The Arctic sea ice cover evolves dramatically through the summer melt season. Floe size distribution (FSD) is a critical parameter used to examine this change as the ice cover transitions from large rectilinear plates in spring to an ensemble of discrete rounded floes by midsummer. The FSD at a given time impacts the dynamic and thermodynamic behavior of the ice cover. Focusing on the seasonal marginal ice zone in the Beaufort and Chukchi Seas from May to September 2014, we present qualitative and quantitative results derived from National Technical Means high‐resolution imagery and supported by ice mass balance buoy data. Results indicate that as melt accelerates, floe breaking pattern, and therefore FSD, is heavily influenced by the distribution of melt ponds. Discrete element model results using morphological conditions derived from analyzed satellite imagery confirmed that breaking occurs along ponds and perpendicular to applied stress.

1 Introduction The Arctic sea ice cover undergoes profound seasonal change transitioning from big, rectilinear plates in spring to an ensemble of discrete rounded floes in the summer. By the end of the melt season, floes in the marginal ice zone (MIZ) have been reduced to scattered fragments surrounded by open water. The floe size distribution (FSD) has been established as an important parameter that impacts both dynamic and thermodynamic behavior of the ice cover throughout this seasonal evolution. For instance, it has been shown that ice velocity decreases for floes smaller than 100 m in diameter [Steele et al., 1989] and that an increase in perimeter associated with a greater number of smaller floes augments the importance of lateral melt in the system [Maykut and Perovich, 1987; Steele, 1992]. Previous studies have determined floe size distributions for different locations in both the Arctic and the Antarctic [Rothrock and Thorndike, 1984; Hudson, 1987; Paget et al., 2001; Holt and Martin, 2001; Toyota et al., 2006, 2011; Steer et al., 2008; Perovich and Jones, 2014]. Wave‐ice interaction has been cited as a primary mechanism of floe breakup in the traditionally defined MIZ, causing flexural failure and breaking of the ice cover [Steer et al., 2008; Bennetts and Squire, 2011; Dumont et al., 2011; Toyota et al., 2011; Williams et al., 2013]. In the interior of the ice pack (e.g., Beaufort Sea) where waves are not prevalent, Perovich et al. [2001] indicate floe features created by winter dynamics as important determinants of breaking patterns in the summer months. They found that a preconditioning of the ice cover by the formation and subsequent refreezing of cracks and leads sets the stage for preferential melting and dynamic failure at these locations during the summer. In this study we will examine floe breakup in the marginal ice zone of the Beaufort Sea during the 2014 melt season. Using high‐resolution satellite imagery taken coincident with a set of Lagrangian measurements collected by an in situ ice mass balance buoy, we track the evolution of ice floe morphology throughout the course of the melt. We revisit the importance of winter dynamics and explore the impact of melt ponds on floe breakup. A discrete element method (DEM) sea ice model is used to support our observations.

2 Approach This work was conducted as part of an initiative sponsored by the Office of Naval Research, aimed at analyzing the physics governing the seasonally emerging Marginal Ice Zone in the Beaufort and Chukchi seas. Imagery used in this study comes from the library of declassified images obtained by National Technical Means (NTM). Each scene covers more than 15 × 15 km in total area and is represented by a gray scale image with 256 intensity levels at 1 m resolution [Kwok et al., 2013]. The acquisition of these satellite images has been coordinated to follow drifting autonomous ice mass balance buoy (IMB) 2014C [Polashenski et al., 2011] for Lagrangian tracking of a specific ensemble of floes. Coincident in situ sea ice mass balance data were collected at our study site by an IMB deployed in early 2014 [http://imb.erdc.dren.mil/]. While imagery was available for 26 different days, only 12 mosaic images contained useful data unobstructed by cloud cover. Five mosaic images of the region of interest were chosen for this study. This subset included four images that represented distinctly different ice cover regimes observed from May to September 2014 and one additional image acquired on 2 August that provided visual, qualitative examples of pronounced thermodynamic control on floe breakup. At the time of image acquisition, the IMB was within image bounds for four out of the five images and 3–10 km from the edge of the May 2014 image. Each mosaic was processed using a histogram equalization transform followed by a median filter with a square kernel of length 25 pixels to highlight distinctive features and smooth the image. We then used the modified image to conduct a qualitative analysis of floe breakup by identifying features that persisted over several images and the patterned breaking of singular floes into smaller components over time. Four images were quantitatively processed to examine distinct features of individual floes, to partition the image into ice, open water, and ponds (when applicable), and to extract relevant descriptive statistics of the segmented features. For a nonponded scene (early or late season) the image was partitioned into open water and ice according to a threshold that maximizes interclass variance of pixel values in a two‐dimensional representation of the image histogram. This 2‐D histogram is calculated by first multiplying the original pixel value with the average value of its 5 × 5 neighborhood, and then normalizing to an 8‐bit scale. Ponded images were partitioned into three classes with a random forest machine‐learning algorithm. After Miao et al. [2015], our algorithm implements the random forest method to sort objects that have been extracted by the Feature Extraction work flow of the image processing software, ENVI. Ice concentration is measured as the ratio of pixels classified as ice or pond to the total number of image pixels. A watershed transform was applied to simplified two‐class images to separate distinct floes and measure their size and perimeter. Ponds are considered part of an ice floe, and perimeter is the length of the pixel vector of the ice class that borders open water. The size parameter of a given floe is computed as the diameter of a circle with an equivalent area.

3 Results We describe changes in first year sea ice in the Beaufort Sea from 6 May to 13 September of 2014. During this time, the region of interest drifts about 335 km northwest beginning 425 km from the ice edge at 73.846°N, −140.068°W and ending 65 km from the ice edge at 75.199°N, −150.924°W. The ice concentration declines from 0.92 to 0.08. We assume free drift in our analysis of 30 July through 13 September with an ice concentration less than 0.80 [Leppäranta, 2005]. Figure 1 shows a sequence of four satellite images illustrating the evolution of the ice cover. The first row displays each image at a scale that shows nearly the entire scene. The second row is a close‐up of the ice conditions in each image. A tight cluster of large angular floes displayed in the 6 May image contrasts the densely ponded and separated ice floes in the 30 July image. The 11 August image represents a third regime that can be characterized by an even larger density of mature ponds and floes that are more rounded in shape. In between round floes are jagged, irregularly shaped ice fragments. The fourth panel shows the remains of the ice cover on 13 September. Small pieces of ice that have so far survived the melt have gathered in clusters created primarily by the wind and ocean currents [Thorndike and Colony, 1982]. Only the thicker, ridged ice survived the summer melt. The thinner ice completely melted predominantly due to bottom melt. When considering the evolution of the floe size distribution it is important to include the loss of floes due to thinning as well as lateral melting and breaking. Figure 1 Open in figure viewer PowerPoint (a–d) The evolution of the ice cover in the study area. Figure 1a was acquired 6 May 2014. Figure 1b is 30 July, Figure 1c is 11 August, and Figure 1d shows an almost completely melted ice cover on 13 September. The red dot (through Figures 1a–1c) shows a distinct feature (indiscernible by time in Figure 1d) recognizable through the time series. The second row is a magnified version of the image above (not following any particular feature). Results from the IMB 2014C provide key information about the progression of melt at the study site. Prior to melt, IMB data show that ice at this location reached a maximum thickness of 1.96 m with a 0.51 m deep snow cover. Results also indicate that air temperature reached the melting point (0°C) in late June and remained there for July and most of August. Bottom melting in this region begins on 6 June, snowmelt on 7 June, and surface ice melt 1 month later on 6 July. A peak ocean water temperature of about −1.1°C in the last week of July is immediately followed by the highest rate of bottom melt (more than 7 cm d−1). We used the IMB observations to estimate the ocean heat flux as a residual of conduction through the ice and changes in specific and latent heat of the ice [McPhee and Untersterner, 1982]. Ocean heat fluxes reached maximum values of 56 Wm−2 during this period. By 24 August, the IMB had completely melted free of the ice. A total of 27 cm of surface ice melt and 15 cm of ice equivalent snowmelt are observed in addition to 116 cm of bottom ice melt. A summary of the quantitative image processing results is provided in Table 1. In May, the region of interest has a floe number density, defined by number per unit area of the scene, of 7 floes km−2 with 27% of floes greater than 100 m in diameter. By mid‐September, density has increased to 73 floes km−2 with only 0.5% of floes greater than 100 m in diameter and an average floe size of only 24 m. Results from the analysis of the two ponded images (Figures 1b and 1c) yields a melt pond concentration of 12.7% of total floe area with 2.9 × 103 ponds km−2 ice on 30 July. By 11 August, ice concentration was unchanged but pond concentration increased to 16.2% of the floe area. The number of ponds per unit area of floe area decreased to 2.3 × 103 km−2 due to ponds becoming open water with breaking. Floe perimeter associated with the two ponded images increases from 9.1 km km−2 to 1.7 km km−2 (normalized by floe area) or from 0.91% to 1.73% of floe area. Floe number density increases from 17 km−2 to 80 km−2 with the simultaneous increase in bottom melt between 30 July and 11 August. Both perimeter and pond fraction in Table 1 are normalized by total floe area to represent change in terms of floes. Table 1. Quantitative Image Processing Results Date Lat. (°N) Lon. (°W) Distance to Ice Edge (km) Image Area (km2) Ice Concentration Floe Number Density (km−2) Average Floe Size Diameter (m) Median Floe Size Diameter (m) Perimeter (m km−2) Pond Fraction 6 May 73.846 140.068 425 km 789 km2 0.92 7 km−2 195 m 115 m 8,431 m km−2 0.00 30 July 74.153 148.888 250 km 776 km2 0.71 17 km−2 59 m 16 m 9,104 m km−2 0.13 11 August 74.201 151.874 150 km 635 km2 0.71 80 km−2 30 m 16 m 17,300 m km−2 0.16 13 September 75.199 150.924 65 km 641 km2 0.08 73 km‐2 b 24 m b 19 m 1.9 × 105 m km−2 0.00 The trend toward a greater perimeter length and a larger number of smaller floes is apparent with these results. A close examination of the imagery indicates the two key mechanisms driving the summer floe breaking. The first mechanism is reported by Perovich et al. [2001]: winter dynamics manifested in frozen cracks that preferentially melt during summer, impacting the summer breakup of floes. An example of breaking at these preconditioned interfaces is shown in Figure 2, where a small piece of ice breaks away from a larger floe along an old dynamic feature, specifically a refrozen lead. The ice in the refrozen lead is thinner than the adjacent ice and therefore is weaker to the point of failure or melts all the way through before the adjacent older ice. Investigation of the images (not shown) also indicates that many floes break along ridge subduction zones where flooding has caused the formation of ponds and a greater absorption of incident solar energy. Figure 2 Open in figure viewer PowerPoint Before (30 July) and after (02 August) of the preferential melting of an old refrozen lead that becomes a new ice‐water interface as a result of breaking. Figure 3 shows the breaking of a particular ice floe between 30 July and 2 August. A portion of the floe breaks along a connected mature pond feature. Melting is greater in ponded ice than unponded [Perovich et al., 2003], causing the ponded ice to become thinner and more porous than unponded ice and, hence, relatively weak. In this case, the ponding has sufficiently weakened the ice and has caused one floe to break into two floes. Results from the IMB show that the highest rate of bottom melt observed all summer in the region of interest occurred between the dates of these images (i.e., 30 July and 2 August). The role of mature melt ponds was confirmed by identifying several parent floes in the 30 July image and then locating the corresponding child floes in the 2 August image (two examples shown in Figures 2 and 3). This allowed us to examine the effects of the large heat flux and subsequent melt that transpired over this 4 day period. As in Figure 3, breaking mainly occurred along interconnected pond network boundaries. Breaking along these features results in smaller floes with jagged edges as seen in Figure 1c and in Figure 3, thus altering both the size and shape characteristics of the floes. In essence, networks of mature ponds act as linked perforations in the ice floes, weak areas where the floes can come apart with minimal forcing. Figure 3 Open in figure viewer PowerPoint Before (30 July) and after (02 August) of a floe breaking event. The floe breaks along pond boundaries. To confirm the impact of mature ponds on floe breakup, we conducted a numerical experiment using a discrete element method (DEM) sea ice model [Hopkins et al., 2004; Wilchinsky et al., 2010]. For these simulations, we used a 500 m × 500 m subimage of the segmented 30 July image (Figure 4). Both the pond fraction of 0.32 and the spatial distribution of ponds were included in the DEM simulation. We set unponded ice to a thickness of 2 m and ponded ice to 0.5 m. We treat the ponded ice as more porous than unponded ice [Eicken et al., 2002] and implement the empirical correlation from Timco and Weeks [2010] to describe the relationship between the tensile strength and the porosity. Model parameters are given in Table 2. The bottom boundary of the domain was fixed, and a stress was applied directed 45° above this boundary, as shown in Figure 4. Figure 4 Open in figure viewer PowerPoint The simulated breakup of ponded ice. The bottom of the domain is fixed, and the ice is subjected to a stress indicated by the red arrows. The lines of fractures tend to be perpendicular to the principal stress direction, indicating that failure happens primarily in tension. Table 2. Summary of Parameters Used in Discrete Element Modeling Parameter Value Unponded ice thickness 2 m Ponded ice thickness 0.5 m Average floe size 4.0 m Pack width/height 500 m Elastic modulus 1 GPa Poisson ratio 0.3 Ice compressive strength 1.285 h3/2 MPa m Kovacs and Sodhi [ 1980 Ice tensile strength 4.278 hv t −0.6455 MPa m Timco and Weeks [ 2010 Shear rupture coefficient 0.6 Wilchinsky et al. [ 2010 Ice sliding friction coefficient 0.3 Wilchinsky et al. [ 2010 The fracture pattern occurs preferentially in the pond locations due to the significantly lower ice strength. Also, the orientation of the fractures is predominantly perpendicular to the principal stress direction, indicating that the ice fails primarily in tension in this example.

4 Discussion By following an ensemble of ice floes, we observe floe breaking during the 2014 summer melt season in the seasonally emerging MIZ in the Beaufort Sea region of the Arctic Ocean. An associated increase in floe perimeter, an increase in number density of floes, and a decrease in average and median size of floes are reported. Floe breakup was observed during a period when Smith and Thompson (unpublished data, 2014) report low levels of wave activity based on wave buoy results from this region. The ice cover was in free drift with small internal ice stress. We conclude that floe breakup was thermodynamically driven in the later part of July as melt accelerated. The network of melt ponds formed weak zones where modest forcing was sufficient for floes to break apart. The observed contribution of the spatial distribution of melt ponds to floe breakup is supported by results from discrete element modeling. To acknowledge the degree to which melt ponds dictate floe breaking patterns is to conclude that ice features and ice types subject to preferential melt and ponding play a large role in the evolution of floe size distribution. As concentration, distribution, and evolution of melt ponds vary between ice types [Webster et al., 2015] and specific floe features, so will breaking patterns along with size and shape parameters of child floes. This knowledge can be used to determine regions where thermodynamic processes exert the greatest control on ice cover characteristics including FSD. For instance, refrozen cracks and leads are areas of preferential melting and are characterized by thinner ice and a lower freeboard, which makes them susceptible to ponding [Perovich et al., 2001]. When surface and bottom melt rates increase, old cracks and leads will fail first and will mark potential breaking locations. Flooded subduction zones along ridges are also another area where ice is likely to become heavily ponded, weaken, and break. These zones form during dynamic events in the winter and subsequently return to play an important role during summer breakup. In addition to predicting breaking at particular features, these findings promote a better understanding of the behavior of Arctic first year ice in a seasonal marginal ice zone, which now develops routinely in the Beaufort Sea region. Individual ice floes display differing degrees of susceptibility to ponding based on ice type, age, and ice thickness. Undeformed first year sea ice is conducive to large highly connected melt ponds that can cover up to 50% or more of the ice surface. Thermodynamically influenced breaking patterns for this type of pond coverage would vary from that of multiyear ice, which tends to have a lower concentration of deeper ponds constrained spatially by dynamic surface topography and deformations [Polashenski et al., 2012; Webster et al., 2015]. As the Arctic shifts toward a higher concentration of first year ice [Maslanik et al., 2011; Xie et al., 2013], thermodynamic forcings are amplified in three important ways: (1) a high concentration of linked dark melt ponds increases absorption of shortwave irradiance in the ice and upper ocean enhancing melt, (2) the thinner first‐year ice causes weaker melt pond perforations, and (3) breaking by mechanisms 1 and 2 creates small thin floes that completely melt away in short periods of time. This loss of smaller floes has a significant impact on ice concentration in the MIZ, as indicated by a drastic change from 92% ice in May to only 8% in September at our study location. Results from this study suggest a melt sequence for Arctic first‐year ice in a seasonal marginal ice zone. Early in the melt season the snowmelts and ponds form. More solar radiation is absorbed in the ponded ice and transmitted through to the upper ocean. As melt progresses the following occur: (i) the pond ice thins faster than the surrounding bare ice forming areas of weaker ice, (ii) the ponds interconnect forming a network of weak perforations in the ice cover, and (iii) the heat content of the upper ocean increases due to transmitted solar radiation. The ponded areas become so weak that even modest tensile stress is enough to break the floes apart, thus increasing the number of floes and decreasing the size of floes. Ice motion during this breakup increases the release of heat stored in the upper ocean. Lateral melting eliminates small floes. Large diameter, but thin floes, are lost due to substantial amounts of bottom melting. Only the thickest, ridged ice survives. Expanding knowledge of the impact of thermodynamic processes with respect to ponded sea ice draws a distinct contrast between the Arctic MIZ and the Antarctic MIZ. In the Antarctic, wave action penetrates hundreds of kilometers into the ice pack [Kohout et al., 2014], whereas in much of the Arctic, the ice cover between the open ocean and continuous ice may not be subject to wave forcings for most of the melt season. In the absence of melt ponds, consistent exposure to waves defines the Antarctic marginal ice zone with rounded floes that tend to be smaller and more uniform than Arctic floes [Toyota et al., 2011; Kohout et al., 2014].

Acknowledgments The NTM images are available at htt://gfl.usgs.gov/.IMB; results can be found at http://imb.erdc.dren.mil/. This study was supported by the Office of Naval Research under awards N000141410176 and N0001413MP20163 and also by the National Science Foundation Arctic Observing Network (NSF‐ 0856376). The Editor thanks Stephen Ackley and an anonymous reviewer for their assistance in evaluating this paper.