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There has been considerable attention focused recently on Marine Heatwaves, which are warm anomalies persisting for days to months and on spatial scales from one to a thousand kilometers or more (Hobday et al., 2016). Further studies have examined the frequency of Marine Heatwaves in the North Pacific and North Atlantic (Scannell et al., 2016), the influence of global warming on the frequency of Marine Heatwaves under various carbon emission scenarios (Frohlicher et al., 2018), significant historical changes globally based on historical data (Oliver et al., 2018), and the contributions of various ocean-atmosphere modes on the global distribution of heatwaves (Holbrook et al., 2019).

While Marine Heatwaves lasting over days may disturb the marine environment, it is the long lasting events that have the most significant impact. The most dramatic recent example in the Northwest Atlantic coastal ocean was the warming of 2012 (Mills et al., 2013; Chen et al., 2014). This event, which lasted over 6 months with ocean temperature anomalies exceeding 2°C, was caused by a northward shift in the atmospheric Jet Stream during winter that resulted in a 50% reduction of heat loss from the ocean to the atmosphere compared to a normal winter (Chen et al., 2014).

As recent technological improvements and deployment of ocean observatories have allowed further examination of shelf and slope processes, it has become apparent that offshore forcing of the continental shelf and slope is becoming more prevalent and that processes driving shelfbreak exchange may be significant contributors to Marine Heatwaves in the Middle Atlantic Bight. Two observational programs that have been particularly important in recent years are the Ocean Observatories Initiative (OOI) Pioneer Array (Gawarkiewicz and Plueddemann, 2019) and the Commercial Fisheries Research Foundation/Woods Hole Oceanographic Institution Shelf Research Fleet (Gawarkiewicz and Malek Mercer, 2019). The Pioneer Array is a multi-scale shelfbreak observatory designed to study shelfbreak exchange processes, while the Shelf Research Fleet utilizes the commercial fishing fleet in Rhode Island to regularly sample the continental shelf using CTDs provided by Woods Hole Oceanographic Institution (WHOI). Data from these two programs have revealed a dramatic warming event in January 2017 south of New England in which warm water fish typically found in the Gulf Stream were caught at the 30 m isobath off Block Island, RI (Gawarkiewicz et al., 2018). The magnitude of the temperature anomaly reached 6°C relative to a monthly climatology of shelf temperature in the region (Fleming, 2016). However, this event was not examined closely in context with historical data from the region, nor was the fate of the warm water flooding the continental shelf established in Gawarkiewicz et al. (2018).

In this study, we examine a broad suite of observations to characterize the origin of the event, compare its statistical properties with a Marine Heatwave definition, and track the fate of the anomalous water mass on the northeast US Shelf. The outline of the paper is as follows. Section “Data Sources and Methods,” describes the data sources used to examine the 2017 event as well as the criteria used to qualify as a Marine Heatwave using limited sub-surface data. The characteristics of the warm and salty anomalies and their duration south of New England are described in section “Shelfbreak Exchange–Initiation of the Marine Heatwave South of New England.” Anomalous conditions subsequently observed over the New Jersey shelf are described in section “The Marine Heatwave Over the New Jersey Continental Shelf.” The fate of the Heatwave is presented in section “The Fate of the Marine Heatwave” using data showing the anomalous water mass over the continental shelf north of Cape Hatteras. A discussion follows, which includes ecosystem effects and directions for future research. The results are briefly summarized in the final section.

Because of the spatial and temporal extent of this Extreme Shelfbreak Exchange Event, a number of different data sources were used to track the thermohaline anomalies along the length of the Middle Atlantic Bight. These include the Commercial Fisheries Research Foundation/Woods Hole Oceanographic Institution (CFRF/WHOI) Shelf Research Fleet, the Ocean Observatories Initiative Pioneer Array, the CMV Oleander ship of opportunity temperature data set, the National Marine Fisheries Service (NMFS) Ecosystem Monitoring (ECOMON) program, and the National Science Foundation Processes driving Exchange At Cape Hatteras (PEACH) experiment. It is interesting to note that the first identification of this Marine Heatwave event was from Captain Michael Marchetti of the F/V Mister G based in Point Judith, Rhode Island, who emailed photographs and descriptions of unusual conditions and catch to Anna Malek Mercer at CFRF and Glen Gawarkiewicz at WHOI. As reported in Gawarkiewicz et al. (2018), Marchetti noted several unusual species in his catch in January 2017, including Gulf Stream flounder and juvenile black sea bass. Marchetti is a participant in the CFRF/WHOI Shelf Research Fleet, and thus has frequent communications with both CFRF and WHOI.

The Warm Core Ring intrusion that initiated this event was briefly described in Gawarkiewicz et al. (2018) and a review of the data presented there sets the stage for the current analysis. Glider data from the OOI Pioneer Array roughly along the 200 m isobath was used to show that Ring water with a temperature of 15°C was present on January 22–24, 2017. Sea Surface Temperature imagery showed the presence of a large Warm Core Ring abutting the continental shelf south of New England on January 25. The glider data from February 11–13 shows temperatures in the range of 9–13°C, and by March 13–16 the temperature in the top 100 m of the water column was approximately 7°C (see Figure 5 in that paper). Two Shelf Research Fleet CTD profiles were presented to further detail the unusual conditions on the continental shelf south of New England. The first profile, from January 29, had a surface temperature of 10°C and a bottom temperature of 13.5°C, while the profile from February 14 had a well-mixed water column with a temperature of 7°C. The surface salinity value from January 29 was 33.9 PSU and the bottom value was 35 PSU. By February 14 the salinity was well mixed with a value of 33.2 PSU. These data all appear in Figure 5 of Gawarkiewicz et al. (2018). Climatological values of temperature and salinity from Fleming (2016) for January south of New England are 5°C and 33 PSU, so that the anomalies are over 5°C at the surface and 8.5°C at the bottom on January 29. Salinity anomalies were 0.9 PSU at the surface and 2.0 PSU at the bottom. Neither the temporal nor the spatial extent of the anomalies were addressed, nor the statistical properties relative to previous measurements.

Eleven profiles were collected by the Shelf Research Fleet in January 2017. The depth-averaged temperature from these 11 profiles relative to the historical distribution by latitude (north of 40°24′ North) appears in Figure 3A. Seven of the 11 profiles from January 2017 exceed 10°C. The 11 profiles were collected between January 1 and January 31. The profile collected at 41°03′ North is particularly striking with a value of 11.6°C while the historical range does not exceed 7.5°C.

The January 2017 values relative to the distribution of historical depth-averaged temperature data (January-March) appear in Figure 3B. The histogram is segmented into 1°C bins centered at integer values, with the most common value being centered at 6°C. The seven warmest profiles, that exceed 10°C, are above the 98th percentile of the historical values. Two of the coldest values of the month appear on January 29 and 30 (6.1°C) indicative of colder shelf water starting to sweep into the Shelf Research Fleet sampling area in 2017.

The distribution of depth-averaged salinity values for January 2017 versus the historical values appears in Figure 4A. Six of the 11 profiles show salinities exceeding 34.0 PSU, with one profile at 41°03′ North with a value of 34.1 PSU. Only two of the historical profiles show values exceeding 34 PSU in this latitude range.

There are a cluster of profiles with salinities above 33.8 PSU that occur above the 97th percentile of the historical values (Figure 4B). For both the temperature and salinity values, the January 2017 profiles exceed the 90th percentile, and the duration extends through all of January. Thus, the event in January 2017 exceeds the criteria established for a Marine Heatwave as defined in Hobday et al. (2016). The statistical thresholds are also exceeded for salinity. The high salinities combined with the warm temperatures strongly suggest that the origin of the anomalies was a Warm Core Ring abutting the continental shelf, as suggested in Gawarkiewicz et al. (2018), with mixing processes reducing salinities to 34 PSU from typical ring values of 35.5 PSU. Temperatures in the intrusion moving across the continental shelf were 10–11°C, cooler than the 15°C temperatures recorded by the Pioneer Array glider on January 22–24. The cooling may have resulted from air-sea interaction or mixing with cooler shelf waters as Ring water moved onshore across the continental shelf.

Two important sources of uncertainty exist relative to the criteria for choosing Marine Heatwave events presented in Hobday et al. (2016). The first is that we have chosen a non-standard time period for the historical archive instead of the 30 years reference period previously suggested. In the WOD archive January profiles prior to 2004 are available from 1940 to 1996 south of New England. The second limitation is the absence of position information for 5 of the 11 profiles collected by the Shelf Research Fleet. These were not recorded at the time of data collection. In order to include these points, we compared the depth of the collected profiles to the depth distribution of profiles collected in December 2016 and February 2017. We used the latitude of the December or February profile that was closest in depth. The points with uncertain positions are marked with blue diamonds in Figures 3A, 4A. This does introduce uncertainty in the exact latitude for the profiles. However, given the difficulty of collecting data in the middle of winter while active fishing activities are underway and rare nature of this event relative to historical data we feel that it is important to include the 5 profiles even with the uncertainty in position.

In order to establish the spatial structure of the warm anomalies as well as establish the temporal evolution of the anomalously warm shelf water in winter (>10°C), a sequence of Sea Surface Temperature images ranging from November 11, 2016 to February 18, 2017 appears in Figure 5. In mid-November 2016, an extremely large Warm Core Ring with surface temperatures of 20°C abutted the shelfbreak from approximately 66°–68° West. Surface water warmer than 10°C extended from Nantucket Shoals to Delaware in the images at least from December 25, 2016 to January 1 2017 across virtually the entire continental shelf (two upper right panels in Figure 5). Cooler water appears near the coast by January 25, and colder shelf water advects into the entire Shelf Research Fleet operational area by February 18, 2017. Because of long periods of cloud cover during the winter, these six images offer the best view of the spatial structure of the Sea Surface Temperature, but do not appear frequently enough to provide a detailed evolution of the spatial structure of the intrusion and surface temperature anomalies.

The normal continental shelf circulation in the Middle Atlantic Bight continues to the southwest from south of New England to the Cape Hatteras region, where the shelf water deflects offshore under the influence of the Gulf Stream (e.g., Gawarkiewicz et al., 2008). As seen in the Sea Surface Temperature imagery, the warm water observed in January 2017 extended a substantial alongshelf distance to the southwest. Two sources of data can be used to examine the sub-surface thermohaline properties of this feature, the temperature data from the CMV Oleander XBT line and the temperature and salinity data from ECOMON surveys. We will use the CMV Oleander XBT data to examine the temperature fields to see if the anomalies reach the threshold of declaring a Marine Heatwave using data collected from the CMV Oleander from 1977 to 2017. We then use near surface data from the thermo-salinograph to further characterize the timing and cross-shelf extent of the anomalies.

The depth-averaged temperatures observed by CMV Oleander during January 2017 appear as filled red circles in Figure 6. These are superimposed on a histogram of temperature values from prior Oleander transects occupied between 1977 to 2017. Only temperature profiles from positions between the 40 and 80 m isobaths over the continental shelf are considered in order to minimize variability from the Shelfbreak Front and influence by the coastal current from the Hudson River. The dashed line in Figure 6 indicates the 90th percentile for temperature for the historical depth-averaged data from January. Half of the XBT profiles in January 2017 exceed the criterion for designation as a Marine Heatwave.

The thermo-salinograph data confirms maximum cross-shelf penetration of warm saline water in late January (Figure 7). The temperature exceeded 10°C and 34.0 PSU as far west as 73°15′ West on January 21, more than 50 km west of the shelfbreak (marked by the vertical dotted line in the figure). The Oleander data show that the Shelfbreak Front extended approximately halfway across the continental shelf in late January. The ECOMON data from February 2017 confirms this onshore displacement of the Shelfbreak Front and allows estimation of the duration of the Heatwave.

A map of the depth-averaged temperature from the ECOMON cruise in February 2017 appears in Figure 8A. The profiles with temperatures that exceed 10°C are generally seaward of the 80 m isobath, in contrast to both the Oleander data in January 2017 as well as observations from the shelf south of New England where 10°C water occurred well shoreward of the shelfbreak. Between the 40 and 80 m isobaths the temperature was generally in the range of 8–10°C (8 of 12 profiles) with the remaining values between 7 and 8°C.

The spatial pattern of the temperature anomalies in February 2017 relative to the climatology of Fleming (2016) appears in Figure 8B. The largest anomalies are located from mid-shelf to the coast, and range in value from 1.5 to 2.0°C.

The depth-averaged salinity distributions from the ECOMON cruise in February 2017 suggest that the Shelfbreak Front migrated well shoreward of its mean position. Typically, the foot of the front (bottom outcrop) is located at the 80 m isobath over the New Jersey shelf (Linder and Gawarkiewicz, 1998), but observations in 2017 show salinities exceeding 34.0 PSU as far inshore as the 40 m isobath with values over 34.5 PSU reaching the 60 m isobath. Lentz et al. (2003) have defined shelf water in the Middle Atlantic Bight as less than 34.0 PSU, with frontal water in the range of 34.0–35.0 PSU. By this definition the Shelfbreak Front did indeed translate onshore to the 40 m isobath, as shown in the map in Figure 8C. It is important to note that the foot of the front is likely further shoreward of the depth-averaged salinity value of 34.0 PSU for individual profiles.

The spatial pattern of the salinity anomalies appears in Figure 8D. The largest salinity anomalies, approaching 1 PSU, are adjacent to the coast extending offshore to the 50 m isobath. This is consistent with the pattern of the temperature anomalies. Combined, the patterns suggest that the thermohaline anomalies that originated up in New England are most unusual at their greatest cross-shelf extent, i.e., closer to the coast.

The ECOMON cruise on the R/V Henry Bigelow sampled the New Jersey continental shelf from February 12 to 16 and thus does not quite reach the 5 days duration used in the Hobday et al. (2016) criteria. The difficulty of collecting data during January and February in this region can be extreme as this particular cruise was cut short due to stormy conditions. However, combined with the near-surface thermo-salinograph data from the CMV Oleander, the duration of the unusual conditions over the New Jersey shelf can be estimated. The time frame is roughly a month between the first CMV Oleander line in 2017 and the end of the ECOMON survey (January 14 to February 16).

The mean flow over the continental shelf in the Middle Atlantic Bight progresses southward to the vicinity of Cape Hatteras where the flow is exported near the Gulf Stream (Savidge and Bane, 2001; Gawarkiewicz and Linder, 2006; Gawarkiewicz et al., 2008). Fortunately, during 2017 a field program, the Processes driving Exchange at Cape Hatteras (PEACH), began on April 12–29 with a mooring deployment and hydrographic sampling cruise on the R/V Neil Armstrong.

The CTDs present near the surface (2 m depth) and bottom at the B1 mooring in the PEACH array (see Figure 1 for location) indicate the arrival of a surge of near bottom Cold Pool water (Houghton et al., 1982) on April 23 2017 (Figure 9). During the initial deployment on April 18 there was a near bottom water mass present with a temperature of 14°C and salinity of 35.0 PSU, indicative of the presence of Slope water over the continental shelf. However, a strong southward flow appeared on April 23 with a velocity of over 0.4 m/s carrying near bottom temperature of 9.8°C and a salinity of 34.0 PSU. Relative to the climatology of Fleming (2016), the anomaly was approximately 1°C and 1 PSU along 35°45′ North.

While the evolution of temperature and salinity near Cape Hatteras (Figure 9) cannot conclusively establish that this is the same water mass initially transported onto the continental shelf in early January south of New England, a crude estimate of the advection rate offers a hint as to how plausible this scenario is. If we assume that the initial onshore transport of Warm Core Ring water occurred near 40°00′ North and 70°00′ West on January 1 2017, the estimated along-isobath distance to the PEACH array is approximately 850 km. The anomalous watermass was observed at the B1 mooring approximately 113 days after the Warm Core Ring intrusion on the New England Shelf, giving an estimated advection rate of 8.7 cm/s. This is in reasonable agreement with estimated flow rates of 5–10 cm/s over the outer continental shelf (e.g., Lentz, 2008). Thus we find it plausible for the pulse recorded at the B1 mooring of the PEACH array on April 23 to be a remnant of the upstream Marine Heatwave as it passed through the Middle Atlantic Bight. Because of the extreme variability of the water mass properties over the continental shelf in the vicinity of Cape Hatteras, it is unlikely that the anomalies recorded in the hydrographic cross-shelf transect on April 25 (1°C and 1 PSU) exceeded the 90th percentile for temperature or salinity. Nevertheless, the similarity of the T/S properties (Figure 10) to those recorded south of New England and over the New Jersey shelf suggests that the ultimate fate of the water mass carried in the Marine Heatwave was to exit the shelf near Cape Hatteras. This is consistent with Lagrangian properties of the flow field near Cape Hatteras (e.g., Gawarkiewicz and Linder, 2006). While the temperature recorded in the hydrographic section and the B1 mooring does not meet the definition of a Marine Heatwave by Hobday et al. (2016), it is suggestive that the Warm Core Ring intrusion that initiated south of New England was carried the length of the Middle Atlantic Bight, influencing the continental shelf in this region over a 4 months period and an along-shelf span of 850 km. A schematic of the Ring Intrusion with the timing and temperature range at various locations is shown in Figure 11.

We suggest that the continental shelf south of Nantucket Shoals has in recent years become a hotspot for shelfbreak exchange, with Warm Core Ring water masses preferentially carried onshore in this region. Recent evidence has shown that the upper continental slope has become considerably more saline, from a 2.5 year record of glider transects from the OOI Pioneer Array (Gawarkiewicz et al., 2018). This is consistent with evidence from Sea Surface Height Anomalies that large amplitude meanders of the Gulf Stream have been more frequent and occur further to the west than in previous decades (Gawarkiewicz et al., 2012; Andres, 2016). There has been a recent regime shift occurring in the year 2000 with a substantial increase in the number of Warm Core Rings forming each year (33 per year from 2000 to 2017 and 18 per year from 1980 to 1999; Gangopadhyay et al., 2019). We hypothesize that the continental shelf south of Nantucket Shoals traps Warm Core Rings. Because of the proximity of Nantucket Shoals, ring water masses that travel onshore here would flow along isobaths that curve well to the north toward Block Island, Rhode Island. Further investigation is warranted to see if other Marine Heatwaves that have occurred in recent years originate south of Nantucket Shoals to establish whether this area is indeed a new hot spot for shelfbreak exchange. We note that this area was not an area of enhanced onshore flow in the late 1990s, based on drifters released in the Gulf of Maine (Limeburner et al., 2000; Lozier and Gawarkiewicz, 2001). It is possible that the greater amplitude of Gulf Stream meanders combined with the more frequent appearance of Warm Core Rings may have resulted in the formation of a new hot spot for enhanced shelfbreak exchange.

A suite of observations is used to establish the occurrence of a Marine Heatwave south of New England in January 2017 and over the New Jersey continental shelf in late January and February 2017. The warm anomalies reached 6°C relative to a long-term climatology and salinity anomalies as large as 1 PSU could be tracked downstream to the continental shelf north of Cape Hatteras in late April.

The Marine Heatwave and associated salinity anomalies traversed the entire Middle Atlantic Bight, moving from the shelf south of Nantucket Shoals to a region just north of Cape Hatteras over a period of 4 months. Much more research is needed to investigate the dynamics of these ring intrusions and their frequency of occurrence as well as their possible effects on shelf and slope ecosystems.

The datasets generated for this study are available on request to the corresponding author.

GG identified Heatwave event initially organized analysis, wrote first draft, directed figures and made some figures, identified possible link to Unusual Mortality Event for Humpback whales, oversaw edits, compiled references. KC analyzed SST data, discussed historical databases and obtained WOD data and analyzed this data, discussed interpretation and overall structure of manuscript at every stage, placed event into larger context of Heatwave literature. JF analyzed Oleander data from New Jersey, made figures, and wrote section. FB processed Shelf Fleet data, made preliminary figures, discussed choices of data presentation, extensively discussed comparisons between Shelf Fleet data and historical observations. AM and AE oversaw collection of Shelf Fleet data, added material on ecosystem impacts, and edited first draft of manuscript. PF provided ECOMON data and assisted in analysis and improvement of figures, provided important feedback and comments on first draft of manuscript. HS, SH, and LH deployed B1 mooring, made figures, commented on first draft, important edits, and feedback on first draft.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.