The Northwest Tropical Atlantic Station (NTAS) : NTAS-17 mooring turnaround cruise report cruise on board FV Pisces May 30 – June 21, 2018 Mayport, FL, USA – Morehead City, NC, USA

The Northwest Tropical Atlantic Station (NTAS) was established to address the need for accurate air-sea flux estimates and upper ocean measurements in a region with strong sea surface temperature anomalies and the likelihood of significant local air–sea interaction on interannual to decadal timescales. The approach is to maintain a surface mooring outfitted for meteorological and oceanographic measurements at a site near 15N, 51W by successive mooring turnarounds. These observations are used to investigate air–sea interaction processes related to climate variability. The NTAS Ocean Reference Station (ORS NTAS) is supported by the National Oceanic and Atmospheric Administration’s (NOAA) Ocean Observing and Monitoring Division. This report documents recovery of the NTAS-16 mooring and deployment of the NTAS-17 mooring at the same site. Both moorings used Surlyn foam buoys as the surface element. These buoys were outfitted with two Air–Sea Interaction Meteorology (ASIMET) systems. Each system measures, records, and transmits via Argos satellite the surface meteorological variables necessary to compute air–sea fluxes of heat, moisture and momentum. The upper 160 m of the mooring line were outfitted with oceanographic sensors for the measurement of temperature, salinity and velocity. The mooring turnaround was done by the Upper Ocean Processes Group of the Woods Hole Oceanographic Institution (WHOI), onboard F/V Pisces, Cruise PC-18-03. The cruise took place between May 30 and June 21 2018. The NTAS-17 mooring was deployed on June 10, and the NTAS-16 mooring was recovered on June 12. No inter-comparison between ship and buoys was performed on this cruise. This report describes these operations, as well as other work done on the cruise and some of the pre-cruise buoy preparations. Other operations during PC-18-03 consisted in the recovery and deployment of the Meridional Overturning Variability Experiment (MOVE) subsurface moorings array (MOVE 1 in the east, and MOVE 3 and 4 in the west near Guadeloupe). Acoustic download of data from Pressure Inverted Echo Sounders (PIES) was also conducted. MOVE is designed to monitor the integrated deep meridional flow in the tropical North Atlantic.Funding was provided by the National Oceanic and Atmospheric Administration under Grant No. NA14OAR432015

Intense oceanic uptake of oxygen during 2014-2015 winter convection in the Labrador Sea

Measurements of near-surface oxygen (O2) concentrations and mixed layer depth from the K1 mooring in the central Labrador Sea are used to calculate the change in column-integrated (0–1700 m) O2 content over the deep convection winter 2014/2015. During the mixed layer deepening period, November 2014 to April 2015, the oxygen content increased by 24.3 ± 3.4 mol m−2, 40% higher than previous results from winters with weaker convection. By estimating the contribution of respiration and lateral transport on the oxygen budget, the cumulative air-sea gas exchange is derived. The O2 uptake of 29.1 ± 3.8 mol m−2, driven by persistent undersaturation (≥5%) and strong atmospheric forcing, is substantially higher than predicted by standard (nonbubble) gas exchange parameterizations, whereas most bubble-resolving parameterizations predict higher uptake, comparable to our results. Generally large but varying mixed layer depths and strong heat and momentum fluxes make the Labrador Sea an ideal test bed for process studies aimed at improving gas exchange parameterizations

Atlantic Meridional Overturning Circulation: Observed Transport and Variability

The Atlantic Meridional Overturning Circulation (AMOC) extends from the Southern Ocean to the northern North Atlantic, transporting heat northwards throughout the South and North Atlantic, and sinking carbon and nutrients into the deep ocean. Climate models indicate that changes to the AMOC both herald and drive climate shifts. Intensive trans-basin AMOC observational systems have been put in place to continuously monitor meridional volume transport variability, and in some cases, heat, freshwater and carbon transport. These observational programs have been used to diagnose the magnitude and origins of transport variability, and to investigate impacts of variability on essential climate variables such as sea surface temperature, ocean heat content and coastal sea level. AMOC observing approaches vary between the different systems, ranging from trans-basin arrays (OSNAP, RAPID 26°N, 11°S, SAMBA 34.5°S) to arrays concentrating on western boundaries (e.g., RAPID WAVE, MOVE 16°N). In this paper, we outline the different approaches (aims, strengths and limitations) and summarize the key results to date. We also discuss alternate approaches for capturing AMOC variability including direct estimates (e.g., using sea level, bottom pressure, and hydrography from autonomous profiling floats), indirect estimates applying budgetary approaches, state estimates or ocean reanalyses, and proxies. Based on the existing observations and their results, and the potential of new observational and formal synthesis approaches, we make suggestions as to how to evaluate a comprehensive, future-proof observational network of the AMOC to deepen our understanding of the AMOC and its role in global climate

Variability in formation, properties, and transport of North Atlantic Deep Water

North Atlantic Deep Water is found in much of the deep Atlantic Ocean, and its formationin the Labrador and Nordic Seas and subsequent southward export are a vital part of globalocean circulation and Earth’s climate system. The overarching goal of this dissertation is tobetter understand the processes controlling variability of North Atlantic Deep Water formation,properties, and transport in the Atlantic Ocean.Chapter 1 uses data from the central Labrador Sea during winter to estimate the uptake of oxygenassociated with deep convection in 2014–15. The results show that intense air-sea exchangeresults in an uptake of 29.1 ± 3.8 mol m^−2 during the convective season, with much of the fluxbeing associated with injection of air bubbles. Chapter 2 looks at lateral fluxes of carbon, oxygen,and nitrate from the Labrador Sea’s boundary current into the center of the basin during thesummertime productive season. Lateral fluxes are found to play an important role for the carbonand nitrate budgets immediately below the mixed layer, with respiration rates underestimated byup to 50% if they are ignored.In chapter 3, gravity measurements from satellites are used to investigate variability in oceancirculation. After trends in the data are validated using independent measurements, they are usedto study decadal circulation changes of North Atlantic Deep Water in the North Atlantic Ocean.The analysis reveals a strengthening of the interior branch of North Atlantic Deep Water flow,with transport increasing by 13.9 ± 3.7 Sv (1 Sv = 10^6 m^3 s^−1 ), balanced by a weaker southwardflow in the Deep Western Boundary Current.A twenty-year record of mooring data is analyzed in chapter 4 to investigate changes in NorthAtlantic Deep Water transport at 16 ◦ N. Multi-decadal variability is observed in the transport timeseries, and is largely associated with density changes in the lower half of the North Atlantic DeepWater layer, which in turn appear to be caused by changes in the source region. The data are alsocompared to another transport time series at 26 ◦ N, and similarities and differences are discussed

Variability in formation, properties, and transport of North Atlantic Deep Water

North Atlantic Deep Water is found in much of the deep Atlantic Ocean, and its formationin the Labrador and Nordic Seas and subsequent southward export are a vital part of globalocean circulation and Earth’s climate system. The overarching goal of this dissertation is tobetter understand the processes controlling variability of North Atlantic Deep Water formation,properties, and transport in the Atlantic Ocean.Chapter 1 uses data from the central Labrador Sea during winter to estimate the uptake of oxygenassociated with deep convection in 2014–15. The results show that intense air-sea exchangeresults in an uptake of 29.1 ± 3.8 mol m^−2 during the convective season, with much of the fluxbeing associated with injection of air bubbles. Chapter 2 looks at lateral fluxes of carbon, oxygen,and nitrate from the Labrador Sea’s boundary current into the center of the basin during thesummertime productive season. Lateral fluxes are found to play an important role for the carbonand nitrate budgets immediately below the mixed layer, with respiration rates underestimated byup to 50% if they are ignored.In chapter 3, gravity measurements from satellites are used to investigate variability in oceancirculation. After trends in the data are validated using independent measurements, they are usedto study decadal circulation changes of North Atlantic Deep Water in the North Atlantic Ocean.The analysis reveals a strengthening of the interior branch of North Atlantic Deep Water flow,with transport increasing by 13.9 ± 3.7 Sv (1 Sv = 10^6 m^3 s^−1 ), balanced by a weaker southwardflow in the Deep Western Boundary Current.A twenty-year record of mooring data is analyzed in chapter 4 to investigate changes in NorthAtlantic Deep Water transport at 16 ◦ N. Multi-decadal variability is observed in the transport timeseries, and is largely associated with density changes in the lower half of the North Atlantic DeepWater layer, which in turn appear to be caused by changes in the source region. The data are alsocompared to another transport time series at 26 ◦ N, and similarities and differences are discussed

Decadal Strengthening of Interior Flow of North Atlantic Deep Water Observed by GRACE Satellites

The Gravity Recovery and Climate Experiment (GRACE) satellite mission provides information on changes to the Earth’s gravity field, including ocean mass. Long-term trends in the GRACE data are often considered unreliable due to uncertainties in the corrections made to calculate ocean mass from the raw measurements. Here, we use an independent estimate of ocean mass from satellite altimetry and in situ density data from five mooring sites and repeat hydrography to validate trends in GRACE over the North Atlantic, finding substantial agreement between the methods. The root mean square difference between ocean mass changes calculated with this method from the mooring data and those measured by GRACE is 3.5 mm/decade, much lower than the mean signal of 15.6±1.8 mm/decade for GRACE and 17.8±5.2 mm/decade for the altimetry-mooring estimate. The GRACE ocean mass data are then used to study the change in the deep circulation of the North Atlantic between the 2002/04/01-2009/03/31 and 2010/04/01-2017/03/31 periods, revealing a large-scale anticyclonic circulation anomaly off the North American coast. The change is associated with an increase of 13.9 ± 3.3 Sv (1Sv = 106 m3 s −1) of southward North Atlantic Deep Water flow in the interior between 30◦N and 40◦N, largely balanced by a northward anomaly of 10.7±3.3 Sv for the boundary circulation.  This implies an increased importance of interior pathways compared to the Deep Western Boundary Current for the spreading of North Atlantic Deep Water, which constitutes the lower limb of the Atlantic Meridional Overturning Circulation

Decadal variability of oxygen uptake, export, and storage in the Labrador Sea from observations and CMIP6 models

The uptake of dissolved oxygen from the atmosphere via air-sea gas exchange and its physical transport away from the region of uptake are crucial for supplying oxygen to the deep ocean. This process takes place in a few key regions that feature intense oxygen uptake, deep water formation, and physical oxygen export. In this study we analyze one such region, the Labrador Sea, utilizing the World Ocean Database (WOD) to construct a 65–year oxygen content time series in the Labrador Sea Water (LSW) layer (0–2200 m). The data reveal decadal variability associated with the strength of deep convection, with a maximum anomaly of 27 mol m–2 in 1992. There is no long-term trend in the time series, suggesting that the mean oxygen uptake is balanced by oxygen export out of the region. We compared the time series with output from nine models of the Ocean Model Intercomparison Project phase 1 in the Climate Model Intercomparison Project phase 6, (CMIP6-OMIP1), and constructed a “model score” to evaluate how well they match oxygen observations. Most CMIP6-OMIP1 models score around 50/100 points and the highest score is 57/100 for the ensemble mean, suggesting that improvements are needed. All of the models underestimate the maximum oxygen content anomaly in the 1990s. One possible cause for this is the representation of air-sea gas exchange for oxygen, with all models underestimating the mean uptake by a factor of two or more. Unrealistically deep convection and biased mean oxygen profiles may also contribute to the mismatch. Refining the representation of these processes in climate models could be vital for enhanced predictions of deoxygenation. In the CMIP6-OMIP1 multi-model mean, oxygen uptake has its maximum in 1980–1992, followed by a decrease in 1994–2006. There is a concurrent decrease in export, but oxygen storage also changes between the two periods, with oxygen accumulated in the first period and drained out in the second. Consequently, the change in oxygen export (5%) is much less than that in uptake (28%), suggesting that newly ventilated LSW which remains in the formation region acts to buffer the linkage between air-sea gas exchange and oxygen export

Oxygen export to the deep ocean following Labrador Sea Water formation

The Labrador Sea in the North Atlantic Ocean is one of the few regions globally where oxygen from the atmosphere can reach the deep ocean directly. This is the result of wintertime deep convection, which homogenizes the water column to a depth of up to 2000 m and brings deep water undersaturated in oxygen into contact with the atmosphere. In this study, we analyze how the intense oxygen uptake during Labrador Sea Water (LSW) formation affects the properties of the outflowing deep western boundary current, which ultimately feeds the upper part of the North Atlantic Deep Water layer in much of the Atlantic Ocean. Seasonal cycles of oxygen concentration, temperature, and salinity from a 2-year time series collected by sensors moored at 600 m nominal depth in the outflowing boundary current at 53∘ N show a cooling, freshening, and increase in oxygen content of the water flowing out of the basin between March and August. Analysis of Argo float data suggests that this is preceded by an increased input of LSW into the boundary current about 1 month earlier. This input is the result of newly ventilated LSW entering from the interior, as well as LSW formed directly within the boundary current. Together, these results imply that the southward export of newly formed LSW primarily occurs in the months following the onset of deep convection, from March to August, and that this direct LSW export route controls the seasonal oxygen increase in the outflow at 600 m depth. During the rest of the year, properties of the boundary current measured at 53∘ N resemble those of Irminger Water, which enters the basin with the boundary current from the Irminger Sea. The input of newly ventilated LSW increases the oxygen concentration from 298 µmol L−1 in January to a maximum of 306 µmol L−1 in April. As a result of this LSW input, an estimated (1.60 ± 0.42) × 1012 mol yr−1 of oxygen are added to the outflowing boundary current, mostly during spring and summer, equivalent to 50 % of the wintertime uptake from the atmosphere in the interior of the basin. The export of oxygen from the subpolar gyre associated with this direct southward pathway of LSW is estimated to supply 42 %–71 % of the oxygen consumed annually in the upper North Atlantic Deep Water layer in the Atlantic Ocean between the Equator and 50∘ N. Our results show that the formation of LSW is important for replenishing oxygen to the deep oceans, meaning that possible changes in its formation rate and ventilation due to climate change could have wide-reaching impacts on marine life

Can Empirical Algorithms Successfully Estimate Aragonite Saturation State in the Subpolar North Atlantic?

17 pages, 4 tables, 8 figuresThe aragonite saturation state (ΩAr) in the subpolar North Atlantic was derived using new regional empirical algorithms. These multiple regression algorithms were developed using the bin-averaged GLODAPv2 data of commonly observed oceanographic variables [temperature (T), salinity (S), pressure (P), oxygen (O2), nitrate (NO−3 ), phosphate (PO3−4 ), silicate (Si(OH)4), and pH]. Five of these variables are also frequently observed using autonomous platforms, which means they are widely available. The algorithms were validated against independent shipboard data from the OVIDE2012 cruise. It was also applied to time series observations of T, S, P, and O2 from the K1 mooring (56.5°N, 52.6°W) to reconstruct for the first time the seasonal variability of ΩAr. Our study suggests: (i) linear regression algorithms based on bin-averaged carbonate system data can successfully estimate ΩAr in our study domain over the 0–3,500 m depth range (R2 = 0.985, RMSE = 0.044); (ii) that ΩAr also can be adequately estimated from solely non-carbonate observations (R2 = 0.969, RMSE = 0.063) and autonomous sensor variables (R2 = 0.978, RMSE = 0.053). Validation with independent OVIDE2012 data further suggests that; (iii) both algorithms, non-carbonate (MEF = 0.929) and autonomous sensors (MEF = 0.995) have excellent predictive skill over the 0–3,500 depth range; (iv) that in deep waters (>500 m) observations of T, S, and O2 may be sufficient predictors of ΩAr (MEF = 0.913); and (iv) the importance of adding pH sensors on autonomous platforms in the euphotic and remineralization zone (<500 m). Reconstructed ΩAr at Irminger Sea site, and the K1 mooring in Labrador Sea show high seasonal variability at the surface due to biological drawdown of inorganic carbon during the summer, and fairly uniform ΩAr values in the water column during winter convection. Application to time series sites shows the potential for regionally tuned algorithms, but they need to be further compared against ΩAr calculated by conventional means to fully assess their validity and performance.MD was supported by an NSERC Discovery Grant, and SKL was supported by the EU H2020 research and innovation project AtlantOS (grant agreement no 633211). The OVIDE-12 cruise and the contribution of FA-P and FP were supported by the Spanish Ministry of Economy and Competitiveness through the project CTM2013-41048-P co-funded by the Fondo Europeo de Desarrollo Regional 2014-2020 (FEDER). K1 mooring is an Oceansites site supported by the EU Horizon 2020 research and innovation program AtlantOS (grant agreement no 633211). LDEO contribution no 8169.Peer reviewe