The Gulf Stream: along-stream evolution of volume transport and water properties observed by underwater gliders

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2021.The Gulf Stream, the western boundary current of the subtropical North Atlantic, plays a key role in the Earth’s climate system with its poleward volume and heat transports being major components of the upper limb of the Atlantic Meridional Overturning Circulation. Extensive observations collected using Spray autonomous underwater gliders from 2004 through 2020 fill a 1500-km-long gap in longer-term sustained subsurface measurements of the Gulf Stream. The gliders provide concurrent, high-resolution measurements of Gulf Stream hydrography and velocity over more than 15 degrees of latitude between Florida and New England. These observations are used to characterize the along-stream evolution of Gulf Stream volume transport; its long-known poleward increase is shown to result primarily from entrainment of subthermocline waters. Antarctic Intermediate Water, which makes up the deepest waters within the Gulf Stream in the Florida Strait, is eroded through both vertical mixing and lateral stirring as it flows downstream. Satellite-based observations of sea surface height coincident with the glider observations are used to evaluate the efficacy of inferring Gulf Stream transport from remotely sensed measurements. The detailed analyses of Gulf Stream transport and water property evolution herein provide targets for regional and global circulation models to replicate.We gratefully acknowledge funding from the National Science Foundation (OCE-0220769, OCE-1633911, OCE-1923362, OCE-1558521), NOAA’s Global Ocean Monitoring and Observing Program (NA14OAR4320158, NA19OAR4320074), the Office of Naval Research (N000141713040), WHOI’s Oceans and Climate Change Institute, Eastman Chemical Company, and the W. Van Alan Clark, Jr. Chair for Excellence in Oceanography at WHOI (awarded to Breck Owens)

Along-stream evolution of Gulf Stream volume transport

Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 50(8), (2020): 2251-2270, doi:10.1175/JPO-D-19-0303.1.The Gulf Stream affects global climate by transporting water and heat poleward. The current’s volume transport increases markedly along the U.S. East Coast. An extensive observing program using autonomous underwater gliders provides finescale, subsurface observations of hydrography and velocity spanning more than 15° of latitude along the path of the Gulf Stream, thereby filling a 1500-km-long gap between long-term transport measurements in the Florida Strait and downstream of Cape Hatteras. Here, the glider-based observations are combined with shipboard measurements along Line W near 68°W to provide a detailed picture of the along-stream transport increase. To account for the influences of Gulf Stream curvature and adjacent circulation (e.g., corotating eddies) on transport estimates, upper- and lower-bound transports are constructed for each cross–Gulf Stream transect. The upper-bound estimate for time-averaged volume transport above 1000 m is 32.9 ± 1.2 Sv (1 Sv ≡ 106 m3 s−1) in the Florida Strait, 57.3 ± 1.9 Sv at Cape Hatteras, and 75.6 ± 4.7 Sv at Line W. Corresponding lower-bound estimates are 32.3 ± 1.1 Sv in the Florida Strait, 54.5 ± 1.7 Sv at Cape Hatteras, and 69.9 ± 4.2 Sv at Line W. Using the temperature and salinity observations from gliders and Line W, waters are divided into seven classes to investigate the properties of waters that are transported by and entrained into the Gulf Stream. Most of the increase in overall Gulf Stream volume transport above 1000 m stems from the entrainment of subthermocline waters, including upper Labrador Sea Water and Eighteen Degree Water.We gratefully acknowledge funding from the Office of Naval Research (N000141713040), the National Science Foundation (OCE-0220769, OCE-1633911, OCE-1923362), NOAA’s Global Ocean Monitoring and Observing Program (NA14OAR4320158, NA19OAR4320074), WHOI’s Oceans and Climate Change Institute, Eastman Chemical Company, and the W. Van Alan Clark, Jr. Chair for Excellence in Oceanography at WHOI (awarded to Breck Owens)

Autonomous and Lagrangian ocean observations for Atlantic tropical cyclone studies and forecasts

Author Posting. © The Oceanography Society, 2017. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 30, no. 2 (2017): 92–103, doi:10.5670/oceanog.2017.227.The tropical Atlantic basin is one of seven global regions where tropical cyclones (TCs) commonly originate, intensify, and affect highly populated coastal areas. Under appropriate atmospheric conditions, TC intensification can be linked to upper-ocean properties. Errors in Atlantic TC intensification forecasts have not been significantly reduced during the last 25 years. The combined use of in situ and satellite observations, particularly of temperature and salinity ahead of TCs, has the potential to improve the representation of the ocean, more accurately initialize hurricane intensity forecast models, and identify areas where TCs may intensify. However, a sustained in situ ocean observing system in the tropical North Atlantic Ocean and Caribbean Sea dedicated to measuring subsurface temperature, salinity, and density fields in support of TC intensity studies and forecasts has yet to be designed and implemented. Autonomous and Lagrangian platforms and sensors offer cost-effective opportunities to accomplish this objective. Here, we highlight recent efforts to use autonomous platforms and sensors, including surface drifters, profiling floats, underwater gliders, and dropsondes, to better understand air-sea processes during high-wind events, particularly those geared toward improving hurricane intensity forecasts. Real-time data availability is key for assimilation into numerical weather forecast models.The NOAA/AOML component of this work was originally funded by the Disaster Relief Appropriations Act of 2013, also known as the Sandy Supplemental, and is currently funded through NOAA research grant NA14OAR4830103 by AOML and CARICOOS, as well as NOAA’s Integrated Ocean Observing System (IOOS). The TEMPESTS component of this work is supported by NOAA through the Cooperative Institute for the North Atlantic Region (NA13OAR4830233) with additional analysis support from the WHOI Summer Student Fellowship Program, Nortek Student Equipment Grant, and the Rutgers University Teledyne Webb Graduate Student Fellowship Program. The drifter component of this work is funded through NOAA grant NA15OAR4320071(11.432) in support of the Global Drifter Program

The Gulf Stream: Along-stream evolution of volume transport and water properties observed by underwater gliders

The Gulf Stream, the western boundary current of the subtropical North Atlantic, plays a key role in the Earth’s climate system with its poleward volume and heat transports being major components of the upper limb of the Atlantic Meridional Overturning Circulation. Extensive observations collected using Spray autonomous underwater gliders from 2004 through 2020 fill a 1500-km-long gap in longer-term sustained subsurface measurements of the Gulf Stream. The gliders provide concurrent, high-resolution measurements of Gulf Stream hydrography and velocity over more than 15 degrees of latitude between Florida and New England. These observations are used to characterize the along-stream evolution of Gulf Stream volume transport; its long-known poleward increase is shown to result primarily from entrainment of subthermocline waters. Antarctic Intermediate Water, which makes up the deepest waters within the Gulf Stream in the Florida Strait, is eroded through both vertical mixing and lateral stirring as it flows downstream. Satellite-based observations of sea surface height coincident with the glider observations are used to evaluate the efficacy of inferring Gulf Stream transport from remotely sensed measurements. The detailed analyses of Gulf Stream transport and water property evolution herein provide targets for regional and global circulation models to replicate.Ph.D

Flow of warm Atlantic Water in the Norske trough on the East Greenland shelf

Local water temperature increases associated with global warming have the potential to melt glaciers and ice sheets, which are major contributors to sea level rise. This Bachelor of Science thesis examines the flow of warm (0 ◦C – 2 ◦C) Atlantic Water (AW) in the Norske trough on the East Greenland continental shelf using an oceanographic section of CTD and ship-board ADCP (SADCP) data collected in June 2014. The Norske trough connects the open ocean to the marine terminating glacier Nioghalvfjerdsfjorden at 79.5◦ N and it is of interest whether warm water can flow through this trough to the floating ice tongue. The CTD data shows that AW with temperatures of 1.5 ◦C and salinities of 34.5 is overlain by Polar Water (PW) with temperatures of -1.5 ◦C and salinities of 31.5. The stratification is strong and the halocline is located at a depth of 100 m. The SADCP data was detided using the barotropic tidal model AOTIM5 and it shows that the flow reaches velocities along the trough of up to 0.15 m s−1 in the upper 250 m. The absolute geostrophic velocities calculated using the SADCP data, as well as the Rossby radius of 12 - 14 km calculated from the stratification, support the assumption that a bi-directional flow with an in- and outflow exists in the trough. Hence it is shown that warm AW reaches the Norske trough and flows into the direction of the 79.5◦ N glacier at a depth where it could potentially melt the glacier or its floating ice tongue. As a precursory step, it was attempted to calibrate lowered ADCP (LADCP) data in order to remove compass deviations that were assumed to be caused by a steel block in the vicinity of the CTD rosette. The data of both the upward and the downward looking LADCP were compared to the SADCP data in order to determine the compass deviation. It was found that in this case this method can- not be used to gain more information about the flow below 250 m depth because the compass deviations vary over time and between stations. It is proposed that this is due to the electromagnetic influence of coiled cables that were located close to the instrument

Transient Response of the Gulf Stream to Multiple Hurricanes in 2017

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 10,509-10,519, doi:10.1029/2018GL079180.Autonomous underwater glider observations collected during and after 2017 Hurricanes Irma, Jose, and Maria show two types of transient response within the Gulf Stream. First, anomalously fresh water observed near the surface and within the core of the Gulf Stream offshore of the Carolinas likely resulted from Irma's rainfall being entrained into the Loop Current‐Gulf Stream system. Second, Gulf Stream volume transport was reduced by as much as 40% for about 2 weeks following Jose and Maria. The transport reduction had both barotropic and depth‐dependent characteristics. Correlations between transport through the Florida Straits and reanalysis winds suggest that both local winds in the Florida Straits and winds over the Gulf Stream farther downstream may have contributed to the transport reduction. To clarify the underlying dynamics, additional analyses using numerical models that capture the Gulf Stream's transient response to multiple tropical cyclones passing nearby in a short period are needed.DOC | NOAA | Climate Program Office (CPO) Grant Number: NA14OAR4320158; DOD | United States Navy | Office of Naval Research (ONR) Grant Number: N000141713040; Eastman Chemical Company; National Science Foundation (NSF) Grant Numbers: OCE-0220769, OCE-1633911; Woods Hole Oceanographic Institution (WHOI)2019-04-0

Transient Response of the Gulf Stream to Multiple Hurricanes in 2017

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 10,509-10,519, doi:10.1029/2018GL079180.Autonomous underwater glider observations collected during and after 2017 Hurricanes Irma, Jose, and Maria show two types of transient response within the Gulf Stream. First, anomalously fresh water observed near the surface and within the core of the Gulf Stream offshore of the Carolinas likely resulted from Irma's rainfall being entrained into the Loop Current‐Gulf Stream system. Second, Gulf Stream volume transport was reduced by as much as 40% for about 2 weeks following Jose and Maria. The transport reduction had both barotropic and depth‐dependent characteristics. Correlations between transport through the Florida Straits and reanalysis winds suggest that both local winds in the Florida Straits and winds over the Gulf Stream farther downstream may have contributed to the transport reduction. To clarify the underlying dynamics, additional analyses using numerical models that capture the Gulf Stream's transient response to multiple tropical cyclones passing nearby in a short period are needed.DOC | NOAA | Climate Program Office (CPO) Grant Number: NA14OAR4320158; DOD | United States Navy | Office of Naval Research (ONR) Grant Number: N000141713040; Eastman Chemical Company; National Science Foundation (NSF) Grant Numbers: OCE-0220769, OCE-1633911; Woods Hole Oceanographic Institution (WHOI)2019-04-0

OceanGliders: A Component of the Integrated GOOS

International audienceThe OceanGliders program started in 2016 to support active coordination and enhancement of global glider activity. OceanGliders contributes to the international efforts of the Global Ocean Observation System (GOOS) for Climate, Ocean Health, and Operational Services. It brings together marine scientists and engineers operating gliders around the world: (1) to observe the long-term physical, biogeochemical, and biological ocean processes and phenomena that are relevant for societal applications; and, (2) to contribute to the GOOS through real-time and delayed mode data dissemination. The OceanGliders program is distributed across national and regional observing systems and significantly contributes to integrated, multi-scale and multi-platform sampling strategies. OceanGliders shares best practices, requirements, and scientific knowledge needed for glider operations, data collection and analysis. It also monitors global glider activity and supports the dissemination of glider data through regional and global databases, in real-time and delayed modes, facilitating data access to the wider community. OceanGliders currently supports national, regional and global initiatives to maintain and expand the capabilities and application of gliders to meet key global challenges such as improved measurement of ocean boundary currents, water transformation and storm forecast