RRS James Cook Cruise JC191 19 January - 1 March 2020 Hydrographic sections from the Florida Straits to the Canaries Current across 24ºN in the Atlantic Ocean

A hydrographic section across the North Atlantic Ocean at a nominal latitude of 24°N was occupied by the RRS James Cook (cruise identifier: JC191) from 19 January to 1 March, 2020. The ship departed from Port Everglades, USA, completing a total of 135 CTD stations over the Florida Straits, the western basin, Mid-Atlantic Ridge, eastern basin and eastern boundary up to Morocco, before ending the cruise in Santa Cruz de Tenerife, Spain. The main objectives of the JC191 research expedition was to collect/measure physical-, chemical-, and biological-ocean data with the purpose of estimating heat, freshwater and carbon budgets on low frequency time scales. All CTD stations had measurements from a CTD rosette equipped with temperature, conductivity, pressure, oxygen sensors, in addition to water captured from 24 niskin bottles fired at varying intervals throughout the full depth water column. The water from the niskin bottles was analysed for dissolved oxygen, carbon (DIC/TA), nutrients, and conductivity. Water for methane (CH4), C14, C13, and pigments (filtered) was collected for onshore analysis. The CTD rosette was also equipped with 2 RBR loggers measuring conductivity, temperature and pressure (up to 6,000m), and a lowered Acoustic Doppler Current Profiler (LADCP) making full depth velocity measurements. The 135 CTD stations include 2 carbon blank stations, and 2 bulk water stations for incubations. In addition to the CTD stations, the RRS James Cook has an underway system, which includes an intake for surface water to be pumped into the water bottle annex and the deck lab; two vessel mounted ADCPs (VMADCPs). A thermosalinograph and a fluorometer, installed in the water bottle annex, continually recorded conductivity, temperature and fluorescence. Water from the CTD was collected to calibrate the ship’s underway TSG. The VMADCPs, 75Hz and 150HZ, mounted on the drop keel record ocean velocities in roughly the top 300- and 600-m, respectively. Surface carbon and methane measurements were also recorded from the underway systems, and surface meteorological variables were monitored via the meteorological sampling system and the pumped water underway system. Finally bathymetric data were recorded an EA640 echosounder and a Kongsberg EM122 multibeam, both of which are mounted on the ship’s hull. Last, 5 Deep Apex Argo floats measuring conductivity, temperature, pressure and oxygen (except for one float not equipped with an optode) were deployed in the western basin. Many of the science party also engaged in extensive outreach via blogs and social media, heightening visibility of the science teams activities to the oceanographic community and the general public. This report summarises the data collected and analysed, and the methodology used for the acquisition and processing of the data onboard the James Cook during the JC191 research expedition

The Icelandic Low as a predictor of the Gulf Stream north wall position

The Gulf Stream’s Northwall east of Cape Hatteras marks the abrupt change in velocity and water properties between Slope Sea to the north and Gulf Stream itself. An index of the Northwall position constructed by Taylor and Stephens (1998), called GSNW, is analyzed in terms of interannual changes in the Icelandic Low (IL) pressure anomaly and longitudinal displacement. Sea Surface Temperature (SST) composites suggest that when IL pressure is anomalously low, there are lower temperatures in the Labrador Sea and south of the Grand Banks. Two years later, warm SST anomalies are seen over the Northern Recirculation Gyre and a northward shift in the GSNW occurs. Similar changes in SSTs occur during winters in which the IL is anomalously west resulting in a northward displacement of the GSNW 3 years later. Although time lags of 2 and 3 years between the IL and the GSNW are used in the calculations, it is shown that lags with respect to each atmospheric variable are statistically significant at 5% level over a range of years. Utilizing the appropriate time lags between the GSNW index and the IL pressure and longitude, as well as the Southern Oscillation index, a regression-prediction scheme is developed for forecasting the GSNW with a lead-time of 1 year. This scheme, which uses only prior information, was used to forecast the GSNW from 1994 to 2015. The correlation between the observed and forecasted values for 1994-2014 was 0.60, significant at the 1% level. The predicted value for 2015 indicates a small northward shift of GSNW from its 2014 position

Secular change and inter-annual variability of the Gulf Stream position, 1993–2013, 70°−55°W

The Gulf Stream (GS) is the northeastward-flowing surface limb of the Atlantic Ocean's meridional overturning circulation (AMOC) “conveyer belt” that flows towards Europe and the Nordic Seas. Changes in the GS position after its separation from the coast at Cape Hatteras, i.e., from 75°W to 50°W, may be key to understanding the AMOC, sea level variability and ecosystem behavior along the east coast of North America. In this study we compare secular change and inter-annual variability (IAV) of the Gulf Stream North Wall (GSNW) position with equator-ward Labrador Current (LC) transport along the southwestern Grand Banks near 52°W using 21 years (1993–2013) of satellite altimeter data. Results at 55°, 60°, and 65°W show a significant southward (negative) secular trend for the GSNW, decreasing to a small but insignificant southward trend at 70°W. IAV of de-trended GSNW position residuals also decreases to the west. The long-term secular trend of annual mean upper layer (200 m) LC transport near 52°W is positive. Furthermore, IAV of LC transport residuals near 52°W along the southwestern Grand Banks are significantly correlated with GSNW position residuals at 55°W at a lag of +1-year, with positive (negative) LC transport residuals corresponding to southward (northward) GSNW positions one year later. The Taylor-Stephens index (TSI) computed from the first principal component of the GSNW position from 79° to 65°W shows a similar relationship with a more distal LC index computed along altimeter ground track 250 located north of the Grand Banks across Hamilton Bank in the western Labrador Sea. Increased (decreased) sea height differences along ground track 250 are significantly correlated with a more southward (northward) TSI two years later (lag of +2-years). Spectral analysis of IAV reveals corresponding spectral peaks at 5–7 years and 2–3 years for the North Atlantic Oscillation (NAO), GSNW (70°−55°W) and LC transport near 52°W for the 1993–2013 period suggesting a connection between these phenomena. An upper-layer (200 m) slope water volume calculation using the LC IAV rms residual of +1.04 Sv near 52°W results in an estimated GSNW IAV residual of 79 km, or 63% of the observed 125.6 km (1.13°) rms value at 55°W. A similar upper-layer slope water volume calculation using the positive long-term, upper-layer LC transport trend accounts for 68% of the mean observed secular southward shift of the GSNW between 55° and 70°W over the 1993–2013 period. Our work provides additional observational evidence of important interactions between the upper layers of the sub-polar and sub-tropical gyres within the North Atlantic over both secular and inter-annual time scales as suggested by previous studies

RRS James Cook Cruise JC159 28 February - 11 April 2018. Hydrographic sections from the Brazil to the Benguela Current across 24S in the Atlantic

A Hydrographic section was occupied at a nominal latitude of 24°S in the Atlantic Ocean during March and April 2018 on Cruise JC159 of RRS James Cook. The primary objective of this cruise was to measure ocean physical, chemical and biological parameters in order to establish regional budgets of heat, freshwater and carbon, and to infer decadal variability. In addition, 371 Niskin Bottles were sampled for microplastics, reflecting increasing awareness of plastics pollution in the oceans. A total of 121 CTD/LADCP stations were conducted, including one test station and two CFC bottle blank stations. In addition to temperature, salinity and oxygen profiles from the sensors on the CTD package, water samples from a 24 x 20 litre rosette were analysed for the following parameters at all stations: salinity; dissolved oxygen; inorganic nutrients; alkalinity and dissolved inorganic carbon; CFCs. Samples were collected for shore analysis for oxygen and carbon isotopes (del-18O, del13C and del-14C). Samples were collected and filtered for pigments (shore analysis) at 44 stations and for microplastics at 45 stations. 8 Argo floats were deployed, including two Bio-PROVOR floats and 2 Deep ARVORs. In addition, samples were collected from the ships’ underway system to calibrate and compliment the data continually collected by the TSG (thermosalinograph). Full depth velocity measurements were made at every station by LADCP (Lowered Acoustic Doppler Current Profiler) mounted on the frame of the rosette. Throughout the cruise, velocity data in the upper few hundred metres of the water column were collected by the ship’s VMADCP (Vessel Mounted Acoustic Doppler Current Profiler) transducers (75Hz and 150Hz) mounted on one of the two drop keels. Meteorological variables were monitored using the onboard surface water and meteorological sampling system (SURFMET). Bathymetric data were collected using the Kongsberg EM122 multibeam system and the EA640 echo sounder. This report describes the methods used to acquire and process the data on board the ship during cruise JC159

Forecasting the Gulf Stream Path using buoyancy and wind forcing over the North Atlantic

Fluctuations in the path of the Gulf Stream (GS) have been previously studied by primarily connecting to either the wind-driven subtropical gyre circulation or buoyancy forcing via the subpolar gyre. Here we present a statistical model for 1 year predictions of the GS path (represented by the GS northern wall—GSNW) between urn:x-wiley:21699275:media:jgrc24667:jgrc24667-math-0001W and urn:x-wiley:21699275:media:jgrc24667:jgrc24667-math-0002W incorporating both mechanisms in a combined framework. An existing model with multiple parameters including the previous year's GSNW index, center location, and amplitude of the Icelandic Low and the Southern Oscillation Index was augmented with basin-wide Ekman drift over the Azores High. The addition of the wind is supported by a validation of the simpler two-layer Parsons-Veronis model of GS separation over the last 40 years. A multivariate analysis was carried out to compare 1-year-in-advance forecast correlations from four different models. The optimal predictors of the best performing model include: (a) the GSNW index from the previous year, (b) gyre-scale integrated Ekman Drift over the past 2 years, and (c) longitude of the Icelandic Low center lagged by 3 years. The forecast correlation over the 27 years (1994–2020) is 0.65, an improvement from the previous multi-parameter model's forecast correlation of 0.52. The improvement is attributed to the addition of the wind-drift component. The sensitivity of forecasting the GS path after extreme atmospheric years is quantified. Results indicate the possibility of better understanding and enhanced predictability of the dominant wind-driven variability of the Atlantic Meridional Overturning Circulation and of fisheries management models that use the GS path as a metric

Intraseasonal variability of air-sea fluxes over the Bay of Bengal during the southwest monsoon

In the Bay of Bengal (BoB), surface heat fluxes play a key role in monsoon dynamics and prediction. The accurate representation of large-scale surface fluxes is dependent on the quality of gridded reanalysis products. Meteorological and surface flux variables from five reanalysis products are compared and evaluated against in situ data from the RAMA moored array in the BoB. The reanalysis products: ERA-Interim (ERA-I), TropFlux, MERRA-2, JRA-55 and CFSR are assessed for their characterisation of air-sea fluxes during the southwest monsoon season (JJAS). ERA-I captured radiative fluxes best while TropFlux captured turbulent and net heat fluxes (Qnet) best, and both products outperformed JRA-55, MERRA-2 and CFSR, showing highest correlations and smallest biases when compared to the in situ data. In all five products, the largest errors were in shortwave radiation (QSW) and latent heat flux (QLH), with non-negligible biases up to ∼75 W m−2. The QSW and QLH are the largest drivers of the observed Qnet variability, thus highlighting the importance of the results from the buoy comparison. There are also spatially coherent differences in the mean basin-wide fields of surface flux variables from the reanalysis products, indicating that the biases at the buoy position are not localized. Biases of this magnitude have severe implications on reanalysis products ability to capture the variability of monsoon processes. Hence, the representation of intraseasonal variability was investigated through the boreal summer intraseasonal oscillation and we found that TropFlux and ERA-I perform best at capturing intraseasonal climate variability during the southwest monsoon season

A dynamically based method for estimating the Atlantic overturning circulation at 26° N from satellite altimetry

The large-scale system of ocean currents that transport warm surface (1000 m) waters northward and return cooler waters southward is known as the Atlantic meridional overturning circulation (AMOC). Variations in the AMOC have significant repercussions for the climate system, hence there is a need for long term monitoring of AMOC fluctuations. Currently the longest record of continuous directly measured AMOC changes is from the RAPID-MOCHA-WBTS programme, initiated in 2004. The RAPID programme, and other mooring programmes, have revolutionised our understanding of large-scale circulation, however, by design they are constrained to measurements at a single latitude. High global coverage of surface ocean data from satellite altimetry is available since the launch of TOPEX/Poseidon satellite in 1992 and has been shown to provide reliable estimates of surface ocean transports on interannual time scales. Here we show that a direct calculation of ocean circulation from satellite altimetry compares well with transport estimates from the 26° N RAPID array on low frequency (18-month) time scales for the upper mid-ocean transport (UMO; r = 0.75), the Gulf Stream transport through the Florida Straits (r = 0.70), and the AMOC (r = 0.83). The vertical structure of the circulation is also investigated, and it is found that the first baroclinic mode accounts for 83 % of the interior geostrophic variability, while remaining variability is explained by the barotropic mode. Finally, the UMO and the AMOC are estimated from historical altimetry data (1993 to 2018) using a dynamically based method that incorporates the vertical structure of the flow. The effective implementation of satellite-based method for monitoring the AMOC at 26° N lays down the starting point for monitoring large-scale circulation at all latitudes

A dynamically based method for estimating the Atlantic meridional overturning circulation at 26° N from satellite altimetry

The large-scale system of ocean currents that transport warm waters in the upper 1000 m northward and return deeper cooler waters southward is known as the Atlantic meridional overturning circulation (AMOC). Variations in the AMOC have significant repercussions for the climate system; hence, there is a need for long-term monitoring of AMOC fluctuations. Currently the longest record of continuous directly measured AMOC changes is from the RAPID-MOCHA-WBTS programme, initiated in 2004. The RAPID programme and other mooring programmes have revolutionised our understanding of large-scale circulation; however, by design they are constrained to measurements at a single latitude and cannot tell us anything pre-2004. Nearly global coverage of surface ocean data from satellite altimetry has been available since the launch of the TOPEX/Poseidon satellite in 1992 and has been shown to provide reliable estimates of surface ocean transports on interannual timescales including previous studies that have investigated empirical correlations between sea surface height variability and the overturning circulation. Here we show a direct calculation of ocean circulation from satellite altimetry of the upper mid-ocean transport (UMO), the Gulf Stream transport through the Florida Straits (GS), and the AMOC using a dynamically based method that combines geostrophy with a time mean of the vertical structure of the flow from the 26∘ N RAPID moorings. The satellite-based transport captures 56 %, 49 %, and 69 % of the UMO, GS, and AMOC transport variability, respectively, from the 26∘ N RAPID array on interannual (18-month) timescales. Further investigation into the vertical structure of the horizontal transport shows that the first baroclinic mode accounts for 83 % of the interior geostrophic variability, and the combined barotropic and first baroclinic mode representation of dynamic height accounts for 98 % of the variability. Finally, the methods developed here are used to reconstruct the UMO and the AMOC for the time period pre-dating RAPID, 1993 to 2003. The effective implementation of satellite-based method for monitoring the AMOC at 26∘ N lays down the starting point for monitoring large-scale circulation at all latitudes

Absence of the Great Whirl giant ocean vortex abates productivity in the Somali upwelling region

Somali upwelling is the fifth largest upwelling globally with high productivity, attracting tuna migratory species. A key control on the upwelling productivity is its interaction with one of the world’s largest oceanic eddies, the Great Whirl inducing a strong downwelling signal. Here, we use satellite-derived observations to determine the Great Whirl impact on the extent of the upwelling-driven phytoplankton bloom. We find that following decreases in upwelling intensity, productivity has declined by about 10% over the past two decades. The bloom extent has also been diminishing with an abrupt decrease around 2006–2007, coinciding with an abrupt increase in the downwelling effect. Absent or weak Great Whirl leads to the occurrence of smaller anticyclonic eddies with a resulting downwelling stronger than when the Great Whirl is present. We suggest that 2006–2007 abrupt changes in the bloom and downwelling extents’ regimes, are likely driven by Indian Ocean Dipole abrupt shift in 2006

The Dynamics of the Southwest Monsoon Current in 2016 from High-Resolution In Situ Observations and Models

The strong stratification of the Bay of Bengal (BoB) causes rapid variations in sea surface temperature (SST) that influence the development of monsoon rainfall systems. This stratification is driven by the salinity difference between the fresh surface waters of the northern bay and the supply of warm, salty water by the Southwest Monsoon Current (SMC). Despite the influence of the SMC on monsoon dynamics, observations of this current during the monsoon are sparse. Using data from high-resolution in situ measurements along an east–west section at 8°N in the southern BoB, we calculate that the northward transport during July 2016 was between 16.7 and 24.5 Sv (1 Sv ≡ 106 m3 s−1), although up to ⅔ of this transport is associated with persistent recirculating eddies, including the Sri Lanka Dome. Comparison with climatology suggests the SMC in early July was close to the average annual maximum strength. The NEMO 1/12° ocean model with data assimilation is found to faithfully represent the variability of the SMC and associated water masses. We show how the variability in SMC strength and position is driven by the complex interplay between local forcing (wind stress curl over the Sri Lanka Dome) and remote forcing (Kelvin and Rossby wave propagation). Thus, various modes of climatic variability will influence SMC strength and location on time scales from weeks to years. Idealized one-dimensional ocean model experiments show that subsurface water masses advected by the SMC significantly alter the evolution of SST and salinity, potentially impacting Indian monsoon rainfall