Several studies have quantified the exchange of sea ice between the Arctic Ocean and the CAA (e.g., Agnew et al., 2008 ; Howell et al., 2013 ; Kwok, 2006 ), but the temporal domain of these previous studies restricted the assessment of long‐term variability and trends in relation to the mechanism put forth by Melling ( 2002 ). Further, the ice area flux into Nares Strait is known to contribute to the loss of Arctic Ocean sea ice, especially during 2007 (Kwok et al., 2010 ), but in 2016, the Arctic Ocean ice area flux into the CAA was almost double that of the 2007 Nares Strait flux. Here, we use a 22 year record (1997–2018) of Arctic Ocean‐CAA ice exchange derived from RADARSAT‐1 and RADARSAT‐2 to (i) analyze and identify the processes responsible for Arctic Ocean‐CAA area and volume flux variability and trends from 1997 to 2018 and (ii) place the large Arctic Ocean ice area flux into the CAA in 2016 within the context of the 22 year record.

Sea ice area within the CAA during the summer months has decreased by 4.8%/decade from 1968 to 2016 (Derksen et al., 2018 ; Mudryk et al., 2018 ), but the effect of warming on its ice dynamic processes could be counterintuitive. Specifically, Melling ( 2002 ) suggested that warming in the CAA could result in the increased transport of ice from the Arctic Ocean into the CAA. The suggested mechanism was that the weakening of the ice arches in the northern CAA that block the southward transport of ice from the Arctic Ocean into the CAA will break up earlier and form later, providing an increased opportunity for the inflow of thick heavily ridged MYI from the Arctic Ocean. Indeed, Haas and Howell ( 2015 ) found that MYI originating from the Arctic Ocean is found in the southern channels of the CAA and Barber et al. ( 2018 ) found that thick Arctic Ocean MYI can be transported from the CAA to the East Coast of Newfoundland in a single season.

Map of the Canadian Arctic Archipelago (inset) with the location of the Arctic Ocean exchange gates. The RADARSAT‐2 imagery is from May 16, 25, and 29 2019. RADARSAT‐2 Data and Products © MacDonald, Dettwiler and Associates Ltd. (2019). All Rights Reserved. RADARSAT is an official trademark of the Canadian Space Agency.

Canada's Changing Climate Report indicates that annual average air temperature in northern Canadian regions have increased 2.3 °C over the period of 1948–2016 and relative to 1986–2005 are projected to increase 7.8 °C by 2081–2100 (Zhang et al., 2019 ). The Canadian Arctic Archipelago (CAA) is a unique sea ice region located in northern Canada, adjacent to the Arctic Ocean (Figure 1 ). How sea ice in the CAA responds to warming is of considerable importance because (i) ice arches across the channels in the CAA have historically blocked the inflow of multiyear ice (MYI) from the Arctic Ocean for most of the year (Melling, 2002 ), (ii) Arctic freshwater either as ice or seawater is transported through the channels of the CAA to the Atlantic where it influences large‐scale ocean circulation (Kuhlbrodt et al., 2007 ), (iii) the shipping lanes of the Northwest Passage are located within the CAA, where it has been found that vessel traffic during the summer months has increased (Pizzolato et al., 2014 ), and (iv) the northern CAA represents a region of the Last Ice Area, which will be the only refuge for sea ice‐dependent species when the majority of the Arctic is sea ice‐free during the summer (Derksen et al., 2018 ; Lange et al., 2019 ; Moore et al., 2019 ).

CAA‐Arctic Ocean ice area flux was calculated at the six exchange gates shown in Figure 1 . The M'Clure gate has a 183 km aperture and the Ballantyne Strait, Wilkins Strait, Prince Gustaf Adolf Sea, Peary Channel, and Sverdrup Channel gates, collectively referred to as the Queen Elizabeth Islands (QEI) gates, have a total aperture 370 km. Ice area flux was calculated from May to November using the method described in Howell et al. ( 2013 ), which is based on Agnew et al. ( 2008 ) and Kwok ( 2006 ). Briefly, sea ice motion from each sequential pair of RADARSAT‐1 or RADARSAT‐2 imagery (~1–3 day time separation) was estimated using the Komarov and Barber ( 2014 ) tracking algorithm, interpolated to a 30 km region surrounding the exchange gate and then sampled at 5 km intervals across the gate. The ice area flux ( F ) was then calculated using F = Σ c i u i Δ x , where c i is the ice concentration obtained from the closest Canadian Ice Service ice chart to the image pair, u i is the ice speed normal to the flux gate at the i th location, and Δ x is the spacing along the gate (5 km). Howell et al. ( 2013 ) estimated that the uncertainty in F at gates shown in Figure 1 is 12–14 km 2 /day. All F values were summed into monthly totals from May to November. The period was restricted from May to November because of landfast conditions persisting within the CAA for most of the year (Canadian Ice Service, 2011 ) resulting in negligible ice exchange from December to April. Monthly ice volume flux was then determined from the product of monthly ice area flux and the average of monthly PIOMAS ice thickness values within the 30 km region surrounding each exchange gate. We note that interannual ice thickness variability from PIOMAS is similar when compared to CryoSat‐2 observations (Moore et al., 2019 ) but PIOMAS has been found to underestimate ice thickness values north of the CAA (Schweiger et al., 2011 ); therefore, we expect our volume flux to be underestimated.

The data used in this study consisted primarily of RADARSAT‐1 and RADARSAT‐2 synthetic aperture radar (SAR) imagery at ~200 m spatial resolution. Supporting data included weekly total, MYI and seasonal first‐year ice (FYI) concentration from the ice charts in the Canadian Ice Service Digital Archive (Tivy et al., 2011 ), monthly ice thickness from Pan‐Arctic Ice Ocean Modeling and Assimilation System (PIOMAS; Zhang & Rothrock, 2003 ), monthly sea level pressure (SLP) from ERA5 (Copernicus Climate Change Service (C3S), 2017 ) and melt season duration determined from the passive microwave algorithm (Markus et al., 2009 ) by taking the difference between the first date of melt and the first date of freeze analogous to Howell et al. ( 2009 ).

3 Results and Discussion

3.1 Variability and Trends in Ice Area and Volume Flux, 1997–2018 Figure 2a illustrates the total May to November ice area and volume flux between the CAA and the Arctic Ocean from 1997 to 2018. Positive/negative sign indicates Arctic Ocean ice inflow/outflow. There has been considerable variability in the area flux over the 22 year record, but the majority of years experienced Arctic Ocean inflow. The 22 year average was 23 × 103 km2 (±44 × 103 km2), which represents ~55% of the average annual ice area flux through Nares Strait (42 × 103 km2; Kwok et al., 2010) and ~3% of the average annual ice area flux through Fram Strait (~793 × 103 km2; Kwok, 2009; Smedsrud et al., 2017). With respect to volume flux, variability was also high with a 22 year average of 61 km3 (±59 km3). This represents ~43% of the average volume flux though Nares Strait (141 km3; Kwok et al., 2010) and ~3% of the average annual volume flux through Fram Strait (~2 × 103 km3; Kwok, 2009; Ricker et al., 2018). Of note in Figure 2a was that in 2016, the ice area flux of 161 × 103 km2 was 7 times larger and the volume flux of 225 km3 was ~4 times larger than their respective 22 year averages. Figure 2 Open in figure viewer PowerPoint Time series of the seasonal (May to November) total area and volume flux for the (a) Canadian Arctic Archipelago, (b) M'Clure Strait, and (c) Queen Elizabeth Islands. Positive and negative flux signs correspond to Arctic Ocean ice inflow and outflow, respectively. Although the Arctic Ocean ice area flux into the CAA has increased since 1997, no statistically significant trend was present over the 22 year record. However, looking at the individual flux gates indicates that the ice area flux at the QEI gates (i.e., northern CAA) has significantly (95% confidence) increased by 103 km2/year from 1997 to 2018 (Figure 2c). In addition, it is apparent that the M'Clure Strait represents the larger component of the CAA area flux but the QEI has the larger volume flux (Figures 2b and 2c). The 22 year average volume flux was 19 km3 (±43 km3) and 42 km3 (±34 km3) at the M'Clure Strait and QEI gates, respectively. The volume flux was larger for the QEI because of thicker ice. The 22 year average ice thickness from May to November at the QEI gates was 3.3 m (±0.7 m) compared to 1.4 m (±0.7 m) at the M'Clure Strait.

3.2 Mechanisms Responsible for Increased Arctic Ocean Ice Inflow What has caused the increased amount of Arctic Ocean inflow into the CAA via the QEI gates over the 22 year record? Large‐scale atmospheric circulation almost always forces Arctic Ocean sea ice up against the northern CAA (Melling, 2002). This is evident from Figure 2, which indicates that Arctic Ocean sea ice inflow dominates the seasonal average for the QEI gates, whereas at the M'Clure Strait gate there is more inflow and outflow variability. Recall that Melling (2002) suggested that increased Arctic Ocean inflow could result from a longer duration of flow because of ice arches in the QEI collapsing earlier and forming later. The timing of collapse, formation, and ice flow duration at the QEI exchange gates from 1997 to 2018 is shown in Table S1 in the supporting information that was determined by visual inspection of the Canadian Ice Service weekly ice charts (e.g., Figures S1 and S2). On average, the arches typically collapsed in early August and formed again in late October. In 1997, the arches at the QEI exchange gates did not collapse, and in 2014, only the Peary Channel collapsed, resulting in negligible ice exchange for both years (Figure 2c). Over the 22 year record, the duration of flow has increased slightly for the ice arches at the QEI exchange gates but not yet at the 95% confidence level. However, there is a statistically significant detrended correlation (99% confidence) between the duration of flow and ice area flux of R = 0.66 (Figure 3a), which provides statistical evidence for the Melling (2002) mechanism. Kwok et al. (2010) also demonstrated that ice flow duration exerts considerable influence on the ice area flux through Nares Strait. Figure 3 Open in figure viewer PowerPoint The relationship between (a) seasonal average flow duration and May to November total ice area flux at the Queen Elizabeth Islands exchange gates from 1997 to 2018 and (b) seasonal average ice thickness and ice speed at Queen Elizabeth Islands exchange gates from 1997 to 2018. Have other factors contributed to the increased ice area flux trend at QEI exchange gates the 22 year record? The detrended correlation between ice speed and ice area flux at the QEI exchange gates is R = 0.62 (99% confidence), and we suggest that the positive trend in ice area flux into the QEI from the Arctic Ocean has also been influenced by the ice speed increasing over the 22 year period. The increase in ice speed has in turn been facilitated by a decrease in the overall ice thickness at the QEI exchange gates. Evidence for this mechanism is provided in Figure 3b, illustrating that the relationship between ice thickness and ice speed at the QEI exchange gates is relatively strong and statistically significant (99% confidence) with a detrended correlation of R = −0.63. In addition, there are statistically significant trends in both ice thickness (−0.06 m/year; 99% confidence) and speed (0.04 km/day/year; 95% confidence) through the QEI exchange gates over the 22 year period. Previous studies have shown that faster ice speed is associated with thinner ice that is more susceptible to geostrophic wind forcing (Kwok et al., 2013; Moore et al., 2019; Olason & Notz, 2014; Zhang et al., 2012). Even with unobstructed flow and faster moving ice, there still must be leeway from open water to facilitate the ice area flux from the Arctic Ocean. We suggest that a changing ice regime, together with a longer melt duration within the CAA, has increased the open water leeway availability such that there is more opportunity for Arctic Ocean sea ice to be transported southward through the CAA during the summer months. Figure 4 confirms that the melt duration within the CAA has increased by 5.1 days/decade from 1979 to 2018 and also illustrates that September MYI has decreased significantly (99% confidence) by 12.9%/decade and July seasonal FYI has increased significantly (95% confidence) by 5.5%/decade indicating that a transition to a younger and thinner seasonal FYI regime has occurred within the CAA from 1979 to 2018. MYI is thicker than FYI and more difficult to melt and together with the shorter melt durations in the early part of the time series contributed to more congested ice conditions within the CAA during the summer months and limited Arctic Ocean inflow. However, in recent years the melt duration has significantly increased and there is significantly more FYI at the start of the melt season, which breaks up earlier and forms later. Together, these recent changes have resulted in more open water to accommodate ice entering the CAA from the Arctic Ocean during the melt season. Figure 4 Open in figure viewer PowerPoint Time series and trends of average July seasonal first‐year (FYI) area, average September multiyear ice (MYI) area and melt season duration anomalies in the Canadian Arctic Archipelago from 1979 to 2018.