From Gawarkiewicz et al., 22 November 2019

Marine heat waves are warm anomalies persisting for days to months and on spatial scales from 1-1000 km (Hobday et al., 2016). These heat waves disturb the marine environment and may have significant feedbacks on the atmosphere. Recent studies suggest that the New England continental shelf is increasingly impacted by Warm Core Rings (WCR) initiating from the Gulf Stream. Gawarkiewicz et al 2019 describe a strong marine heatwave in early 2017 that was apparently initiated by a WCR intrusion onto the shelf.

The first indication of the 2017 heatwave was from a Rhode Island fisherman who noticed unusual species (typically found in Gulf Stream waters) in his catch from the New England shelf. This indicated unusually warm waters on the shelf, and researchers began to investigate the regional extent. A number of different data sources, including OOI Pioneer Array assets, were necessary to track the anomalies along the length of the Middle Atlantic Bight (MAB).

Temperature anomalies measuring up to 6 C and salinity anomalies exceeding 1 PSU were found. The duration of the heatwave was approximately 4 months, initiating near Nantucket Shoals in Jan 2017, traversing the entire MAB, and dissipating offshore of Cape Hatteras in April. The advective path of the heatwave extended roughly 850 km.

Comparison with historical records from 1940 to 1996 showed that the 2017 heatwave was an unprecedented event in terms of temperature and salinity anomalies. The heat wave had significant impacts, including shoreward shift of the shelfbreak front, lowered chlorophyll concentrations, and the presence of warm water fish in New England coastal waters.

Blow down of the 200-meter Shallow Profiler Mooring Platform 2019

K. Roseburg and W. Ruef, Regional Cabled Array and P. MacCready, University of Washington, 20 August 2019

In June and August, 2019, the Slope Base Shallow Profiler Mooring was impacted by two substantial “blow down” events: these were the first observances of blow downs since the moorings were installed in 2014. The largest of these events, which occurred near the end of July and which lasted over two weeks, resulted in a deepening of the 12 ft across, 7,000 lb. platform nearly 50 m and migration of the platform ~ 150 m to the west. Results from pressure, ADCP, temperature and oxygen data pulled from CI using M2M, indicate that as the currents strengthened, the float was depressed into ~ 0.25-0.50°C cooler water with slightly decreased oxygen concentrations. An evaluation of the eddy kinetic energy calculated from the ADCP data, does not support eddy-driven water mass transport. The lower oxygen concentrations could reflect lower amounts of photosynthesis as the platforms are pushed deeper into the water column. We do not know of other documented blow down events such as these in the PNW.

Deep temperature modulations at Axial Base

From Interactive Oceans Data Portal, 21 July 2019

Axial Base is the only site in the world’s oceans where continuous full water column measurements are being made, spanning >2600 m to ~ 5 m beneath the oceans’ surface (a & b). The Axial Base Deep Profiler (DP) Mooring has been operational for >17 months, completing two profiles a day from ~2650 m to 127 m (b). Since August 2019, DP vehicle measurements overlap with those made by instruments on the Shallow Profiler Mooring, which conducts nine profiles a day (a): since 2015, the three Shallow Profiler science pods have conducted >40,000 profiles. The high-resolution temperature measurements delineate fine scale structure both in shallow waters and below 1000 m water depth (c&d) marked by regular vertical striations . These perturbations may result from the vertical advection of the temperature field by passing semidiurnal internal tides (D. Luther, pers. com). Some modulations may be wind-driven inertial waves. Examination of dissolved oxygen concentrations during this same time period are not consistent with drawdown caused vehicle movement. Additional analyses are required, however, to unequivocally determine the processes responsible for these perturbations. A Jupyter notebook on the UW interactiveoceans data portal is now developed to analyze MODIS satellite data (chlorophyll) with measurements being made by instruments on the Shallow Profiler mooring (interactive oceans.washington.edu).

From Hopkins et al., 23 April 2019

Hopkins et al., 2019 use data from the OOI Irminger Sea flanking moorings to create the longest continuous record to date of Deep Western Boundary Current (DWBC) volume transport in the region. This study, part of the Overturning in the Subpolar North Atlantic Program (OSNAP), used data from two OOI flanking moorings, along with three U.S. OSNAP moorings, and five U.K. OSNAP moorings to determine the 22-month mean DWBC volume transport, and its spatial structure off Greenland (Figure above).

Determining DWBC properties is critical to understanding the transport of heat, salt, nutrients, and carbon by the Atlantic Meridional Overturning Circulation (AMOC), part of a system of currents that form the global thermohaline circulation. The combined OSNAP/OOI mooring array deployed at 60°N in the Irminger Sea during 2014 – 2015 provides the longest continuous record of DWBC volume transport at this latitude. This enables not only the most reliable estimate available of the mean transport, but the ability to investigate temporal variability.

Several key points are made by the authors. First, the average volume transport of deep water was 10.8 ± 4.9 Sv (mean ± 1 std) to the south. Of the total transport, North East Atlantic Deep Water accounted for about 6.5 Sv while about 4.1 Sv was associated with the Denmark Strait Overflow. Second, the long record allows the first systematic investigation of DWBC variability. The observed transport shows a shift from high to low frequency fluctuations with increasing distance from the East Greenland coast. High‐frequency fluctuations (2–8 days) dominate close to the continental slope, likely associated with topographic Rossby waves and/or cyclonic eddies. In deeper water, transport variance at 55 days dominates. Finally, the results indicate a modest (1.8 Sv) increase in total transport since 2005–2006, but this difference can be accounted for by a range of methodological and data limitation biases. This is of interest because although AMOC variability related to climate change is expected to be reflected in DWBC transport and properties, conclusive observational evidence of transport change has been elusive.

From Josey et al., 18 December 2018

A recent paper by Josey et al. [1] use data from the OOI Irminger Sea surface mooring to present the first characterization of multi-winter surface heat exchange at a high latitude North Atlantic site based on in-situ measurements. Heat loss in the North Atlantic is a primary determinant of how much deep water is formed in a given winter, which ultimately influences the strength of the Atlantic circulation. The Irminger Sea is a key deep convection site [2], [3], but the amount of heat lost is poorly known because winter measurements are difficult to obtain in the harsh environment [1].

Data from the first four deployments of the Irmginer surface mooring provided early winter observations in three consecutive years – 2014, 2015 and 2016. Strong variations on times scales of days to weeks are apparent in the air-sea flux components. These combine to create net heat losses exceeding 400 W/m^2 on at least one occasion each winter (Fig. 1 d).

The integrated effects of the anomalously strong heat loss events in 2014–2015 result in cumulative net heat loss significantly greater than following years (Fig. 1 e,f) This is primarily due to variations in frequency of intense short timescale (1–3 days) forcing (Fig. 1g) that would be difficult to detect without the mooring observations.

Interpreting the observations in the context of a high-resolution atmospheric model (European Centre for Medium Range Weather Forecasts Reanalysis 5) shows that the main source of multi-winter variability is changes in the frequency of Greenland tip jets that can result in hourly mean heat loss exceeding 800 W/m2 (Fig. 1g). The tip-jet events result from the mountainous Greenland terrain, which focuses winds into narrow, very strong jets over the ocean [4], [5]. Josey et al. [1] suggest how changing atmospheric circulation may influence the number of events and hence the ocean heat loss.

Improved understanding of Irminger Sea winter heat loss is likely to be critical to reliable projections of future changes in both the North Atlantic overturning circulation and climate variability.

[1] Josey, S.A., M.F. de Jong, M. Oltmanns, G.K. Moore and R.A. Weller, 2019. Extreme variability in Irminger Sea winter heat loss revealed by ocean observatories initiative mooring and the ERA5 reanalysis, Geophys. Res. Lett., 46. https://doi.org/10.1029/ 2018GL080956

[2] de Jong, M.F. and L. de Steur, 2016. Strong winter cooling over the Irminger Sea in winter 2014–2015, Geophys. Res. Lett., 43, 106–107. https://doi.org/10.1002/2016GL069596

[3] de Jong, M.F., M. Oltmanns, J. Karstensen, and L. de Steur, 2018. Deep convection in the Irminger Sea observed with a dense mooring array. Oceanography, 31(1), 50–59. https://doi.org/10.5670/oceanog.2018.109

[4] Doyle, J.D. and M.A. Shapiro, M. A., 1999. Flow response to large-scale topography: The Greenland tip jet. Tellus A: Dynamic Meteorol. and Oceanog., 51(5), 728–748. https://doi.org/10.3402/tellusa.v51i5.14471

[5] Moore, G. W. K., & Renfrew, I. A. (2005). Tip jets and barrier winds: A QuickSCAT climatology of high wind speed events around Greenland, J. Climate, 18(18), 3713-3725, https://doi.org/10.1175/JCLI3455.1.

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From Zhang and Partida, 15 October 2018

The shelf break front is a persistent feature in the Middle Atlantic Bight, with the foot of the front found near the 100 m isobath. Warm core rings pinched off from the Gulf Stream are found over the continental slope, and have significant impacts on the frontal system when they impinge on the shelf. Zhang and Partida, 2018 use data from OOI Pioneer Array moorings and gliders to document a unique shelf/slope interaction related to the presence of a warm core ring.

Satellite SST imagery shows a well-formed ring impinging on the shelf south of New England during June 2014. Observations from the Pioneer Offshore and Central Offshore moorings are able to identify relatively cold, fresh shelf water that migrates downward with time during the period that ring water is at the surface. Relatively high dissolved oxygen concentration indicates that the shelf water was recently exposed to the surface. This suggests that shelf water filaments are being subducted at the edge of the ring. A cross-shelf section from the Pioneer EB glider (Figure above) clearly shows the cold, fresh shelf water beneath warm, salty ring water. The shelf water is seen well offshore of 100 m where the frontal boundary would be expected in the absence of interaction with the ring.

Computer simulations reproduce the observed phenomenon and reveal the underlying mechanism. The onshore movement of the warm-core ring sharpens the interface between the ring and shelf waters. The subducted flow results from circulation trying to relax the sharpening interface and serves to transport sub mesoscale filaments of shelf water offshore. This offshore flow of subducted shelf water may be a significant exchange mechanism, but has not been well studied. It does not have a surface signature and cannot be seen by satellites.