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Observing the Seasonal Ice Zone in the Western Arctic SPECIAL ISSUE ON AUTONOMOUS AND LAGRANGIAN PLATFORMS AND SENSORS (ALPS) An Autonomous Approach to Observing the Seasonal Ice Zone in the Western Arctic By Craig M. Lee, Jim Thomson, and the Marginal Ice Zone and Arctic Sea State Teams 56 Oceanography | Vol.30, No.2 ABSTRACT. The Marginal Ice Zone and Arctic Sea State programs examined the MIZ is a region of complex atmosphere- processes that govern evolution of the rapidly changing seasonal ice zone in the Beaufort ice-ocean dynamics that varies with sea Sea. Autonomous platforms operating from the ice and within the water column ice properties and distance from the ice collected measurements across the atmosphere-ice-ocean system and provided the edge (Figure 2; e.g., Morison et al., 1987). persistence to sample continuously through the springtime retreat and autumn advance Additionally, the northward retreat of of sea ice. Autonomous platforms also allowed operational modalities that reduced the sea ice exposes an increasing expanse of field programs’ logistical requirements. Observations indicate that thermodynamics, open water south of the ice edge, eventu- especially the radiative balances of the ice-albedo feedback, govern the seasonal cycle ally providing sufficient fetch for the gen- of sea ice, with the role of surface waves confined to specific events. Continuous eration of long-period, large-amplitude sampling from winter into autumn also reveals the imprint of winter ice conditions and waves (e.g., Thomson and Rogers, 2014). fracturing on summertime floe size distribution. These programs demonstrate effective Such waves are capable of propagating use of integrated systems of autonomous platforms for persistent, multiscale Arctic north and penetrating into the pack to observing. Networks of autonomous systems are well suited to capturing the vast scales effect mechanical breakup of floes, greatly of variability inherent in the Arctic system. accelerating melt. Accurate characteriza- tion of MIZ processes becomes increas- INTRODUCTION dramatic sea ice decline has occurred in ingly important as the Beaufort MIZ Dramatic changes in summertime Arctic the Beaufort Sea and the Canada Basin gains prominence. sea ice motivated two process studies that (Figure 1a; Shimada et al., 2006), result- In addition to the increasing areal relied on recent advances in autonomous ing in the loss of thick, multiyear ice extent of open water, the duration of the observing to collect atmosphere, ice, and (e.g., Maslanik et al., 2007) and the north- open water season is also increasing. This ocean measurements across the necessary ward retreat of the summertime ice edge is particularly notable in the autumn; span of temporal and spatial scales. The from the shelf into the deep basin. the freeze-up in much of the Beaufort Arctic is warming at over twice the rate Climate models have successfully cap- and Chukchi Sea region is now one full observed at lower latitudes (Overland tured the overall trend in summertime sea month delayed from historic timing et al., 2016), with pronounced impacts ice extent; however, they under-predict (Thomson et al., 2016). This delay pro- on the timing and extent of sea ice. Arctic the observed rate of decline (Figure 1c; vides more opportunity for the ocean Ocean sea ice follows a seasonal cycle dic- Jeffries et al., 2013). Observed summer- to receive heat directly from solar radi- tated by incoming solar radiation, with time minimum sea ice extent varies sig- ation, as well as more opportunity for sea ice advancing southward in autumn, nificantly year to year, but the underlying autumn storms to mix that heat far below as insolation drops with the approaching rapid decline is just within the one stan- the canonical depths of the near-surface Arctic night, and retreating northward in dard deviation bound of predictions gen- temperature maximum (Jackson et al., spring as insolation increases. This sea- erated by an ensemble of climate models. 2010). The delay in ice formation thus sonality has changed in recent decades, This suggests that simulations may fail feeds back to affect the type of ice that with a trend toward greater ice retreat to properly represent the processes and is formed and the persistence of that ice each summer and a smaller, but signif- feedbacks that govern sea ice evolution. into the next seasonal cycle. For example, icant, trend of decreasing wintertime Limited understanding of the processes increasing surface wave activity is more maximum sea ice extent. The timing of that govern sea ice evolution in the mar- likely to produce pancake ice, which until these extrema has also shifted (Perovich ginal ice zone (MIZ) may contribute to the recently was rarely observed in the Arctic et al., 2016). Summertime minimum inability of models to reproduce the steep (Thomson et al., 2017). The formation of Arctic sea ice extent has been in decline decline in sea ice. Summertime open- pancake ice can actually accelerate the for nearly 40 years (Perovich et al., 2012), ing of the Beaufort and Chukchi Seas has autumn ice advance, but the ability to which, along with a reduction in thick- amplified the extent and influence of the forecast this process is extremely limited. ness, has led to an overall decrease in sea seasonal MIZ, the region of fractional ice Rapid changes in sea ice can have pro- ice volume (e.g., Kwok and Rothrock, cover that forms the transition between found impacts on human subsistence and 2009; Schweiger et al., 2011). The most open water and pack ice (Figure 1a). The commercial activities. Arctic ecosystems Oceanography | June 2017 57 responding to changing sea ice affect the persistent sampling over many months that govern sea ice evolution during the timing and availability of subsistence while resolving scales of kilometers and highly dynamic period that spans melt- hunting. Declining sea ice has also gener- hours. Relative to conventional sampling out to freeze-up. Alongside their scientific ated increased interest in activities such as (Figure 3), autonomous observations can objectives, these programs also focused transpolar shipping, resource extraction, span seasonal cycles with much greater on advancing methodologies for autono- and tourism, with implications for safety temporal coverage. mous observing, developing and demon- and national security. Coastal commu- Motivated by an overall need to improve strating new approaches for conduct- nities suffer from accelerated erosion as predictability in the western Arctic on both ing sustained observations in ice-covered sea ice retreats northward, leaving shore- operational and climate time scales, the environments. Here, we report on the lines exposed to increased summertime US Office of Naval Research (ONR) sup- approaches and early combined findings wave activity. Improved sea ice predic- ported two large, integrated observational of both programs. tions are needed to inform planning and programs focused on the Beaufort Sea. In formulate responses to these challenging 2014, the Marginal Ice Zone (http://apl. AUTONOMOUS PLATFORMS developments. uw.edu/miz) program employed a large AND EXPERIMENT DESIGN Recent advances in autonomous plat- array of autonomous platforms to study The dynamics associated with summer- forms are providing new perspectives on the seasonal ice retreat. In 2015, the Sea time sea ice retreat and autumn advance the processes that govern Arctic sea ice State (http://apl.uw.edu/arcticseastate) pose significant observational challenges. evolution because they capture spatial and program used a research vessel and auton- Coincident measurements of the atmo- temporal scales that previously had been omous platforms to study the seasonal ice sphere, sea ice, and ocean must resolve challenging to sample. Robotic instru- advance. Although conducted in different a broad range of spatial and temporal ments that operate from the sea ice and years, taken together the two programs scales to document how the balance of in the water column have demonstrated provide a novel picture of the processes processes evolves in response to chang- ing surface forcing, ice cover, and upper- ocean stratification. During the transition a September 1980 b September 2007 periods, rapid shifts in sea ice extent and properties demand mobile approaches that are capable of tracking the ice edge as it moves. Continuous measurements that span months to years are needed to illu- minate the feedbacks that unfold at longer time scales, such as the sequestration of summertime solar heating and its influ- ence on the timing and spatial variability of freeze-up (Timmermans et al., 2014). Previous and ongoing Arctic research programs, such as the Beaufort Gyre 12 c NSIDC Observing System (e.g., Proshuntinsky Mean 10 Median et al., 2009a, 2009b) and the International ±1 SD Arctic Buoy Programme (http://iabp.apl. 8 washington.edu), have demonstrated the 6 value of autonomous sampling in this complex region. 4 The seasonal distribution of ice thick- Sea-Ice Extent (10 km) 2 ness measurements in the western Arctic reflects the challenges of making mea- 0 surements in this difficult environment 1900 1920 1940 1960 1980 2000 2020 2040 2060 2080 2100 Year (Figure 3). The two largest concentrations FIGURE 1. September sea ice cover in 1980 (a) and in 2007 (b). The red outline shows the differ- of measurements center on late sum- ence in ice coverage, which is most notable in the western Arctic. Credit: NSIDC (c) Time series of mer, when maximal open water offers the sea ice area at the September minimum each year. The black line is a satellite data product from the best access for ships, and spring, the US National Snow and Ice Data Center. The yellow and blue lines mark the mean and median of September sea ice minimum predictions from an ensemble of climate models, with gray shading when the ice pack and operating condi- marking the one standard deviation bounds. From Jeffries et al. (2013) tions allow researchers to access the ice 58 Oceanography | Vol.30, No.2 using aircraft.
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