Multiyear ice replenishment in the Canadian ... - Wiley Online Library

PUBLICATIONS Journal of Geophysical Research: Oceans RESEARCH ARTICLE 10.1002/2015JC010696 Key Points: Construction of 17 year MYI replenishment time series within the CAA FYI aging is the primary source of CAA MYI replenishment 55% decrease in MYI replenishment from 2005–2012 relative to 1997– 2004

Correspondence to: S. E. L. Howell, [email protected]

Citation: Howell, S. E. L., C. Derksen, L. Pizzolato, and M. Brady (2015), Multiyear ice replenishment in the Canadian Arctic Archipelago: 1997–2013, J. Geophys. Res. Oceans, 120, 1623–1637, doi:10.1002/2015JC010696. Received 2 JAN 2015 Accepted 29 JAN 2015 Accepted article online 6 FEB 2015 Published online 14 MAR 2015

Multiyear ice replenishment in the Canadian Arctic Archipelago: 1997–2013 Stephen E. L. Howell1, Chris Derksen1, Larissa Pizzolato2, and Michael Brady3 1

Climate Research Division, Environment Canada, Toronto, Ontario, Canada, 2Department of Geography, University of Ottawa, Ottawa, Ontario, Canada, 3Department of Geography and Environmental Management, University of Waterloo, Waterloo, Ontario, Canada

Abstract In the Canadian Arctic Archipelago (CAA), multiyear ice (MYI) replenishment from first-year ice aging (CAAMYI-Oct-1) and Arctic Ocean MYI exchange (CAAMYI-exchange) contribute to the CAA’s relatively heavy sea ice conditions at the end of the summer melt season. We estimate these components using RADARSAT and the Canadian Ice Service Digital Archive and explore processes responsible for interannual variability from 1997 to 2013. CAAMYI-Oct-1 (52 6 36 3 103 km2) provides a larger contribution than CAAMYI-exchange (13 6 11 3 103 km2). CAAMYI-Oct-1 represents 10% of the amount that occurs in the Arctic Ocean. CAAMYIexchange represents 50% of Nares Strait MYI export to Baffin Bay and 12% of Fram Strait MYI export to the Greenland Sea. CAAMYI-Oct-1 exhibits dependence on warmer (cooler) summers that increase (decrease) melt evident from strong relationships to surface air temperature (SAT), albedo and total absorbed solar radiation (Qtotal). CAAMYI-exchange is influenced by summer sea level pressure (SLP) anomalies over the Beaufort Sea and Canadian Basin which shifts the primary source of CAAMYI-exchange between less obstructed M’Clure Strait (low SLP anomalies) and the more obstructed Queen Elizabeth Islands (high SLP anomalies). Over the 17record, appreciable replenishment occurred for most years from 1997 to 2004, reduced replenishment from 2005 to 2012, and large replenishment in 2013. The reduced replenishment period was associated with positive SAT, negative albedo, and positive Qtotal anomalies that facilitated more melt and less CAAMYI-Oct-1, together with high SLP anomalies that facilitated less CAAMYI-exchange. Large replenishment in 2013 was primarily from CAAMYI-Oct-1 attributed to strongly negative SAT and Qtotal anomalies and strongly positive albedo that impeded melt.

1. Introduction Over the passive microwave satellite record, multiyear ice (MYI) area within the Arctic has declined by 14% decade [Comiso, 2012]. As a result of this decline, the Arctic is gradually shifting toward a more seasonal first-year ice (FYI) regime [Maslanik et al., 2011]. MYI within the Arctic is replenished when FYI survives the melt season and is promoted to MYI thus, replacing MYI lost due to melt. When MYI replenishment is constrained to the domain of the Arctic Ocean, FYI aging replaces MYI lost from melt as well as MYI export to the Greenland Sea via Fram Strait [Kwok et al., 2004], Baffin Bay via Nares Strait [Kwok, 2005] and the Canadian Arctic Archipelago (CAA) via the Queen Elizabeth Islands (QEI), and the M’Clure Strait [Kwok, 2006; Agnew et al., 2008; Howell et al., 2013a]. Regionally, the export of Arctic Ocean MYI into the CAA (Figure 1a) provides it with an additional source of MYI that complements FYI aging. Together, these two sources of MYI maintain relatively stable and heavy sea ice conditions within the CAA, even during the summer months [Melling, 2002; Howell et al., 2009]. This is further demonstrated through studies investigating summer season MYI trends within the CAA, which have only reported small declines [e.g., Tivy et al., 2011; Derksen et al., 2012].

Reproduced with the permission of the Minister of Environment.

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Knowledge of MYI within the CAA is important given, it is the most significant hazard to navigation within the Northwest Passage, even when present only in small amounts. There is a strong link between the decreased presence of MYI within the CAA and a longer navigation season for the Parry Channel route of Northwest Passage, that is especially pronounced since 2007 [Howell et al., 2013b]. Despite a longer navigation season, the Parry Channel route is only temporarily clear of MYI for a few weeks during the melt season as the predominant modes of atmospheric circulation facilitate MYI inflow from the higher latitudes [Howell et al., 2009, 2013b]. The latter process is of concern because certain types of vessel traffic (i.e., nonice

C 2015 Her Majesty the Queen in Right of Canada V C 2015 American Geophysical Union Journal of Geophysical Research: Oceans V

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Figure 1. (a) Map of the Canadian Arctic Archipelago and (b) time series of mean September total ice within the Canadian Arctic Archipelago from 1968 to 2013. The vertical dashed lines indicate the 2006–2012 period of low multiyear ice replenishment.

strengthened pleasure crafts) within the Canadian Arctic have increased significantly since 1990 [Pizzolato et al., 2014]. In addition, the continued decline of Arctic sea ice projected by global climate models has led to the suggestion of a more navigable Parry Channel route of the Northwest Passage by 2050 [Smith and Stephenson, 2013]. The coarse spatial resolution of global climate models, however, limits the representativeness of these projections within the narrow channels of the CAA. Howell et al. [2009] used the Canadian Ice Service Digital Archive (CISDA) operational ice charts to provide estimates of the CAA’s two sources of MYI replenishment from 1979 to 2008 and reported insignificant declines in September MYI because of the balance between FYI aging and Arctic Ocean MYI inflow. However, there is some uncertainty with these estimates because they are based on differences between weekly ice charts and do not explicitly track ice exchange in and out of the CAA. Moreover, the time series of mean September sea ice area within the CAA from 1968 to 2013 illustrates the heavy and stable ice conditions that were apparent in the early period of the record have relaxed from 2006 to 2012 (Figure 1b). Notably, 2011 and 2012 broke all previous record low ice years within the CAA with September ice conditions falling 60% lower than the 1981–2010 climatology (Figure 1b) [Howell et al., 2013b]. A more direct and up-todate assessment of MYI replenishment within the CAA is necessary. The availability of RADARSAT data since 1997 allows for improved estimates of MYI replenishment within the CAA that can be constructed over a 17 year time period. The objective of this study is to quantify the components of MYI replenishment within the CAA at the end of the summer melt season from 1997 to 2013 and explore factors contributing to interannual variability.

2. Data and Methods 2.1. Data Description Sea ice concentration (i.e., total, MYI, and FYI) estimates were obtained from the CISDA. The CISDA is a compilation of Canadian Ice Service (CIS) regional weekly ice charts that integrate all available real-time sea ice information from various satellite sensors, aerial reconnaissance, ship reports, operational model results, and the expertise of experienced ice forecasters, spanning 1968 to present [Canadian Ice Service, 2007]. The CISDA contains a technological bias that is related to advances in sensor technology and changes in regional focus due to the emergence of important shipping routes [Tivy et al., 2011]. This study is focused entirely within the RADARSAT era (i.e., 1997-present) of CIS ice chart production therefore the impact of any technological and methodological changes with respect to CISDA is negligible. Monthly surface air temperature (SAT), incoming solar radiation at the surface (K#), and monthly broadband albedo at the surface (a) were obtained from the extended Advanced Very High Resolution Radiometer

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(AVHRR) Polar Pathfinder (APP-x) [Wang and Key, 2005a]. The APP-x data set consists of surface, cloud, and radiation parameters over the Arctic at a 25 km spatial resolution from 1982 to 2013. The APP-x data set has been validated with in situ data from the Surface Heat Budget of the Arctic Ocean (SHEBA) field experiment [Maslanik et al., 2001] and has been used to assess trends and spatiotemporal variability in surface parameters for Arctic regions [e.g., Wang and Key, 2003; Wang and Key, 2005b; Fernandes et al., 2009; Stroeve et al., 2014]. The root-mean-square error (RMSE) for SAT, K#, and a is 1.98 K, 34.4 W m22, and 0.10, respectively [Wang and Key, 2005a]. RADARSAT-1 synthetic aperture radar (SAR) imagery in ScanSAR mode at HH (horizontal transmit and receive) polarization sampled to a spatial resolution of 200 m pixel21 were acquired from the Alaska Satellite Facility (ASF; https://www.asf.alaska.edu/) from 1997 to 2007. RADARSAT-2 ScanSAR images at HH polarization also sampled to a spatial resolution of 200 m pixel21 were acquired from National Earth Observation Data Framework Catalogue of Natural Resources Canada (NEODF; https://neodf.nrcan.gc.ca/) from 2008 to 2014. RADARSAT-1 and RADARSAT-2 ScanSAR images consist of a series of merged beams that are aggregated to produce a swath of 460 and 500 km, respectively. Monthly sea level pressure (SLP) was obtained from the National Centers for Environmental PredictionNational Center for Atmospheric Research (NCEP-NCAR) Reanalysis [Kalnay et al., 1996; Kistler et al., 2001]. 2.2. Estimating MYI Replenishment Within the CAA The amount of MYI that is replenished after each melt season in the CAA (CAAMYI) is the cumulative sum of its net MYI area exchange between the Arctic Ocean and Baffin Bay (CAAMYI-exchange) plus the amount of FYI that survived the melt season within the CAA and is promoted to MYI, which by definition occurs on October 1 (CAAMYI-Oct-1). CAAMYI over a certain time period (t) can be estimated using the following equation: CAAMYI ðtÞ5CAAMYI2exchange ðtÞ1CAAMYI2Oct21

(1)

CAAMYI-exchange can be further expressed as: CAAMYI2ecchange ðtÞ5MSMYI2exchange ðtÞ1QEIMYI2exchange ðtÞ1 AGMYI2exchange ðtÞ1JSMYI2exchange ðtÞ1LSMYI2exchange ðtÞ

(2)

where MSMYI-exchange is the net MYI area exchange between the M’Clure Strait and Arctic Ocean, QEIMYIis the net MYI area exchange between the QEI and the Arctic Ocean, AGMYI-exchange is the net MYI area exchange between the Amundsen Gulf and the Arctic Ocean, JSMYI-exchange is the net MYI area exchange between the Jones Sound and Baffin Bay, and LSMYI-exchange is the net MYI area exchange between the Lancaster Sound and the Baffin Bay. All the aforementioned exchange gates are all illustrated in Figure 1a.

exchange

The CAA remains landfast for the majority of the winter months [Canadian Ice Service, 2011] and in this study we are only concerned with melt season processes. As a result, for each year, we estimate CAAMYIexchange from the period of May 1 (t 5 0) to October 1. This time period effectively represents MYI conditions prior to (i.e., landfast) and throughout the melt season. We assume that MYI entering the CAA at the exchange gates does not melt, which likely leads to some over estimation of the CAAMYI-exchange term. In the calculation of CAAMYI-exchange, we also assume that the AGMYI-exchange, JSMYI-exchange, and LSMYI-exchange terms all equal 0 and provide a negligible contribution to CAAMYI-exchange. This is a reasonable assumption during the May 1 to October 1 time period because based on the ice climatology these regions are almost entirely made up of FYI and always become ice-free during the summer months (Figure 2). This considerably limits MYI exchange at these gates as noted by previous studies [e.g., Kwok, 2006; Agnew et al., 2008]. Furthermore, the majority of MYI migrating through the CAA during the melt season accumulates in the southern channels [Howell et al., 2008] which is also evident in Figure 2. When MYI was present in these regions during the May 1 through October 1 period from 1997 to 2013, it was almost always less than 1/10th in concentration (not shown) and this amount would exert a very small impact on the overall value of CAAMYI-exchange. We estimated MSMYI-exchange and QEIMYI-exchange using the approach described by Howell et al. [2013a] based on earlier work by Kwok [2006] and Agnew et al. [2008]. Sea ice motion was derived from sequential RADARSAT-1 or RADARSAT-2 image pairs with a temporal resolution of 2–5 days using the CIS-Automated Sea Ice Tracking System (CIS-ASITS). Full details of the CIS-ASITS are available in Wohlleben et al. [2013] and

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Figure 2. (left) 1981–2010 median multiyear ice (MYI) concentration for selected weeks during the melt season within the Canadian Arctic Archipelago and (right) 1981–2010 frequency of MYI occurrence for MYI concentrations greater than 4 tenths.

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Komarov and Barber [2013]. Sea ice motion for each image pair was then interpolated to the M’Clure Strait and QEI exchange gates (Figure 1) that includes a buffer region of 30 km on each side of the gate and sampled at 5 km intervals across the gate. The MYI area flux (F) at each gate was calculated using the following equation: X F5 ci ui Dx (3) where ci is the MYI concentration, ui is the ice motion normal to the flux gate at the ith location, and Dx is the spacing along the gate (5 km). For the M’Clure Strait and QEI gates, F, from all available image pairs were summed from May 1 to October 1. The uncertainty in F at the M’Clure Strait and QEI gates is 12– 14 km2 day21 [Howell et al., 2013a]. CAAMYI-Oct-1 is difficult to accurately estimate and as a result we evaluated three approaches to determine the most representative estimate using the CISDA: 1. Based on Howell et al. [2009], the difference in MYI area within the CAA across the October first promotion week. Specifically, the amount of MYI within the CAA on the second week of October (CAAMYI-Oct-8) minus the amount of MYI within the CAA on last week of September (CAAMYI-Sep-24) as given by the following equation: CAAMYI-Oct-1 5CAAMYI2Oct28 2CAAMYI2Sep224

(4)

2. The sum of all FYI within the CAA as classified by the CIS ice chart on the week before 1 October (CAAFYISep-24) as given by the following equation: X CAAMYI-Oct-1 5 CAAFYI2Sep224 (5) 3. The sum of all second-year ice (SYI) within the CAA as classified by the CIS ice chart issued on the 1 October (CAASYI-Oct-1) as given by the following equation: X CAAMYI-Oct-1 5 CAASYI2Oct21 (6) It should be noted that CIS ice charts are issued weekly therefore, their dates vary slightly depending on the year (Table 1). As shown in Figure 3, the three methods are similar and highly correlated with each other (r 5 0.9220.95) however, the limitation with approaches (i) and (ii) is that melt and/or exchange can take place during this one 1 week time window over the FYI promotion period leading to under or overestimations. We deem approach (iii) to be the most representative estimate of CAAMYI-Oct-1 considering the availability of RADARSAT imagery since 1997 improves upon the ability of CIS analysts to manually identify SYI on the imagery. Figure 4 shows several good examples of how SYI generation within the CAA can be manually identified with RADARSAT imagery. Note how the brighter MYI (i.e., high backscatter) can be visually separated from the darker SYI and the darker still FYI (Figure 4). Approach (iii) does not experience any dynamic or thermodynamic over or underestimations that are problematic to approaches (i) and (ii). However, approach (iii) only represents the inventory of FYI at the end of melt season and does not differentiate between FYI that could have originated outside the CAA.

3. Multiyear Ice Replenishment 1997–2013 The time series of CAAMYI from 1997 to 2013 is shown in Figure 5. The mean was 65 3 103 km2 or 8% of the CAA’s total ice covered domain. Variability was high and ranged from 2 3 103 km2 (