4.1. Spatiotemporal Distribution of Melt Onset, Freeze Onset, and Melt Duration
 The date of MO, FO and MD length varied among and between regions of the CAA from 2000 to 2007. Within the CAA the average date of MO occurred on day 150, the average FO occurred on day 266, and 116 was the average number of days of melt (Table 2). On average, MD was the in shortest in the CAA at 103 days in 2004 and the longest in 2006 at 136 days (Table 2). Air temperature over Resolute Bay, Nunavut located in the central CAA (Figure 1) that has previously been used as a proxy indicator of the climate regime of the CAA [e.g., Falkingham et al., 2001; Atkinson et al., 2006] shows that 2004 experienced the coldest temperatures and 2006 experienced the warmest temperatures in almost every month from 2000 to 2007 (Table 3).
Table 2. Mean Year Day of Sea Ice Melt Onset, Freeze Onset, and Melt Duration in the Canadian Arctic Archipelago as Detected by the QuikSCAT σ° for 2000–2007a
|Year||Melt Onset (day)||Freeze Onset (day)||Melt Duration (days)|
|2000||153 (13.4)||260 (20)||107 (28.2)|
|2001||148 (11.9)||266 (19)||118 (27.2)|
|2002||152 (8.8)||267 (14.3)||115 (19.5)|
|2003||152 (13.1)||264 (20.1)||112 (29.0)|
|2004||154 (11.5)||257 (16.3)||103 (23.4)|
|2005||145 (9.7)||264 (16.4)||119 (22.1)|
|2006||141 (9.9)||277 (21)||136 (26)|
Table 3. Monthly Air Temperatures From the Environment Canada Meteorological Weather Station Located at Resolute Bay, Nunavut for 2000–2007a
 Spatially, the average MO date occurred first and average FO date occurred last within the Amundsen, Western Arctic Waterway and Eastern Parry Channel regions (Figures 6 and 7) . The QEI regions were the last to experience MO (Figure 6) but the first to experience FO (Figure 7). As a result, the QEI experienced shorter MDs between 79 and 110 days, compared to longer MDs in the more southerly regions of the CAA from 2000 to 2007 (Figure 8). Both Smith [1998b] and Belchansky et al. [2004b] found that the shortest sea ice MDs in the Arctic occur immediately north of the QEI. The M'Clure and Viscount-Melville regions also experienced shorter average FO dates (Figure 7) as well as reduced MDs (Figure 8). Regions where reoccurring polynyas can form early in the season (i.e., Eastern Parry Channel, Amundsen, and Jones Sound) typically experienced longer MDs (Figure 8). These observations are in agreement with the results of Stroeve et al. .
 MD from 2000 to 2002 exhibited a similar spatial distribution but MD was shortest for the QEI in 2000 (Table 2 and Figure 8). The QEI region experienced longer MDs for 2001–2002 (91 to 99 days) compared to a shorter MD observed in 2000 (79 days). Although MO occurred later than most other years it is the early FO date (day 239) observed in 2000 that is largely responsible for the shorter MD. Alt et al.  reported that FO occurred early in 2000 for the high-latitude regions of QEI because of northerly atmospheric flow off the Arctic Ocean.
 MD for 2003 was shorter than previous years within the QEI, M'Clure, Viscount-Melville, and Peel Sound regions at 94, 110, 107, and 121 days, respectively. While MO timing was fairly similar compared to previous years, FO dates are noticeably earlier (Figure 7). MO occurred later than average for most regions within the CAA for 2004 (Figure 6) and FO also occurred earlier than previous years (Figure 7). Temperatures were lower for 2004 compared to 2000–2003 for almost every month (Table 3).
 The year 2005 had the second longest MD within the CAA at 119 days (Table 2) and experienced a more homogenous spatial distribution for MO and MD compared to previous years (Figures 6 and 8) whereas the FO was early and similar to 2004 (Figure 7). From 1979 to 2005, Stroeve et al.  found that in 2005 the Arctic experienced its longest melt season. The year 2006 had an even longer MD (136 days) compared to 2005 and temperatures during the melt season for 2006 were warmer than 2005, especially during the shoulder seasons within the CAA (Table 3).
 The year 2007 experienced a later than average MO date (154) and the second latest FO date resulting in a just slightly longer than average MD of 117 days. Temperatures were low during the start and end of the melt season but compared to previous years higher temperatures were found during the peak melt months of July and August (Table 3). Comiso et al.  found anomalously warmer temperatures over the central Arctic for 2007 during these months.
 Overall the CAA has experienced considerable spatiotemporal variability in melt season parameters over the past 8 years. It has previously been observed that the CAA has experienced MYI increases from 2000 to 2004 followed by decreases in 2005–2006 [Howell et al., 2008]. MYI has different physical and thermodynamic properties than seasonal FYI [Maykut and Untersteiner, 1971] and we would expect changes in MYI to be reflected in melt season parameters. Moreover, despite the 2007 extreme record low sea ice in the Arctic, 1998 still represents record low sea ice conditions within the CAA. It therefore seems important to examine the linkages between melt season parameters in relation to MYI within the CAA.
4.2. Can MYI Within the Canadian Arctic Archipelago Survive Longer Melt Seasons?
 Figure 9 depicts the MD in relation to the observed MYI coverage on the last week of September (i.e., the amount that survived the melt season), the amount of FYI promoted to MYI, and the amount of MYI dynamically imported within the CAA from 2000 to 2007. Indeed, the CAA has accumulated increased amounts of MYI from both FYI promotion and dynamic import from 2000 to 2004. The increases are related to the recovery from record minimum MYI conditions in 1998 caused by an anomalous warming event within the CAA [Atkinson et al., 2006; Howell et al., 2008]. FYI to MYI promotion accounted for slightly more of the increases compared to MYI dynamic import (Figure 9).
Figure 9. Bar graph illustrating multiyear ice coverage on the last week of September, first-year ice promoted to multiyear ice, multiyear ice dynamically imported, and melt duration within the Canadian Arctic Archipelago from 2000 to 2007.
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 Longer MDs are apparent from 2005 to 2007 and both MYI dynamic import and FYI promotion almost “shut down” but the resulting MYI decreases being a function of a longer MD are only strongly apparent in 2006 (Figure 9). The spatial distribution of MYI on the first week of April (i.e., immobile landfast conditions) and the last week of September are shown in Figures 10 and 11, respectively. On average, high concentrations of MYI are present in the QEI and the M'Clure regions that extend to Viscount-Melville and further south to the M'Cintock and Larsen Sound regions for 2005–2006, despite these regions experiencing longer MDs (Figures 8, 10, and 11). On the basis of these observations, it seems apparent that MYI can survive longer melt seasons even in the southern regions without replenishment. This raises the more compelling question of how this process is realized.
Figure 10. Spatial distribution of multiyear ice concentration in the Canadian Arctic Archipelago on the first week of April from 2000 to 2007. Legend is ice concentration in tenths.
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Figure 11. Spatial distribution of multiyear ice concentration in the Canadian Arctic Archipelago on the last week of September from 2000 to 2007. Legend is ice concentration in tenths.
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 The answer lies in the relationship between the increased thickness of MYI and the oceanic heat flux. Specifically, surface ablation from solar radiation cannot be the dominant heat source that ablates MYI in the CAA. Instead, it must slowly ablate from the underside, even as it slowly migrates to the southern regions of the CAA. Although only focused within the QEI region, Melling  hypothesized this process by demonstrating that for MYI within the QEI the small 5–15 W m2 oceanic heat flux is responsible for ablating the MYI. MYI is very thick within the QEI (3.4 m on average) and its coverage (and concentration) remains high during the summer melt season. Therefore, the ocean is not likely to be warmed as much by solar radiation and accelerate the ablation process. Maykut and McPhee  found that most of the ocean heat flux is solar energy deposited through leads (88%) and only 12% was transmitted through MYI less than 1.8 m thick. Higher-magnitude values were reported by Perovich  that was attributed to a thinner (1.58 m) MYI cover. Thicker MYI results in less solar energy input to the sea ice–ocean system and this shallower energy input slows the positive sea ice–albedo feedback [Perovich, 2005]. As a result, even if MO occurs early within the CAA, atmospheric forcing from solar radiation is still likely insufficient to significantly ablate the MYI cover.
 Looking at the time series of MYI coverage in the QEI along with the APP-x derived Q* from 2000 to 2004 illustrates that MYI experiences only subtle decreases during the peak times of the melt season whereas most of the FYI completely ablates (Figure 12). Even the longer melt season in the QEI for 2002 resulted in minimal MYI losses during the melt season (Figures 8 and 12). An examination of MYI conditions from 2000 to 2004 within Viscount-Melville, M'Clintock, and Larsen Sound regions that are outside of the QEI reveals that a similar process occurs as MYI migrates southward. The aforementioned regions contained a mix of MYI and FYI with MYI eventually dominating the Viscount-Melville and M'Clintock icescape in 2003 (Figures 13 and 14). Within these regions MYI remains relatively stable but the FYI completely ablates and MYI increases typically occurred following FYI reductions (Figures 13, 14, and 15). While some of this MYI was FYI promoted to MYI a considerable portion has in fact slowly migrated from the QEI (Figures 13, 14, 15, and 16). This loss of MYI in the QEI and subsequent increase in Viscount-Melville for 2001 and 2002 (which also exhibited a long MD in the QEI) are evidence of this (Figures 12 and 13). The increases were not from the Arctic Ocean via the M'Clure Strait because Kwok , using tracked RADARSAT-1 data, found that no MYI import occurred for 2001 and 2002. The southward migration of MYI from the QEI through the southern channels (i.e., M'Clintock, Peel Sound, and Baffin Inlet) of the CAA was particularly evident for 2003 and 2004 (Figure 16). Both the M'Clintock and Larsen Sound regions exhibited MYI increases and decreases when nonlandfast conditions persist, providing more evidence for this dynamic process (Figures 14 and 15). The ocean heat flux also depends on latitude such that a lower solar angle will increase the ocean heat flux [Krishfield and Perovich, 2005] and as a result greater melt is expected in regions south of the QEI, especially in Larsen Sound. Moreover, thinner MYI is expected as it migrates southward which results in more solar radiation transmitted through the ice, thus increasing the rate of under ice ablation. Nevertheless, MYI has slowly migrated to southerly regions of the CAA from 2000 to 2004 and has not completely ablated while in transit.
Figure 12. Time series of multiyear ice coverage, first-year ice coverage, and net all wave radiation (Q*) within the Queen Elizabeth Island region for 2000–2004. QuikSCAT σ°-detected melt onset is indicated by the first vertical dashed line, and freeze onset is indicated by the second vertical dashed line. The bold vertical dashed line indicates the date (1 October) when first-year ice is promoted to multiyear ice.
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Figure 16. Spatial distribution of multiyear ice dynamic movement within the Canadian Arctic Archipelago from 2000–2007. A negative value represents dynamic export or melt, and a positive value indicates dynamic import. Legend is concentration in tenths.
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 More compelling evidence for the oceanic heat flux slowly ablating MYI within the CAA can be found by looking at MYI conditions following the longer melt season of 2005–2006. During this time period, regions of the CAA are not subject to significant replenishment from dynamic import and/or FYI promotion, yet considerable MYI was still present following the melt season (Figure 11). The year 2005 exhibited a long MD of 119 days (Table 2) but this was probably attributed more to the early MO because FO was similar to previous years, especially in regions of high concentrations of MYI (Figures 7 and 11). Specifically, temperatures were warmer during August and September for 2005 (Table 3) but the FO dates over the MYI regions were similar to 2004, which was a much colder year (Figures 7 and 11). This supports the notion that even with earlier MO dates the influence of solar radiation is reduced over MYI because early FO transitions (hence slower sea ice albedo feedback) still have occurred over the thicker MYI. MYI losses are apparent within the QEI for 2005 (Figure 17) and the Viscount-Melville, M'Clintock, and Larsen Sound regions also all experience losses of MYI but remnant amounts still remain following the melt season (Figures 18, 19, and 20). Slight MYI increases in the northern part of Viscount-Melville were from the QEI which exhibited some southward export of MYI (Figures 16, 17, and 18). Almost no positive dynamic import occurred in the M'Clintock and Larsen Sound regions (Figures 16, 19, and 20).
Figure 17. Time series of multiyear ice coverage within the Queen Elizabeth Island region for 2005–2007. QuikSCAT σ°-detected melt onset is indicated by the first vertical dashed line, and freeze onset is indicated by the second vertical dashed line. The bold vertical dashed line indicates the date (1 October) when first-year ice is promoted to multiyear ice.
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 In 2006, on average the CAA experienced both the latest mean FO date (277) and longest MD (136) of the years considered (Table 2). Spatially, the southerly regions contained appreciable amounts of MYI at the onset of the 2006 melt season (Figure 10) and MO began earlier in 2006 compared to 2005. The long MD of 2006, especially in regions south of the QEI, is a combination of both an early MO and later FO whereas the long MD in 2005 is primarily attributed to an earlier MO. The longer MD for 2006 was a function of warmer temperatures (Table 3) but was also influenced by MYI in the southern regions experiencing net ablation for over 2 years with almost zero replenishment. This reduced MYI concentration and thickness allowed surface heating to have more of an impact which eventually facilitated complete ablation following the 2006 melt season (Figure 11). Notice how the QEI experienced MYI increases for 2006, but very little MYI was exported to the southern regions (Figure 17). Subtle increases of MYI are observed in Viscount-Melville during 2006 that can be attributed to small export from the QEI and M'Clure regions (Figures 16 and 18). The slight MYI increases in M'Clintock and Larsen Sound regions during the season represent MYI drifting southward from Viscount-Melville (Figures 19 and 20). Examining the situation within the CAA for 2007 finds that it experienced a similar MD compared to 2005 at 117 days but 2007 began the melt season with small concentrations of MYI in the M'Clure, Viscount-Melville, and M'Clintock regions (Figure 10). Interestingly, although no net positive FYI to MYI promotion occurred, the CAA once again exhibited a positive net dynamic import of MYI for the first time since 2004 (Figure 9).
 Very little replenishment of MYI has occurred in the M'Clure, Viscount-Melville, M'Clintock, Larsen Sound, and Baffin Inlet regions of the CAA since 2004. However, the increased thickness of MYI only permitted its slow ablation from the underside, even as it migrated southward. Despite the longer melt seasons for 2005 and 2006 and little MYI replenishment, considerable amounts of MYI survived. Therefore MYI can survive long melt seasons even in the southern regions of the CAA. This now brings us to the question of whether or not MYI decreases are expected to continue within the CAA with the advent of longer melt seasons.
4.3. Will MYI in the CAA Continue to Decrease?
 The longer melt seasons of 2005–2007 within in the CAA seem to be responsible for reducing the amount of FYI promoted to MYI compared to 2000–2004 (Figure 9). Should this trend continue, MYI conditions in the CAA should reduce especially in the southern regions, however, MYI dynamic import is likely to play an important role. The regional icescape of the CAA was heavily congested with MYI at the onset of the 2005 melt season (Figure 10), thus restricting dynamic ice motion. Note that the spatial distribution of MYI at the start and end of the 2005 melt season changed very little (especially in the southern regions) (Figures 10 and 11). For southward drift from the QEI to occur there must be leeway in Viscount-Melville or in Larsen Sound and blockage in Viscount-Melville or Larsen Sound also restrict import from the Arctic Ocean via the M'Clure Strait [Marko, 1977]. No net dynamic import of MYI occurred for 2005 because Viscount-Melville and southern Larsen Sound were heavily congested with MYI, limiting free drift conditions thus preventing intrusions of MYI (Figure 10). This likely even prevented import from the Arctic Ocean into the QEI, as 2005 was the only year that the QEI exhibited a significant MYI deficit (Figures 12 and 17).
 The short MDs observed within the QEI restrict MYI mobility to only a few months a year and therefore it can take several years for MYI to migrate through the QEI into the southern regions of the CAA. Historically speaking (dating back to the early 1960s), following low-MYI years there has always been a 2–5 year period of MYI recovery within the CAA [Melling, 2002; Atkinson et al., 2006; Howell et al., 2008]. Large-scale sea ice dynamics continuously force MYI up against the QEI, and until this process stops, thick MYI from the Arctic Ocean can still flow through the regions of the QEI and then into more southern regions of the CAA. If the reduced MYI concentrations at the end of 2006 in the southern regions of the CAA can be interpreted as part of this process, then movement should begin to reinitialize within the next few years. Witness the QEI region exhibited MYI losses (mostly attributed to melt) in 2005 but considerable dynamic import occurred in 2006 (Figure 17). This suggests that the losses of 2005 provided room for MYI to enter the QEI in 2006. Subsequently in 2007 MYI was exported from the QEI and began to accumulate once again in the Viscount-Melville following the melt season (Figures 11 and 16) which contributed to a net dynamic MYI increase within the CAA (Figure 9). The cyclic nature of MYI within the CAA will therefore still facilitate its presence despite longer melt seasons.