In this study, the continental shelf-slope region of the NW Atlantic between 36°N and 46°N was examined for MHW occurrences and eddy activity. The shelf break is located at the 200-meter isobath ( Figure 1 ), and the slope region is defined as the area between the shelf and the average path of the Gulf Stream. The average path of the Gulf Stream was identified using the 25-centimeter sea surface height (SSH) contour of monthly SSH maps spanning 2000-2020 (19 years). This method of identifying the average path of the Gulf Stream with satellite altimetry is used in multiple previous works (Lillibridge and Mariano, 2013; Rossby et al., 2014; Andres, 2016). Two geographic subregions in the NW Atlantic defined by Richaud et al. (2016), the Gulf of Maine and the Middle Atlantic Bight, were used as literature describes significant MHW events occurring there (Mills et al., 2013; Chen et al., 2014; Pershing et al., 2018; Gawarkiewicz et al., 2019; Perez et al., 2021; Schlegel et al., 2021; Großelindemann et al., 2022).

In the MHW active years (2012, 2016, 2017, and 2020), there are 475 distinct eddy absorption events within the NW Atlantic ( Figure 1 ). The GOM had between 45-60 AE absorptions with an average of 566 detected AEs during these MHW active years. The MAB has on average 3 AE absorptions and a range of 8-22 detected eddies. The MAB had significantly less eddies than the GOM. Total number of absorption eddy events in the NW Atlantic was 128 for 2012, 114 for 2016, 103 for 2017, and 130 for 2020 ( Figure 1 ). Perez et al. (2021) describes 2016 as a year with abnormally more WCR absorptions, yet 2012 and 2020 had more total absorptions within the NW Atlantic. 2020 had the most eddies in the NW Atlantic and both subregions, there is not a significant difference between these years. 2017 and 2016 have fewer eddies that tend to be longer lived than 2012 and 2020.

In the NW Atlantic, there were 43355 MHW events detected in the four MHW active years ( Figure 2 ). 2017 had the least events, while the other years were closer together with 2016 having the most events. On average the GOM had 2080 MHWs and the MAB had 1140 ( Figure 2 ). The GOM experiences more MHW events in these four years than the MAB. The 2012 MHWs overall had a high frequency near the coastline, especially in the GOM and the Scotian shelf ( Figure 2A ). The 2016 MHWs were widespread throughout the NW Atlantic ( Figure 2B ). Both 2017 and 2020 had a high frequency of events along the GS but 2020 also had events concentrated on the shelf near Cape Cod ( Figures 2C, D ).

In 2012, the MAB experienced the maximum mean MHW SSTAs of 2.3°C in July, with the maximum SSTA peaking in April ( Figure 3A ). SSSAs at the maximum mean SSTA is low, while SSSAs during the maximum SSTA is high ( Figure 3A ). The 2012 GOM had low SSSA with little variability throughout the year ( Figure 3B ). The mean and maximum SSTAs in the GOM increased from January until May, reaching a peak at 1.7°C ( Figure 3B ). These GOM MHWs found in the early part of the year are atmospherically forced (Chen et al., 2014). The 2016 MAB maximum mean SSTA is 2.4°C in August, which aligns with a minimum in maximum SSTA and mean SSSA ( Figure 3C ). This month had low intensity events with widespread atmospherically forcings ( Figure 3C ; Perez et al., 2021). March through April experiences another peak SSTA with a high mean SSSA, aligning with AE driven events ( Figure 3C ; Perez et al., 2021). The maximum mean MHW SSTA in the GOM occurs in September (1.7°C) after increasing since May ( Figure 3D ). The maximum SSSA peaks are in August and February which coincide with maximum SSTA peaks ( Figure 3D ). The MAB and GOM both have high SSTA and SSSA in August; this is when Perez et al. (2021) observe that much of the NW Atlantic was experiencing widespread MHWs ( Figures 3C, D ). The 2017 MAB experiences a peak mean and maximum SSTA (0.9°C; 1.3°C) in March, which occurs simultaneously with a high mean SSSA (1 psu; Figure 3E ). From January through April, a MHW driven by a WCR passed through the MAB, peaking in intensity during March ( Figure 3E ; Gawarkiewicz et al., 2019). The 2017 GOM has on average a high mean SSSA ( Figure 3F ). There is a peak in mean SSTA in June that occurs with high SSSA, but the maximum mean SSTA (2.2°C) occurs at a minimum SSSA in November 2017, following the pattern of 2012’s atmospherically driven MHW event ( Figures 3B, F ). The 2020 MAB peak mean SSTA (1.2°C) in February aligns with maximum SSSA peak but mean SSSA is low, indicating no widespread ocean-driven event ( Figure 3G ). The MAB has low SSSA and SSTA from June through September ( Figure 3G ). The GOM maximum mean SSTA (1.8°C) in August ( Figure 3H ). Low mean SSSAs are prevalent throughout the year but summer months experience high maximum SSSAs in June that align with maximum SSTA ( Figure 3H ).

The maximum mean SSTA intensities in the GOM occur in May 2012, September 2016, November 2017, and August 2020 ( Figure 3 ). May 2012 had a high number of AEs around the GOM ( Figure 4A ). September 2016 had more AEs further off the shelf ( Figure 4B ). November 2017 follows a similar distribution to 2012, with more AEs also along the GS ( Figure 4C ). August 2020 has a similar spatial distribution of AEs to 2016 ( Figure 4D ). The similar distribution of AEs in 2012 and 2017 lead to peak intensities in the GOM by moving warmer waters into a region already in a MHW state, thereby extending the duration of the state. 2016 and 2020 GOM mean intensities quickly decrease after the peak ( Figure 3 ), indicating processes encouraging MHW decay in the absence of AEs. Low AE radius and low amplitude was seen between these selected months ( Figures 4E–L ). Overall low SSTA and SSSA are also present in these months ( Figures 4M–T ). Between 40-42°N, higher AE radius and amplitude and more extreme SSTA and SSSA are observed. The higher selected AE radii exceeds 90 km and amplitude exceeds 3.3 cm, with 2020 having the highest radii and amplitude and 2017 having the lowest. The mean SSTA between 40-42°N was 0.2°C across these four months ( Figures 4M–P ). Below 40°N, 2012 experiences unusually strong negative AE SSTAs, indicating that these AEs have moved into an area with higher average SST than where they originated, such as the GS or Sargasso Sea, leading to the negative SSTAs ( Figure 4M ). Negative AE SSSA are present around the GOM in 2012, the Scotian shelf in 2016 and 2017, and above the MAB in 2020 ( Figures 4Q–T ).

From 2012 to 2021, there were 4.7-8 AEs and 6.8-10.4 CEs, with no overall trend ( Figure 5A ). During the 2012 MHW period (February-July) AEs decreased, while CEs are consistent ( Figure 5A ). 2016’s MHW period (April-September) has less variability than 2012 but follow a similar pattern ( Figure 5A ). In 2017’s MHW period (March-November) and 2020’s MHW period (January-September) had a consistent values of AEs and CEs ( Figure 5A ). The average amplitude (6.8 cm), radius (57.3 km), and EKE (204.3 cm² s^-2) of AEs and CEs in the MHW periods of 2012, 2016, and 2017 are noticeably lower and have less variability than the rest of the time series ( Figures 5B–D ). 2020 MHW period has a minimum of AE amplitude, radius, and EKE in late March, with maximum of AE and minimum of CE in amplitude, radius, and EKE in September ( Figures 5B–D ). Similar peaks are seen in September 2013, October 2014, November 2015, April 2018, and September 2021 ( Figures 5B–D ).

Prior to the interaction between MHWs and AEs, temperature intensity increases to an average peak of 3.3°C before declining ( Figure 6A ). RMSE is inverse to the mean time series, indicating that the peak temperature intensity is better at representing MHW dynamics at the interaction peak ( Figure 6A ). SSSA and the RSME have little variability ( Figure 6B ). OHCA peaks one day prior to the MHW-AE interaction, and rapidly declines one day after ( Figure 6C ). Between five days prior and 3 days after the interaction, the RMSE decreases and remains constant to the end of the observed time series ( Figure 6C ). MLD and the RMSE reach a peak 3 days after the interaction ( Figure 6D ). The temperature and salinity anomalies composites with depth show the average changes that occur during these events from 1993 through 2020. The temperature anomaly maximum at 13.5 m persists through the event (6.4°C; Figure 6E ). High temperature anomalies (above 3.9°C) were in the upper 100 m prior to day 0 of this interaction ( Figure 6E ). Maximum temperature intensities of the MHW events become slightly deeper after interacting with AEs. Salinity anomalies from the surface to 40.3 m change only slightly through these events, remaining below 2.0 psu ( Figure 6F ). Days prior to the event had consistent salinity anomalies between 2.1-2.3 psu below 50 m ( Figure 6F ). The maximum salinity anomaly was 2.3 psu 5 days prior to the event. After the event, salinity anomalies tended to decrease with depth but remained above 2.1 psu ( Figure 6F ). Overall, AEs interacting with MHWs increase OHC leading up to the event, deepen the range the MHW impacts, and affect the surface temperature variability ( Figure 6 ).

The number of AEs for each MHW active year is relatively consistent, with a mean in the GOM at 6.7 eddies and in the MAB at 7.4 eddies ( Figures 7A–D ). AE numbers in the MAB and the GOM decreased throughout the active MHW years, except in the 2020 in the GOM, which did not have a decreasing trend ( Figures 7A–D ). While the decreasing trend in the subregions reflect the overall NW Atlantic trends for 2012, 2016, and 2017, this trend is not seen during the observed years in Figure 5 . The GOM AE numbers do not relate to GOM MHW intensity ( Figures 7A–D ). The high MAB AE numbers, above 7.5 eddies, in spring of 2012, 2016, and 2017 have a concurrence of high MAB MHW intensity above 3.5°C ( Figures 7A–C ). The AE radius in the GOM for all four years averages to 62.8 km with low variability and are larger than MAB AEs ( Figures 7E–H ). 2020 experiences smaller AEs, an effect which is pronounced in the MAB ( Figure 7H ). 2016 and 2017 AE radii increased throughout their respective years ( Figures 7F–G ). There was no relationship between AE radius and MHW intensity ( Figure 7E–H ). EKE also had no relationship with MHW intensity ( Figures 7I–L ). EKE is more variable than other eddy characteristics. Unlike in Figure 5 , simultaneous AE radius and EKE peaks are not evident in Figure 7E–L . 2012 and 2016 have higher average EKEs in the MAB than the GOM, while 2017 and 2020 show the opposite relationship ( Figures 7I–L ).

To further understand the relationship between MHWs and their possible drivers, they are compared in a Lead-Lag Cross-Correlation in both subregions for each MHW active year ( Figure 8 ). The MAB consistently showed low correlation values, below 0.3, in the following pairs: MAB AE EKE and MHW events, AE EKE and MHW intensity, and GS EHFC and AE EKE ( Figure 8 ). Values below 0.44 with less than 15 lag days in 2012, 2016, and 2020 were observed in the GoM for AE EKE and MHW intensity as well as GS EHFC and AE ( Figures 8A, B, D ). 2017 has correlations for these same parameters above 0.65 and has the largest lag for AE EKE and MHW intensity ( Figure 8C ). 2012 and 2016 share a pattern of GS EHFC lagging AE EKE, while in 2017 and 2020 GS EHFC leads ( Figure 8 ). The shift between 2016 and 2017 is likely from the increased GS variability that started in late fall 2016, but this does not explain the 2020 pattern (Andres, 2021; Perez et al., 2021). In all MHW active years, GS EHFC and MHW intensity have correlations above 0.75 with lag days between -15 and 15 for both subregions ( Figure 8 ). 2017 has the strongest correlation of this parameter pair and 2016 has correlations between each subregion closer than any other parameter pair ( Figures 8B, C ). GS EHFC is a metric of near-surface transport of heat by the GS, and MHWs are intensifying from the heat brought by the GS to these subregions in these four MHW active years (Zheng et al., 2022).

High temperature anomalies above 2.2°C exceed 100 m in May through July 2012 during the duration of the atmospherically forced MHW (Chen et al., 2014) as shown in Figure 9A . Additional high temperature anomalies are found between 55-77m in September and December; this subsurface heating may indicate peaks in subsurface MHW intensities that go undetected from surface observations ( Figure 9A ). During high temperature anomalies in May through July 2012 ( Figure 9A ), a high salinity anomaly forms below 40 m, reaching a maximum of 0.5 psu ( Figure 9B ). 2016 high temperature anomalies above 1.6°C are present January through March throughout the upper 100 m and in June and October at 55.8 m ( Figure 9C ). GOM 2016 positive salinity anomalies are observed to exceed 100 m in January through March and August through September, aligning with high subsurface temperature anomalies ( Figure 9D ). January through March and fall 2016 were recorded as months when subregions in the NW Atlantic experienced MHWs that coincided with an increase in AE absorption and GS variability (Perez et al., 2021). 2017 GOM temperature anomalies above 1°C are present through the upper 100 m in February, June through August, and October through December ( Figure 9E ). Salinity anomalies in 2017 are low, averaging 0.1 psu from January through November ( Figure 9F ). The GOM in 2017 contained greater than average SSSAs in MHW grid points among the other MHW active years but the GOM as a whole did not have higher than average salinities ( Figure 9F ). 2020 temperature anomalies in the GOM are the lowest in these four years ( Figure 9G ). The notable high temperature anomalies in 2020 that exceed 1.0°C were in June through August above the mixed layer or below 80m, February below 80m, and December exceeding 100m ( Figure 9G ). Salinity anomalies outside of December in 2020 GOM average below 0 psu ( Figure 9H ).

2012 MAB has high temperature anomalies above 2.2°C in January through July ranging between 80 m and 100 m ( Figure 10A ). Higher salinity anomalies are seen in January between the surface and 80 m, in April through May between 40 m and 100 m, in June within the MLD, and in July through mid-August below the MLD ( Figure 10B ). 2016 has high temperature anomalies above 1.4°C extending from the surface or the mixed layer depth through the upper 100 m for most of the year, with maximums in March through April ( Figure 10C ). The 2016 MAB has high salinity anomalies through most of the year coinciding with high temperature anomalies ( Figure 10D ). Negative surface salinity anomalies occur in July ( Figure 10D ). The MAB in spring 2016 experienced several WCR absorption events, leading to warmer and saltier waters (Perez et al., 2021). High temperature anomalies occur in January through April and October 2017, while June through September has negative temperature anomalies ( Figure 10E ). Salinity anomalies exceed 0.3 psu at the surface in January through April and in December ( Figure 10F ). These results are contextualized by Gawarkiewicz et al. (2019), who recorded the MAB MHW from January to April following a WCR intrusion of the MAB shelf with a distinct salinity anomaly remaining after the MHW’s decline. In 2020, high temperature anomalies above 1.0°C extended from the surface to 100 m in February and March, from the surface to 51 m in November through December, and below 40.3 m in June ( Figure 10G ). A subsurface low anomaly (-1.4°C) is present from 30-100 m in July through September ( Figure 10G ). The only overlap of high salinity and temperature anomalies is in November through December above the MLD ( Figure 10H ). Negative salinity anomalies occur at the surface in March, May, and July and below 50 m from July into October ( Figure 10H ). The 2020 MAB has the lowest temperature and salinity anomalies in these MHW active years ( Figures 9 , 10 ).

The 2012 GOM MHWs remain above 2.0°C in the upper 60 m but quickly decrease and reach a minimum of 1°C at 186.1 m ( Figure 11A ). The 2012 GOM MHW salinity anomalies remain around 0.4 psu from the surface until 130.7 m ( Figure 11B ). The most extreme 2012 GOM MHW events occurred below the surface and the lack of abrupt changes in the oceanic profiles reflect the dynamics of widespread atmospheric forcings that are present in 2012 ( Figures 11A, B ) as also observed by Chen et al. (2014). The 2016 GOM MHWs remained near 2.0°C above 70 m before decreasing ( Figure 11A ). 2016 has strong surface GOM MHW salinity anomalies that decrease to 0.3 psu at 155.9 m ( Figure 11B ). 2017 has the highest GOM MHW temperature anomalies in the surface mixed layer and overall, reaching 3.5°C at 34.4 m ( Figure 11A ). 2017 MHW salinity anomalies decrease from 1.0 psu at the surface to 0.3 psu at 100 m ( Figure 11B ). The peak of the average MHW temperature anomaly aligns with the peak temperatures in October through December in Figure 9E , further indicating these months as the most extreme MHW events in 2017 GOM. These 2017 MHWs also experience more abrupt changes in temperature and salinity at shallow depths than other profiles ( Figures 11A, B ). 2020 MHW temperature anomalies have a subsurface peak of 2.4°C at 109.7 m ( Figure 11A ). 2020 salinity anomalies also have a subsurface maximum of 0.5 psu at 130.7 m, slightly below the temperature anomaly ( Figure 11A, B ). The lower temperature and salinity anomalies for 2020 MHWs in the upper 100 m are consistent with the low average temperature and salinity anomaly averages demonstrated in Figure 9G and 13H. The peak 2020 MHW temperature and salinity intensity occurs below 100 m, the depth considered to have the most intense MHWs (Großelindemann et al., 2022).

The 2012 MAB MHW temperature anomalies increase from 2.0°C to 2.5°C, peaking at 55 m before steeply dropping ( Figure 11C ). 2012 MHW salinity anomalies reach a maximum at 0.5 psu at the same time as the peak in temperature anomalies ( Figure 11D ). The MHW temperature and salinity anomaly maxima reflect the depth where the major anomalies are in Figures 10A, B ( Figures 11C, D ). As previously noted, the 2012 MHWs were largely atmospherically forced (Chen et al., 2014), but AEs were also intensifying events in the MAB, specifically during January, May, and July ( Figure 10B ). The 2016 MAB MHW temperature anomalies have a maximum of 2.8°C at 66 m and stay above 2.0°C until 250 m ( Figure 11C ). MHW salinity anomalies decreased from 0.6 psu to below 0.4 psu throughout the water column ( Figure 11D ). 2016 MHW temperature and salinity profiles mirror the change in slope in each subregion, indicating the drivers of 2016 MHWs influencing both temperature and salinity but with different strengths across the subregions (Perez et al., 2021). For MAB MHWs, 2016 has the most intense MHWs across these four MHW active years ( Figures 11C, D ). The 2017 MHW temperature anomalies remain around 1.7°C but reach a subsurface maximum of 2.0°C at 78 m ( Figure 11C ). The salinity anomalies have very little variability but decrease from 0.4 psu at the surface to 0.2 psu below the surface layer ( Figure 11D ). Fewer 2017 MHW events occurred throughout the year, which those that did form tending to have low surface temperature and salinity intensity ( Figures 2 , 3 ). The MAB also experienced low temperature and salinity anomalies throughout the summer and early fall ( Figures 10E, F ), leading to MHWs not having a strong influence upon the vertical profiles. Surface MHWs anomalies are influenced by WCRs in January through April, which caused warm waters to cross the 100 m isobath. However, due to additional MHWs of lower temperature intensity, the average profile has little overall intensity (Gawarkiewicz et al., 2019). 2020 MHW events in the MAB had relatively uniform temperature anomalies with depth and salinity anomalies were similarly constant below the mixed layer ( Figures 11C, D ).