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The lower limb of the Atlantic meridional overturning circulation (AMOC) is the equatorward flow of dense waters formed through the cooling and freshening of the poleward-flowing upper limb. In the subpolar North Atlantic (SPNA), upper limb variability is primarily set by the North Atlantic Current, whereas lower limb variability is less well understood. Using observations from a SPNA mooring array, we show that variability of the AMOC's lower limb is connected to poleward flow in the interior Irminger Sea. We identify this poleward flow as the northward branch of the Irminger Gyre (IG), accounting for 55% of the AMOC's lower limb variability. Over 2014-2018, wind stress curl fluctuations over the Labrador and Irminger Seas drive this IG and AMOC variability. On longer (>annual) timescales, however, an increasing trend in the thickness of intermediate water, from 2014 to 2020, within the Irminger Sea coincides with a decreasing trend in IG strength.

Understanding the variability of the Atlantic Meridional Overturning Circulation is essential for better predictions of our changing climate. Here we present an updated time series (August 2014 to June 2020) from the Overturning in the Subpolar North Atlantic Program. The 6-year time series allows us to observe the seasonality of the subpolar overturning and meridional heat and freshwater transports. The overturning peaks in late spring and reaches a minimum in early winter, with a peak-to-trough range of 9.0 Sv. The overturning seasonal timing can be explained by winter transformation and the export of dense water, modulated by a seasonally varying Ekman transport. Furthermore, over 55% of the total meridional freshwater transport variability can be explained by its seasonality, largely owing to overturning dynamics. Our results provide the first observational analysis of seasonality in the subpolar North Atlantic overturning and highlight its important contribution to the total overturning variability observed to date.

The Irminger Current (IC) on the western flank of the Reykjanes Ridge is an important contributor to the northward transport in the Atlantic Meridional Overturning Circulation. Here, we combine 28 years of Copernicus Marine Environment Monitoring Service (CMEMS) ocean reanalysis data with 6 years of mooring data to investigate variability in volume transport of the IC. We found a mean volume transport for the IC of 11.6 Sv between 1993 and 2020 revealing the dominance of the IC in the total Irminger Sea northward volume transport (20.3 Sv). We found a significant decrease (−3.7 Sv) in volume transport of the IC until 2011 followed by a steeper (but shorter, leading to +2.7 Sv) increase until 2020. These changes across the Irminger Sea section are dominated by the IC, which in turn are driven by changes in the density gradient across the Irminger Sea related to convection. On decadal and interannual time scales the IC volume transport is well correlated with the decrease in the sea surface height difference over the basin. In 2019, a temporary intensification of the western IC core led to an exceptionally strong volume transport of the IC with 20 Sv. This was caused by density changes within the IC boundaries due to the presence of mesoscale eddies. Thus, IC transport variability is a superposition of basin-wide to local processes that influence the velocity field on different time scales.

Shipboard hydrography along the A25-Ovide section (2002 – 2018) is combined with a high-resolution mooring array (2014 – 2020) and a regional fleet of Deep-Argo floats (2016 – 2021) to describe temperature changes of overflow-derived water masses in the Irminger Sea. Removing dynamical influences enables to identify a new statistically-significant trend reversal in Iceland Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW) core temperatures in the mid-2010s. A basin-wide cooling trend of -16 ± 6 m°C yr-1 during 2016 – 2021 – but reaching as strong as -44 ± 13 m°C yr-1 for DSOW in recent years – is found to interrupt a warming phase that was prevailing since the late 1990’s. The absence of an apparent reversal in the Nordic Seas and the faster changes detected in DSOW compared to ISOW point out the entrainment of subpolar signals within the overflows near the Greenland-Iceland-Scotland sills as a most likely driver.

The Irminger Current (IC), flowing northeastward along the western flank of the Reykjanes Ridge, is an important component in the overturning of the North Atlantic subpolar gyre. A 2‐year (2014–2016) time series from moored observations shows that the IC consists of two highly variable current cores. A subsampling experiment, using an ocean reanalysis, showed that this current's variability is adequately captured by the array. The two current cores contribute nearly equally to the mean volume transport. The total 2‐year mean transport was found to be 10.6 Sv with a standard deviation of daily (monthly) values of 9.2 Sv (4.4 Sv) and a standard error of 1.4 Sv. Mean heat and freshwater transport were 0.21 PW and −22.5 mSv, respectively (Sref = 34.92). The volume transport is strongest in spring, and the average over the first year (8.6 Sv) was lower than that of the second year (12.4 Sv), coinciding with an increase in the density gradient over the array in the second year. The variability of the total transport is dominated by variations in the western core, while the warmer, saltier eastern core contributes more to the heat and (negative) freshwater transport. During the two observed winters, which were marked by exceptional deep convection in the central Irminger Sea, mixed layer deepening down to 400 m depth and outcropping of the 27.7 kg m3 isopycnal were observed within the IC.

Hermetically sealed sodium-sulfur cells intended to operate at 300 C in atmospheric environment and to deliver 10 A for 2 hr above 1.0 V for 100 cycles with a specific power and specific energy of 110 W/kg and 110 W hr/kg, respectively, were designed and fabricated. The cells were to withstand five thermal cycles from operating temperature to room temperature without loss of electrical performance. These cells utilized a disk shaped magnesium oxide- enriched beta-alumina electrolyte with low cost materials for cell containers. The seals used in the cell were all based on state-of-the-art technology and comprised a glass sealant between the beta-alumina electrolyte and the alpha- alumina rings, copper braze between the alpha-alumina rings and the Kovar cell containers, and laser welds between the Kovar cell containers and the stainless steel cell cover. Chromium plasma coating was used to protect the copper braze and Kovar against corrosion by sulfur and/or sodium polysulfide. No catastrophic cell failure due to cell design or materials of construction has been observed during testing of a cell for over 800 hr.

The NASAL notched flat method for determining compressive strength of filament wound materials is described. Data are presented for a series of tests, in which a number of different methods of grip construction were employed, indicating good reproducibility, with an overall coefficient of variation of 4.9%. For a material of 2:1 orthogonal construction, with S glass reinforcement, an average compressive strength value of 157,800 psi was obtained in the stronger ('2') direction. This value is within 5% of the average value reported for tests on cylindrical specimens, of similar construction, tested hydrostatically. Very recent data for a material of superior quality, tested by the NASL notched flat method, gave an average value of compressive strength in the '2' direction of 178,000 psi, with an individual high of 192,800 psi. Based on these data, it is believed that laminates of this type may be produced with present day materials which will have compressive strengths of 200,000 psi or higher. (Author)

An international effort, Overturning in the Subpolar North Atlantic Program (OSNAP), is a partnership among oceanographers from the US, UK, Germany, the Netherlands, Canada and China whose goal is to measure and understand what drives the Atlantic Meridional Overturning Circulation (MOC) and its variability. OSNAP is consisted of more than 53 moorings that stretch from Labrador to Greenland to Scotland, providing a continuous record of the full water column, trans-basin volume transports in the subpolar North Atlantic. The first 4 years of data (August 2014 - May 2018) from the full OSNAP array has been used to produce the 30-day mean estimates of the MOC at OSNAP. All data are freely available from www.o-snap.org.

An international effort, Overturning in the Subpolar North Atlantic Program (OSNAP), is a partnership among oceanographers from the US, UK, Germany, the Netherlands, Canada and China whose goal is to measure and understand what drives the Atlantic Meridional Overturning Circulation (MOC) and its variability. OSNAP is consisted of more than 53 moorings that stretch from Labrador to Greenland to Scotland, providing a continuous record of the full water column, trans-basin volume transports in the subpolar North Atlantic. The first 4 years of data (August 2014 - May 2018) from the full OSNAP array has been used to produce the 30-day mean estimates of the MOC at OSNAP. All data are freely available from www.o-snap.org.