Harley E. Hurlburt

An optimal definition for ocean mixed layer depth

A new method is introduced for determining ocean isothermal layer depth (ILD) from temperature profiles and ocean mixed layer depth (MLD) from density profiles that can be applied in all regions of the worlds oceans. This method can accommodate not only in situ data but also climatological data sets that typically have much lower vertical resolution. The sensitivity of the ILD and MLD to the temperature difference criteria used in the surface layer depth definition is discussed by using temperature and density data, respectively: (1) from 11 ocean weather stations in the northeast Pacific and (2) from the World Ocean Atlas 1994. Using these two data sets, a detailed statistical error analysis is presented for the ILD and MLD estimation by season. MLD variations with location due to temperature and salinity are properly accounted for in the defining density (Δσt) criterion. Overall, the optimal estimate of turbulent mixing penetration is obtained using a MLD definition of ΔT =0.8°0, although in the northeast Pacific region the optimal MLD criterion is found to vary seasonally. The method is shown to produce layer depths that are accurate to within 20 m or better in 85% or more of the cases. The MLD definition presented in this investigation accurately represents the depth to which turbulent mixing has penetrated and would be a useful aid for validation of one-dimensional bulk mixed layer models and ocean general circulation models with an embedded mixed layer.

A Numerical Study of Loop Current Intrusions and Eddy Shedding

Abstract The dynamics of the eddy shedding by the Loop Current in the Gulf of Mexico have been investigated using three nonlinear numerical models: two-layer, barotropic and reduced gravity. The barotropic and reduced gravity models demonstrate the individual behavior of the external and internal modes, and provide insight into how they interact in the two-layer model. Because of the economy of the semi-implicit free surface models, it was possible to perform over 100 experiments to investigate the stability properties of the Loop Current. Typically, the models were integrated 3–5 years to statistical equilibrium on a 1600 km×900 km rectangular domain with a resolution of 20 km×18.75 km. Prescribed inflow through the model Yucatan Channel was compensated by outflow through the Florida Straits. A long-standing hypothesis is that the Loop Current sheds eddies in response to quasi-annual variations in the inflow. We find that the Loop Current can penetrate into the Gulf, bend westward, and shed realistic ant...

Coupled dynamics of the South China Sea, the Sulu Sea, and the Pacific Ocean

The complex geometry, the seasonally reversing monsoon winds, and the connectivity with the Pacific Ocean all contribute to the coupled dynamics of the circulation in the South China Sea (SCS), the Sulu Sea, and the region around the Philippine Islands. The 1/2°, 1.5-layer global reduced gravity thermodynamic Navy layered ocean model (NLOM) is used to separate these components and to investigate the role of each one. When forced by the Hellerman and Rosenstein [1983] (HR) monthly wind stress climatology, the basic features of the model solution compare well with observations, and with higher-resolution NLOM versions. The dynamics of the flow from the Pacific Ocean into the SCS via the Luzon Strait are emphasized. The effects of Ekman suction/pumping due to wind curl are examined by forming monthly spatial averages of the winds over the SCS/Sulu Sea basins. This maintains a monthly varying stress but with a region of zero curl. Forcing the model with these modified winds leaves the mean Luzon Strait transport unchanged, and the variability actually increases slightly. These results suggest that it is the pressure head created by the pileup of water from the monsoonal wind stress that controls the variability of the Luzon Strait transport. The forcing for wind stress pileup effects could be either internal or external to the SCS/Sulu Sea basin. The effects of internal forcing are studied by applying monthly winds within this basin but annual HR winds outside the region. With this forcing the mean Luzon Strait transport is essentially unchanged, but the variability is only 44% of the standard case value. The external forcing is defined as zero stress in the SCS/Sulu Sea basins and HR monthly winds outside. Again, the mean Luzon Strait transport is unchanged, and here the variability is 60% of the standard case. The mean Luzon Strait transport is largely a function of the model geometry. When the Sulu archipelago is opened, a net cyclonic flow develops around the Philippines, which is essentially an extension of the northern tropical gyre. The bifurcation latitude of the North Equatorial Current (NEC) at the Philippine coast is also affected by the amount of transport through the Sulu archipelago. Opening this archipelago causes the NEC split point to move southward and increases the transport of the Kuroshio east of Luzon while decreasing the Mindanao Current. Opening or closing the Sunda Shelf/Java Sea or the Sulu archipelago does not affect the transport of the Pacific to Indian Ocean throughflow.

Dynamics of the Kuroshio/Oyashio current system using eddy-resolving models of the North Pacific Ocean

A set of numerical simulations is used to investigate the Pacific Ocean circulation north of 20°S, with emphasis on the Kuroshio/Oyashio current system. The primitive equation models used for these simulations have a free surface and realistic geometry that includes the deep marginal seas, such as the Sea of Japan. Most of the simulations have 1/8° resolution for each variable but range from 1/2°, 1.5-layer reduced gravity to 1/16°, six layer with realistic bottom topography. These are used to investigate the dynamics of the Kuroshio/Oyashio current system and to identify the processes that contribute most to the realism of the simulations. This is done by model-data comparisons, by using the modularity of layered ocean models to include/exclude certain dynamical processes, by varying the model geometry and bottom topography, and by varying model parameters, such as horizontal grid resolution, layer structure, and eddy viscosity. In comparison with observational data, the simulations show that the barotropic mode, at least one internal mode, nonlinearity, high “horizontal” resolution (1/8° or finer), the regional bottom topography, and the wind forcing are critical for realistic simulations. The first four are important for baroclinic instability (eddy-mean energetics actually show mixed barotropic-baroclinic instability), the wind curl pattern for the formation and basic placement of the current system, and the bottom topography for the distribution of the instability and for influences on the pathways of the mean flow. Both the Hellerman and Rosenstein (1983) (HR) monthly wind stress climatology and 1000-mbar winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) have been used to drive the model. East of about 150°E, they give a mean latitude for the Kuroshio Extension that differs by about 3°, approximately 34°N for HR, 37°N for ECMWF, and 35°N observed. The subarctic front is the northern boundary of the subtropical gyre. It is associated with the annual and April–September mean zero wind stress curl lines (which are similar), while the Kuroshio Extension is associated with wintertime zero wind stress curl. This means that part of the flow from the Kuroshio must pass north of the Kuroshio Extension and connect with the Oyashio and subarctic front. Realistic routes for this connection are flow through the Sea of Japan, a nonlinear route separated from the east coast of Japan, and bifurcation of the Kuroshio at the Shatsky Rise. In addition, the six-layer simulations show a 3-Sv meridional overturning cell with southward surface flow and northward return flow centered near 400 m depth. Baroclinic instability plays a critical role in coupling the shallow and abyssal layer circulations and in allowing the bottom topography to strongly influence the shallow circulation. By this means, the Izu Ridge and Trench and seamounts upstream and downstream of these have profound influence on (1) the mean path of the Kuroshio and its mean meanders south and east of Japan and (2) on separating the northward flow connecting the Kuroshio and the Oyashio/subarctic front from the east coast of Japan. Without the topographic influence, the models show an unrealistic northward current along the east coast of Japan. In essence, the topography regulates the location and strength of the baroclinic instability. The baroclinic instability gives eddy-driven deep mean flows that follow the f/h contours (where f is the Coriolis parameter and h is the depth of the water column) of the bottom topography. These abyssal currents then strongly influence the pathway for subtropical gyre flow north of the Kuroshio Extension and steer the mean meanders in the Kuroshio south and east of Japan. This is corroborated by current meter data from the Kuroshio Extension Regional Experiment (World Ocean Circulation Experiment line PCM 7). The meander path south of Japan depends on the occurrence of baroclinic instability west of the Izu Ridge; otherwise, a straight path occurs. The pathway shows little sensitivity to the Tokara Strait transport over the range simulated (36–72 Sv in yearly means). However, interannual increases in wind forcing or Tokara Strait transport give rise to a predominant meander path, while decreases yield a predominant straight path. Resolution of 1/8° in an ocean model is comparable to the 2.5° resolution used in atmospheric forecast models in the early 1980s based on the first internal mode Rossby radius of deformation. Model comparisons at 1/8° and 1/16° resolution and comparisons with current meter data and Geosat altimeter data show that 1/16° resolution is needed for adequate eastward penetration of the high eddy kinetic energy associated with the Kuroshio Extension.

The Nondeterministic Nature of Kuroshio Penetration and Eddy Shedding in the South China Sea

Abstract 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 ...

Impact of 1/8° to 1/64° resolution on Gulf Stream model-data comparisons in basin-scale subtropical Atlantic Ocean models

Abstract We investigate the impact of 1/8°, 1/16°, 1/32°, and 1/64° ocean model resolution on model–data comparisons for the Gulf Stream system mainly between the Florida Straits and the Grand Banks. This includes mean flow and variability, the Gulf Stream pathway, the associated nonlinear recirculation gyres, the large-scale C-shape of the subtropical gyre and the abyssal circulation. A nonlinear isopycnal, free surface model covering the Atlantic from 9°N to 47°N or 51°N, including the Caribbean and Gulf of Mexico, and a similar 1/16° global model are used. The models are forced by winds and by a global thermohaline component via ports in the model boundaries. When calculated using realistic wind forcing and Atlantic model boundaries, linear simulations with Munk western boundary layers and a Sverdrup interior show two unrealistic mean Gulf Stream pathways between Cape Hatteras and the Grand Banks, one proceeding due east from Cape Hatteras and a second one continuing northward along the western boundary until forced eastward by the regional northern boundary. The northern pathway is augmented when a linear version of the upper ocean global thermohaline contribution to the Gulf Stream is added as a Munk western boundary layer. A major change is required to obtain a realistic pathway in nonlinear models. Resolution of 1/8° is eddy-resolving but mainly gives a wiggly version of the linear model Gulf Stream pathway and weak abyssal flows except for the deep western boundary current (DWBC) forced by ports in the model boundaries. All of the higher resolution simulations show major improvement over the linear and 1/8° nonlinear simulations. Additional major improvement is seen with the increase from 1/16° to 1/32° resolution and modest improvement with a further increase to 1/64°. The improvements include (1) realistic separation of the Gulf Stream from the coast at Cape Hatteras and a realistic Gulf Stream pathway between Cape Hatteras and the Grand Banks based on comparisons with Gulf Stream pathways from satellite IR and from GEOSAT and TOPEX/Poseidon altimetry (but 1/32° resolution was required for robust results), (2) realistic eastern and western nonlinear recirculation gyres (which contribute to the large-scale C-shape of the subtropical gyre) based on comparisons with mean surface dynamic height from the generalized digital environmental model (GDEM) oceanic climatology and from the pattern and amplitude of sea surface height (SSH) variability surrounding the eastern gyre as seen in TOPEX/Poseidon altimetry, (3) realistic upper ocean and DWBC transports based on several types of measurements, (4) patterns and amplitude of SSH variability which are generally realistic compared to TOPEX/Poseidon altimetry, but which vary from simulation to simulation for specific features and which are most realistic overall in the 1/64° simulation, (5) a basin wide explosion in the number and strength of mesoscale eddies (with warm core rings (WCRs) north of the Gulf Stream, the regional eddy features best observed by satellite IR), (6) realistic statistics for WCRs north of the Gulf Stream based on comparison to IR analyses (low at 1/16° resolution and most realistic at 1/64° resolution for mean population and rings generated/year; realistic ring diameters at all resolutions), and (7) realistic patterns and amplitude of abyssal eddy kinetic energy (EKE) in comparison to historical measurements from current meters.

The connectivity of eddy variability in the Caribbean Sea, the Gulf of Mexico, and the Atlantic Ocean

A set of numerical simulations is used to investigate the connectivity of mesoscale variability in the Atlantic Ocean, the Caribbean, and the Gulf of Mexico. The primitive equation models used for these simulations have a free surface and realistic coastline geometry including a detailed representation of the Lesser Antilles island arc. Two simulations have 1/4° resolution and include a 5.5-layer reduced gravity and a 6-layer model with realistic bottom topography. Both are wind forced and include the global thermohaline circulation. The third simulation is from a 1/2° linear wind-driven model. In the two nonlinear numerical simulations, potential vorticity from decaying rings shed by the North Brazil Current retroflection can be advected through the Lesser Antilles. This potential vorticity acts as a finite amplitude perturbation for mixed barotropic and internal mode baroclinic instabilities, which amplify mesoscale features in the Caribbean. The eddies associated with the Caribbean Current are primarily anticyclonic and transit a narrow corridor across the Caribbean basin along an axis at 14° to 15°N with an average speed of 0.15 m/s. It takes them an average of 10 months to transit from the Lesser Antilles to the Yucatan Channel. Along the way, many of the eddies intensify greatly. The amount of intensification depends substantially on the strength of the Caribbean Current and is greatest during a multiyear period when the current is anomalously strong owing to interannual variation in the wind forcing. Some Caribbean eddies squeeze through the Yucatan Channel into the Gulf of Mexico, where they can influence the timing of Loop Current eddy-shedding events. There is a significant correlation of 0.45 between the Loop Current eddy shedding and the eddies near the Lesser Antilles with a time lag of 11 months. However, Caribbean eddies show no statistically significant net influence on the mean eddy-shedding period nor on the size and strength of shed eddies in the Gulf of Mexico. Additionally, no significant correlation is found between eddy shedding in the Gulf of Mexico and transport variations in the Florida Straits, although transport fluctuations in the Florida Straits at 27°N and the Yucatan Channel and showed a correlation of about 0.7 with a lag of 15 days. The linear solution exhibited a multiyear anomaly in the strength of the Caribbean circulation that was concentrated in the central and eastern Caribbean due to a multiyear anomaly in the wind field over the basin. In the nonlinear simulation this anomaly extended into the western Caribbean and across the entire Gulf of Mexico. This westward extension resulted from the nonlinearity and instability of the Caribbean Current, the westward propagation of the eddies, and the passage of Caribbean eddies through the Yucatan Channel into the Gulf of Mexico.

A Numerical Model of Coastal Upwelling

Abstract A wind-driven model of coastal upwelling induced into a stratified, rotating ocean is solved numerically. The circulation is on an f plane and longshore variations are neglected. A multilevel model is derived, but only solutions for a two-layer model are discussed. A longshore baroclinic surface jet is discovered. The time-dependent geostrophic jet is dynamically explained by conservation of potential vorticity. The existence of the jet depends critically on stratification and non-zero wind stress at the coast. Coastal upwelling is confined to within 30 km of the shore. The model exhibits no deep countercurrent during active coastal upwelling. A time scale of the order of 10 days or longer is required for a pycnocline at 50 m depth to penetrate the surface. Solutions for a wide (>300 km) coastal shelf, an irregular shallow shelf, and a continental slope region are illustrated. A secondary upwelling region is found offshore at sharp breaks in the shelf topography. In all cases, the offshore flow i...

The Dynamics of the East Australian Current System: The Tasman Front, the East Auckland Current, and the East Cape Current

Abstract The dynamics of the flow field surrounding New Zealand are investigated using a series of global ocean models. The physical mechanisms governing the direction, magnitude, and location of the East Australian Current (EAC), the Tasman Front, the East Auckland Current (EAUC), and the East Cape Current (ECC) are studied using numerical simulations whose complexity is systematically increased. As new dynamics are added to each successive simulation, their direct and indirect effects on the flow field are examined. The simulations have horizontal resolutions of 1/8°, 1/16°, or 1/32° for each variable, and vertical resolutions ranging from 1.5-layer reduced gravity to 6-layer finite depth with realistic bottom topography. All simulations are forced by the Hellerman and Rosenstein monthly wind stress climatology. Analysis of these simulations shows that several factors play a critical role in governing the behavior of the examined currents. These factors include 1) mass balance of water pathways through ...

Efficient and Accurate Bulk Parameterizations of Air–Sea Fluxes for Use in General Circulation Models

Abstract 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 ...