Using colored dissolved organic matter fluorescence to trace Pacific-derived water in the Eastern Canadian Arctic

Abstract

Low-salinity Pacific Water (PW) entering the Arctic Ocean through the Bering Strait is the second largest source of freshwater to the Arctic Ocean and dominates the freshwater inventory of Baffin Bay due to PW advection through Nares Strait and the Canadian Arctic Archipelago (CAA). PW carries nutrients vital for Arctic primary production and the marine ecosystem. Yet, the relative contribution of these pathways to PW advection remains unclear. Here, we focus on PW transport through the northernmost oceanographic passageway of the CAA, Nansen and Eureka Sounds, where observations have been particularly sparse. Using profiles of temperature, salinity and fluorescence of colored dissolved organic matter (FCDOM) collected during the summer of 2024 we contrast PW outflow through Nansen and Eureka Sounds against outflow through Nares Strait and northern Baffin Bay. We find that the subsurface FCDOM maximum successfully traces the PW flow through the CAA and Nares Strait. However, the FCDOM peak of PW erodes when PW passes over the rough bottom topography of the CAA channels and Nares Strait. Further downstream, when PW reaches Jones Sound, Smith Sound, and northern Baffin Bay, lateral interactions with water masses from the West Greenland current and vertical mixing both disrupt the subsurface FCDOM maximum of PW. Overall, our findings indicate that FCDOM assessed by optical sensors is a powerful tool for tracing PW in the CAA, Nares Strait and the adjoining Canada Basin providing valuable insights into water mass mixing processes over the CAA continental shelf and downstream in northern Baffin Bay.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-026-38848-2.

Introduction

The subsurface water column of the Canada Basin (CB) is comprised of water of Pacific and Atlantic origin. Relatively fresh Pacific-derived Arctic seawater is the second main source of freshwater to the Arctic Ocean (AO) freshwater budget after river runoff¹. Pacific-modified polar water (PW) is formed from waters entering the AO via the Bering Strait. It spreads over the AO following two major pathways, a Transpolar branch crossing the AO toward Fram Strait and an Alaskan branch along the Beaufort Sea continental slope^2–4 (Fig. 1a). Along with the Arctic freshwater flux originating from net precipitation, river runoff and sea ice melt, PW contributes to freshwater export from the AO through the Canadian Arctic Archipelago (CAA) and Nares Strait to Baffin Bay and the North Atlantic. The northernmost passageways transporting PW into Baffin Bay consist of (i) Nares Strait located between Ellesmere Island and northern Greenland (hereinafter the northeastern passage) and (ii) Nansen and Eureka Sounds, Norwegian Bay and Jones Sound (hereinafter the northwestern passage) (Fig. 1). PW experiences significant modifications while transiting along these two passageways due to enhanced vertical mixing over the shoals and sills in Eureka Sound, Cardigan Strait, and Fram Sound⁵ as well as in Kane Basin of Nares Strait⁶. In northern Baffin Bay, PW is also modified due to lateral interaction with water transported by the West Greenland Current⁷ (WGC). This water circulates cyclonically (anticlockwise) in Baffin Bay and joins the southward outflow from Nares Strait, Jones Sound and Lancaster Sound (Fig. 1a). As a result, in northern Baffin Bay the original thermohaline signature of PW becomes significantly altered, and tracing the PW modifications using thermohaline characteristics alone becomes difficult. Therefore, we are looking for additional indicators to assess the contribution of PW to the freshwater outflow from the Arctic Ocean to northern Baffin Bay.

Fig. 1.

(a) Bathymetric chart of the western Arctic adopted from the International Bathymetric Chart of the Arctic Ocean, Version 3.0⁴³. The schematic circulation of Pacific and Atlantic water in the Arctic Ocean and adjoining Canadian Arctic Archipelago (CAA) is depicted by white and red arrows, respectively^2–4. Blue dots depict CTD profiles taken by the Ice-Tethered Profiler (ITP) #64 over the Canada Basin in August-September 2012. Black rectangle highlights the northeastern CAA and northern Baffin Bay enlarged in (b). (b) Oceanographic stations occupied from the CCGS Des Groseilliers in 12–22 August 2024, and from CCGS Amundsen on 23 August to 2 September 2024, and in 8–26 September 2024 are indicated with red, black, and blue crosses, respectively. Black dashed rectangle highlights the eastern Jones Sound captured by satellite imagery in Fig. S1.

Here we use fluorescence of colored dissolved organic matter (FCDOM) to trace the southward flow of PW through the northernmost passageways from the AO to Baffin Bay, and to assess modifications of PW occurring “en route” and in northern Baffin Bay due to interactions with waters of the WGC. We build upon the approach⁸ that allows to use of the FCDOM as a supplementary tracer for PW. Pacific Winter Water (PWW) in the CB is characterized by a sub-surface peak in marine dissolved organic matter due to interaction with shelf sea sediments in the Chukchi and Northern Bering seas, where this water mass is formed⁸. In fact, using FCDOM offers a new approach for tracing the distribution of PW and its export from the AO, by utilizing combined relationships of seawater temperature, salinity, and FCDOM. Implementing this approach⁸, for the first time we contrast the PW outflow through Nares Strait against that through Nansen and Eureka Sounds using oceanographic observations collected in August-September 2024 from the CCGSs Amundsen and Des Groseilliers (Fig. 1b).

Results

The water mass assembly of AO outflow into the CAA and Nares Strait consists of water masses with varying origins. At the surface, polar surface water is present, followed by a subsurface Pacific-derived water layer, which overlays deeper Atlantic-modified and Atlantic water layers. In this study we focus on the latter three layers approximately summarized in Table S1.

PW and CDOM at the gateways

The thermohaline characteristics of PW leaving the AO through Nansen Sound and Robeson Channel are modified during their passage. PW at st. 7 (Nansen Sound entrance) and st. 34 (Robeson Channel entrance) occupies depths of ~ 50–180 m (Figs. 2a–c). The Nansen Sound PW with salinities 32–34 and temperatures from − 0.5 °C to − 1.35 °C is on average saltier (~ 0.5) and warmer (0.2 °C) compared to Robeson Channel (Fig. 2a and b). The upper 20-m portion of the PW layer in Robeson Channel at 50–75 m depth with salinities < 32 suggests entrainment of freshwater (Fig. 4a). Another distinct difference between Robeson Channel and Nansen Sound is that the upper portion of the PW layer in Robeson Channel for ~ 70–110 m depth shows the subsurface temperature maximum with temperature slightly increasing from − 1.65 °C to ~ − 1.1 °C (Figs. 2a and 3b, and 4b). The AW layer with temperatures exceeding 0 °C (the commonly accepted upper boundary of the AW) in both Nansen Sound and Robeson Channel is observed at depths > 240 m. However, temperature and salinity of AW in Nansen Sound relative to those in Robeson Channel is slightly warmer (by ~ 0.1 °C) and saltier (by ~ 0.15).

Fig. 2.

Vertical profiles of (a) in situ temperature (°C), (b) practical salinity, and (c) FCDOM (mg m^–3) taken at the entrances of Nansen Sound (st. 7, red) and Robeson Channel (st. 34, green), in northeastern Baffin Bay (st. 120, blue), and over the eastern CB (ITP-64, black). Blue and pink shading highlights the Pacific-derived and Atlantic‐derived water, respectively, at the entrances of Nansen Sound and Robeson Channel. (d-g) Property-property scatterplots of (d, e) FCDOM, salinity, and water temperature (in color) and (f, g) in situ water temperature, salinity, and FCDOM (in color) for all profiles taken from (d, f) Nansen Sound and (e, g) Nares Strait to Baffin Bay and at st. 120. Deviations attributed to the WGC are highlighted with dashed blue ovals. Blue and pink shading highlight the Pacific-derived and Atlantic‐derived water, respectively.

Fig. 4.

(a, d) Salinity, (b, e) temperature (°С), and (c, f) FCDOM (mg m^–3) transects taken from (a − c) the mouth of Nares Strait to Lancaster Sound and (d − f) across the mouth of Lancaster Sound in August-September 2024. Only the upper 500-m portion is shown. Red, black, and blue arrows at the top indicate stations occupied from CCGSs Des Groseilliers (22 August 2024) and Amundsen during 23 August to 2 September, and 8–26 September 2024, respectively. Blank areas represent missing data.

Fig. 3.

(a) Salinity, (b) temperature (°С), and (c) FCDOM (mg m^–3) transects taken from Nansen to Lancaster Sounds in August-September 2024. Only the upper 500-m portion is shown. Red and black arrows at the top indicate stations occupied from CCGSs Des Groseilliers (12–22 August 2024) and Amundsen (2 September 2024), respectively. Blank areas represent missing data.

There is a FCDOM subsurface maximum between ~ 50 and 180 m depth at the entrance to Nansen Sound and Nares Strait (Fig. 2c). In Nansen Sound, FCDOM increases to ~ 4.5 mg m^–3 at 50–110 m depth, coinciding with relatively low water temperatures of ~ − 1.3 °C (Fig. 2a and c). For Robeson Channel, the upper 20 m portion of this layer with salinity < 32 shows relatively low FCDOM < 3.7 mg m^–3 that rapidly increases with depth at salinities > 32 to 6.25 mg m^–3 at 135 m depth (Fig. 2b and c). The coldest portion of the PW layer in Robeson Channel, displaying temperatures of ~ − 1.3 °C, achieves maximum CDOM values exceeding 5.5 mg m^–3 (Fig. 2a and c).

In northern Baffin Bay, the surface water layer of the WGC down to ~ 140 m depth is more saline compared to water exiting the AO, which has been freshened by river discharge and PW contributions (Fig. 2b). For depths > 140 m, salinity of the AO outflow (st. 34) starts to exceed that of the WGC, and between 200 and 500 m the salinity difference ranges from 0.3 to 0.7 (Fig. 2b). In the WGC (st. 120), the AW with temperatures above 0 °C is observed at depths > 110 m increasing with depth to 1.5 °C at 500 m (Fig. 2a). At 230–500 m depth, AW of the WGC shows temperature intrusions of cooler water (Fig. 2a). This indicates a lateral interaction with cooler AW flowing southward along the western coast of Baffin Bay. In contrast to the Pacific-derived subsurface FCDOM maximum for the AO outflow, the WGC shows gradual increase of FCDOM with depth from 2.7 mg m^–3 at the 50-m surface layer to 4.1 mg m^–3 at 500 m depth (Fig. 2c).

Northwestern passage throughflow

The CTD and FCDOM sections in Figs. 3 and 4 show how the PW signature transforms while transiting through the CAA and Nares Strait, respectively. Following the northwestern passageway, FCDOM is diluted in Eureka Sound southward of st. 5 located near the sill north of Stor Island with bottom depths between 70 and 85 m (Fig. 3c). It seems that tidally driven vertical mixing over the sill reduces the FCDOM values to < 4.5 mg m^–3 (Fig. 3c) and diffuses the subsurface temperature minimum, which in Nansen Sound is associated with the FCDOM maximum (Figs. 2d, and 2f, and 3b, and 3c).

Further downstream in Jones Sound, the subsurface FCDOM maximum is eroded after passing the sills in Cardigan Strait and Fram Sound (Fig. 3c). This seems to be attributed to the enhanced tidally driven vertical mixing over the sills^5,9. Moreover, in Jones Sound the depth range of salinities 32–34 is significantly expanded compared to the upstream (Fig. 3a). In Nansen and Eureka Sounds and Norwegian Bay, the water layer within this salinity range occupies depths from ~ 50 to 180 m. In contrast, in Jones Sound this layer expands from ~ 30 m to 270 m depth (Fig. 3a) overlapping the AW layer traced by the 0 °C isotherm at 215–280 m depth (Fig. 3b). Furthermore, st. 65 in Jones Sound is different from those occupied upstream and downstream. Instead of the subsurface FCDOM maximum, it shows FCDOM gradually increasing with depth to > 4.5 mg m^–3 at 450 m depth (Fig. 3c). Note that st. 1, 65, and 18 were taken on 12 August, 2 September, and 22 August 2024, respectively. It is very likely that the spatial variability captured in Jones Sound is attributed to synoptic eddies frequently observed in this area using satellite imagery (Fig. S1). This is supported by numeric simulations demonstrating persistent eddies in the area¹⁰. Finally, the Baffin Bay portion of this section (st. 64) down to ~ 120 m depth shows the salinity and FCDOM patterns like those of the lower Eureka Sound and Norwegian Bay. However, for depths > 200 m, temperatures up to ~ 0.5 °C and salinities > 34.3 clearly indicate that this water is generated by lateral interactions with water of the WGC. FCDOM in this water is also increased to > 4 mg m^–3 indicating contributions from the WGC and the high CDOM bottom water of Baffin Bay.

Northeastern passage throughflow

Passing the northeastern passageway, PW undergoes gradual transformation due to lateral interaction with the water mass assembly of the WGC and vertical mixing (Fig. 4). PW maintains its thermohaline and FCDOM signature through Robeson Chanel, Hall Basin and Kennedy Channel until Kane Basin where the water column > 100 m depth starts to become fresher (Fig. 4a). This freshening extends downstream to Smith Sound and northern Baffin Bay. Moreover, in Smith Sound the subsurface FCDOM maximum spreads over a greater depth range (Fig. 4c). Overall, at ~ 300 m depth, the salinity decrease and FCDOM increase from Robeson Chanel to Smith Sound is ~ 0.6 (Fig. 4a) and 0.7 mg m^–3 (Fig. 4c), respectively. In northern Baffin Bay, the subsurface water layer from ~ 50 m to 120 m depth at salinities between ~ 31.5 to 33 and FCDOM > 4 mg m^–3 can still be attributed to PW outflow through Nares Strait. However, the underlying water layer with FCDOM from 3.7 to 4.2 mg m^–3 is strongly impacted by freshened and CDOM elevated water of the WGC, containing a fraction of AW resided below 200–220 m depth (Fig. 4).

Lancaster sound

Two sections along the northeastern and northwestern passageways overlap in northwestern Baffin Bay at st. 64 (Fig. 1b). Another transect of stations across the mouth of Lancaster Sound (st. 19–23) shows interactions between the Nares Strait PW, the Lancaster Sound PW flow through the CAA, and the recirculating water of the WGC. The water mass assembly from northern Baffin Bay (including Nares Strait PW and WGC AW) flows into the northern rim of Lancaster Sound before meeting a shallow sill at Barrow Strait and recirculating back towards Baffin Bay along the southern rim^11,12. The Lancaster Sound inflow is traced by enhanced FCDOM values up to 3.7 mg m^–3 in the subsurface layer (the Nares Strait PW) and in the underlying layer primarily composed of the WGC AW (Fig. 4f). The Lancaster Sound outflow is relatively fresh (< 33.5) and traced by elevated FCDOM values from 3.5 to 4 mg m^–3 down to ~ 200 m depth. The underlying water of the recirculating WGC centered at ~ 370 m depth shows elevated FCDOM values of about the same magnitude as PW (Fig. 4f). The majority of this water with temperatures from 0℃ up to 1.2℃ is of Atlantic origin (Fig. 4e).

Discussion

In the AO, CDOM can be treated as a relatively conservative tracer for PW and its advection^8,13. CDOM in the Arctic originates predominantly from two sources: (i) terrigenous organic matter primarily attributed to the Eurasian and American continental runoff water^14,15, and (ii) subsurface interactions with organic-rich sediments on the Arctic shelves^8,15,16. In the CB, the FCDOM subsurface maximum originates from interaction with shelf sediments in the Chukchi Sea during the formation of PWW^8,16. We note, however, that CDOM cannot be fully treated as a conservative tracer. In the surface layer, CDOM can be derived from high primary production¹⁷ being slowly mixed downward to the upper portion of PW.

First, we focus on the properties of water exiting the AO through Nansen Sound and Nares Strait and water entering northern Baffin Bay via the WGC. Second, we discuss outflow of PW through the northernmost passageways from the AO to Baffin Bay.

In the CB, the shallow temperature maximum ( > − 1.2 °C, < 100 m depth) coinciding with salinities of ~ 31 is usually assigned to Pacific Summer Water^18–20 (PSW; Figs. 2a and S2b). This water with intermediate values of FCDOM < 4.24 mg m^–3 (Fig. 2c) is formed by the physical mixing of surface freshwater and subsurface PW^8,21. PSW was captured by CTD and FCDOM profiles taken in the CB about 900 km from the entrance of Nansen Sound by Ice Tethered Profiler (ITP) #64 in August-September 2012 (Figs. 1a, S2, and black lines in Figs. 2a–c). The remnants of PSW can still be traced by the shallow temperature maximum at salinities 31 − 32 at the entrances of Nansen Sound and Robeson Channel (Fig. 2), but this signal gradually disappears further downstream (Figs. 3 and 4, and S2).

The underlying upper halocline water of the CB with temperatures down to − 1.5°С and salinities of ~ 32–33.5 (blue shading in Fig. 2), is PWW originating from the Chukchi Sea shelf²². This water mass is the most common winter product formed during freezing and brine rejection in the Bering and Chukchi seas^23,24. A subsurface maximum in FCDOM observed at the gateways of the northernmost passageways to Baffin Bay at intermediate salinities between 32 and 33.75 (Fig. 2d and e) is consistent with previously documented salinity-FCDOM relationships in the CB^8,16 (Fig. S2a). The temperature increase from − 1.4 °C in the CB (Fig. S2b) to − 1°С in Nansen Sound and Robeson Channel (Fig. 2a) indicates significant warming of PWW “en route” to the CAA. We attribute this warming to enhanced vertical mixing with underlying warm AW over the rough bottom topography of the CAA continental margins^25–27. This is also confirmed by the dispersion of the lower portion of the FCDOM maximum in Nansen Sound and Robeson Channel compared to the CB (Figs. 2c and S2). Moreover, the level of dispersion consistently increasing from the CB to Robeson Channel and Nansen Sound (Fig. 2c) suggests that PWW in Nansen Sound originates from the Transpolar branch of the PW flow that partially turns westward when reaching the northern CAA. In this context, the longer transit time to Nansen Sound compared to Nares Strait also contributes to modifications of PW such as the gradual weakening of the subsurface FCDOM maximum associated with PWW (Fig. 2c) and the subsurface temperature maximum attributed to PSW (Fig. 2a).

In northeastern Baffin Bay, the water column is composed of waters from the WGC that circulate cyclonically and join the southward flowing water from Nares Strait, Jones Sound and Lancaster Sound to form the Baffin Island Current²⁸ (Fig. 1a). The WGC comprises AO outflow waters from Fram Strait that are advected south in the East Greenland Current (EGC), joined by glacier meltwater from Greenland, and AW supplied by the Irminger Current (IC; Fig. 1a). The EGC supplies cooler and relatively fresh Arctic water, while the IC recirculates warm and saline AW. As a result of their interactions, the water column of northern Baffin Bay below 150 m depth resembles that of the WGC²⁸. It is fresher and significantly warmer compared to the AO gateway in Robeson Channel²⁸ (Fig. 2). For example, at 240 m depth, the salinity and temperature differences reach 0.6 and 1.7 °C, respectively (Figs. 2a and b). Instead of the subsurface FCDOM maximum at the AO gateways, FCDOM of the WGC gradually increases with depth from ~ 2.65 mg m^–3 to ~ 4 mg m^–3 at 500 m depth (Fig. 2c). Apparently, in the WGC, the limited photodegradation in the Baffin Bay deep water²⁹ and the vertical mixing and upwelling over the northwest Greenland continental slope and the North Water polynya³⁰ both contribute to a gradual increase in FCDOM concentrations with depth. Note that the bathymetry of the northwest Greenland shelf impacted by shelf-break glaciation during the Last Glacial Maximum is very rough^31–33 favouring upwelling and mixing at depth. High CDOM in the deep water of Baffin Bay is due to the very long residence time and a gradual degradation of organic matter^34–36. We use the distinct difference in temperature, salinity and FCDOM between the AO outflow and the WGC (Fig. 2) for tracing transformations of the PW flow through the CAA and Nares Strait passageways to Baffin Bay.

Our results show that outflow of PW through the northernmost passageways from the AO to Baffin Bay remains traceable using FCDOM until this signal becomes disrupted by rough bottom topography and interaction with water of the WGC.

Following the northwestern passageway, a rough bottom topography and associated vertical mixing attenuates and/or disperses the consolidated FCDOM maximum. It first becomes weakened bypassing the sill in Eureka Sound north of Stor Island (Figs. 1b and 3c). Still the Pacific-derived FCDOM signal is partially maintained downstream in Norwegian Bay by the inflow of PW through Peary and Sverdrup Channels (Figs. 1a and 3c). Further downstream in Jones Sound, water flowing through the shallow Cardigan Strait (and Fram Sound) is believed to consist primarily of PW³⁷. However, our data shows that the FCDOM subsurface maximum is disrupted passing through Cardigan Strait and Fram Sound. We explain this pattern by water dynamics through these straits. First, water entering Jones Sound via Cardigan Strait and Fram Sound occupies only the top 65 m of the water column, and below this depth outflow dominates¹⁰. Thus, it is not very surprising that in western Jones Sound the FCDOM maximum associated with PW is weakened and dispersed (Fig. 3c). Second, significant freshening of the Jones Sound water column below 100 m depth (Fig. 3b) indicates vertical mixing down to ~ 200 m depth (Fig. S3). Finally, freshening below 200 m depth is likely linked to fresher waters transported by the WGC into Jones Sound through Lady Ann Strait (Figs. 1b and S3). This freshening is also consistent with water temperature and FCDOM gradually increasing with depth at st. 64 and 65 (Figs. 3c and S3c) that is a characteristic pattern of FCDOM vertical distribution in the WGC (Fig. 2c). Because of the fact that the FCDOM below 200 m depth at st. 64 and 65 is still greater than that observed in the WGC waters (st. 120), we suggest additional contribution from the high CDOM bottom water of Jones Sound and northwest Baffin Bay.

In the northeastern passageway, the FCDOM maximum retains its identity from Robeson Channel to Smith Sound (Fig. 4c). In Kane Basin, the consolidated FCDOM maximum > 5 mg m^–3 remains apparent (Fig. 4c). Downstream in Smith Sound, a rough bottom topography and associated vertical mixing over the Kane Basin sill⁶ seem to disperse the FCDOM maximum downward, elevating FCDOM values below the PW layer down to the seafloor (Fig. 4c). Moreover, significant freshening > 100–150 m depth (Fig. 4a) clearly indicates the lateral mixing with the WGC water with FCDOM gradually increasing with depth > 100 m (Figs. 2f and g, and 4c). Downstream in northwestern Baffin Bay, the upper portion of water column is conditioned by the PW outflow through Nares Strait merging with the WGC. The PW signal remains traceable, but is strongly attenuated by vertical mixing dispersing the subsurface FCDOM maximum downward (Figs. 2e and 4c). The lower portion of water column is comprised by warm, relatively fresh and CDOM elevated water of the WGC mixed with the Baffin Bay deep water enriched with CDOM (Fig. 2g).

In summary, our data shows that the subsurface FCDOM maximum traces the throughflow of PW from the AO to Baffin Bay until this signal is disrupted by vertical mixing and lateral mixing with water of the WGC. Moreover, compared to conditions observed in the CB, the lower portion of PW undergoes transformations before passing through the CAA and Nares Strait. It seems to be due to enhanced vertical mixing over rough bottom topography of the CAA continental margins. Consequently, waters of Pacific and Atlantic origin are transformed through warming and cooling, respectively, as they pass over the outer margins of the CAA shelf. This also disperses the lower portion of FCDOM maximum. A consecutive dispersion of the FCDOM maximum from the CB to Robeson Channel and Nansen Sound indicates that PW in Nansen Sound likely originated from the Transpolar branch of the PW flow in the AO that partially turns westward by reaching the northern CAA. This supports simulations revealing that PW entering the CAA can be originated from the Transpolar branch³⁸.

Overall, our results confirm suggestions that FCDOM assessed by optical sensors is a powerful tool for tracing water mass origins and circulation patterns^8,35. FCDOM can act as a tracer of PW revealing how PW transits within the AO, flows through the CAA and Nares Strait, and interacts with water of the WGC. The combined use of CTD and FCDOM provides valuable insights into deciphering water mass mixing in northern Baffin Bay, Nares Strait, the narrow and shallow channels of the CAA, and the CB adjoining the CAA.

Methods

Between 12 and 22 August 2024, 18 conductivity-temperature-depth (CTD) and FCDOM profiles were collected in Nansen and Eureka Sounds, Norwegian Bay and Jones Sound from the CCGS Des Groseilliers (st. 1–18 in Fig. 1b). Among all these stations, only st. 7 was located in the marginal ice zone where the landfast ice edge resided during winter time until late July. This data set was complemented by st. 65 taken in Jones Sound, and st. 64 taken just outside Lady Ann Strait on 2 September 2024 from the CCGS Amundsen (Fig. 1b). All these stations along with st. 19–23 across Lancaster Sound (22 August, CCGS Des Groseilliers) make up the oceanographic section representing the northwestern passageway of PW into the CAA (Figs. 1b and 3).

From 12 August to 4 September 2024, 40 CTD and FCDOM profiles were measured from the CCGS Amundsen (Leg 3) in northern Baffin Bay, Nares Strait and the adjoining fjord systems. Among them, we selected 5 stations (34, 50, 63, 64, and 70) to compile the oceanographic section following the Nares Strait passageway of PW (Fig. 1b). Among these stations, only st. 34 was taken in the marginal ice zone extended over the entire Robeson Channel. These profiles were complemented by 8 stations carried out from the CCGS Amundsen (Leg 4) between 8 and 26 September 2024 comprising the oceanographic section following the northeastern passageway of PW to Baffin Bay (Figs. 1b and 4). Note that Lancaster Sound represents another separate passageway for PW to enter Baffin Bay. However, this passageway is intentionally excluded from the scope of this study due to insufficient data coverage.

Additional CTD and FCDOM profiles are used to provide context for the water masses of the WGC. We selected st. 120, taken from the CCGS Amundsen (Leg 4) on 22 September 2024 in northeastern Baffin Bay, as being representative of water column conditions influenced by the northward flowing WGC (Figs. 1b and 2).

The CTD observations from CCGS Des Groseilliers were carried out with a Sea-Bird Scientific SBE-19plus CTD that was accurate to ± 0.005 °C and ± 0.0005 S m^–1. The CTD observations aboard CCGS Amundsen were taken using a Sea-Bird 9plus CTD that was accurate to ± 0.001 °C and ± 0.0003 S m^–1. CTDs were calibrated prior to the expedition. Throughout the manuscript, we used practical salinity calculated directly from the conductivity and temperature of seawater as defined by Practical Salinity Scale 1978³⁹. All CTD casts were taken through the full water column (CCGS Amundsen) or to a maximum depth of 520 m (CCGS Des Groseilliers). All CTD data were 1-meter binned.

The CTDs aboard CCGSs Des Groseilliers and Amundsen were outfitted with a Wet Labs ECO fluorometer for measuring FCDOM for EX/EM = 370/460 nm. The FCDOM sensor sensitivity is 0.09 parts per billion (ppb). The FCDOM sensor aboard CCGS Des Groseilliers was calibrated prior to the expedition. However, the resulting measurements were not post-calibrated against discrete samples. The FCDOM sensor aboard CCGS Amundsen was not calibrated resulting in data offset relative to that of CCGS Des Groseilliers. For overlapping, the FCDOM profiles taken from CCGS Amundsen were adjusted by 1.77 mg m^–3 assuming that for depths > 500 m, the FCDOM values at the Nansen Sound entrance are consistent with that at the Nares Strait entrance. FCDOM profiles were averaged with a 7-m moving window. We limited our analysis based on FCDOM data only to PW qualitative tracing.

An additional set of complementary CTD and FCDOM profiles from the CB is used to provide context for the water masses we observed in Robeson Channel and Nansen Sound. We adopted data collected by an ITP⁴⁰ over the CB at ~ 78°N, 136°W (Fig. 1a). We specifically use the mean of profiles #1–125 from ITP-64⁴¹ occupied in August-September 2012. Note that among all ITPs in the CB adjoining the CAA, only ITP-64 carried the FCDOM optical sensor.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

IAD: Conceptualization, Methodology, Supervision, Investigation, Writing – original draft, Visualization, Formal analysis, Data curation. CAS: Methodology, Investigation, Visualization, Validation, Formal analysis, Writing – review & editing. DB: Data curation, Writing – review & editing. BR: Methodology, Writing – review & editing. SAK: Data curation. JE: Data curation, Writing – review & editing. TB: Methodology, Investigation, Writing – review & editing. DDJ: Project administration, Supervision, Funding acquisition.

Funding

Funding for this research was provided by the Canada Excellence Research Chairs program led by Dorthe Dahl-Jensen. Oceanographic observations aboard CCGSs Des Groseilliers and Amundsen were also funded by the Canada Foundation for Innovation through the Amundsen Science Corporation.

Data availability

All data that support the findings of this study are openly available⁴² at the following URL/DOI: https://doi.org/10.17632/zx2y57zjbk.1.

Declarations

Competing interests

The authors declare no competing interests.