ITP data give continuous, year-round conditions in the Arctic Ocean, but only where sufficient sea ice persists to support the system. Here, we use summer 2005 to summer 2008 ITP data to investigate the seasonal evolution of the NSTM. Initial observations showed that the depth of the NSTM was closely linked to the depth of a near-surface pycnocline, which we call the summer halocline (Figure 2). Thus, the Brunt-Väisälä frequency (N) was calculated as
to compare the strength of the stratification. For this equation, a value of 9.8 m s−2 was used for gravity (g), ρ represents the observed density and z is the vertical distance measured upward. Once the Brunt-Väisälä frequency was calculated, the depth of the maximum Brunt-Väisälä value was used to represent the most stratified region of the water column.
 Here, we describe the seasonal evolution of the summer halocline. The summer halocline is formed when enough sea ice melt water is injected into the ocean to stratify the near-surface waters, thereby becoming a barrier between the fresh surface mixed layer and the saltier waters below. The summer halocline continues to strengthen until either solar radiation weakens and there is not enough heat in the surface waters to melt sea ice or until all the ice has melted. At this time, the summer halocline stops forming and is either entrained into the surface mixed layer if there is enough mixing from air-sea and ice-water stress to break down the stratification or, if not, the halocline remains intact below the descending surface mixed layer. At this time, the summer halocline represents a remnant of the previous summer's stratification.
 In the following evaluation of the annual progression of the NSTM based on ITP data, we will call each summer halocline according to the year in which it was formed. It is also important to note here that we neglect advective effects in the following section. While lateral advection is no doubt important in the Canada Basin, we instead focus on the average seasonal evolution of the NSTM. How advection, and in particular baroclinic advection, affects the NSTM would be an important topic for future research.
3.1. Seasonal Progression of the NSTM From ITP Data
 From August 2005 to August 2006, ITP1 moved from the northwest to the east part of the Canada Basin, within the range of 75°–80°N, 131°–153°W (Figure 3). From at least mid-August through late October 2005, a NSTM was present in the salinity range of 28–30 and was generally deeper than 20 m. The water at 10 m reached the freezing temperature in early October, indicating the onset of freezing and the end of summer 2005 halocline formation. The NSTM gradually cooled and deepened, with average temperatures above the freezing temperature of 0.32°C, 0.27°C and 0.24°C and depths of 20 m, 24 m and 29 m in August, September and October, respectively. During this period, the summer 2005 halocline (denoted by the Brunt-Väisälä frequency maximum) was always above the NSTM, with an average depth between the two features of 3 m (Figure 3b). From the beginning of November, a NSTM was observed sporadically until mid-June. During this period, the NSTM deepened to an average depth of 41 m yet the temperature varied, with the warmest NSTMs observed at the beginning of February and in early May. Higher temperatures coincided with the most southwesterly location of ITP1. The NSTM first reappeared near the surface in mid-July and persisted above 22 m through at least mid-August. In comparison, the maximum Brunt-Väisälä frequency remained deep, below 35 m, until mid-August, when the summer 2006 halocline formed at 12 m. It is interesting to note that while the summer 2006 halocline appears to have formed intermittently in August 2006 (Figure 3b), the depth of the maximum in Brunt-Väisälä frequency alternated between two stratified waters: the summer 2006 halocline and the deeper summer 2005 halocline.
Figure 3. Results from ITP1. (a) A contour plot of the observed temperature above the freezing temperature from 10 to 80 m at ITP1. The black lines indicate the salinity contours at 1 salinity unit intervals. (b) A comparison of the depth of the Brunt-Väisälä Frequency maximum (blue line with blue crosses that denote the sample date) and the depth of the NSTM (red dashed line with red dots that denote the sample date) over the sample period. (c) The movement of ITP1 over the sample period. Data from every 3 days were plotted from 16 August 2005 to 15 August 2006.
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 From September 2006 to August 2007, ITP6 moved south in the central region of the Canada Basin, in the range of 74°–79°N, 136°–145°W (Figure 4). ITP6 was deployed later than other ITPs and the NSTM was already deep at the beginning of September. The water at 8 m reached the freezing temperature in late September. A progressively deepening NSTM was observed consistently until the end of January and then sporadically until the middle of April. Again, the NSTM cooled and deepened through fall, with average temperatures above the freezing temperature of 0.32°C, 0.26°C, and 0.26°C and depths of 26 m, 33 m, and 37 m in September, October, and November, respectively. The depth of the summer 2006 halocline was again on average just 3 m shallower than the NSTM until the end of January and there were no instances when the NSTM was above this stratified water. A deeper temperature maximum that could be PSW was present throughout the study period and was cooler and deeper until the end of January, then became warmer and shallower as ITP6 moved south from May through August. The NSTM first reappeared near the surface in mid-June and persisted above 21 m until at least mid-August. The summer 2007 halocline was first observed in mid-July at 11 m, however the most stratified water was associated with the PSW so this feature is not evident in Figure 4b. The warmest NSTM was 0.95°C above the freezing temperature on 8 August at 18 m, more than two times the temperature of the NSTMs observed by ITP1 in 2006.
 From August 2007 to July 2008, ITP8 moved from the northwest to the northeast region of the Canada Basin, within the range of 78°–81°N, 130°–154°W (Figure 5). In the summer of 2007, the NSTM was relatively shallow until the end of August and the warmest NSTM was 0.73°C above the freezing temperature at 16 m on 30 August. The NSTM was observed, for the first time, year-round from ITP8. The NSTM again cooled and deepened through the fall, with average temperatures above the freezing temperature of 0.57°C, 0.49°C, and 0.49°C and depths of 24 m, 31 m, and 31 m in September, October, and November, respectively. The depth of the summer 2007 halocline was just above the NSTM, with an average difference of 4 m, until the beginning of February, when these features, with an average difference of 10 m, became less correlated. The NSTM first reformed at the end of June 2008, and the summer 2008 halocline formed in early July.
 From August 2007 to July 2008, ITP18 followed the anticyclonic path of sea ice in the Beaufort Gyre [Proshutinsky et al., 2002], within the range of 74°–79°N, 131°–145°W (Figure 6). Overall, the near-surface water properties were different at ITP18 than other ITPs and three features were of note. First, the NSTM was shallower throughout late summer to early fall 2007, and did not consistently descend below 20 m until late October. It is interesting to note that the water at 8 m reached the freezing temperature on 9 October, thus for almost 3 weeks ice was forming less than 20 m above water that was up to 0.7°C above the freezing temperature. Second, the NSTM was on average 6 m deeper than the summer 2007 halocline from mid-August to the beginning of December, which was more than the 3–4 m difference observed from the other ITPs. Third, the depth of the NSTM and summer 2007 halocline were much more variable from December through June than at the other ITPs. Some of the latter variability can be explained by the movement of ITP18. At the end of December, ITP18 moved south, into a region which contained a warm, shallow NSTM below the freshest observed surface water. From February to mid-March, ITP18 moved west, into a region with a deeper NSTM below relatively unstratified water. In late March, near 145°W, ITP18 again moved into a region with a relatively shallow NSTM. The NSTM was then only seen intermittently from mid-April through the end of May. Overall, there were multiple pycnoclines in the upper water column through this winter. Thus, our method of simply choosing the maximum Brunt-Väisälä frequency to represent the summer 2007 halocline does not distinguish between this and other regions stratified by eddies and other water masses. Likewise, there were several instances, for example in early March, where the NSTM may have merged with a deeper temperature maximum that could be PSW, thus our analysis may have identified a NSTM when one was not present. We left these examples to show the complex near-surface structure of the south central Canada Basin in early 2008. The NSTM reformed in late June, and the summer 2008 halocline formed in early July. Both these features were observed at a shallow depth through the remainder of our study period.
3.2. Changes to the Upper Ocean Heat Content
 In the previous section, we found that (1) the NSTM warmed from 2005 to 2008, (2) the NSTM was warmer at more southerly locations, and (3) the NSTM was a year-round feature in the winter of 2007–2008. This variability would be expected to have a significant impact on heat stored in the upper waters of the Canada Basin. To examine these changes, we calculated the heat content (HC) relative to the freezing temperature above PWW. HC was calculated as:
where ρo, the reference density, was 1027 kg m−3, Cp, the specific heat of seawater, was 3986 J°C−1kg−1, and (T − Tf) (S, P) was the temperature relative to the freezing temperature interpolated in 1 m intervals. The base of the initial calculation was chosen to be PWW since this is a relatively consistent feature whose presence can be approximated based on temperature and salinity profiles. Our initial findings suggested that the depth of PWW varied considerably from about ∼130 m in 2005 to ∼190 m in 2008. To correct for this, we calculated the average heat content per meter instead of the total heat content above PWW. Also, the initial examination found that changes to the heat content of the NSTM were often dwarfed by variations in the temperature maxima below the remnant of the winter mixed layer (rML, temperature minimum below the NSTM). Thus, to highlight near-surface changes, we separated the water column into two different layers and calculated the average heat content per meter in each layer. The first layer was from 10 m to the rML (Figure 7a) and the second layer was from rML to PWW (Figure 7b). There were a few instances when no rML was present, for example in May 2007 from ITP6. In these cases, we instead plotted the average heat content from PWW to 10 m in Figure 7b.
Figure 7. The average heat content per meter (in MJ m−3) for two different layers. (a) The first layer is between 10 m and the temperature minimum below the NSTM that we call the remnant winter mixed layer (rML). (b) The second is between the rML and Pacific Winter Water (PWW). Here we calculated the heat content relative to the freezing temperature. If no rML was present, the average heat content for Figure 7b was calculated from PWW to 10 m. A 9 day centered running mean was applied to the average heat content in both Figures 7a and 7b to smooth the data.
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 An important difference between the upper and lower layers was that, with the exception of ITP18, the heat content in water above the rML had a distinct seasonal cycle. This cycle showed that at ITP1, ITP6 and ITP8, the heat content decreased around the end of October and was generally less than 1 MJ m−3 until the beginning of June, when the heat content began to increase. This closely matched the timing of the NSTM formation described in section 3.1. As previously discussed, near-surface observations from ITP18 were different. In particular, while the average summer heat content was similar at ITP6, ITP8 and ITP18 (∼1.5–3 MJ m−3), the average winter heat content above the rML at ITP18 was greater than at other ITPs, indicating that about 1 MJ m−3 more heat was being stored there during the winter. Below the rML, about 2–3.5 MJ m−3 of heat were stored year-round at all ITPs. We suggest that the amount of heat stored here was dependent on both advective and vertical processes. Advective processes would be the shelf to basin transport of modified Pacific water into the Canada Basin and the circulation of the Beaufort Gyre while vertical processes would be the quasi-perennial storage of heat below the rML that is trapped by the summer halocline. Further investigation of the different temperature maxima below the rML is needed to determine how these are changing the structure of the upper ocean and how these may impact the sea ice cover.
3.3. Ekman Pumping, Winter Storms, and Advection
 In this paper, we propose that the seasonal progression of the NSTM is primarily forced by the 1-D thermodynamics dominated by the annual cycle of solar radiation. However, it is evident from the variable depth and temperature of the NSTM, especially in fall, that other processes, including Ekman pumping, winter storms, and advection, affect the behavior of the NSTM. To examine these processes, we plotted the average monthly downwelling rate (based on calculations by Yang ) with the Brunt-Väisälä frequency and the depth of the NSTM (Figure 8). The downwelling rates we used were 0.9 m month−1 for September, 3.6 m month−1 for October, 4.5 m month−1 for November and December, 3.6 m month−1 for January, 2.7 m month−1 for February, and 1.8 m month−1 for March through July. Based on ITP observations, McPhee et al.  estimated downwelling rates of 3.5 m month−1 from September through May, indicating that an NSTM at 20 m at the end of September would descend to 48 m by the end of May compared to 44 m based on the values of Yang .
Figure 8. A comparison of the strength of the Brunt-Väisälä frequency (s−2) between the four ITPs: (a) ITP1, (b) ITP6, (c) ITP8, and (d) ITP18. Here, the black dots represent the depth of the NSTM, and the white line shows the average rate each month of downwelling from Ekman pumping as calculated by Yang . White regions denote times when no data were available.
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 Overall, the depth of the NSTM closely followed the average downwelling rate at all ITPs. However, there were some instances when the NSTM was deeper or shallower than can be explained by Ekman pumping alone. Storm-driven vertical mixing can cause the NSTM to deepen. Based on buoy data analyzed from April 1996 to April 1997 in the Canada Basin, Yang et al.  found that winter and spring storms can cause ice to move up to 40 cm s−1, thereby amplifying ice-ocean stress and deepening the surface mixed layer to at least 45 m, even under full winter ice cover. The average number of stormy days (defined as daily average geostrophic winds above 15 m s−1) in the Canada Basin is 8–16 d yr−1 [Yang et al., 2004]. In addition, since there is variable sea ice cover, sea ice melt, and incoming solar radiation throughout the Canada Basin, the NSTM is not formed at a uniform depth. Thus, wind-driven advection of sea ice can cause the ITP to drift into a region with either a shallower or deeper NSTM.
 At ITP18, the NSTM was less correlated with the downwelling rate than the other ITPs. For example, the decreased stratification observed in mid-December 2007 was correlated with a very deep NSTM that was followed 3 days later by a very shallow NSTM located just below a near-surface halocline. At the same time, the salinity decreased by about 0.5. It is possible that this near-surface variability was due to mixing from a winter storm that caused sea ice to melt, freshen the upper waters, and form a near-surface halocline. However, based on 6-hourly data from the NCEP/NCAR reanalysis 1 [Kalnay et al., 1996], no major storms occurred near ITP18 in mid-December. Thus, it is likely that ITP18 drifted across a front in mid-December that separated saltier near-surface water with a deep NSTM from fresher near-surface water with a shallow NSTM. Given the shallow depth of the NSTM during the winter of 2007–2008, it is possible that heat from the NSTM could melt ice during winter if there was sufficient mixing to erode the halocline and we suggest this is an important subject for future research.