The summer extent of the Arctic sea ice cover, widely recognized as an indicator of climate change, has been declining for the past few decades reaching a record minimum in September 2007. The causes of the dramatic loss have implications for the future trajectory of the Arctic sea ice cover. Ice mass balance observations demonstrate that there was an extraordinarily large amount of melting on the bottom of the ice in the Beaufort Sea in the summer of 2007. Calculations indicate that solar heating of the upper ocean was the primary source of heat for this observed enhanced Beaufort Sea bottom melting. An increase in the open water fraction resulted in a 500% positive anomaly in solar heat input to the upper ocean, triggering an ice–albedo feedback and contributing to the accelerating ice retreat.

[1] Over the past few decades the Arctic sea ice cover has decreased in areal extent. This has altered the solar radiation forcing on the Arctic atmosphere-ice-ocean system by decreasing the surface albedo and allowing more solar heating of the upper ocean. This study addresses how the amount of solar energy absorbed in areas of open water in the Arctic Basin has varied spatially and temporally over the past few decades. A synthetic approach was taken, combining satellite-derived ice concentrations, incident irradiances determined from reanalysis products, and field observations of ocean albedo over the Arctic Ocean and the adjacent seas. Results indicate an increase in the solar energy deposited in the upper ocean over the past few decades in 89% of the region studied. The largest increases in total yearly solar heat input, as much as 4% per year, occurred in the Chukchi Sea and adjacent areas.

The summer extent of the Arctic sea-ice cover has decreased in recent decades and there have been alterations in the timing and duration of the summer melt season. These changes in ice conditions have affected the partitioning of solar radiation in the Arctic atmosphere-ice-ocean system. The impact of sea-ice changes on solar partitioning is examined on a pan-Arctic scale using a 25 km  25 km Equal-Area Scalable Earth Grid for the years 1979-2007. Daily values of incident solar irradiance are obtained from NCEP reanalysis products adjusted by ERA-40, and ice concentrations are determined from passive microwave satellite data. The albedo of the ice is parameterized by a five-stage process that includes dry snow, melting snow, melt pond formation, melt pond evolution, and freeze-up. The timing of these stages is governed by the onset dates of summer melt and fall freeze-up, which are determined from satellite observations. Trends of solar heat input to the ice were mixed, with increases due to longer melt seasons and decreases due to reduced ice concentration. Results indicate a general trend of increasing solar heat input to the Arctic ice-ocean system due to declines in albedo induced by decreases in ice concentration and longer melt seasons. The evolution of sea-ice albedo, and hence the total solar heating of the ice-ocean system, is more sensitive to the date of melt onset than the date of fall freeze-up. The largest increases in total annual solar heat input from 1979 to 2007, averaging as much as 4% a -1 , occurred in the Chukchi Sea region. The contribution of solar heat to the ocean is increasing faster than the contribution to the ice due to the loss of sea ice.

There has been a marked decline in the summer extent of Arctic sea ice over the past few decades. Data from autonomous ice mass-balance buoys can enhance our understanding of this decline. These buoys monitor changes in snow deposition and ablation, ice growth, and ice surface and bottom melt. Results from the summer of 2008 showed considerable large-scale spatial variability in the amount of surface and bottom melt. Small amounts of melting were observed north of Greenland, while melting in the southern Beaufort Sea was quite large. Comparison of net solar heat input to the ice and heat required for surface ablation showed only modest correlation. However, there was a strong correlation between solar heat input to the ocean and bottom melting. As the ice concentration in the Beaufort Sea region decreased, there was an increase in solar heat to the ocean and an increase in bottom melting.

The Arctic sea ice cover undergoes large changes over an annual cycle. In winter and spring, the ice cover consists of large, snow-covered plate-like ice floes, with very little open water. By the end of summer, the snow cover is gone and the large floes have broken into a complex mosaic of smaller, rounded floes surrounded by a lace of open water. This evolution strongly affects the distribution and fate of the solar radiation deposited in the ice-ocean system and consequently the heat budget of the ice cover. In particular, increased floe perimeter can result in enhanced lateral melting. We attempt to quantify the floe evolution process through the concept of a floe size distribution that is modified by lateral melting and floe breaking. A time series of aerial photographic surveys made during the SHEBA field experiment is analyzed to determine evolution of the floe size distribution from spring through summer. Based on earlier studies, we assume the floe size cumulative distribution could be represented by a power law D 2a , where D is the floe diameter. The exponent a as well as the number density of floes N tot are estimated from measurements of total ice area and perimeter. As summer progressed, there was an increase in a as the size distribution shifted toward smaller floes and the number of floes increased. Lateral melting causes the distribution to deviate from a power law for small floe sizes.

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[1] The sea spray generation function dF/dr 0 predicts the rate at which droplets of initial radius r 0 are produced at the sea surface. Because this function is not readily measurable in the marine environment, however, it is often inferred from measurements of the near-surface droplet concentration, C(r 0 ), through an assumed velocity scale, the effective spray production velocity. This paper proceeds in reverse, though: It uses a reliable estimate of dF/dr 0 and 13 sets of measurements of C(r 0 ) over the ocean to calculate the implied effective production velocity, V eff , for droplets with initial radii r 0 from 5 to 300 mm. It then compares these V eff values with four candidate expressions for this production velocity: the dry-deposition velocity, V Dh ; the mean wind speed at the significant wave amplitude (A 1/3 ), U A 1/3 ; the standard deviation in vertical droplet velocity, s wd ; and laboratory measurements of the ejection velocity of jet droplets, V ej . The velocity scales U A 1/3 and V ej agree best with the implied V eff values for 20 ≤ r 0 ≤ 300 mm. The deposition velocity, V Dh , which is the velocity most commonly used in this application, agrees worst with the V eff values. For droplets with r 0 less than about 20 mm, the analysis also rejects the main hypothesis: that dF/dr 0 and C(r 0 ) can be related through a velocity scale. These smaller droplets simply have residence times that are too long for spray concentrations to be in local equilibrium with the spray production rate.

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