[1] We present our best estimate of the thickness and volume of the Arctic Ocean ice cover from 10 Ice, Cloud, and land Elevation Satellite (ICESat) campaigns that span a 5-year period between 2003 and 2008. Derived ice drafts are consistently within 0.5 m of those from a submarine cruise in mid-November of 2005 and 4 years of ice draft profiles from moorings in the Chukchi and Beaufort seas. Along with a more than 42% decrease in multiyear (MY) ice coverage since 2005, there was a remarkable thinning of $0.6 m in MY ice thickness over 4 years. In contrast, the average thickness of the seasonal ice in midwinter ($2 m), which covered more than two-thirds of the Arctic Ocean in 2007, exhibited a negligible trend. Average winter sea ice volume over the period, weighted by a loss of $3000 km 3 between 2007 and 2008, was $14,000 km 3 . The total MY ice volume in the winter has experienced a net loss of 6300 km 3 (>40%) in the 4 years since 2005, while the first-year ice cover gained volume owing to increased overall area coverage. The overall decline in volume and thickness are explained almost entirely by changes in the MY ice cover. Combined with a large decline in MY ice coverage over this short record, there is a reversal in the volumetric and areal contributions of the two ice types to the total volume and area of the Arctic Ocean ice cover. Seasonal ice, having surpassed that of MY ice in winter area coverage and volume, became the dominant ice type. It seems that the near-zero replenishment of the MY ice cover after the summers of 2005 and 2007, an imbalance in the cycle of replenishment and ice export, has played a significant role in the loss of Arctic sea ice volume over the ICESat record.

we estimate their ice thicknesses using constructed fields of daily snow depth and compare them with ice drafts from moored upward-looking sonars. The methodologies, considerations, and assumptions used in the conversion of freeboard to ice thickness are discussed. The thickness distributions of the Arctic multiyear and seasonal ice covers are contrasted. Broadly, the resulting fields seem seasonally and interannually consistent in terms of thickness, growth and ice production. We find mean thicknesses of 2.15/2.46 m in ON05/FM06 and an overall thinner ice cover of 1.96/2.37 m in ON06/MA07. This represents a growth of $0.3 m and $0.4 m during the $4-month intervals of the ON05-FM06 and ON06-MA07 campaigns, respectively. After accounting for data gaps, we compute overall Arctic Ocean ice volumes of 11,318, 14,075, 10,626, and 13,891 km 3 for the ON05, FM06, ON06, and MA07 campaigns, respectively. The higher total volume in ON05 (versus ON06) can be attributed to the higher multiyear ice coverage that fall: 37% versus 31%. However, the higher estimated ice production (less export) during the second year (3265 versus 2757 km3) is likely due to the higher growth rate over the larger expanse of seasonal sea ice during the fall and winter. Inside a 25-km radius of two mooring locations in the Beaufort Sea, the estimated mean ICESat ice drafts from ON05 and FM06 are within 0.5 m of those measured at the moorings.Citation: Kwok, R., and G. F. Cunningham (2008), ICESat over Arctic sea ice: Estimation of snow depth and ice thickness,

One contribution of 9 to a discussion meeting issue 'Arctic sea ice reduction: the evidence, models and impacts (part 1)' . We present our estimates of the thickness and volume of the Arctic Ocean ice cover from CryoSat-2 data acquired between October 2010 and May 2014. Average ice thickness and draft differences are within 0.16 m of measurements from other sources (moorings, submarine, electromagnetic sensors, IceBridge). The choice of parameters that affect the conversion of ice freeboard to thickness is discussed. Estimates between 2011 and 2013 suggest moderate decreases in volume followed by a notable increase of more than 2500 km 3 (or 0.34 m of thickness over the basin) in 2014, which could be attributed to not only a cooler summer in 2013 but also to large-scale ice convergence just west of the Canadian Arctic Archipelago due to wind-driven onshore drift. Variability of volume and thickness in the multiyear ice zone underscores the importance of dynamics in maintaining the thickness of the Arctic ice cover. Volume estimates are compared with those from ICESat as well as the trends in ice thickness derived from submarine ice draft between 1980 and 2004. The combined ICESat and CryoSat-2 record yields reduced trends in volume loss compared with the 5 year ICESat record, which was weighted by the record-setting ice extent after the summer of 2007.

[1] We summarize 24 years of ice export estimates and examine, over a 9-year record, the associated variability in the time-varying upward-looking sonar (ULS) thickness distributions of the Fram Strait. A more thorough assessment of the PMW (passive microwave) ice motion with 5 years of synthetic aperture radar (SAR) observations shows the uncertainties to be consistent with that found by Kwok and Rothrock [1999], giving greater confidence to the record of ice flux calculations. Interesting details of the cross-strait motion profiles and ice cover characteristics revealed by high-resolution SAR imagery are discussed. The average annual ice area flux over the period is 866,000 km 2 /yr. Between the 1980s and 1990s, the decadal difference in the net exported ice area is $400,000 km 2 , approximately half the annual average. Except for the years with extreme negative NAO, correlation of winter ice area export with the NAO index remains high (R 2 = 0.62). With thickness estimates from ULS moorings, we estimate the average annual ice volume flux (8 years) to be $2218 km 3 /yr ($0.07 Sv). Over the $9-year ULS ice thickness data set, there is an overall decrease of 0.45 m in the mean ice thickness over the entire time series and a decrease of 0.23 m over the winter months (December through March). Correspondingly, the mode of the MY ice thickness exhibits an overall decrease of 0.55 m and a winter decrease of 0.42 m. These are significant trends. Whether these trends are indicative of the thickness trends of the Arctic Ocean is examined, as the time-varying behavior of the monthly ULS thickness distributions can be related not only to the seasonal cycle in the basal growth and melt, but also to the magnitude and pattern of ice motion in the Arctic Ocean, and the proximity of the ULS moorings to the ice edge.

[1] Total freeboard (snow and ice) of the Arctic Ocean sea ice cover is derived using Ice, Cloud, and land Elevation Satellite (ICESat) data from two 35-day periods: one during the fall (OctoberÀNovember) of 2005 and the other during the winter (FebruaryÀMarch) of 2006. Three approaches are used to identify near-sea-surface tiepoints. Thin ice or open water samples in new openings, typically within 1À2 cm of the sea surface, are used to assess the sea surface estimates. Results suggest that our retrieval procedures could provide consistent freeboard estimates along 25-km segments with uncertainties of better than 7 cm. Basin-scale composites of sea ice freeboard show a clear delineation of the seasonal ice zone in the fall. Overall, the mean freeboards of multiyear (MY) and first-year (FY) ice are 35 cm and 14 cm in the fall, and 43 cm and 27 cm in the winter. The increases of $9 cm and $12 cm on MY and FY sea ice are associated with the 4 months of ice growth and snow accumulation between data acquisitions. Since changes in snow depth account for >90% of the seasonal increase in freeboard on MY ice, it dominates the seasonal signal. Our freeboard estimates are within 10 cm of those derived from available snow/ice thickness measurements from ice mass balance buoys. Examination of the two residual elevations fields, after the removal of the sea ice freeboard contribution, shows coherent spatial patterns with a standard deviation (S.D.) of $23 cm. Differencing them reduces the variance and gives a near random field with a mean of $2 cm and a standard deviation of $14 cm. While the residual fields seem to be dominated by the static component of unexplained sea surface height and mean dynamic topography (S.D. $23 cm), the difference field reveals the magnitude of the time-varying components as well as noise in the ICESat elevations (S.D. $10 cm).

This study combines sea surface height (SSH) estimates of the ice‐covered Southern Ocean with conventional open‐ocean SSH estimates from CryoSat‐2 to produce monthly composites of dynamic ocean topography (DOT) and sea level anomaly (SLA) on a 50 km grid spanning 2011–2016. This data set reveals the full Southern Ocean SSH seasonal cycle for the first time; there is an antiphase relationship between sea level on the Antarctic continental shelf and the deeper basins, with coastal SSH highest in autumn and lowest in spring. As a result of this pattern of seasonal SSH variability, the barotropic component of the Antarctic Slope Current (ASC) has speeds that are regionally up to twice as fast in the autumn. Month‐to‐month circulation variability of the Ross and Weddell Gyres is strongly influenced by the local wind field, and is correlated with the local wind curl (Ross: −0.58; Weddell: −0.67). SSH variability is linked to both the Southern Oscillation and the Southern Annular Mode, dominant modes of southern hemisphere climate variability. In particular, during the strong 2015–2016 El Niño, a sustained negative coastal SLA of up to −6 cm, implying a weakening of the ASC, was observed in the Pacific sector of the Southern Ocean. The ability to examine sea level variability in the seasonally ice‐covered regions of the Southern Ocean—climatically important regions with an acute sparsity of data—makes this new merged sea level record of particular interest to the Southern Ocean oceanography and glaciology communities.

For the summers of 1993 through 2009, we estimate the loss of multiyear sea ice (MYI) area in the Beaufort Sea due to melt. Parcels of MYI in April are traced into the Beaufort Sea where they melt as the ice edge retreats. Net loss of area (with fractional MYI coverage >50%) over the 17‐year period is ∼900 × 103 km2. Three‐quarters of that area, ∼10% of the area of the Arctic Ocean, was lost after 2000. There is a clear positive trend in the record, with a distinct peak of 213 × 103 km2 in 2008; this is twice the summer outflow at the Fram Strait that year. The net melt area of 490 × 103 km2 between 2005 and 2008 accounts for nearly 32% of the net loss of 1.54 × 106 km2 of Arctic Ocean MYI coverage over the same period. Volume loss, for the years with ICESat thickness (2004–2009), is highest at 473 km3 in 2008 followed by 320 km3 in 2007. Net loss in MYI volume for the six summers is ∼1400 km3. This is ∼20% of the loss in MYI volume of 6300 km3 during 2004–2008. This adds to the freshwater content of the Arctic Ocean and locally to the freshening of the Beaufort Gyre.

We investigate the shape of near-Earth asteroid (101955) Bennu by constructing a high-resolution (20 cm) global digital terrain model from laser altimeter data. By modeling the northern and southern hemispheres separately, we find that longitudinal ridges previously identified in the north extend into the south but are obscured there by surface material. In the south, more numerous large boulders effectively retain surface materials and imply a higher average strength at depth to support them. The north has fewer large boulders and more evidence of boulder dynamics (toppling and downslope movement) and surface flow. These factors result in Bennu’s southern hemisphere being rounder and smoother, whereas its northern hemisphere has higher slopes and a less regular shape. We infer an originally asymmetric distribution of large boulders followed by a partial disruption, leading to wedge formation in Bennu’s history.