Generalized Digital Environmental Model Research Articles

World Ocean Isopycnal Level Absolute Geostrophic Velocity (WOIL-V) Inverted from GDEM with the P-Vector Method

Three-dimensional dataset of world ocean climatological annual and monthly mean absolute geostrophic velocity in isopycnal level (called WOIL-V) has been produced from the United States (U.S.) Navy’s Generalized Digital Environmental Model (GDEM) temperature and salinity fields (open access from the website http://data.nodc.noaa.gov/cgi-bin/iso?id=gov.noaa.nodc:9600094) using the P-vector method. The data have horizontal resolution of 0.5° × 0.5°, and 222 isopycnal-levels. The total 13 data files include annual and monthly mean values. The WOIL-V is the only dataset of absolute geostrophic velocity in isopycnal level compatible to the GDEM (T, S) fields, and provides background ocean currents for oceanographic and climatic studies, especially in ocean modeling with the isopycnal coordinate system.

Kinematic parameters of internal waves of the second mode in the South China Sea

Abstract. Spatial distributions of the main properties of the mode function and kinematic and non-linear parameters of internal waves of the second mode are derived for the South China Sea for typical summer conditions in July. The calculations are based on the Generalized Digital Environmental Model (GDEM) climatology of hydrological variables, from which the local stratification is evaluated. The focus is on the phase speed of long internal waves and the coefficients at the dispersive, quadratic and cubic terms of the weakly non-linear Gardner model. Spatial distributions of these parameters, except for the coefficient at the cubic term, are qualitatively similar for waves of both modes. The dispersive term of Gardner's equation and phase speed for internal waves of the second mode are about a quarter and half, respectively, of those for waves of the first mode. Similarly to the waves of the first mode, the coefficients at the quadratic and cubic terms of Gardner's equation are practically independent of water depth. In contrast to the waves of the first mode, for waves of the second mode the quadratic term is mostly negative. The results can serve as a basis for expressing estimates of the expected parameters of internal waves for the South China Sea.

Sensitivity of acoustic propagation modeling to variations in ocean environmental conditions

A parabolic equation acoustic model has been used to estimate transmission loss for a mid-frequency sound source to assist in marine mammal behavioral response studies. The sound propagation is modeled for a range-dependent environment using the Navy Digital Bathymetric Data Base and Bottom Sediment Type database, NOAA Global Ocean Sediment Thickness Dataset, and the High-Resolution Global Sea Surface Wind Speed monthly climatology. The sound speed along the propagation path is modeled using two ocean temperature/salinity fields of different spatial and temporal resolution: monthly 0.25 deg outputs of the Generalized Digital Environmental Model (GDEM), and the daily 1/12 deg outputs of the regional Hybrid Coordinate Ocean Model (HYCOM). The modeling was done for two operational areas with strikingly different ocean environments, the Southern California Bight and the Gulf Stream region off Cape Hatteras. The latter represents a much more dynamic ocean environment with profound temperature fronts, highly variable meso- and submesoscale eddy activity, and sharp bathymetry gradients along the shelf break. Received sound levels measured on tagged marine mammals, and synchronized sound source/sound receiver tracklines are used to estimate uncertainty of the acoustic modeling results, and the non-linear sensitivity of the model to wide-range variations in the ocean environmental conditions.

Propagation regimes and populations of internal waves in the Mediterranean Sea basin

The geographical and seasonal distributions of kinematic and nonlinear parameters of long internal waves are derived from the Generalized Digital Environmental Model (GDEM) climatology for the Mediterranean Sea region, including the Black Sea. The considered parameters are phase speed of long internal waves and the coefficients at the dispersion, quadratic and cubic terms of the weakly-nonlinear Korteweg–de Vries-type models (in particular, the Gardner model). These parameters govern the possible polarities and shapes of solitary internal waves, their limiting amplitudes and propagation speeds. The key outcome is an express estimate of the expected parameters of internal waves for different regions of the Mediterranean basin.

Development and Analysis of the Systematically Merged Atlantic Regional Temperature and Salinity Climatology for Oceanic Heat Content Estimates

Abstract An oceanic climatology to calculate upper-ocean thermal structure was developed for application year-round in the North Atlantic Ocean basin. The Systematically Merged Atlantic Regional Temperature and Salinity (SMARTS) Climatology is used in a two-layer model to project sea surface height anomalies (SSHA) into the water column at ¼° resolution. SMARTS blended monthly temperature and salinity fields from the World Ocean Atlas 2001 (WOA01) and Generalized Digital Environmental Model (GDEM) version 3.0 based on their performance compared to in situ measurements. Daily mean isotherm depths of 20°C (D20) and 26°C (D26) (and their mean ratio), reduced gravity, and mixed layer depth (MLD) were estimated from the climatology. This higher-resolution climatology resolves features in the Gulf of Mexico (GOM), including the Loop Current (LC) and eddy shedding region. Using SMARTS with satellite-derived SSHA and SST fields, daily values of isotherm depths, mixed layer depths, and ocean heat content (OHC) were calculated from 1998 to 2012. OHC is an important scalar when determining the ocean’s impact on tropical cyclone intensification, because it is a better predictor of SST cooling during hurricane passage. Airborne- and ship-deployed expendable bathythermographs (XBT), long-term moorings, and Argo profiling floats provided over 50 000 thermal profiles to assess satellite retrievals of isotherm depths and OHC using the SMARTS Climatology. The OHC calculation presented in this document reduces errors basinwide by 20%, with a 35% error reduction in the GOM.

The destiny of the North Pacific Intermediate Water in the South China Sea

The previous studies show that the spreading path of the subtropical salinity minimum of the North Pacific Intermediate Water (NPIW) is southwestward pointing to the Luzon Strait. Based on the P-vector method and generalized digital environmental model (GDEM) data, the volume transport of NPIW through Luzon Strait and the upward transport on the NPIW lower and upper boundaries are calculated to examine the destiny of NPIW in the South China Sea (SCS). On the annual mean, the estimation of NPIW transport into the SCS through the Luzon Strait is 1.72 Sv (1Sv=106 m3/s). The upward transport over the SCS is 0.31 Sv on the NPIW upper boundary and 1.31 Sv on the NPIW lower boundary. There is no strait or passage deeper than the surface for the NPIW to extend, except for the Luzon Strait. For the volume balance in the SCS NPIW, the volume transport of 2.72 Sv has to flow out of the SCS NPIW layer through the Luzon Strait.

Variability of Surface Water Properties in the Japan/East Sea on Different Time Scales

This study examined the multi-scale variabilities of sea surface temperature (SST) and salinity in the Japan/East Sea (JES) based on statistical analyses of observational data, with a focus on the northwestern part of the sea. The regionality of JES SST variability was estimated for different frequency ranges on semimonthly (11-17 days), monthly to seasonal (30-90 days), quasi-semiannual (157-220 days), and quasi-biennial (1.5-3 years) time scales using cluster analyses of daily gridded SST data for 1996 to 2007 from the Japan Meteorological Agency (JMA). Several significant peaks and regional cores were found in each frequency range of the SST anomaly (SSTA) oscillations. Quasi-semiannual SSTA oscillations with high amplitude were found in the south-southwestern part of the Japan Basin (<TEX>$41-43^{\circ}N$</TEX>) and were amplified in the area adjacent to Peter the Great Bay. Oscillations with periods of 79 and 55 days also prevailed over the southwest Japan Basin between the Yamato Rise and the continental slope. A similar method was applied to classify SST and the annual cycle of surface salinity using Generalized Digital Environmental Model (GDEM) gridded data. The Tatarskii Strait and adjacent area showed the most specific annual cycles and variability in salinity on interannual to interdecadal time scales. The most significant inverse relationship between surface salinity in the Tatarskii Strait and southern JES areas was found on the interdecadal time scale. Linkages of sea water salinity in the Tatarskii Strait with Amur River discharge and wind velocity over Amurskii Liman were also revealed.

Thermal and haline fronts in the Yellow/East China Seas: Surface and subsurface seasonality comparison

Seasonal variability of surface and subsurface thermal/haline fronts in the Yellow/East China Seas (YES) has been investigated using three-dimensional monthly-mean temperature and salinity data from U.S. Navy’s Generalized Digital Environmental Model (Version 3.0). The density-compensated Cheju-Yangtze Thermal/Haline Front has (northern and southern) double-tongues. The northern tongue is most evident throughout the depth from December to April. The southern tongue is persistent at the subsurface with conspicuous haline fronts. The thermal (haline) frontal intensity of the northern tongue is controlled mainly by the temperature (salinity) variation on the shoreward (seaward) side of the front. The cold water over the Yangtze Bank is influential in generating the southern tongue and intensifying the Tsushima Thermal Front. The year-round Cheju-Tsushima Thermal Front is evident throughout the depth and intensifies from July to December. The northern arc of the Yangtze Ring Haline Front is manifest in spring and is sustained until summer, whereas the southern one is fully developed in summer because of eastward migration of the Yangtze Diluted Water. The area showing strong frontal intensity in the Chinese Coastal Haline Front shifts seasonally north and south along the Zhejiang-Fujian coast. The Generation and evolution of YES fronts are closely associated with YES circulation (inferred from the linkage of the water masses). Moreover, the subsurface temperature/salinity evolution on the fronts in the Yellow Sea differs from that in the East China Sea owing to local factors such as wintertime vertical mixing and a summertime strong thermocline above the Yellow Sea Bottom Cold Water.

Comparison of RIAMOM and MOM in Modeling the East Sea/Japan Sea Circulation

The seasonal variations in the circulation of the water mass in the East Sea/Japan Sea have been simulated using a free surface primitive ocean model, RIAMOM (RIAM Ocean Model), comparing the results from GFDL-MOM1 (Geophysical Fluid Dynamics Laboratory Modular Ocean Model, version 1.1, hereafter MOM) with the GDEM (Generalized Digital Environmental Model) data. Both models appear to successfully reproduce the distinct features of circulation in the East Sea/Japan Sea, such as the NB (Nearshore Branch) flowing along the Japanese coast, the EKWC (East Korean Warm Current) flowing northward along the Korean coast, and the NKCC/LCC (North Korean Cold Current/Liman Cold Current) flowing southwestward along Korean/Russian coast. RIAMOM has shown better performance, compared to MOM, in terms of the realistic simulation of the flow field in the East Sea/Japan Sea; RIAMOM has produced more rectified flows on the coastal region, for example, the narrower and stronger NKCC/LCC than MOM has. There is however obvious differences between the model results and the GDEM data in terms of the calculation of the water mass; both models have shown a tendency to overpredict temperature and underpredict salinity below 50m; more diffusive forms of thermocline and halocline have been simulated than noted in GDEM data.

Determination of the current system on isopycnal surface between Mindanao and New Guinea from GDEM

In this study, we used the Navy’s Generalized Digital Environmental Model (GDEM) climatological temperature and salinity data on a 0.5°×0.5° grid to investigate the seasonal variabilities of the southwest Philippines Sea (0.5°–9°N, 123.5°–136.5°) thermohaline structure and circulation. The GDEM for the area was built up on historical (1930–1997) temperature and salinity profiles. A three-dimensional estimate of the absolute geostrophic velocity field on isopycnal surface was obtained from the GDEM temperature and salinity fields using the P-vector method. The seasonal variabilities of the thermohaline structure and currents (obtained from the inverse method) such as the Mindanao Current, Mindanao Undercurrent, North Equatorial Counter Current, New Guinea Coastal Undercurrent, and dual-eddies (cyclinic Mindanao Eddy and anticyclonic Halmahera Eddy) are identified.

Seasonal variability of thermohaline front in the central South China Sea

An upper layer thermohaline front across the South China Sea (SCS) basin from the South Vietnamese coast (around 15°N) to Luzon Island (around 19°N) has been identified using the Navy's open domain Generalized Digital Environmental Model (GDEM) monthly mean temperature and salinity data on a 0.5° × 0.5° grid. This front does not occur at the surface in summer. The strength of this front is around 1°C/100 km at the surface and 1.4°C/100 km at the subsurface (50 m deep). A cross-basin current, inverted using the P-vector method, is associating with the front. Meandering and eddies have been identified along this current. Seasonal and vertical variabilities of the thermohaline structure across this front are reported in this paper.

Northwest Pacific subtropical countercurrent on isopycnal surface in summer

A three‐dimensional absolute velocity field on the isopycnal surface in the northwest Pacific subtropical region in the summer is calculated from the Navy's Generalized Digital Environmental Model (GDEM) climatological temperature and salinity data on a 0.5° × 0.5° grid using the P‐vector method. GDEM for the area was built from historical (1930–1997) temperature and salinity profiles. The basic current system, consisting of North Equatorial Current, Kuroshio and its recirculation, Subtropical Countercurrent (STCC), is identified. STCC occurs from the surface to the level of σθ = 25.8 between 18°N and 23.5°N, with a maximum velocity around 0.1 m/s. At σθ = 23.5, STCC originates east of Luzon Strait at around 122.5°E and meanders eastward. As σθ increases, its origin shifts toward northeast. Comparison between velocity and potential vorticity fields leads to the conclusion that the STCC core is above the southern boundary of the subtropical mode water.

The Modular Ocean Data Assimilation System (MODAS)*

The Modular Ocean Data Assimilation System (MODAS) is used by the U.S. Navy for depiction of three- dimensional fields of temperature and salinity over the global ocean. MODAS includes both a static climatology and a dynamic climatology. While the static climatology represents the historical averages, the dynamic cli- matology assimilates near-real-time observations of sea surface height and sea surface temperature and provides improved temperature and salinity fields. The methodology for the construction of the MODAS climatology is described here. MODAS is compared with Levitus and Generalized Digital Environmental Model climatologies and with temperature and salinity profiles measured by SeaSoar in the Japan/East Sea to illustrate MODAS capabilities. MODAS with assimilated remotely sensed data is able to portray time-varying dynamical features that cannot be represented by static climatologies.

Japan Sea Thermohaline Structure and Circulation. Part II: A Variational P-Vector Method

Abstract The second part of this work investigates the seasonal variabilities of the Japan/East Sea (JES) circulation using the U.S. Navy Generalized Digital Environmental Model (GDEM) climatological temperature and salinity dataset (public domain) on a 0.5° × 0.5° grid. A variational P-vector method was developed to invert the velocity field. The GDEM for the JES was built up on historical (1930–97) 136 509 temperature and 52 572 salinity profiles. The climatological mean and seasonal variability of the current systems are well inverted, especially the Tsushima Warm Current and its bifurcation, the East Korean Warm Current (EKWC), the Japan nearshore branch, the confluence of the EKWC, and the North Korean Cold Current near the Korean coast and flows northeastward along the subpolar front, and a mesoscale anticyclonic eddy in the Ulleng/Tsushima Basin. Furthermore, this method has the capability to invert flow reasonably well across the shallow straits such as the Tsushima/Korea, Tsugaru, and Soya Strait...

Japan Sea Thermohaline Structure and Circulation. Part I: Climatology

In this study, the U.S. Navy’s Generalized Digital Environmental Model (GDEM) climatological temperature and salinity data on a 0.5° × 0.5° grid is used to investigate the seasonal variabilities of the Japan/East Sea (JES) thermohaline structure and circulations. The GDEM for the JES was built up on historical (1930–97) 136 509 temperature and 52 572 salinity profiles. A three-dimensional estimate of the absolute geostrophic velocity field was obtained from the GDEM temperature and salinity fields using the P-vector method. The seasonal variabilities of the thermohaline structure and the inverted currents such as the Subpolar Front, the salinity minimum and maximum in the Japan Sea Intermediate Water, and the Tsushima Warm Current and its bifurcation are identified.

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.

Empirical orthogonal function representations of stochastic sound-speed ensembles

The representation of stochastic ensembles of sound-speed profiles by convenient and efficient means is examined. The procedure is based on using discrete empirical orthogonal functions (EOFs) with random coefficients. The eventual objective is to use the EOF representations in random wave propagation models, for which their efficiency is expected to reduce significantly the computational expense. To illustrate the procedure, two specific model ensembles are considered. The first consists of deep-water (Munk) sound-speed profiles, and the second has shallow-water profiles obtained from temperature profiles generated by a portion of the generalized digital environmental model (GDEM). After a statistical distribution of stochastic environmental input parameters is assumed, EOFs are calculated for ensembles of up to 10 000 profile samples. Relations between the statistics of the input parameters and the sound speed profiles that are obtained from EOF representations are investigated. In each case examined, the number of EOFs needed to account for virtually all of the sample variance is at most four. Additional cases are considered using Continental Shelf data by assuming measured profiles represent ensemble samples. [Work supported by ONR.]

A Comparison Between the Generalized Digital Environmental Model and Levitus climatologies

Two ocean climatologies of temperature and salinity, the Generalized Digital Environmental Model (GDEM) and the Climatological Atlas of the World Ocean, are compared. Dynamic height fields are computed by season from each climatology for the North Atlantic, North Pacific, and Indian oceans and are compared on a 1° latitude‐longitude grid. Large‐scale oceanographic features are generally found to be similarly represented in both climatologies. GDEM appears to render better representations of seasonal variability and regions of high current shear, such as the Gulf Stream, because of a different smoothing method and a finer grid spacing. Maps of dynamic heights from both climatologies are presented, and their similarities and differences are discussed. The methodology for the construction of GDEM is also described in detail.