E. Joseph Metzger

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 ...

South China Sea throughflow impact on the Indonesian throughflow

Received 16 April 2012; revised 2 May 2012; accepted 3 May 2012; published 2 June 2012. [1 ]I n 2008 –2009 the Makassar throughflow profile changed dramatically: the characteristic thermocline velocity maximum increased from 0.7 to 0.9 m/sec and shifted from 140 m to 70 m, amounting to a 47% increase in the transport of warmer water between 50 and 150 m during the boreal summer. HYCOM output indicates that ENSO induced change of the South China Sea (SCS) throughflow into the Indonesian seas is the likely cause. Increased SCS throughflow during El Nino with a commensurate increase in the southward flow of buoyant surface water through the Sulu Sea into the northern Makassar Strait, inhibits tropical Pacific surface water injection into Makassar Strait; during La Nina SCS throughflow is near zero allowing tropical Pacific inflow. The resulting warmer ITF reaches into the Indian Ocean, potentially affecting regional sea surface temperature and climate. Citation: Gordon, A. L., B. A. Huber, E. J. Metzger, R. D. Susanto, H. E. Hurlburt, and T. R. Adi (2012), South China Sea throughflow impact on the Indonesian throughflow, Geophys. Res. Lett., 39, L11602, doi:10.1029/2012GL052021.

An operational Eddy resolving 1/16° global ocean nowcast/forecast system

The first real-time eddy resolving nearly global ocean nowcast/forecast system has been running daily at the Naval Oceanographic Office (NAVOCEANO) since 18 October 2000 and it became an operational system on 27 September 2001. Thirty-day forecasts are made once a week. The system, which was developed at the Naval Research Laboratory (NRL), uses the NRL Layered Ocean Model (NLOM) with 1/16j resolution and seven layers in the vertical, including a Kraus–Turner type bulk mixed layer. Sea surface temperature (SST) from satellite IR and satellite altimeter sea surface height (SSH) data from TOPEX/ POSEIDON (T/P), ERS-2 and Geosat-Follow-On (GFO), provided via NAVOCEANO’s Altimeter Data Fusion Center (ADFC), are assimilated into the model. The large size of the model grid (4096 � 2304 � 7) and operational requirements make it necessary to use a computationally efficient ocean model and data assimilation scheme. The assimilation consists of an optimum interpolation (OI) based scheme that uses an OI deviation analysis with the model as a first guess, a statistical inference technique for vertical mass field updates, geostrophic balance for the velocity updates outside of the equatorial region and incremental updating of the model fields to further reduce inertia–gravity wave generation. A spatially varying mesoscale covariance function determined from T/P and ERS-2 data is used in the OI analysis. The SST assimilation consists of relaxing the NLOM SST to the Modular Ocean Data Assimilation System (MODAS) SSTanalysis, which is performed daily at NAVOCEANO. Realtime and archived results from the model can be viewed at the NRL web site http://www.ocean.nrlssc.navy.mil/global_nlom. This includes many zoom regions, nowcasts and forecasts of SSH, upper ocean currents and SST, forecast verification statistics, subsurface temperature cross-sections, the amount of altimeter data used for each nowcast from each satellite and nowcast comparisons with unassimilated data. The results show that the model has predictive skill for mesoscale and other types of variability lasting at least 1 month in most regions and when calculated globally. D 2003 Elsevier Science B.V. All rights reserved.

Mesoscale variability in the boundary currents of the Alaska Gyre

Abstract Measurements of sea-surface height anomalies acquired during the GEOSAT, ERS-1, and TOPEX altimeter missions show that the boundary currents of the Alaska gyre exhibit interannual variability with respect to the occurrence, size, and propagation of mesoscale, eddy-like features. Observations and model results suggest that eddies are generated in the Alaska Current during years in which the wind forcing in the eastern Gulf of Alaska promotes strong downwelling along the British Columbia–Alaska coast. Wind forcing conditions that support eddy formation and intensification often occur in years that coincide with El Nino–Southern Oscillation events. Eddy variability is significantly more deterministic in the Alaska Current than in the Alaskan Stream.

The importance of high horizontal resolution and accurate coastline geometry in modeling South China Sea Inflow

As resolution is increased from 1/2° to 1/32° in Pacific Ocean simulations using the NRL Layered Ocean Model, marked changes are found in the Kuroshios mean pathway as it intrudes into the South China Sea (SCS) via the Luzon Strait. With increased horizontal resolution comes a more accurate representation of the coastline geometry associated with the Batan/Babuyan Islands within the strait, and a reduction in the modeled westward intrusion of the Kuroshio into the SCS. The 1/16° model is extremely sensitive to two very small scale shoals (Calayan Bank and a shoal north of Calayan Island) that are resolvable at this grid spacing. The exclusion of these three model gridpoints significantly alters the mean Kuroshio pathway to resemble the pathway from the 1/8° model. In addition, excluding all islands within the Luzon Strait in the 1/16° model gives a deep intrusion mean pathway as found in the 1/2° model.

Bifurcation of the Kuroshio Extension at the Shatsky Rise

A 1/16° six-layer Pacific Ocean model north of 20°S is used to investigate the bifurcation of the Kuroshio Extension at the main Shatsky Rise and the pathway of the northern branch from the bifurcation to the subarctic front. Upper ocean-topographic coupling via a mixed barotropic-baroclinic instability is essential to this bifurcation and to the formation and mean pathway of the northern branch as are several aspects of the Shatsky Rise complex of topography and the latitude of the Kuroshio Extension in relation to the topography. The flow instabilities transfer energy to the abyssal layer where it is constrained by geostrophic contours of the bottom topography. The topographically constrained abyssal currents in turn steer upper ocean currents, which do not directly impinge on the bottom topography. This includes steering of mean pathways. Obtaining sufficient coupling requires very fine resolution of mesoscale variability and sufficient eastward penetration of the Kuroshio as an unstable inertial jet. Resolution of 1/8° for each variable was not sufficient in this case. The latitudinal extent of the main Shatsky Rise (31°N–36°N) and the shape of the downward slope on the north side are crucial to the bifurcation at the main Shatsky Rise, with both branches passing north of the peak. The well-defined, relatively steep and straight eastern edge of the Shatsky Rise topographic complex (30°N–42°N) and the southwestward abyssal flow along it play a critical role in forming the rest of the Kuroshio northern branch which flows in the opposite direction. A deep pass between the main Shatsky Rise and the rest of the ridge to the northeast helps to link the northern fork of the bifurcation at the main rise to the rest of the northern branch. Two 1/16° “identical twin” interannual simulations forced by daily winds 1981–1995 show that the variability in this region is mostly nondeterministic on all timescales that could be examined (up to 7 years in these 15-year simulations). A comparison of climatologically forced and interannual simulations over the region 150°E–180°E, 29°N–47°N showed greatly enhanced abyssal and upper ocean eddy kinetic energy and much stronger mean abyssal currents east of the Emperor Seamount Chain (about 170°E) in the interannual simulations but little difference west of 170°E. This greatly enhanced the upper ocean-topographic coupling in the interannual simulations east of 170°E. This coupling affected the latitudinal positioning of the eastward branches of the Kuroshio Extension and tended to reduce latitudinal movement compared to the climatologically forced simulation, including a particularly noticeable impact from the Hess Rise. Especially in the interannual simulations, effects of almost all topographic features in the region could be seen in the mean upper ocean currents (more so than in instantaneous currents), including meanders and bifurcations of major and minor currents, closed circulations, and impacts from depressions and rises of large and small amplitudes.

Ocean current and wave effects on wind stress drag coefficient over the global ocean

[1] The effects of ocean surface currents and dominant waves on the wind stress drag coefficient (CD) are examined over the global ocean. Major findings are as follows: (1) the combination of both ocean wave and current speeds can result in reductions in daily CD (>10%), but the notable impact of the latter is only evident in the tropical Pacific Ocean; (2) the presence of waves generally makes winds weaker and CD lower almost everywhere over the global ocean; (3) strong ocean currents near the western boundaries (Kuroshio and Gulf Stream) do not substantially influence CD since the winds and currents are not always aligned; and (4) the change in speed used in bulk flux parameterization also causes large changes in fluxes. Globally, the combined outcome of ocean currents and waves is to reduce CD by about (2%), but spatial variations (0% to 14%) do exist. Citation: Kara, A. B., E. J. Metzger, and M. A. Bourassa (2007), Ocean current and wave effects on wind stress drag coefficient over the global ocean, Geophys. Res. Lett., 34, L01604, doi:10.1029/2006GL027849.

Skill testing a three‐dimensional global tide model to historical current meter records

[1] We apply several skill tests to assess tidal currents within a three-dimensional, eddy resolving, global ocean circulation model compared to over 5000 observational velocity records spanning 40 years. We examine the skill of the HYbrid Coordinate Ocean Model (HYCOM) on a regional, basin, and global scale and in deep versus shallow water. On a global scale, we examine the model tidal kinetic energy (KE) compared to the tidal KE estimated from the observational velocity records. We examine the vertical structure of the model tidal KE by averaging over predetermined depth bins. We also investigate the ability of the model to satisfy the 95% confidence intervals of the individual tidal ellipse parameters. On a basin scale, we determine if any bias exists in model performance with regards to a particular part of the global ocean and further investigate if any variability of model skill exists within the ocean basins by testing the model against smaller subsets of the observations. Our results show that the skill of the nondata assimilative HYCOM is comparable to the skill of the altimetric-constrained model TPXO7.2. HYCOM is shown to have up to 20% higher skill in resolving the Greenwich phase of the tides on a global basis and demonstrates moderate skill in replicating the vertical structure of the tidal currents as represented by the current meters. HYCOM demonstrates up to 20% higher skill than TPXO7.2 for some ocean basins and some ocean regions but exhibits up to 20% weaker skill in the Southern Ocean.