125° 15-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-15) and 1.0° × 1.0° Navy Operational Global Atmospheric Prediction System (NOGAPS)—are used here. A fine-resolution [1/25° × 1/25° cos(lat)], (longitude × latitude) (∼3.2 km) eddy-resolving Black Sea Hybrid Coordinate Ocean Model (HYCOM) is then forced with/without sea-filled atmospheric variables from these gridded products to simulate monthly mean climatological sea surface temperature (SST). Using only over-ocean values from atmospheric forcing fields in the ocean model simulations significantly reduces the climatological mean SST bias (by ∼1°–3°C) and rms SST difference over the seasonal cycle (by ∼2°–3°C) in coastal regions. Performance of the creeping sea-fill methodology is also directly evaluated using measurements of wind speed at 10 m above the surface from the SeaWinds scatterometer on the NASA Quick Scatterometer (QuikSCAT) satellite. Comparisons of original monthly mean wind speeds from operational ECMWF and NOGAPS products with those from QuikSCAT give basin-averaged rms differences of 1.6 and 1.4 m s−1, respectively, during 2000–03. Similar comparisons performed with sea-filled monthly mean wind speeds result in a much lower rms difference (0.7 m s−1 for both products) during the same time period, clearly confirming the accuracy of the methodology even on interannual time scales. Most of the unrealistically low wind speeds from ECMWF and NOGAPS near coastal boundaries are appropriately corrected with the use of the creeping sea fill. Wind speed errors for ECWMF and NOGAPS (mean bias of ≥ 2.5 m s−1 with respect to QuikSCAT during 2000–03) are substantially eliminated (e.g., almost no bias) near most of the land–sea boundaries. Finally, ocean, atmosphere, and coupled atmospheric–oceanic modelers need to be aware that the creeping sea fill is a promising methodology in significantly reducing the land contamination resulting from an improper land–sea mask existing in gridded coarse-resolution atmospheric products (e.g., ECMWF).

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A. Birol Kara, Peter A. Rochford, and Harley E. Hurlburt

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Efficient and computationally inexpensive simple bulk formulas that include the effects of dynamic stability are developed to provide wind stress, and latent and sensible heat fluxes at the air–sea interface in general circulation models (GCMs). In these formulas the exchange coefficients for momentum and heat (i.e., wind stress drag coefficient, and latent and sensible heat flux coefficients, respectively) have a simple polynomial dependence on wind speed and a linear dependence on the air–sea temperature difference that are derived from a statistical analysis of global monthly climatologies according to wind speed and air–sea temperature difference intervals. Using surface meteorological observations from a central Arabian Sea mooring, these formulas are shown to yield air–sea fluxes on daily timescales that are highly accurate relative to those obtained with the standard algorithm used by the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE), where the latter includes the effect of dynamic stability in calculating wind stress and air–sea heat fluxes. Direct comparisons in calculating the wind stress, and latent and sensible heat fluxes with these formulas and the TOGA COARE algorithm demonstrate that the methodology presented here is computationally inexpensive because iterative calculations are not required in the present methodology. Wind stress and air–sea fluxes can be calculated ≈30 times faster with these bulk formulas than by using the TOGA COARE algorithm. This methodology is of direct practical value for GCMs of high spatial resolution, where the severe computational demands of performing GCM simulations encourage computing air–sea fluxes in the most computationally efficient manner possible. The combination of accuracy and ease of computation of this method makes it the preferred one for computing air–sea fluxes in such GCMs.

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A. Birol Kara, Harley E. Hurlburt, and Alan J. Wallcraft

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This study introduces exchange coefficients for wind stress (CD), latent heat flux (CL), and sensible heat flux (CS) over the global ocean. They are obtained from the state-of-the-art Coupled Ocean–Atmosphere Response Experiment (COARE) bulk algorithm (version 3.0). Using the exchange coefficients from this bulk scheme, CD, CL, and CS are then expressed as simple polynomial functions of air–sea temperature difference (TaTs)—where air temperature (Ta) is at 10 m, wind speed (Va) is at 10 m, and relative humidity (RH) is at the air–sea interface—to parameterize stability. The advantage of using polynomial-based exchange coefficients is that they do not require any iterations for stability. In addition, they agree with results from the COARE algorithm but at ≈5 times lower computation cost, an advantage that is particularly needed for ocean general circulation models (OGCMs) and climate models running at high horizontal resolution and short time steps. The effects of any water vapor flux in calculating the exchange coefficients are taken into account in the polynomial functions, a feature that is especially important at low wind speeds (e.g., Va < 5 m s−1) because air–sea mixing ratio difference can have a major effect on the stability, particularly in tropical regions. Analyses of exchange coefficients demonstrate the fact that water vapor can have substantial impact on air–sea exchange coefficients at low wind speeds. An example application of the exchange coefficients from the polynomial approach is the recalculation of climatological mean wind stress magnitude from 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data in the North Pacific Ocean over 1979–2002. Using ECMWF 10-m winds and the authors’ methodology provides accurate surface stresses while largely eliminating the orographically induced Gibb’s waves found in the original ERA-40 surface wind stresses. These can have a large amplitude near mountainous regions and can extend far into the ocean interior. This study introduces exchange coefficients of air–sea fluxes, which are applicable to the wide range of conditions occurring over the global ocean, including the air–sea stability differences across the Gulf Stream and Kuroshio, regions which have been the subject of many climate model studies. This versatility results because CD, CL, and CS are determined for Va values of 1 to 40 m s−1, (TaTs), intervals of −8° to 7°C, and RH values of 0% to 100%. Exchange coefficients presented here are called the Naval Research Laboratory (NRL) Air–Sea Exchange Coefficients (NASEC) and they are suitable for a wide range of air–sea interaction studies and model applications.

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