Oceanic fronts are regions of rapid change in seawater properties. We examine a collection of approaching 10,000 oceanic sound speed profiles from the surface to 1000 m measured by autonomous underwater gliders within 200 km of the Gulf Stream front between 35 and 41 °N in the North Atlantic Ocean. The underwater gliders sample with approximately 5 km resolution in the cross-front direction and allow us to study subsurface acoustic ducts across the front. We algorithmically detect ducts in the sound speed profiles and examine statistics of subsurface duct axial sound speed, width, and cutoff frequency depending on depth and distance from the Gulf Stream. We associate the formation of subsurface sound speed ducts with ocean water mass properties and dynamics. In particular, we examine the case of ducts formed by cool, fresh water from the continental shelf that is exported, entrained, and subducted beneath the Gulf Stream.

The Gulf Stream separates cooler, fresher waters of subpolar origin from warmer, saltier subtropical waters, with a notably deeper sound speed minimum on the warm side of the front. North and east of its separation from the continental margin at Cape Hatteras, the Gulf Stream meanders substantially, leading to large variability in acoustic properties on time scales of days to weeks. Despite its varying position and orientation, the structure of the Gulf Stream front is known to be quite stable. Here, satellite altimetry is used to determine the location and orientation of the Gulf Stream at particular times, and a seasonal, stream-coordinate climatology developed from nearly a decade of underwater glider sampling is used to infer corresponding subsurface sound speed structure. Resulting predictions are compared to measured sound speed profiles and predictive skill is compared to an operational numerical simulation.

The Gulf Stream transports heat from the tropics poleward and is an integral part of how the planet redistributes heat. Studies of the variability of the Gulf Stream have suggested that the seasonal cycle of ocean mixed layer heat content in the Gulf Stream may be driven by the local net atmospheric heat flux or by oceanic advection of heat in the strong western boundary current. Here we use sustained underwater glider observations of the Gulf Stream from 80W to 67W during 2015–2023 to show that oceanic advection heats the ocean mixed layer in spring when the upper ocean switches from cooling to warming. Estimated terms of the mixed layer temperature budget demonstrate that the net atmospheric heat flux cools the upper ocean in all seasons except summer and that mixed layer warming in spring is due to ocean advection of heat from farther south.

A variety of near-real-time observations are routinely assimilated into operational numerical simulations of the ocean. This analysis focuses on how subsurface profiles from autonomous underwater gliders and Argo profiling floats have varying impact in the operational Navy Coastal Ocean Model (NCOM) for the US East Coast region depending on the proximity of the profiles to the Gulf Stream. Changes in the model's representation of the ocean state from 24-hour-ahead forecasts to the following days' nowcast runs are used to evaluate the impact of observations made available within the final day before the model valid time during 2017. In general, this metric of observation impact decays over a spatial scale of O(100) km, consistent with covariance scales in the data assimilation scheme. However, observations within and near the Gulf Stream are associated with forecast-to-nowcast changes in the model that are about twice as large as for observations far from the Gulf Stream. Moreover, the strongly advective nature of the Gulf Stream leads to elevated downstream impact of observations within the current. For constraining ocean models, these results suggest that autonomous underwater gliders may be most effectively used to target regions with strong gradients, such as are common along oceanic boundaries.

Ocean boundary currents are complex and highly variable systems that play key roles in connecting the open and coastal ocean through cross-slope circulation and upwelling of nutrient-rich water. The structure, strength, and variability of boundary currents are associated with a broad range of spatial and temporal scales. For that reason, long-term boundary current monitoring is challenging and requires the use of complementary observing platforms and sensors coupled with numerical simulations. The Ocean Observations Physics and Climate Panel Boundary Systems Task Team recently held a virtual dialogue series to discuss six mature boundary current monitoring systems. The goal of the series was to examine strategies for developing a conceptual design for sustained observing activities applicable to a wide range of boundary current systems. This article provides a brief overview of the six systems, including users and the observational and modeling components needed to achieve scientific, operational, and societal goals. Ocean observing best practices and recommendations are shared to provide guidance for the coordination and sustainability of observing systems at ocean boundaries and to strengthen and integrate partnerships across and within the global observing networks.

The southeastern United States (SEUS), from North Carolina through southern Florida, has the most extensive cold-water coral reefs, including coral mounds and coral gardens, in the US Exclusive Economic Zone. In fact, a region extending from off central Florida to Georgia has been named the "Million Mounds" area and is estimated to contain tens-of-thousands of coral mounds. Oceanographic patterns in this region are dominated by the Gulf Stream and associated oceanographic events (i.e., eddies, meanders, intrusions). Therefore, corals in the SEUS often experience strong currents and rapid changes in environmental conditions, such as temperature and salinity. The Gulf Stream also serves as a conduit for dispersal of larvae, and thus coral populations throughout the area are highly connected, although there are some signatures of differentiation across a bathymetric gradient. The faunal community in the region is diverse and includes a characteristic deep-reef fauna that differs from areas off reef. While resource extraction activities in deep water are present in the region, the most significant anthropogenic threat to cold-water corals in the SEUS is climate change. Future research efforts should focus on our understanding of the interplay between changing environmental conditions and coral development, growth, physiology, and persistence. In addition, we need to better understand both abiotic and biotic processes that govern cold-water coral ecosystems in the region.

An advanced data-assimilative ocean circulation model is used to investigate Gulf Stream (GS) variability during 2017-2018. The modeling system applies a strong-constraint, 4D variational data assimilation algorithm. It assimilates satellite-based sea surface height and sea surface temperature measurements and in situ temperature and salinity profiles. Model skill assessment metrics along with comparisons of GS position and GS's threedimensional mean kinetic energy with historical observations are applied to validate the data-assimilative model. The resulting time- and space-continuous ocean state estimates are used to diagnose eddy kinetic energy conversion and cross-stream eddy heat and salt fluxes over the two-year study period. The processes leading to kinetic energy conversion are primarily due to GS meanders. Significant inverse energy cascading (EKE & RARR;MKE and EKE & RARR;EPE) can occur during GS-eddy interactions, particularly during onshore intrusions or offshore meanderings of the GS. Throughout the two-year study period, the cross-stream eddy heat and salt fluxes off Cape Hatteras were predominantly positive (onshore). Both GS offshore meandering (occurring 44% of the time and associated with shelf/slope water export) and GS intrusion (occurring 56% of the time) contribute to onshore heat and salt transport. Improved understanding of these processes and dynamics requires strong integration of an advanced observational infrastructure that combines remote sensing; fixed, mobile, and shore-based observing components; and high-resolution data assimilative models.

As the poleward-flowing western boundary current of the North Atlantic ocean, the Gulf Stream plays a key role in the climate system. Here we show that from 2001 to 2023, the Gulf Stream west of 68° W has experienced both surface-intensified warming due to heat uptake at a rate exceeding the global average and a bulk lateral shift towards its cooler shoreward side at a rate of about 5 ± 2 km per decade. The Gulf Stream west of 68° W now has an O (10)-m-thick surface layer of warmer (by ~ 1 °C) and lighter (by ~ 0.3 kg m −3 ) water, contributing to increased upper ocean stratification. Our results rely on over 25,000 temperature and salinity profiles collected by autonomous profiling floats and underwater gliders in the region, allowing robust estimation of trends and clear attribution of observed changes to both ocean heat uptake and a lateral shift of the Gulf Stream.

The Galapagos Archipelago lies on the equator in the path of the eastward flowing Pacific Equatorial Undercurrent (EUC). When the EUC reaches the archipelago, it upwells and bifurcates into a north and south branch around the archipelago at a latitude determined by topography. Since the Coriolis parameter (f) equals zero at the equator, strong velocity gradients associated with the EUC can result in Ertel potential vorticity (Q) having sign opposite that of planetary vorticity near the equator. Observations collected by underwater gliders deployed just west of the Galapagos Archipelago during 2013-16 are used to estimate Q and to diagnose associated instabilities that may impact the Galapagos Cold Pool. Estimates of Q are qualitatively conserved along streamlines, consistent with the 2.5-layer, inertial model of the EUC by Pedlosky. The Q with sign opposite of f is advected south of the Galapagos Archipelago when the EUC core is located south of the bifurcation latitude. The horizontal gradient of Q suggests that the region between 2 degrees S and 2 degrees N above 100 m is barotropically unstable, while limited regions are baroclinically unstable. Conditions conducive to symmetric instability are observed between the EUC core and the equator and within the southern branch of the undercurrent. Using 2-month and 3-yr averages, e-folding time scales are 2-11 days, suggesting that symmetric instability can persist on those time scales. Significance StatementThe Pacific Ocean contains fast-moving currents near the equator and below the surface that result in instabilities and mixing. The Galapagos Archipelago lies directly in the path of the eastward-flowing Pacific Equatorial Undercurrent. There are few observations of what happens to the current when it reaches the Galapagos Archipelago, so theories and models of the instabilities and mixing resulting from these strong currents have not been well verified. The Repeat Observations by Gliders in the Equatorial Region (ROGER) project deployed autonomous underwater gliders to observe the current system in this region. The results show that a range of instabilities may be responsible for the cold sea surface temperature of the Galapagos Cold Pool and the generation of tropical instability waves.

The Processes driving Exchange At Cape Hatteras (PEACH) program seeks to better understand seawater exchanges between the continental shelf and the open ocean near Cape Hatteras, North Carolina. This location is where the Gulf Stream transitions from a boundary-trapped current to a free jet, and where robust along-shelf convergence brings cool, relatively fresh Middle Atlantic Bight and warm, salty South Atlantic Bight shelf waters together, forming an important and dynamic biogeographic boundary. The magnitude of this convergence implies large export of shelf water to the open ocean here. Background on the oceanography of the region provides motivation for the study and gives context for the measurements that were made. Science ques-tions focus on the roles that wind forcing, Gulf Stream forcing, and lateral density gra-dients play in driving exchange. PEACH observational efforts include a variety of fixed and mobile observing platforms, and PEACH modeling included two different resolu-tions and data assimilation schemes. Findings to date on mean circulation, the nature of export from the southern Middle Atlantic Bight shelf, Gulf Stream variability, and position variability of the Hatteras Front are summarized, together with a look ahead to forthcoming analyses.

By Travis N. Miles, Dongxiao Zhang, Gregory R. Foltz, Jun A. Zhang, Christian Meinig, Francis Bringas, Joaquin Triñanes, Matthieu Le Hénaff, Maria F. Aristizabal Vargas, Sam Coakley, Catherine R. Edwards, Donglai Gong, Robert E. Todd, Matthew J. Oliver, W. Douglas Wilson, Kerri Whilden, Barbara Kirkpatrick, Patricia Chardon-Maldonado, Julio M. Morell, Debra Hernandez, Gerhard Kuska, Cheyenne D. Stienbarger, Kathleen Bailey, Chidong Zhang, Scott M. Glenn, and Gustavo J. Goni

Abstract The strong, meandering, and eddy‐shedding Gulf Stream is a large oceanic reservoir of both mean and eddy kinetic energy in the northwestern Atlantic. Since 2015, underwater gliders equipped with Doppler current profilers have collected over 20,000 absolute velocity profiles in and near the Gulf Stream along the US East Coast. Those observations are used to make three‐dimensional estimates of mean and eddy kinetic energy, substantially expanding the geographic coverage of prior estimates of subsurface kinetic energy in the Gulf Stream. Glider observations are combined via weighted least squares fitting with anisotropic and inhomogeneous length scales to reflect both circulation and sampling density; this averaging technique can be applied to other quantities measured by the gliders. Mean and eddy kinetic energy decay approximately exponentially away from the surface. Vertical decay scales are longest within the high‐speed core of the Gulf Stream and somewhat shorter on the flanks of the Gulf Stream.

The strong El Nino of 2014-16 was observed west of the Galapagos Islands through sustained deployment of underwater gliders. Three years of observations began in October 2013 and ended in October 2016, with observations at longitudes 93 degrees and 95 degrees W between latitudes 2 degrees N and 2 degrees S. In total, there were over 3000 glider-days of data, covering over 50 000 km with over 12 000 profiles. Coverage was superior closer to the Galapagos on 93 degrees W, where gliders were equipped with sensors to measure velocity as well as temperature, salinity, and pressure. The repeated glider transects are analyzed to produce highly resolved mean sections and maps of observed variables as functions of time, latitude, and depth. The mean sections reveal the structure of the Equatorial Undercurrent (EUC), the South Equatorial Current, and the equatorial front. The mean fields are used to calculate potential vorticity Q and Richardson number Ri. Gradients in the mean are strong enough to make the sign of Q opposite to that of planetary vorticity and to have Ri near unity, suggestive of mixing. Temporal variability is dominated by the 2014-16 El Nino, with the arrival of depressed isopycnals documented in 2014 and 2015. Increases in eastward velocity advect anomalously salty water and are uncorrelated with warm temperatures and deep isopycnals. Thus, vertical advection is important to changes in heat, and horizontal advection is relevant to changes in salt. Implications of this work include possibilities for future research, model assessment and improvement, and sustained observations across the equatorial Pacific.

Carbon-rich Middle Atlantic Bight (MAB) and South Atlantic Bight (SAB) shelf waters typically converge on the continental shelf near Cape Hatteras. Both are often exported to the adjacent open ocean in this region. During a survey of the region in mid-January 2018, there was no sign of shelf water export at the surface. Instead, a subsurface layer of shelf water with high chlorophyll and dissolved oxygen was observed at the edge of the Gulf Stream east of Cape Hatteras. Strong cooling over the MAB and SAB shelves in early January led to shelf waters being denser than offshore surface waters. Driven by the density gradient, the denser shelf waters cascaded beneath the Gulf Stream and were subsequently entrained into the Gulf Stream, as they were advected northeastward. Underwater glider observations 80 km downstream of the export location captured 0.44 Sv of shelf waters transported along the edge of the Gulf Stream in January 2018. In total, as much as 7×10 6 kg of carbon was exported from the continental shelf to a greater depth in the open ocean during this 5-day-long cascading event. Earlier observations of near-bottom temperature and salinity at a depth of 230 m captured several multiday episodes of shelf water at a location that was otherwise dominated by Gulf Stream water, indicating that the January 2018 cascading event was not unique. Cascading is an important, yet little-studied pathway of carbon export and sequestration at Cape Hatteras.

In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air-sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the United States, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air-sea response during a transition from active-to-break conditions in the central BoB. The active phase of the similar to 20-day research cruise was characterized by warm sea surface temperature (SST > 30 degrees C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s(-1)). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10-12 m s(-1) wind and evaporation of 0.2 mm h(-1). The evolving environmental state included a deepening ocean mixed layer (from similar to 20 to 50 m), cooling SST (by similar to 1 degrees C), and warming/drying of the lower to midtroposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29 degrees C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air-sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.

Citation: Testor P, Young Bd, Rudnick DL, Glenn S, Hayes D, Lee CM, Pattiaratchi C, Hill K, Heslop E, Turpin V, Alenius P, Barrera C, Barth JA, Beaird N, Bécu G, Bosse A, Bourrin F, Brearley JA, Chao Y, Chen S, Chiggiato J, Coppola L, Crout R, Cummings J, Curry B, Curry R, Davis R, Desai K, DiMarco S, Edwards C, Fielding S, Fer I, Frajka-Williams E, Gildor H, Goni G, Gutierrez D, Haugan P, Hebert D, Heiderich J, Henson S, Heywood K, Hogan P, Houpert L, Huh S, Inall ME, Ishii M, Ito S-i, Itoh S, Jan S, Kaiser J, Karstensen J, Kirkpatrick B, Klymak J, Kohut J, Krahmann G, Krug M, McClatchie S, Marin F, Mauri E, Mehra A, Meredith MP, Meunier T, Miles T, Morell JM, Mortier L, Nicholson S, O’Callaghan J, O’Conchubhair D, Oke P, Pallàs-Sanz E, Palmer M, Park J, Perivoliotis L, Poulain P-M, Perry R, Queste B, Rainville L, Rehm E, Roughan M, Rome N, Ross T, Ruiz S, Saba G, Schaeffer A, Schönau M, Schroeder K, Shimizu Y, Sloyan BM, Smeed D, Snowden D, Song Y, Swart S, Tenreiro M, Thompson A, Tintore J, Todd RE, Toro C, Venables H, Wagawa T, Waterman S, Watlington RA and Wilson D (2021) Corrigendum: OceanGliders: A Component of the Integrated GOOS. Front. Mar. Sci. 8:696100. doi: 10.3389/fmars.2021.696100 Corrigendum: OceanGliders: A Component of the Integrated GOOS

The Equatorial Undercurrent (EUC) is a vital component of the coupled ocean-atmosphere system in the tropical Pacific. The details of its termination near the Galapagos Islands in the eastern Pacific have an outsized importance to regional circulation and ecosystems. Subject to diverse physical processes, the EUC is also a rigorous benchmark for global climate models (GCMs). Simulations of the EUC in three generations of GCMs are evaluated relative to recent underwater glider observations along 93 degrees W. Simulations of the EUC have improved, but a slow bias of similar to 36% remains in the eastern Pacific, along with a dependence on resolution. Additionally, the westward surface current is too slow, and stratification is too strong (weak) by similar to 50% above (within) the EUC. These biases have implications for mixing in the equatorial cold tongue. Downstream lies the Galapagos, now resolved to varying degrees by GCMs. Properly representing the Galapagos is necessary to avoid new biases as the EUC improves. Plain Language Summary The Equatorial Undercurrent (EUC) is a swift current that flows eastward along the equator in the Pacific Ocean, about 100 m below the surface. This current is just as challenging to observe as it is to simulate with models-after all, it was only discovered in the 1950s. One of the interesting aspects of the Undercurrent is how it is diverted by the Galapagos Islands when it encounters them in the eastern Pacific. The resultant upwelling is responsible for the remarkably productive and diverse ecosystem of the Galapagos. This paper takes advantage of a unique set of observations from a recent, successful field campaign using underwater gliders to measure the Undercurrent just before it reaches the Galapagos, in order to evaluate how the latest generations of global climate models simulate this current and its neighboring features. Models have steadily improved, but they still struggle to capture the high speed of the EUC. Models are also being run at finer spatial resolution, which enables islands like the Galapagos to be included in some of the model grids. A sampling of islands in different models, and how they interact with the EUC, demonstrates the importance of a proper representation of the Galapagos in models.

The Equatorial Undercurrent (EUC) encounters the Galápagos Archipelago on the equator as it flows eastward across the Pacific. The impact of the Galápagos Archipelago on the EUC in the eastern equatorial Pacific remains largely unknown. In this study, the path of the EUC as it reaches the Galápagos Archipelago is measured directly using high-resolution observations obtained by autonomous underwater gliders. Gliders were deployed along three lines that define a closed region with the Galápagos Archipelago as the eastern boundary and 93°W from 2°S to 2°N as the western boundary. Twelve transects were simultaneously occupied along the three lines during 52 days in April–May 2016. Analysis of individual glider transects and average sections along each line show that the EUC splits around the Galápagos Archipelago. Velocity normal to the transects is used to estimate net horizontal volume transport into the volume. Downward integration of the net horizontal transport profile provides an estimate of the time- and areal-averaged vertical velocity profile over the 52-day time period. Local maxima in vertical velocity occur at depths of 25 and 280 m with magnitudes of (1.7 ± 0.6) × 10−5 m s−1 and (8.0 ± 1.6) × 10−5 m s−1, respectively. Volume transport as a function of salinity indicates that water crossing 93°W south (north) of 0.4°S tends to flow around the south (north) side of the Galápagos Archipelago. Comparisons are made between previous observational and modeling studies with differences attributed to effects of the strong 2015/16 El Niño event, the annual cycle of local winds, and varying longitudes between studies of the equatorial Pacific.

Cross‐equator transects occupied by an underwater glider and a research vessel in the western Indian Ocean captured the evolution of equatorial circulation during onset of the boreal summer monsoon in 2018. At the end of the winter monsoon in March, surface currents were westward, while the equatorial undercurrent carried salty Arabian Sea High‐Salinity Water eastward. As winds transitioned from westward to eastward during April, an eastward near‐surface Wyrtki Jet developed, while the equatorial undercurrent weakened, vanishing by May. A first‐mode baroclinic Kelvin wave propagated through the survey region after westward winds relaxed. However, the vertical structure of the evolving circulation was inconsistent with the first baroclinic mode, suggesting the influence of higher modes in setting observed vertical structure. The strong equatorial undercurrent at the end of the winter monsoon allowed high‐salinity waters from the western equatorial Indian Ocean to reach the southern Bay of Bengal in summer 2018.