RRS James Cook Cruise JC192, 9-28 March 2020 RAPID cruise report for Cruise JC192

The purpose of RRS James Cook cruise JC192 was to refurbish the RAPID 26°N array of moorings that span the Atlantic from the Bahamas to the Canary Islands. The cruise started in Santa Cruz de Tenerife on Monday 9th March 2020 and ended on Saturday 28th November at Southampton, UK. The moorings are part of a purposeful Atlantic wide array that observes the Atlantic Meridional Overturning Circulation and the associated heat and freshwater transports. The RAPID-MOCHA-WBTS array is a joint UK- US programme. During JC192 moorings were serviced at sites: EBH4, EBH4L, EBH3, EBH2, EBH1, EBH1L, EBHi, EB1, EB1L. Sites with suffix ‘L’ denote landers fitted with bottom pressure recorders. Two Pressure Inverted Echo Sounder (PIES) landers were deployed for NOAA AOML. Moorings were equipped with instruments to measure temperature, conductivity and pressure, and a number of moorings were also equipped with current meters and/or oxygen sensors. The ABC Fluxes project extends the measurements on the RAPID 26°N array to include biological and chemical measurements. CTD stations were conducted throughout the cruise for purposes of providing pre- and post- deployment calibrations for mooring instrumentation (including oxygen and carbonate chemistry sampling) and for testing mooring releases prior to deployment. Shipboard underway measurements were systematically logged, processed and calibrated, including: surface meteorology, 5m depth sea temperatures and salinities, water depth, and navigation. Water velocity profiles from 15 m to approximately 800 m depth were obtained using two vessel mounted Acoustic Doppler Current Profilers (one 75 kHz and one 150 kHz)

RRS Discovery Cruise DY129, 8 December 2020 - 26 January 2021, RAPID cruise report for Cruise DY129

The purpose of RRS Discovery cruise DY129 was to refurbish the RAPID 26°N array of moorings that span the Atlantic from the Bahamas to the Canary Islands. The cruise started in Southampton on Tuesday 8th December 2020 and ended on Tuesday 26th January 2021 at Southampton, UK. The moorings are part of a purposeful Atlantic wide array that observes the Atlantic Meridional Overturning Circulation and the associated heat and freshwater transports. The RAPID-MOCHA-WBTS array is a joint UK- US programme. During DY129 moorings were removed at sites: MAR3, MAR3L, NOG, MAR1, MAR0 and WB6. Moorings were serviced at sites: WB4, WB4L, WBH2, WB2, WB2L, WB1, WBADCP and WBAL. Sites with suffix ‘L’ denote landers fitted with bottom pressure recorders. Moorings were equipped with instruments to measure temperature, conductivity and pressure, and a number of moorings were also equipped with current meters and/or oxygen sensors. Chemical sensors used in the ABC fluxes project were recovered and not redeployed. The RAPID telemetry system deployed adjacent to mooring WB2 was recovered. CTD stations were conducted throughout the cruise for purposes of providing pre- and post- deployment calibrations for mooring instrumentation (including oxygen and carbonate chemistry sampling). Shipboard underway measurements were systematically logged, processed and calibrated, including: surface meteorology, 5m depth sea temperatures and salinities, water depth, and navigation. Water velocity profiles from 15 m to approximately 800 m depth were obtained using two vessel mounted Acoustic Doppler Current Profilers (one 75 kHz and one 150 kHz)

CLASS and OSNAP report for JC238

The aim of this expedition was to recover and redeploy all moorings in the OSNAP array and to obtain a CTD section of hydrographic, nutrients, dissolved oxygen and dissolved organic carbon observations along the OSNAP section. The specific measurement objectives of the cruise were: 1. Recover and redeploy 4 existing mooring along the Ellett Array (EB1, WB1, WB2, RH ADCP) and 3 existing moorings in the Iceland Basin (IB5, IB4, IB3) 2. Deploy new drift-free pressure recorder (Sonardyne Fetch AZA) at EB1. 3. Conduct CTD stations and capture water samples for oxygen, total carbon, alkalinity and nutrients analysis. Data from the CTD stations was provided in near-real time (<12 hours) to the UK Met. Office to be assimilated into their short-range ocean forecast models: global 0.25 degree, North Atlantic 1/12th degree and AMM15 (European NW Shelf, ~ 1.5 km). 4. Recover and redeploy a sediment trap mooring on the Darwin Sea Mounds (DMLTM) as part of the UK Marine Protected Area habitat monitoring programme. 5. Deploy three BGC-Argo floats to maintain the UK contribution to the global ARGO network. 6. Using ship-based instrumentation to measure underway meteorology, sea-surface temperature and salinity, ocean currents from the surface to ~400m depth and waterdepth using a Kongsberg multibeam echo sounding system

Synthesis and evaluation of historical meridional heat transport from midlatitudes towards the Arctic

Meridional Energy Transport (MET), both in the atmosphere (AMET) and ocean (OMET), has significant impact on the climate in the Arctic. In this study, we quantify AMET and OMET at subpolar latitudes from six reanalyses datasets. We investigate the differences between the datasets and we check the coherence between MET and the Arctic climate variability from annual to interannual scales. The results indicate that, although the mean transport in all datasets agree well, the spatial distribution and temporal variations of AMET and OMET differ substantially among the reanalysis datasets. For the ocean, only after 2010 the low-frequency signals for all reanalyses products agree well. A further comparison with observed heat transports at 26.5° N and the subpolar Atlantic, and a high-resolution ocean model hindcast confirm that the OMET estimated from reanalyses are consistent with independent observations. For the atmosphere, the variations among reanalyses datasets are large. This can be attributed to differences in temperature transport. A further analysis of linkages between the Arctic climate variability and AMET shows that atmospheric reanalyses differ substantially from each other. Among all the chosen atmospheric products, ERA-Interim results are most consistent with results obtained with coupled climate models. For the ocean, ORAS4 and SODA3 agree well on the relation between OMET and sea ice concentration (SIC), while GLORYS2V3 deviates from those data sets. Our study suggests, since the reanalyses products are not designed for the quantification of energy transport, the AMET and OMET estimated from reanalyses should be used with great care, especially when studying variability and interactions between the Arctic and midlatitudes beyond annual time scales

Large-scale forcing of the European Slope Current and associated inflows to the North Sea

Drifters drogued at 50?m in the European Slope Current at the Hebridean shelf break follow a wide range of pathways, indicating highly variable Atlantic inflow to the North Sea. Slope Current pathways, timescales and transports over 1988–2007 are further quantified in an eddy-resolving ocean model hindcast. Particle trajectories calculated with model currents indicate that Slope Current water is largely ''recruited'' from the eastern subpolar North Atlantic. Observations of absolute dynamic topography and climatological density support theoretical expectations that Slope Current transport is to first order associated with meridional density gradients in the eastern subpolar gyre, which support a geostrophic inflow towards the slope. In the model hindcast, Slope Current transport variability is dominated by abrupt 25–50?% reductions of these density gradients over 1996–1998. Concurrent changes in wind forcing, expressed in terms of density gradients, act in the same sense to reduce Slope Current transport. This indicates that coordinated regional changes of buoyancy and wind forcing acted together to reduce Slope Current transport during the 1990s. Particle trajectories further show that 10–40?% of Slope Current water is destined for the northern North Sea within 6 months of passing to the west of Scotland, with a clear decline in this Atlantic inflow over 1988–2007. The influence of variable Slope Current transport on the northern North Sea is also expressed in salinity, which declines through the hindcast period, and there is evidence for a similar freshening trend in observational records. A proxy for Atlantic inflow may be found in sea level records. Variability of Slope Current transport is implicit in mean sea level differences between Lerwick (Shetland) and Torshavn (Faeroes), in both tide gauge records and a longer model hindcast spanning 1958–2013. Potential impacts of this variability on North Sea biogeochemistry and ecosystems, via associated changes in seasonal stratification and nutrient fluxes, are discussed

Large-scale forcing of the European Slope Current and associated inflows to the North Sea

Drifters drogued at 50?m in the European Slope Current at the Hebridean shelf break follow a wide range of pathways, indicating highly variable Atlantic inflow to the North Sea. Slope Current pathways, timescales and transports over 1988–2007 are further quantified in an eddy-resolving ocean model hindcast. Particle trajectories calculated with model currents indicate that Slope Current water is largely ''recruited'' from the eastern subpolar North Atlantic. Observations of absolute dynamic topography and climatological density support theoretical expectations that Slope Current transport is to first order associated with meridional density gradients in the eastern subpolar gyre, which support a geostrophic inflow towards the slope. In the model hindcast, Slope Current transport variability is dominated by abrupt 25–50?% reductions of these density gradients over 1996–1998. Concurrent changes in wind forcing, expressed in terms of density gradients, act in the same sense to reduce Slope Current transport. This indicates that coordinated regional changes of buoyancy and wind forcing acted together to reduce Slope Current transport during the 1990s. Particle trajectories further show that 10–40?% of Slope Current water is destined for the northern North Sea within 6 months of passing to the west of Scotland, with a clear decline in this Atlantic inflow over 1988–2007. The influence of variable Slope Current transport on the northern North Sea is also expressed in salinity, which declines through the hindcast period, and there is evidence for a similar freshening trend in observational records. A proxy for Atlantic inflow may be found in sea level records. Variability of Slope Current transport is implicit in mean sea level differences between Lerwick (Shetland) and Torshavn (Faeroes), in both tide gauge records and a longer model hindcast spanning 1958–2013. Potential impacts of this variability on North Sea biogeochemistry and ecosystems, via associated changes in seasonal stratification and nutrient fluxes, are discussed

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

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 clearer view of Southern Ocean air–sea interaction using surface heat flux asymmetry

Progress in understanding Southern Ocean heat exchange and wind forcing is discussed and new results presented. These include a metric of the zonal asymmetry between surface ocean heat gain in the Atlantic/Indian sector and heat loss in the Pacific sector. The asymmetry arises from an intersector variation in the humidity gradient between the sea surface and near-surface atmosphere. This gradient increases by 60% in the Pacific sector enabling a 20 Wm−2 stronger latent heat loss compared with the Atlantic/Indian sector. The new metric is used for intercomparison of atmospheric reanalyses and CMIP6 climate simulations. CMIP6 has weaker Atlantic/Indian sector heat gain compared with the reanalyses primarily due to Indian Ocean sector differences. The potential for surface flux buoys to provide an observation-based counterpart to the asymmetry metric is explored. Over the past decade, flux buoys have been deployed at two sites (south of Tasmania and upstream of Drake Passage). The data record provided by these moorings is assessed and an argument developed for a third buoy to sample the Atlantic/Indian sector of the asymmetry metric. To close, we assess evidence that the main westerly wind belt has strengthened and moved southward in recent decades using the ERA5 reanalysis