Jay F. Shriver

Accuracy assessment of global barotropic ocean tide models

The accuracy of state-of-the-art global barotropic tide models is assessed using bottom pressure data, coastal tide gauges, satellite altimetry, various geodetic data on Antarctic ice shelves, and independent tracked satellite orbit perturbations. Tide models under review include empirical, purely hydrodynamic (“forward”), and assimilative dynamical, i.e., constrained by observations. Ten dominant tidal constituents in the diurnal, semidiurnal, and quarter-diurnal bands are considered. Since the last major model comparison project in 1997, models have improved markedly, especially in shallow-water regions and also in the deep ocean. The root-sum-square differences between tide observations and the best models for eight major constituents are approximately 0.9, 5.0, and 6.5 cm for pelagic, shelf, and coastal conditions, respectively. Large intermodel discrepancies occur in high latitudes, but testing in those regions is impeded by the paucity of high-quality in situ tide records. Long-wavelength components of models tested by analyzing satellite laser ranging measurements suggest that several models are comparably accurate for use in precise orbit determination, but analyses of GRACE intersatellite ranging data show that all models are still imperfect on basin and subbasin scales, especially near Antarctica. For the M2 constituent, errors in purely hydrodynamic models are now almost comparable to the 1980-era Schwiderski empirical solution, indicating marked advancement in dynamical modeling. Assessing model accuracy using tidal currents remains problematic owing to uncertainties in in situ current meter estimates and the inability to isolate the barotropic mode. Velocity tests against both acoustic tomography and current meters do confirm that assimilative models perform better than purely hydrodynamic models.

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 ...

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 contribution of the global thermohaline circulation to the Pacific to Indian Ocean Throughflow via Indonesia

World ocean simulations are used to investigate the pathways feeding the Indonesian throughflow as a function of depth, including the role of the global thermohaline (“conveyor belt”) circulation. The simulations use a horizontal resolution of 1/2° for each variable and the vertical resolution ranges from 1.5-layer reduced gravity to six layers with realistic bottom topography. They are forced by the Hellerman and Rosenstein [1983] monthly wind stress climatology. Contrary to the classical theory of Stommel and Arons [1960], the Naval Research Laboratory model shows the Antarctic Circumpolar Current (ACC) region as the main region of abyssal to upper ocean water upwelling which compensates for the deep water formation in the far North Atlantic, a result corroborated by recent observational evidence [Toggweiler and Samuels, 1993]. We examine the contribution of the global conveyor belt circulation to the throughflow by systematically varying the model dynamics (e.g., by disabling the far North Atlantic ports which parameterize deep water formation in that region). The model simulations show a global conveyor belt circulation contribution of 5.7 Sv to the throughflow, a contribution provided mainly by wind-driven upwelling in the Indo-Pacific ACC region. This is due to a cooperative interaction between the thermohaline and wind-driven circulations. The thermohaline circulation makes the throughflow more surface trapped and less subject to topographic blocking in the Indonesian passageways, while the wind-driven circulation provides the Indonesian throughflow pathway for the thermohaline flow upwelled in the ACC region. Mean layer transport fields, cross-layer mass transfer fields, and Lagrangian tracers are used to identify pathways feeding the Pacific to Indian Ocean throughflow via Indonesia. Starting from the ACC, Sverdrup flow shows a circuitous route that is northward in the eastern South Pacific, then westward in the South Equatorial Current (SEC). The SEC retroflects into the North Equatorial Countercurrent (NECC) followed by cyclonic flow around the Northern Tropical Gyre and into the North Equatorial Current (NEC), then into the Mindanao Current, the Sulawesi Sea, the Makassar Strait, and the Indian Ocean. The depth-integrated pathways from nonlinear simulations show the retroflection from the SEC into the NECC as a secondary route and retroflection into the Equatorial Undercurrent (EUC) as the primary route. The EUC connects with the NECC by westward and then northward flow on the northside of the EUC. The pathways as a function of depth can be presented in three layers: a surface layer, the layer containing the EUC, and layers below the EUC. In the top layer the EUC to NECC connection is via upwelling from the EUC in the central and east-central equatorial Pacific. Some of this upwelled water is returned to the EUC layer via downwelling at midlatitudes where it feeds into the NEC or SEC. Very little water in the South Pacific EUC layer passes into the Indian Ocean without upwelling into the surface layer first. While the pathways in the top two layers are complex and strongly coupled and enter the Indonesian Archipelago from the northern hemisphere, below the EUC layer a very direct Pacific to Indian Ocean route is found: SEC → Sulawesi Sea → Makassar Strait.

The effects of Halmahera on the Indonesian throughflow

The pathways of the Pacific to Indian Ocean throughflow and the relative contributions of North Pacific (NP) and South Pacific (SP) water to the throughflow are examined using the Navy Layered Ocean Model. The roles of Halmahera Island in directing flow along the pathways and determining the composition of the throughflow are also studied. The global ocean simulations use a horizontal resolution of up to 1/4° between like variables and have a vertical resolution ranging from one and a half layer reduced gravity to six active layers with realistic bottom topography. All of the simulations are forced by the Hellerman and Rosenstein [1983] monthly wind stress climatology. The predominant throughflow pathway consists of NP water traveling through the Celebes Sea, Makassar Strait, Flores Sea, and to the Indian Ocean through the Timor, Savu, and Lombok Straits. Model results show that the island of Halmahera is responsible for preventing a flow of SP water into the Celebes Sea and for diverting some SP water southward through the Seram and Banda Seas. The island impacts the lower thermocline and intermediate water pathways throughout the entire year and affects the surface layer during the boreal spring through fall. To estimate the relative contributions of the NP and SP surface water to the throughflow, Lagrangian drifters are advected backward in time from near the exit to the throughflow region to their respective sources. By tracking these buoys, it is found that the presence of Halmahera changes the throughflow composition in the surface layer from ∼69% NP and 31% SP to 92% NP and 8% SP. Halmahera does not change the composition of the throughflow in the undercurrent layer, which is fed by the NP, or in the lower thermocline and intermediate water layers, which are fed by water from the SP.

How stationary are the internal tides in a high‐resolution global ocean circulation model?

The stationarity of the internal tides generated in a global eddy-resolving ocean circulation model forced by realistic atmospheric fluxes and the luni-solar gravitational potential is explored. The root mean square (RMS) variability in the M2 internal tidal amplitude is approximately 2 mm or less over most of the ocean and exceeds 2 mm in regions with larger internal tidal amplitude. The M2 RMS variability approaches the mean amplitude in weaker tidal areas such as the tropical Pacific and eastern Indian Ocean, but is smaller than the mean amplitude near generation regions. Approximately 60% of the variance in the complex M2 tidal amplitude is due to amplitude-weighted phase variations. Using the RMS tidal amplitude variations normalized by the mean tidal amplitude (normalized RMS variability (NRMS)) as a metric for stationarity, low-mode M2 internal tides with NRMS < 0.5 are stationary over 25% of the deep ocean, particularly near the generation regions. The M2 RMS variability tends to increase with increasing mean amplitude. However, the M2 NRMS variability tends to decrease with increasing mean amplitude, and regions with strong low-mode internal tides are more stationary. The internal tide beams radiating away from generation regions become less stationary with distance. Similar results are obtained for other tidal constituents with the overall stationarity of the constituent decreasing as the energy in the constituent decreases. Seasonal variations dominate the RMS variability in the Arabian Sea and near-equatorial oceans. Regions of high eddy kinetic energy are regions of higher internal tide nonstationarity.

Assessment of Data Assimilative Ocean Models in the Gulf of Mexico Using Ocean Color

Abstract : This paper illustrates the value of SeaWiFS ocean color imagery in assessing the ability of three data-assimilative ocean models (configured in five prediction systems) to map mesoscale variability in the Gulf of Mexico (i.e., the Loop Current and associated warm and cold eddies) and in helping to diagnose specific strengths and weaknesses of the systems. In addition, the study clearly illustrates that biological responses of the surface waters are strongly linked to the physical events and processes.

Low-Frequency Variability of the Equatorial Pacific Ocean Using a New Pseudostress Dataset: 1930–1989

Abstract Interannual and interdecadal variability of the equatorial Pacific are examined using a new pseudostress dataset. The monthly mean pseudostress fields (1930–89)are derived from Comprehensive Ocean-Atmosphere Data Set (COADS) pseudostresses using climate basis functions obtained from the Florida State University pseudostress product (1966–90). To validate the new wind fields, a two-tier validation scheme was used. The new wind fields were first examined to see if they exhibited characteristics that have been shown to be important in terms of exciting El Nino events. Next, the new wind fields are used to force an ocean model, thereby obtaining model estimates of tropical Pacific currents and model upper-layer thickness (ULT). Observed sea level and spatially averaged SST anomalies are used to validate the hindcasts. The new wind fields were found to have a significant El Nino mode (accounting for 41% of the variance), which possessed features consistent with those that theory and numerical simulati...

Impact of Parameterized Internal Wave Drag on the Semidiurnal Energy Balance in a Global Ocean Circulation Model

AbstractThe effects of a parameterized linear internal wave drag on the semidiurnal barotropic and baroclinic energetics of a realistically forced, three-dimensional global ocean model are analyzed. Although the main purpose of the parameterization is to improve the surface tides, it also influences the internal tides. The relatively coarse resolution of the model of ~8 km only permits the generation and propagation of the first three vertical modes. Hence, this wave drag parameterization represents the energy conversion to and the subsequent breaking of the unresolved high modes. The total tidal energy input and the spatial distribution of the barotropic energy loss agree with the Ocean Topography Experiment (TOPEX)/Poseidon (TPXO) tidal inversion model. The wave drag overestimates the high-mode conversion at ocean ridges as measured against regional high-resolution models. The wave drag also damps the low-mode internal tides as they propagate away from their generation sites. Hence, it can be considered...