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Greenland Sea Project Group.  1990.  Greenland Sea Project: a venture toward improved understanding of the oceans' role in climate. EOS Trans. AGU. 71:750-751and754-755. AbstractWebsite

The Greenland Sea is one of the few major areas where convective renewal of intermediate and deep waters contribute to world-ocean ventilation. Basin-scale cyclonic circulation, boundary currents advecting of Atlantic and Polar origin, mixing across the fronts related to the boundary currents, wintertime heat loss to the atmosphere, ice formulation and related brine release and sequences of penetrative plumes control the renewal. The field work of the Greenland Ice Project began in the summer of 1987. This paper presents and explains the objectives of the Greenland Sea Project and summarizes preliminary results from the first intense field phase which covered the seasonal cycle from summer 1988 to summer 1989. The paper discusses the following: large-scale circulation; convection; circulation and convection variability; exchange across fronts; sea ice; sea ice interactions and copepod life cycles in the European Arctic seas.

Gopalakrishnan, G, Cornuelle BD, Gawarkiewicz G, McClean JL.  2013.  Structure and evolution of the cold dome off northeastern Taiwan: A numerical study. Oceanography. 26:66-79. Abstract

Numerous observational and modeling studies of ocean circulation surrounding Taiwan have reported occurrences of cold water and doming of isotherms (called the cold dome) that result in the formation of coastal upwelling on the northeastern Taiwan shelf. We use a high-resolution (1/24°) ocean model based on the Massachusetts Institute of Technology general circulation model to study the evolution of this distinct shelf-slope circulation phenomenon. We performed a number of model simulations spanning a five-year period (2004–2008) using realistic atmospheric forcing and initial and open boundary conditions. The model solutions were compared with satellite measurements of sea surface height (SSH), sea surface temperature (SST), and historical temperature and salinity observations. The model showed a realistically shaped cold dome with a diameter of ~ 100 km and temperature of ~ 3°C below the ambient shelf waters at 50 m depth. The occurrences of simulated cold dome events appeared to be connected with the seasonal variability of the Kuroshio Current. The model simulations showed more upwelling events during spring and summer when the core of the Kuroshio tends to migrate away from the east coast of Taiwan, compared to fall and winter when the core of the Kuroshio is generally found closer to the east coast of Taiwan. The model also reproduced weak cyclonic circulation associated with the upwelling off northeastern Taiwan. We analyzed the spatio-temporal variability of the cold dome using the model solution as a proxy and designed a "cold dome index" based on the temperature at 50 m depth averaged over a 0.5° × 0.5° box centered at 25.5°N, 122°E. The cold dome index correlates with temperature at 50 m depth in a larger region, suggesting the spatial extent of the cold dome phenomenon. The index had correlation maxima of 0.78 and 0.40 for simulated SSH and SST, respectively, in and around the cold dome box region, and we hypothesize that it is a useful indicator of upwelling off northeastern Taiwan. In addition, both correlation and composite analysis between the temperature at 50 m depth and the East Taiwan Channel transport showed no cold dome events during low-transport events (often in winter) and more frequent cold dome events during high-transport events (often in summer). The simulated cold dome events had time scales of about two weeks, and their centers aligned roughly along a northeastward line starting from the northeastern tip of Taiwan.

Gopalakrishnan, G, Cornuelle BD, Hoteit I.  2013.  Adjoint sensitivity studies of loop current and eddy shedding in the Gulf of Mexico. Journal of Geophysical Research: Oceans. 118:3315-3335.   10.1002/jgrc.20240   AbstractWebsite

Adjoint model sensitivity analyses were applied for the loop current (LC) and its eddy shedding in the Gulf of Mexico (GoM) using the MIT general circulation model (MITgcm). The circulation in the GoM is mainly driven by the energetic LC and subsequent LC eddy separation. In order to understand which ocean regions and features control the evolution of the LC, including anticyclonic warm-core eddy shedding in the GoM, forward and adjoint sensitivities with respect to previous model state and atmospheric forcing were computed using the MITgcm and its adjoint. Since the validity of the adjoint model sensitivities depends on the capability of the forward model to simulate the real LC system and the eddy shedding processes, a 5 year (2004–2008) forward model simulation was performed for the GoM using realistic atmospheric forcing, initial, and boundary conditions. This forward model simulation was compared to satellite measurements of sea-surface height (SSH) and sea-surface temperature (SST), and observed transport variability. Despite realistic mean state, standard deviations, and LC eddy shedding period, the simulated LC extension shows less variability and more regularity than the observations. However, the model is suitable for studying the LC system and can be utilized for examining the ocean influences leading to a simple, and hopefully generic LC eddy separation in the GoM. The adjoint sensitivities of the LC show influences from the Yucatan Channel (YC) flow and Loop Current Frontal Eddy (LCFE) on both LC extension and eddy separation, as suggested by earlier work. Some of the processes that control LC extension after eddy separation differ from those controlling eddy shedding, but include YC through-flow. The sensitivity remains stable for more than 30 days and moves generally upstream, entering the Caribbean Sea. The sensitivities of the LC for SST generally remain closer to the surface and move at speeds consistent with advection by the high-speed core of the current, while sensitivities to SSH generally extend to deeper layers and propagate more slowly. The adjoint sensitivity to relative vorticity deduced from the sensitivities to velocity fields suggests that advection of cyclonic (positive) relative vorticity anomalies from the YC or the LCFEs accelerate the LC eddy separation. Forward model perturbation experiments were performed to complement and check the adjoint sensitivity analysis as well as sampling the predictability and nonlinearity of the LC evolution. The model and its adjoint can be used in four-dimensional variational assimilation (4D-VAR) to produce dynamically consistent ocean state estimates for analysis and forecasts of the circulation of the GoM.

Gopalakrishnan, G, Cornuelle BD, Hoteit I, Rudnick DL, Owens BW.  2013.  State estimates and forecasts of the loop current in the Gulf of Mexico using the MITgcm and its adjoint. Journal of Geophysical Research: Oceans. 118:3292-3314.   10.1002/jgrc.20239   AbstractWebsite

An ocean state estimate has been developed for the Gulf of Mexico (GoM) using the MIT general circulation model and its adjoint. The estimate has been tested by forecasting loop current (LC) evolution and eddy shedding in the GoM. The adjoint (or four-dimensional variational) method was used to match the model evolution to observations by adjusting model temperature and salinity initial conditions, open boundary conditions, and atmospheric forcing fields. The model was fit to satellite-derived along-track sea surface height, separated into temporal mean and anomalies, and gridded sea surface temperature for 2 month periods. The optimized state at the end of the assimilation period was used to initialize the forecast for 2 months. Forecasts explore practical LC predictability and provide a cross-validation test of the state estimate by comparing it to independent future observations. The model forecast was tested for several LC eddy separation events, including Eddy Franklin in May 2010 during the deepwater horizon oil spill disaster in the GoM. The forecast used monthly climatological open boundary conditions, atmospheric forcing, and run-off fluxes. The model performance was evaluated by computing model-observation root-mean-square difference (rmsd) during both the hindcast and forecast periods. The rmsd metrics for the forecast generally outperformed persistence (keeping the initial state fixed) and reference (forecast initialized using assimilated Hybrid Coordinate Ocean Model 1/12° global analysis) model simulations during LC eddy separation events for a period of 1̃2 months.

Gopalakrishnan, G, Hoteit I, Cornuelle BD, Rudnick DL.  2019.  Comparison of 4DVAR and EnKF state estimates and forecasts in the Gulf of Mexico. Quarterly Journal of the Royal Meteorological Society. 145:1354-1376.   10.1002/qj.3493   AbstractWebsite

An experiment is conducted to compare four-dimensional variational (4DVAR) and ensemble Kalman filter (EnKF) assimilation systems and their predictability in the Gulf of Mexico (GoM) using the Massachusetts Institute of Technology general circulation model (MITgcm). The quality of the ocean-state estimates, forecasts, and the contribution of ensemble prediction are evaluated. The MITgcm-Estimating the Circulation and Climate of the Ocean (ECCO) 4DVAR (MITgcm-ECCO) and the MITgcm-Data Assimilation Research Testbed (DART) EnKF (MITgcm-DART) systems were used to compute two-month hindcasts (March-April, 2010) by assimilating satellite-derived along-track sea-surface height (SSH) and gridded sea-surface temperature (SST) observations. The estimates from both methods at the end of the hindcast period were then used to initialize forecasts for two months (May-June, 2010). This period was selected because a loop current (LC) eddy (Eddy Franklin: Eddy-F) detachment event occurred at the end of May 2010, immediately after the Deepwater Horizon (DwH) oil spill. Despite some differences between the setups, both systems produce analyses and forecasts of comparable quality and both solutions significantly outperformed model persistence. A reference forecast initialized from the 1/12 degrees Hybrid Coordinate Ocean Model (HYCOM)/NCODA global analysis also performed well. The EnKF experiments for sensitivity to filter parameters showed enhanced predictability when using more ensemble members and stronger covariance localization, but not for larger inflation. The EnKF experiments varying the number of assimilation cycles showed enhanced short-term (long-term) predictability with fewer (more) assimilation cycles. Additional hindcast and forecast experiments at other times of significant LC evolution showed mixed performance of both systems, which depends strongly on the background state of the GoM circulation. The present work demonstrates a practical application of both assimilation methods for the GoM and compares them in a limited number of realizations. The overall conclusion showing improved short-term (long-term) predictability for EnKF (4DVAR) carries an important caveat that the results from this study are specific to a few 4DVAR and EnKF LC eddy separation experiments in the GoM and cannot be generalized to conclude the relative performance of both methods, especially in other applications. However, some of the concepts and methods should carry over to other applications.

Gilson, J, Roemmich D, Cornuelle B, Fu LL.  1998.  Relationship of TOPEX/Poseidon altimetric height to steric height and circulation in the North Pacific. Journal of Geophysical Research-Oceans. 103:27947-27965.   10.1029/98jc01680   AbstractWebsite

TOPEX/Poseidon altimetric height is compared with 20 transpacific eddy-resolving realizations of steric height. The latter are calculated from temperature (expendable bathythermograph (XBT)) and salinity (expendable conductivity and temperature profiler (XCTD)) profiles along a precisely repeating ship track over a period of 5 years. The overall difference between steric height and altimetric height is 5.2 cm RMS. On long wavelengths (lambda < 500 km), the 3.5 cm RMS difference is due mainly to altimetric measurement errors but also has a component from steric variability deeper than the 800 m limit of the XBT. The data sets are very coherent in the long wavelength band, with coherence amplitude of 0.89. This band contains 65% of the total variance in steric height. On short wavelengths (lambda > 500 km), containing 17% of the steric height variance, the 3.0 cm RMS difference and lowered coherence are due to the sparse distribution of altimeter ground tracks along the XBT section. The 2.4 cm RMS difference in the basin-wide spatial mean appears to be due to fluctuations in bottom pressure. Differences between steric height and altimetric height increase near the western boundary, but data variance increases even more, and so the signal-to-noise ratio is highest in the western quarter of the transect. Basin-wide integrals of surface geostrophic transport from steric height and altimetric height are in reasonable agreement. The 1.9 x 10(4) m(2) s(-1) RMS difference is mainly because the interpolated altimetric height lacks spatial resolution across the narrow western boundary current. A linear regression is used to demonstrate the estimation of subsurface temperature from altimetric data. Errors diminish from 0.8 degrees C at 200 m to 0.3 degrees C at 400 m. Geostrophic volume transport, 0-800 m, shows agreement that is similar to surface transport, with 4.8 Sverdrup (Sv) (10(6) m(3) s(-1)) RMS difference. The combination of altimetric height with subsurface temperature and salinity profiling is a powerful tool for observing variability in circulation and transport of the upper ocean. The continuing need for appropriate subsurface data for verification and for statistical estimation is emphasized. This includes salinity measurements, which significantly reduce errors in specific volume and steric height.

Giglio, D, Roemmich D, Cornuelle B.  2013.  Understanding the annual cycle in global steric height. Geophysical Research Letters. 40:4349-4354.   10.1002/grl.50774   AbstractWebsite

Steric variability in the ocean includes diabatic changes in the surface layer due to air-sea buoyancy fluxes and adiabatic changes due to advection, which are dominant in the subsurface ocean. Here the annual signal in subsurface steric height (eta' below 200 db) is computed on a global scale using temperature and salinity profiles from Argo floats. The zonal average of over a season (e.g., eta'(March) - eta'(December)) is compared to the wind-forced vertical advection contribution (Delta eta'(w)) both in the global ocean and in different basins. The results show agreement that extends beyond the tropics. The estimate of Delta eta'(w) is based on the Ekman pumping and assumes that the seasonal vertical velocity is constant over the depth range of interest. This assumption is consistent with annual isopycnal displacements inferred from Argo profiles. The contribution of horizontal advection to Delta eta' is significant in some regions and consistent with differences between Delta eta' and Delta eta'(w).

Gemba, KL, Sarkar J, Cornuelle B, Hodgkiss WS, Kuperman WA.  2018.  Estimating relative channel impulse responses from ships of opportunity in a shallow water environment. The Journal of the Acoustical Society of America. 144:1231-1244.   10.1121/1.5052259   Abstract

The uncertainty of estimating relative channel impulse responses (CIRs) obtained using the radiated signature from a ship of opportunity is investigated. The ship observations were taken during a 1.4 km (11 min) transect in a shallow water environment during the Noise Correlation 2009 (NC09) experiment. Beamforming on the angle associated with the direct ray-path yields an estimate of the ship signature, subsequently used in a matched filter. Relative CIRs are estimated every 2.5 s independently at three vertical line arrays (VLAs). The relative arrival-time uncertainty is inversely proportional to source bandwidth and CIR signal-to-noise ratio, and reached a minimum standard deviation of 5 μs (equivalent to approximately 1 cm spatial displacement). Time-series of direct-path relative arrival-times are constructed for each VLA element across the 11 min observation interval. The overall structure of these time-series compares favorably with that predicted from an array element localization model. The short-term standard deviations calculated on the direct-path (7 μs) and bottom-reflected-path (17 μs) time-series are in agreement with the predicted arrival-time accuracies. The implications of these observed arrival-time accuracies in the context of estimating sound speed perturbations and bottom-depth are discussed.

Gawarkiewicz, G, Jan S, Lermusiaux PFJ, McClean JL, Centurioni L, Taylor K, Cornuelle B, Duda TF, Wang J, Yang YJ, Sanford T, Lien RC, Lee C, Lee MA, Leslie W, Haley PJ, Niiler PP, Gopalakrishnan G, Velez-Belchi P, Lee DK, Kim YY.  2011.  Circulation and Intrusions Northeast of Taiwan: Chasing and Predicting Uncertainty in the Cold Dome. Oceanography. 24:110-121. AbstractWebsite

An important element of present oceanographic research is the assessment and quantification of uncertainty. These studies are challenging in the coastal ocean due to the wide variety of physical processes occurring on a broad range of spatial and temporal scales. In order to assess new methods for quantifying and predicting uncertainty, a joint Taiwan-US field program was undertaken in August/September 2009 to compare model forecasts of uncertainties in ocean circulation and acoustic propagation, with high-resolution in situ observations. The geographical setting was the continental shelf and slope northeast of Taiwan, where a feature called the "cold dome" frequently forms. Even though it is hypothesized that Kuroshio subsurface intrusions are the water sources for the cold dome, the dome's dynamics are highly uncertain, involving multiple scales and many interacting ocean features. During the experiment, a combination of near-surface and profiling drifters, broad-scale and high-resolution hydrography, mooring arrays, remote sensing, and regional ocean model forecasts of fields and uncertainties were used to assess mean fields and uncertainties in the region. River runoff from Typhoon Morakot, which hit Taiwan August 7-8, 2009, strongly affected shelf stratification. In addition to the river runoff, a cold cyclonic eddy advected into the region north of the Kuroshio, resulting in a cold dome formation event. Uncertainty forecasts were successfully employed to guide the hydrographic sampling plans. Measurements and forecasts also shed light on the evolution of cold dome waters, including the frequency of eddy shedding to the north-northeast, and interactions with the Kuroshio and tides. For the first time in such a complex region, comparisons between uncertainty forecasts and the model skill at measurement locations validated uncertainty forecasts. To complement the real-time model simulations, historical simulations with another model show that large Kuroshio intrusions were associated with low sea surface height anomalies east of Taiwan, suggesting that there may be some degree of predictability for Kuroshio intrusions.

Gasparin, F, Roemmich D, Gilson J, Cornuelle B.  2015.  Assessment of the upper-ocean observing system in the equatorial Pacific: The role of Argo in resolving intraseasonal to interannual variability*. Journal of Atmospheric and Oceanic Technology. 32:1668-1688.   10.1175/jtech-d-14-00218.1   AbstractWebsite

Using more than 10 years of Argo temperature and salinity profiles (2004-14), a new optimal interpolation (OI) of the upper ocean in the equatorial Pacific is presented. Following Roemmich and Gilson's procedures, which were formulated for describing monthly large-scale anomalies, here every 5 days anomaly fields are constructed with improvements in the OI spatial covariance function and by including the time domain. The comparison of Argo maps with independent observations, from the TAO/TRITON array, and with satellite sea surface height (SSH), demonstrates that Argo is able to represent around 70%-80% of the variance at intraseasonal time scales (periods of 20-100 days) and more than 90% of the variance for the seasonal-to-longer-term variability. The RMS difference between Argo and TAO/TRITON temperatures is lower than 1 degrees C and is around 1.5 cm when the Argo steric height is compared to SSH. This study also assesses the efficacy of different observing system components and combinations, such as SSH, TAO/TRITON, and Argo, for estimating subsurface temperature. Salinity investigations demonstrate its critical importance for density near the surface in the western Pacific. Objective error estimates from the OI are used to evaluate different sampling strategies, such as the recent deployment of 41 Argo floats along the Pacific equator. Argo's high spatial resolution compared with that of the moored array makes it better suited for studying spatial patterns of variability and propagation on intraseasonal and longer periods, but it is less well suited for studying variability on periods shorter than 20 days at point locations. This work is a step toward better utilization of existing datasets, including Argo, and toward redesigning the Tropical Pacific Observing System.

Gaillard, F, Cornuelle B.  1987.  Improvement of Tomographic Maps by Using Surface-Reflected Rays. Journal of Physical Oceanography. 17:1458-1467.   10.1175/1520-0485(1987)017<1458:iotmbu>2.0.co;2   AbstractWebsite

The results of the experiment conducted in the northwest Atlantic in 1981 have demonstrated the possibilities of acoustic tomography. The first maps, based only on purely refracted rays, showed the evolution of a cold eddy, confirmed by direct measurement of temperature and salinity. A more complete use of the 1981 dataset, with incorporation of surface-reflected rays, is proposed here. The addition of new data reduces the statistical error on the estimation of the sound speed field. Resolution at levels already well estimated in the earlier computations is improved, and individual maps exhibit a better continuity. Information is now available about the average properties of the upper layers of the ocean, which could not be monitored with purely refracted rays.