RV Knorr Cruise KN200-4, 13 Apr-03 May 2011. RAPID Mooring Cruise

This report describes the mooring operations conducted during RV Knorr cruise KN200-4 between 13 April and 3 May 2011. These mooring operations were completed as part of the United Kingdom Natural Environment Research Council (NERC) funded RAPID-WATCH Programme to monitor the Atlantic Meridional Overturning Circulation (MOC) at 26.5°N. The primary purpose on this cruise for the UK team was to service the RAPID Western Boundary moorings while the US teams worked on the Western Boundary Time Series project and the RAPID-MOCHA Western Boundary moorings. Cruise KN200-4 was from Port Everglades, Florida to Port Everglades, Florida and covered the Western Boundary moorings deployed on RB0901 and OC459. This cruise was the ninth annual refurbishment of the Western Boundary section of an array of moorings deployed across the Atlantic in order to continuously observe the MOC. This array will be further refined and refurbished during subsequent years. The instruments deployed on the array consist of a variety of current meters, bottom pressure recorders, and CTD loggers, which, combined with time series measurements of the Florida Straits Current and wind stress estimates, will be used to determine the strength and structure of the MOC at 26.5°N. (http://www.noc.soton.ac.uk/rapid

RV Walton Smith Cruise WS17305, 31 Oct-10 Nov 2017, Miami – Miami. MerMEED microstructure cruise report.

The MerMEED (Mechanisms responsible for Mesoscale Eddy Energy Dissipation) project is a NERC funded project (NE/N001745/1, 2015–2018) to investigate the levels of dissipation associated with eddies at a western boundary, in order to identify the mechanisms responsible. Mesoscale eddies are ubiquitous in the worlds oceans, and can be found in the subtropical Atlantic travelling slowly westward (at 4–5 cm/s), with a radius of about 100 km. These eddies are formed through baroclinic instability or wind forcing across the Atlantic, but when they reach the western boundary (east coast of the USA), they disappear from the satellite altimetry record. This disappearance of eddies occurs throughout the worlds oceans at western boundaries, but from altimetry alone, it is not known whether they disappear because energy is transferred to other wave modes or the mean flow, or whether it is locally dissipated through eddy-topography interactions. This is the second cruise of the MerMEED project, with the previous being detailed in [Frajka-Williams, 2017]. The purpose of this cruise was to (1) make microstructure temperature and shear measurements in order to measure dissipation at the intersection of an anticyclonic eddy and the steep topography to the east of Abaco, Bahamas, and (2) deploy standard and microstructure Seagliders. Of these, the standard Seagliders were intended to remain in the area for 4 months. During the 10 day cruise, 112 profiles of microstructure data were collected using a tethered microstructure profiler, and a shipboard 75 kHz ADCP collected concurrent measurements of ocean currents. This cruise is the second of several planned cruises for the MerMEED project, and the data collected are intended to complement additional field operations, including moored instruments added to the RAPID array (thermistors and ADCPs on the WB1 mooring) and a second glider deployment in the spring of 2018

TERIFIC project, 26 November – 10 December 2019

The purpose of the fieldwork activities detailed in this report was to deploy a range of autonomous platforms to measure physical oceanographic properties at the west Greenland margin and Labrador Sea. The land-based fieldwork spanned the dates in this report, while the seagoing activities were accomplished with 1 day of work onboard the Adolf Jensen, a 30m Greenlandic vessel. The autonomous platforms used included: two Seagliders (sg602 and sg638) equipped with CTDs, oxygen and biooptics (WETlabs triple puck); an autonomous surface vehicle (Sailbuoy Artemis) measuring surface temperature and salinity, surface wind speed and direction and air temperature, and a wave sensor; 50 standard Global Drifter Program drifters measuring temperature and their position; and 3 drifters measuring surface temperature and salinity sensors and barometric pressure. The drifters were deployed at the continental shelf edge offshore of Qaqortoq, Greenland on December 4. The gliders and autonomous surface vehicle were deployed on the shelf and transited offshore to the central Labrador Sea

RV Walton Smith Cruise WS16336, 01 - 07 Dec 2016, Miami to Miami, USA. MerMEED microstructure cruise report

The MerMEED (Mechanisms responsible for Mesoscale Eddy Energy Dissipation) project is a NERC funded project (NE/N001745/1, 2015-2018) to investigate the levels of dissipation associated with eddies at a western boundary, in order to identify the mechanisms responsible. Mesoscale eddies are ubiquitous in the worlds oceans, and can be found in the subtropical Atlantic travelling slowly westward (at 4-5 cm/s), with a radius of about 100 km. These eddies are formed through baroclinic instability or wind forcing across the Atlantic, but when they reach the western boundary (east coast of the USA), they disappear from the satellite altimetry record. This disappearance of eddies occurs throughout the worldsí oceans at western boundaries, but from altimetry alone, it is not known whether they disappear because energy is transferred to other wave modes or the mean flow, or whether it is locally dissipated through eddy-topography interactions. The purpose of this cruise was to make microstructure temperature and shear measurements in order to measure dissipation at the intersection of an anticyclonic eddy and the steep topography to the east of Abaco, Bahamas. During the 7 day cruise, 70 profiles of microstructure data were collected using a tethered microstructure profiler, and a shipboard 75 kHz ADCP collected concurrent measurements of ocean currents. This cruise is the first of several planned cruises for the MerMEED project, and the data collected are intended to complement additional field operations, including moored instruments added to the RAPID array (thermistors and ADCPs on the WB1 mooring) and glider deployments planned for the 2017/18 year

Emerging negative Atlantic Multidecadal Oscillation index in spite of warm subtropics

Sea surface temperatures in the northern North Atlantic have shown a marked decrease over the past several years. The sea surface in the subpolar gyre is now as cold as it was during the last cold phase of the Atlantic Multidecadal Oscillation index in the 1990s. This climate index is associated with shifts in hurricane activity, rainfall patterns and intensity, and changes in fish populations. However, unlike the last cold period in the Atlantic, the spatial pattern of sea surface temperature anomalies in the Atlantic is not uniformly cool, but instead has anomalously cold temperatures in the subpolar gyre, warm temperatures in the subtropics and cool anomalies over the tropics. The tripole pattern of anomalies has increased the subpolar to subtropical meridional gradient in SSTs, which are not represented by the AMO index value, but which may lead to increased atmospheric baroclinicity and storminess. Here we show that the recent Atlantic cooling is likely to persist, as predicted by a statistical forecast of subsurface ocean temperatures and consistent with the irreversible nature of watermass changes involved in the recent cooling of the subpolar gyre

Estimating the Atlantic overturning at 26°N using satellite altimetry and cable measurements

Climate simulations predict a slowing of the Atlantic meridional overturning circulation (MOC), a key oceanic component of the climate system, while continuous observations of the MOC from boundary arrays demonstrate substantial variability on weekly to interannual time scales. These arrays are necessarily limited to individual latitudes. A potential proxy for the MOC covering longer time scales and larger spatial scales is desirable. Here we use sea surface height data from satellites to estimate the interannual variability of transbasin ocean transports at 26°N. Combining this estimate with surface Ekman transport and cable measurements of the Florida Current, we construct a time series of the MOC from 1993 to 2014. This satellite‐based estimate recovers over 90% of the interannual variability of the MOC measured by the RAPID 26°N array. This analysis complements in situ observational efforts to measure the MOC at multiple latitudes and opens the door to a broader spatial understanding of the Atlantic circulation variability

Technicalities: Exploring the Labrador sea with autonomous vehicles

The Labrador Sea is a fascinating and difficult environment in which to work. In the winter, wind speeds can gust upwards of 200 km/hr, while 10-m wave heights and below freezing temperatures (-20°C) are not unheard off, making it an inhospitable area for field work. Indeed, few ships are present in the Labrador Sea during the winter. However, the same harsh conditions have made the Labrador Sea a key region for Earth’s climate, with the wintertime conditions resulting in localized deep mixing of waters and carbon to great depths (2 km) in the ocean [Lazier, 1980; Pickart, 1997]. As a consequence, in-situ observations in the Labrador Sea are critical to advancing scientific knowledge on past and future climate change scenarios. Previous attempts to use ships for wintertime work required long expeditions at sea, but often with little data collected due to unworkable conditions. Autonomous marine vehicles provide an obvious solution to collecting in-situ data in the wintertime, as they can operate in extreme conditions yet still give us the flexibility to adapt our sampling during the mission [deYoung et al., 2018; Testor et al., 2019]

Observed basin-scale response of the North Atlantic Meridional Overturning Circulation to wind stress forcing

The response of the North Atlantic Meridional Overturning Circulation (MOC) to wind stress forcing is investigated from an observational standpoint, using four time series of overturning transports below and relative to 1000 m, overlapping by 3.6 years. These time series are derived from four mooring arrays located on the western boundary of the North Atlantic: the RAPID WAVE array (42.5°N), the Woods Hole Oceanographic Institution Line W array (39°N), the RAPID MOC/MOCHA array (26.5°N), and the MOVE array (16°N). Using modal decompositions of the analytic cross-correlation between transports and wind stress, the basin-scale wind stress is shown to significantly drives the MOC coherently at four latitudes, on the timescales available for this study. The dominant mode of covariance is interpreted as rapid barotropic oceanic adjustments to wind stress forcing, eventually forming two counter-rotating Ekman overturning cells centered on the tropics and subtropical gyre. A second mode of covariance appears related to patterns of wind stress and wind stress curl associated with the North Atlantic Oscillation, spinning anomalous horizontal circulations which likely interact with topography to form overturning cells

Generation of Internal Waves by Eddies Impinging on the Western Boundary of the North Atlantic

This is the final version. Available from the American Meteorological Society via the DOI in this recordDespite the major role played by mesoscale eddies in redistributing the energy of the large-scale circulation, our understanding of their dissipation is still incomplete. This study investigates the generation of internal waves by decaying eddies in the North Atlantic western boundary. The eddy presence and decay are measured from the altimetric surface relative vorticity associated with an array of full-depth current meters extending ~100 km offshore at 26.5°N. In addition, internal waves are analyzed over a topographic rise from 2-yr high-frequency measurements of an acoustic Doppler current profiler (ADCP), which is located 13 km offshore in 600-m deep water. Despite an apparent polarity independence of the eddy decay observed from altimetric data, the flow in the deepest 100 m is enhanced for anticyclones (25.2 cm s−1) compared with cyclones (−4.7 cm s−1). Accordingly, the internal wave field is sensitive to this polarity-dependent deep velocity. This is apparent from the eddy-modulated enhanced dissipation rate, which is obtained from a finescale parameterization and exceeds 10−9 W kg−1 for near-bottom flows greater than 8 cm s−1. The present study underlines the importance of oceanic western boundaries for removing the energy of low-mode westward-propagating eddies to higher-mode internal waves.The RAPID-WATCH MOC monitoring project is funded by the U.K. Natural Environment Research Council, the U.S. National Science Foundation, and the U.S. National Oceanic and Atmospheric Administration. L. Clément was supported by NERC Grant NE/I528626/1. The participation of K. L. Sheen and J. A. Brearley in this study was supported by NERC Grants NE/E007058/1 and NE/E005667/1. A.C.N.G. acknowledges the support of a Philip Leverhulme Prize