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The US Navy's operational global ocean nowcast/forecast system is presently comprised of the 0.08° HYbrid Coordinate Ocean Model (HYCOM) and the Navy Coupled Ocean Data Assimilation (NCODA). Its high horizontal resolution and adaptive vertical coordinate system make it capable of producing nowcasts (current state) and forecasts of oceanic \"weather,\" which includes three-dimensional ocean temperature, salinity, and current structure; surface mixed layer depth; and the location of mesoscale features such as eddies, meandering currents, and fronts. It runs daily at the Naval Oceanographic Office and provides seven-day forecasts that support fleet operations, provide boundary conditions to higher resolution regional models, and are available to the community. Using a data-assimilative hindcast and series of 14-day forecasts for 2012, the system is shown to have forecast skill of the oceanic mesoscale out to about 10 days for the Gulf Stream region and to 14+ days for the global ocean and other selected subregions. Forecast skill is sensitive to the type of atmospheric forcing (i.e., operational vs. analysis quality). Subsurface temperature bias is small (< 0.25°C) and root mean square error peaks at the depth range of the mixed layer and thermocline. Coupled to the Community Ice CodE (CICE) on the same grid, the HYCOM/CICE/NCODA system (initially restricted to the Arctic) provides sea ice nowcasts and forecasts. Ice edge location errors are improved from the previous sea ice prediction system but are limited in part by the accuracy of the satellite observations it assimilates.

Internal tides (ITs) play a critical role in ocean mixing, and have strong signatures in ocean observations. Here, global IT sea surface height (SSH) in nadir altimetry is compared with an ocean forecast model that assimilates de‐tided SSH from nadir altimetry. The forecast model removes IT SSH variance from nadir altimetry at skill levels comparable to those achieved with empirical analysis of nadir altimetry. Accurate removal of IT SSH is needed to fully reveal lower‐frequency mesoscale eddies and currents in altimeter data. Analysis windows of order 30–120 days, made possible by the frequent (hourly) outputs of the forecast model, remove more IT SSH variance than longer windows. Forecast models offer a promising new approach for global internal tide mapping and altimetry correction. Because they provide information on the full water column, forecast models can also help to improve understanding of the underlying dynamics of ITs. Plain Language Summary Tidal flow over topographic features on the seafloor generates vertical displacements along the interfaces of ocean layers that have different densities. These vertical displacements at tidal frequencies are known as internal tides. Internal tide displacements are largest well below the sea surface, but also display a sea surface height (SSH) signature that is large enough to be measured by satellite altimeters. Removing internal tide signals from satellite altimeter SSH allows for a more accurate accounting of non‐tidal features, including slowly evolving ocean currents and eddies, that are also measured by altimeters. Here, we show that supercomputer ocean forecast simulations of the global internal tide field are able to remove internal tide SSH from satellite altimeter measurements with a skill level that is comparable to the skill of internal tide SSH removal based upon analysis of the satellite altimeter data itself. Thus, forecast models offer a complementary method for this important task. In addition, forecast models provide information on the entire ocean water column, not just the sea surface. Finally, the hourly outputs of forecast models allow for a greater variety of tidal analysis record lengths than can be achieved with altimeter outputs, which report sea surface height fields much less frequently. Key Points Global ocean forecast models can accurately simulate both long‐term (phase‐locked) internal tides and their short‐term modulations Ocean forecast models offer a useful complement to empirical models for mapping internal tides and correcting altimetry for internal tides In regions of strong internal tides, optimal variance reduction in nadir altimetry is attained through short‐term tidal analyses (∼60 days)

High‐resolution numerical simulations of the northern Gulf of Mexico region using the Hybrid Coordinate Ocean Model (HYCOM) were employed to investigate the dynamical processes controlling the fate of the Mississippi River plume, in particular the conditions that favor cross‐marginal transport. The study focuses on the effects of topography, wind‐driven and eddy‐driven circulation on the offshore removal of plume waters. A realistically forced simulation (nested in a data‐assimilative regional Gulf of Mexico HYCOM model) reveals that the offshore removal is a frequent plume pathway. Eastward wind‐driven currents promote large freshwater transport toward the shelf break and the DeSoto Canyon, where eddies with diameters ranging from 50 to 130 km interact with the buoyant plume and effectively entrain the riverine waters. Our estimates show that the offshore removal by eddies can be as large as the wind‐driven shelf transport. The proximity of eddies to the shelf break is a sufficient condition for offshore removal, and shelf‐to‐offshore interaction is facilitated by the steep bottom topography near the delta. Strong eddy‐plume interactions were observed when the Loop Current System impinged against the shelf break, causing the formation of coherent, narrow low‐salinity bands that extended toward the gulf interior. The offshore pathways depend on the position of the eddies near the shelf edge, their life span and the formation of eddy pairs that generate coherent cross‐shelf flows. This study elucidates the dynamics that initiate a unique cross‐marginal removal mechanism of riverine low‐salinity, nutrient‐rich waters, allowing their export along connectivity pathways, induced by a large‐scale current system. Key Points The offshore removal is a frequent pathway of the Mississippi River plume Strong eddy‐plume interactions happen when the LC system approaches the shelf Offshore pathway is dependent on the eddy dynamics

Lagrangian Coherent Structures (LCS) are the hidden fluid flow skeletons that provide meaningful information about the Lagrangian circulation. In this study, we computed the monthly climatological LCSs (cLCS) maps utilizing 24 years (1994–2017) of HYbrid Coordinate Ocean Model (HYCOM) currents and ECMWF re-analysis winds in the Bay of Bengal (BoB). The seasonal reversal of winds and associated reversal of currents makes the BoB dynamic. Therefore, we primarily aim to reveal the cLCSs associated with seasonal monsoon currents and mesoscale (eddies) processes over BoB. The simulated cLCS were augmented with the complex empirical orthogonal functions to confirm the dominant lagrangian transport pattern features better. The constructed cLCS patterns show a seasonal accumulation zone and the transport pattern of freshwater plumes along the coastal region of the BoB. We further validated with the satellite imagery of real-time oil spill dispersion and modelled oil spill trajectories that match well with the LCS patterns. In addition, the application of cLCSs to study the transport of hypothetical oil spills occurring at one of the active oil exploration sites (Krishna-Godavari basin) was described. Thus, demonstrated the accumulation zones in the BoB and confirmed that the persistent monthly cLCS maps are reasonably performing well for the trajectory prediction of pollutants such as oil spills. These maps will help to initiate mitigation measures in case of any occurrence of oil spills in the future.

During the past five to ten years, a broad partnership of institutions under NOPP sponsorship has collaborated in developing and demonstrating the performance and application of eddy-resolving, real-time global- and basin-scale ocean prediction systems using the HYbrid Coordinate Ocean Model (HYCOM). The partnership represents a broad spectrum of the oceanographic community, bringing together academia, federal agencies, and industry/commercial entities, and spanning modeling, data assimilation, data management and serving, observational capabilities, and application of HYCOM prediction system outputs. In addition to providing real-time, eddy-resolving global- and basin-scale ocean prediction systems for the US Navy and NOAA, this project also offered an outstanding opportunity for NOAA-Navy collaboration and cooperation, ranging from research to the operational level. This paper provides an overview of the global HYCOM ocean prediction system and highlights some of its achievements. An important outcome of this effort is the capability of the global system to provide boundary conditions to even higher-resolution regional and coastal models.

The dynamics associated with the Loop Current (LC) variability in the Gulf of Mexico (GoM) are studied using a 5‐year, free‐running numerical simulation with the Hybrid Coordinate Ocean Model (HYCOM). The dynamics of major GoM circulation features are represented: the extension of the LC and the associated anticyclonic, warm core Loop Current Eddies (LCEs) and cyclonic Loop Current Frontal Eddies (LCFEs). The study focuses on the dynamics of the LCFEs and their role during the LCEs shedding, which dramatically affects the GoM circulation. We analyze several characteristics of the LC frontal dynamics. Modeled LCFEs have a coherent vertical structure, which extends to the deep layers of the GoM. They may split in two separate upper and lower layer eddies. Deep and surface remnants from different frontal eddies are able to align to form new, coherent structures. LCFEs intensify along the extended LC northern edge when flowing over the deep northern GoM shelf slope that forms the Mississippi Fan, through a “promontory effect” in which the incoming cyclone aggregates positive potential vorticity anomalies in lower layers, leading to the intensification of the whole vortex structure. LCFEs may also expand further along the LC path by horizontal vortex merging, when they are blocked between the LC and the northeast corner of the continental shelf in the GoM. The intensification and merging due to topographic effects explain the enlarged frontal eddies observed on the eastern side of the Loop Current. These larger eddies further migrate along the LC front and may play a role in the shedding sequence. Key Points Loop Current Frontal Eddies have deep extension They may split and rearrange They are intensified when the Loop Current flows along the deep shelf slope

Frontogenesis and frontal instabilities in the mixed layer are known to be important processes in the formation of submesoscale features. We study the seasonality of such processes in the Gulf Stream (GS) region. To approach this problem, a realistic simulation with the Hybrid Coordinate Ocean Model is integrated for 18 months at two horizontal resolutions: a high-resolution (1/48°) simulation able to resolve part of the submesoscale regime and the full range of mesoscale dynamics, and a coarser resolution (1/12°) case, in which submesoscales are not resolved. Results provide an insight into submesoscale dynamics in the complex GS region. A clear seasonal cycle is observed, with submesoscale features mostly present during winter. The submesoscale field is quantitatively characterized in terms of deviation from geostrophy and 2D dynamics. The limiting and controlling factor in the occurrence of submesoscales appears to be the depth of the mixed layer, which controls the reservoir of available potential energy available at the mesoscale fronts that are present most of the year. Atmospheric forcings are the main energy source behind submesoscale formation, but mostly indirectly through mixed layer deepening. The mixed layer instability scaling suggested in the (Fox-Kemper et al., J Phys Oceanogr 38:1145–1165, 2008 ) parametrization appears to hold, indicating that the parametrization is appropriate even in this complex and mesoscale dominated area.

Ocean currents play an important role in the movement and distribution of organisms and for small animals it is often assumed that their movements in the ocean are determined by passive drift. Here we challenge this assumption by conducting an experiment at the scale of an entire ocean basin to test whether small (~35 cm) juvenile loggerhead sea turtles Caretta caretta move independently of ocean currents. By comparing the trajectories of 46 satellite tracked turtles (11502 positions, 12 850 tracking days) with Lagrangian drifters (3 716 303 positions, 927 529 tracking days) and virtual particles tracked within the Hybrid Coordinate Ocean Model (HYCOM), we found that in certain areas turtles moved in a similar manner to ocean currents, but in other areas turtle movement was markedly different from ocean currents, with turtles moving to areas thousands of kilometres from where they would have drifted passively. We further found that turtles were distributed in more-productive areas than would be expected if their movement depended on passive transport only. These findings demonstrate that regional variation in directional swimming contributes to young sea turtles reaching more favourable developmental habitats and supports laboratory work suggesting that young turtles have a map sense to determine their location in a seemingly featureless ocean.

Warmer (>28°C) sea surface temperature (SST) occurs in the South Eastern Arabian Sea (SEAS, 5°N–13°N, 65°E–76°E) during March–April, and is known as the Arabian Sea Mini Warm Pool (ASMWP). In this study, we address the role of salinity and the upper layer heat and salt budgets in the formation and collapse of this ASMWP. An assessment of Level 3 sea surface salinity (SSS) data from the Soil Moisture and Ocean Salinity (SMOS) satellite mission for the year 2010 shows that SMOS is able to capture the SSS variability in the SEAS. Analysis of temperature, salinity and currents from the Hybrid Coordinate Ocean Model during 2003–06, and, in situ temperature and salinity data from Argo floats during 2003–06 for the SEAS revealed that low salinity waters cap the top 60 m of the SEAS in January–February. This minimum salinity was concurrent with the formation of a barrier layer and with the time when the SEAS gained little net heat flux and the equatorward flowing East India Coastal Current (EICC) fed low saline waters into the SEAS. Subsequently, the net heat flux increased to a peak value under the increased salinity stratification, leading to the formation of the ASMWP in March–April. The ASMWP collapsed by May due to increase in SSS and the associated weakening of the salinity stratification. The monsoon onset vortex in May 2004 could be related to the minimum SSS that occurred in February 2004, followed by higher SST and heat content of the ASMWP in April 2004. Key Points Detection of fresh water plumes using SMOS salinity Dynamics of Arabian Sea warm pool using HYCOM Connection of Arabian Sea warm pool and monsoon vortex

The Arabian Sea oxygen minimum zone (OMZ) has the smallest horizontal area of all open ocean OMZs but is the world's third most intense OMZ with the largest vertical extent. This study used a regional physical-biogeochemical coupled model, HYbrid Coordinate Ocean Model- ECOSystem (HYCOM-ECOSMO), to investigate the variations in Arabian Sea OMZ and deoxygenation for the period 2000–2020. The model was evaluated against BGC-Argo data and World Ocean Atlas 2018 (WOA18) to check the consistency of eddy-permitting simulation. It accurately simulated dissolved oxygen (DO) profiles, with RMSE of 16.5 µmol kg−1 for WOA18 and 21 µmol kg−1 for BGC-Argo. Model efficiency was estimated at 0.81 with percentage bias of 31% indicating that model performs well with observations. Interannual and seasonal variabilities showed good agreement with BGC-Argo profiling floats, but with slight DO overestimation. Despite a slight decreasing trend in the model's OMZ DO, Argo data indicated a minor increase. Sensitivity experiments identified detritus remineralization and sinking rates as key factors influencing DO levels. Surface DO, temperature, and Brunt-Väisälä frequency showed spatial warming impacts. OMZ exhibited seasonal variation, with higher DO concentrations during the winter monsoon due to convective mixing. ENSO and Indian Ocean Dipole (IOD) phases minimally influence surface DO levels, with lagged impacts (1–10 months) on temperature, productivity, organic matter sinking, and microbial respiration. The system responds more promptly to IOD than to ENSO. These findings establish a baseline for future research on marine ecosystems, fishery and regional climate projections in the Arabian Sea.