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All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

The effects of preconditioning on the summer sea ice thickness evolution during MOSAiC

The central role of sea ice within the Arctic climate system is clear, as is the importance of sea ice properties in the Arctic Ocean’s evolution and response to climate change. Less clear is how the history of the sea ice floe (in winter and spring) precondition the summer thickness evolution. Accurate accounting for the collective influences of processes that affect the summer sea ice thickness evolution is necessary for determining sea ice survivability and the rate of future sea ice loss. In this study, we use a satellite-derived sea ice parcel database co-located with the MOSAiC floe to explore the factors that precondition the summer sea ice thickness evolution. First. we compare the >500 collocated satellite-tracked sea ice parcels that intersected the MOSAiC drift track with observations to evaluate the satellite-derived surface energy budget and surface properties, finding agreement with correlations >0.9. Approximately 60% of the satellite tracked parcels survived the summer melt season. Secondly, we explore the spread in sea ice thickness and survivability across the satellite-tracked parcels, finding contributions from spring/summer surface albedo evolution, surface temperature, clouds, and parcel sea ice concentration. The satellite tracked sea ice parcel analysis provides insights into the spatial coherence of the factors that influence sea ice thickness evolution.

Arctic↗
DOE OSTI2014

Theory versus practice in Strategic Environmental Assessment (SEA)

Could the theory of Strategic Environmental Assessment (SEA) be ahead of its time and decoupled from its practice? This paper evolved in search for this leading research question. Over the years the discourse on SEA experienced a gradual shift from the technocratic and rationalist thinking that supported its origin to more strategic approaches and integrated concepts, suggested since the mid 1990's. In this paper we share the results of our analysis of international thinking and practical experience with SEA. Results reveal that SEA practice changes very slowly when compared to advanced thinking supporting the noted shift. Current SEA practice shows to be still predominantly rooted in the logic of projects' environmental impact assessment (EIA). It is strongly bound to legal and regulatory requirements, and the motivation for its application persists being the delivery of environmental (or final) reports to meet legal obligations. Even though advanced SEA theoretical thinking claim its potential to help decisions to look forward, change mind-sets and the rationale of decision-making to meet sustainability challenges and enhance societal values, we note a weak relationship between the theoretical development of SEA and its practice. Why is this happening? Which factors explain this apparent inertia, resistance to change, in the SEA practice? Results appear to demonstrate the influence of assumptions, understandings, concepts, and beliefs in the use of SEA, which in turn suggest the political sensitivity of the instrument. - Highlights: • Theoretical thinking in SEA is ahead of its time. • SEA international practice reveals inertia to move out of project’ EIA comfort zone. • World current SEA practice show similar understandings of 30 years ago. • 100 world reports and survey of practitioners supported world review. • SEA great challenge is to change paradigms into new scientific complexity theories.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2013

Review of critical factors for SEA implementation

The implementation process involved in translating Strategic Environmental Assessment (SEA) intention into action is vital to an effective SEA. Many factors influence implementation and thus the effectiveness of an SEA. Empirical studies have identified and documented some factors influencing the implementation of an SEA. This research is fragmented, however, and it is still not clear what are the most critical factors of effective SEA performance, and how these relate to different stages of the implementation process or other contextual circumstances. The paper takes its point of departure in implementation theory. Firstly, we introduce implementation theory, and then use it in practice to establish a more comprehensive model related to the stages in the implementation process. Secondly, we identify the critical factors in order to see how they are related to the different stages of SEA or are more general in character. Finally we map the different critical factors and how they influence the overall results of an SEA. Based on a literature review, we present a comprehensive picture of the critical factors and where they are found in the process. We conclude that most of the critical factors identified are of a more general character influencing the SEA process as such, while only one out of four of these factors relates to the specific stages of the SEA. Based on this mapping we can sketch a picture of the totality of critical factors. In this study 266 notions of critical factors were identified. Seen at the level of notions of critical factors, only 24% of these relate to specific stages while for 76% the critical factors are of a more general nature. These critical factors interact in complex ways and appear in different combinations in different stages of the implementation process so tracing the cause and effect is difficult. The pervasiveness of contextual and general factors also clearly suggests that there is no single way to put SEA into practice. The paper identifies some of the critical factors for effective SEA implementation, but further research is still needed to conclude which factors are more critical than others, just as the contingencies on which they depend are not easy to unravel. - Highlights: Black-Right-Pointing-Pointer The research on critical factors influencing SEA implementation is fragmented. Black-Right-Pointing-Pointer The critical factors are used to discuss 'hot-spots' in the implementation process. Black-Right-Pointing-Pointer Critical factors are just as broad as the concept of effectiveness. Black-Right-Pointing-Pointer Both stage and general factors are relevant in explaining the effectiveness of SEA.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1993

Observation of pressure ridges in SAR images of sea ice: Scattering theory and comparison with observations

Ridges and keels (hummocks and bummocks) in sea ice flows are important in sea ice research for both scientific and practical reasons. Sea ice movement and deformation is driven by internal and external stresses on the ice. Ridges and keels play important roles in both cases because they determine the external wind and current stresses via drag coefficients. For example, the drag coefficient over sea ice can vary by a factor of several depending on the fluid mechanical roughness length of the surface. This roughness length is thought to be strongly dependent on the ridge structures present. Thus, variations in ridge and keel structure can cause gradients in external stresses which must be balanced by internal stresses and possibly fracture of the ice. Ridging in sea ice is also a sign of fracture. In a practical sense, large ridges form the biggest impediment to surface travel over the ice or penetration through sea ice by ice-strengthened ships. Ridges also play an important role in the damage caused by sea ice to off-shore structures. Hence, observation and measurement of sea ice ridges is an important component of sea ice remote sensing. The research reported here builds on previous work, estimating the characteristics of ridges and leads in sea ice from SAR images. Our objective is to develop methods for quantitative measurement of sea ice ridges from SAR images. To make further progress, in particular, to estimate ridge height, a scattering model for ridges is needed. Our research approach for a ridge scattering model begins with a survey of the geometrical properties of ridges and a comparison with the characteristics of the surrounding ice. For this purpose we have used airborne optical laser (AOL) data collected during the 1987 Greenland Sea Experiment. These data were used to generate a spatial wavenumber spectrum for height variance for a typical ridge - the typical ridge is the average over 10 large ridges. Our first-order model radar scattering includes both the quasi-specular and Bragg resonant scatter mechanisms. This model is extended to include contributions from volume scatter and scatter from discrete objects. Geometrical characteristics from the AOL survey and model calculations imply that for radar wavelengths and observation geometries that are dominated by the quasi-specular scattering mechanism radar backscatter from a ridge is a measure of peak ridge height. We present scattering model results and compare them with ridges observed during the LEADEX experiment of March-April 1992 when both X, C, and L-band aircraft SAR and the ERS-1 satellite SAR observed a region in the Beaufort Sea near 86 deg N, 10 deg W. Data were also collected documenting ridge characteristics on the surface. The surface data are used to generate a SAR signature via the scattering model described above. The predicted SAR signatures compare well with the SAR observations.

Vesecky, J. F.↗
NASA NTRS1994

Mean sea surface and geoid gradient comparisons with TOPEX altimeter data

Cycles 4 to 54 of TOPEX data have been analyzed through comparisons with the mean sea surface given on the disturbed geophysical data record (GDR). Two inverted barometer correction procedures were considered for the data reduction. One used a constant atmospheric pressure for all data while the one adopted for use, for most computations, introduced a cycle average pressure. The maximum difference between the two estimates was 3.0 cm with a clear annual signal. With the modified correction the TOPEX sea surface was compared to The Ohio State University (OSU) mean sea surface, given on the GDR, to estimate three translations ( delta x = -2.3 cm; delta y = 25.0 cm; delta z = -0.3 cm) and a bias (43.3 cm) between the two surfaces. The only significant translation is delta y which indicates the reference frame of the TOPEX system differs from that used in the OSU mean sea surface system. The bias between the TOPEX mean sea surface and the OSU mean sea surface was used to estimate an equatorial radius of 6,378,136.55 m based on an 18-cm biased estimate of the TOPEX altimeter. Examination of the average difference, by cycle, between the TOPEX sea surface and the OSU mean sea surface suggested a bias change of 3.1 +/- 2.2 mm/yr with a positive sign indicating the average ocean surface is rising or the altimeter measured distance is decreasing. Models were implemented that solved directly for a bias, bias rate annual/semiannual, and tide correction terms. The computations indicated that a simultaneous solution for this bias, bias rate, and annual/semiannual terms gave the most accurate results. Nonsimultaneous solutions led to slightly different bias rate values. The root mean square difference between the TOPEX sea surface and OSU sea surface, after translation and bias correction, was +/- 17 cm for a typical cycle. Some locations were indentified where the difference could reach 2.3 cm and were repeated over several cycles indicating errors in the mean sea surface. Most of the large differences occur in regions lacking altimeter data prior to the TOPEX/POSEIDON mission and/or areas of significant bathymetric signature. Geoid gradients are needed for the reduction of individual track data to a reference track. The accuracy of the determination of such gradients was determined through the comparison of predicted along-track gradients to the observed gradients. Among four mean sea surfaces tested the best agreement was found with the OSU mean sea surface placed on the TOPEX geophysical data record.

Rapp, Richard H.↗
NASA NTRS1997

The Role of Sea Ice in 2 x CO2 Climate Model Sensitivity: Hemispheric Dependencies - Part 2

How sensitive are doubled CO2 simulations to GCM control-run sea ice thickness and extent? This issue is examined in a series of 10 control-run simulations with different sea ice and corresponding doubled CO2 simulations. Results show that with increased control-run sea ice coverage in the Southern Hemisphere, temperature sensitivity with climate change is enhanced, while there is little effect on temperature sensitivity of (reasonable) variations in control-run sea ice thickness. In the Northern Hemisphere the situation is reversed: sea ice thickness is the key parameter, while (reasonable) variations in control-run sea ice coverage are of less importance. In both cases, the quantity of sea ice that can be removed in the warmer climate is the determining factor. Overall, the Southern Hemisphere sea ice coverage change had a larger impact on global temperature, because Northern Hemisphere sea ice was sufficiently thick to limit its response to doubled CO2, and sea ice changes generally occurred at higher latitudes, reducing the sea ice-albedo feedback. In both these experiments and earlier ones in which sea ice was not allowed to change, the model displayed a sensitivity of -0.02 C global warming per percent change in Southern Hemisphere sea ice coverage.

Rind, D.↗
NASA NTRS2001

Trends in the Length of the Southern Ocean Sea Ice Season: 1979-1999

Satellite data can be used to observe the sea ice distribution around the continent of Antarctica on a daily basis and hence to determine how many days a year have sea ice at each location. This has been done for each of the 21 years 1979-1999. Mapping the trends in these data over the 21-year period reveals a detailed pattern of changes in the length of the sea ice season around Antarctica. Most of the Ross Sea ice cover has undergone a lengthening of the sea ice season, whereas most of the Amundsen Sea ice cover and almost the entire Bellingshausen Sea ice cover have undergone a shortening of the sea ice season. Results around the rest of the continent, including in the Weddell Sea, are more mixed, but overall, more of the Southern Ocean experienced a lengthening of the sea ice season than a shortening. For instance, the area experiencing a lengthening of the sea ice season by at least 1 day per year is 5.8 x 10(exp 6) sq km, whereas the area experiencing a shortening of the sea ice season by at least 1 day per year is less than half that, at 2.8 x 10(exp 6) sq km. This contrasts sharply with what is happened over the same period in the Arctic, where, overall, there has been some depletion of the ice cover, including shortened sea ice seasons and decreased ice extents.

Parkinson, Claire L.↗
NASA NTRS2021

Assessment of ICESat-2 Sea Ice Surface Classification with Sentinel-2 Imagery: Implications for Freeboard and New Estimates of Lead and Floe Geometry

NASA's Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) mission launched in September 2018 and is now providing high-resolution surface elevation profiling across the entire globe, including the sea ice cover of the Arctic and Southern Oceans. For sea ice applications, successfully discriminating returns between sea ice and open water is key for accurately determining freeboard (the extension of sea ice above local sea level) and new information regarding the geometry of sea ice floes and leads. We take advantage of near-coincident optical imagery obtained from the European Space Agency (ESA) Sentinel-2 (S-2) satellite over the Western Weddell Sea of the Southern Ocean in March 2019 and the Lincoln Sea of the Arctic Ocean in May 2019 to evaluate the surface classification scheme in the ICESat-2 ATL07 and ATL10 sea ice products. We find a high level of agreement between the ATL07 (specular) lead classification and visible leads in the S-2 imagery in these two coincident images across all six ICESat-2 beams, increasing our confidence in the freeboard products and deriving new estimates of the sea ice state. The S-2 overlays provide additional, albeit limited, evidence of the misclassification of dark leads due to clouds. Dark leads are no longer used to derive sea surface and thus freeboard as of the third release (r003) of the ICESat-2 sea ice products. We show estimates of lead fraction and more preliminary estimates of chord length (a proxy for floe size) using two metrics for classifying sea surface (lead) segments across both the Arctic and Southern Ocean for the first winter season of data collection.

A. A. Petty↗
DOE OSTI2013

SEA screening of voluntary climate change plans: A story of non-compliant discretion

Screening within Strategic Environmental Assessment (SEA) is the first critical stage involving considerations on whether an assessment is carried out or not. Although legislation and guidance offer practitioners a legal and logical approach to the screening process, it is inevitable that discretionary judgement takes place and will impact on the screening decision. This article examines the results of discretion involved in screening of climate change plans (CCPs) in a Danish context. These years voluntary CCPs are developed as a response to the global and local emergence of both mitigation and adaptation, and the voluntary commitment by the local authorities is an indication of an emerging norm of climate change as an important issue. This article takes its point of departure in the observation that SEA is not undertaken for these voluntary CCPs. The critical analysis of this phenomenon rests upon a documentary study of Danish CCPs, interviews with a lawyer and ministerial key person and informal discussions between researchers, practitioners and lawyers on whether climate change plans are covered by SEA legislation and underlying reasons for the present practice. Based on a critical analysis of mandatory SEA and/or obligation to screen CCPs according to significance criteria, the authors find that 18 out of the 48 CCPs are mandatory to SEA and 9 would require a screening of significance and thereby potentially be followed by a SEA. In practice only one plan was screened and one was environmentally assessed. The legal, democratic and environmental consequences of this SEA practice are critically discussed. Hereunder is the missed opportunity to use the broad environmental scope of SEA to avoid a narrow focus on energy and CO{sub 2} in CCPs, and the question whether this practice in Denmark complies with the EU Directive. -- Highlights: ► It is inevitable that discretionary judgement takes place and will impact on the screening decision. ► The article hereby calls for greater awareness of the discretion and judgement involved in SEA screening. ► Practice seemingly reveals a lack of application of SEA within climate change planning and a lack of explicit screening. ► Absence of SEAs of voluntary plans such as CCPs, have legal consequences, as well as democratic and environmental consequences. ► The vague definition of ‘administrative provision’ seems to impose the lacking SEA.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS2018

The Arctic Sea Ice Cover of 2016: A Year of Record-Low Highs and High-Than-Expected Lows

The Arctic sea ice cover of 2016 was highly noteworthy, as it featured record low monthly sea ice extents at the start of the year but a summer (September) extent that was higher than expected by most seasonal forecasts. Here we explore the 2016 Arctic sea ice state in terms of its monthly sea ice cover, placing this in the context of the sea ice conditions observed since 2000. We demonstrate the sensitivity of monthly Arctic sea ice extent and area estimates, in terms of their magnitude and annual rankings, to the ice concentration input data (using two widely used datasets) and to the averaging methodology used to convert concentration to extent (daily or monthly extent calculations). We use estimates of sea ice area over sea ice extent to analyse the relative “compactness†of the Arctic sea ice cover, highlighting anomalously low compactness in the summer of 2016 which contributed to the higher-than-expected September ice extent. Two cyclones that entered the Arctic Ocean during August appear to have driven this low-concentration/compactness ice cover but were not sufficient to cause more widespread melt-out and a new record-low September ice extent. We use concentration budgets to explore the regions and processes (thermodynamics/ dynamics) contributing to the monthly 2016 extent/area estimates highlighting, amongst other things, rapid ice intensification across the central eastern Arctic through September. Two different products show significant early melt onset across the Arctic Ocean in 2016, including record-early melt onset in the North Atlantic sector of the Arctic. Our results also show record-late 2016 freeze-up in the central Arctic, North Atlantic and the Alaskan Arctic sector in particular, associated with strong sea surface temperature anomalies that appeared shortly after the 2016 minimum (October onwards). We explore the implications of this low summer ice compactness for seasonal forecasting, suggesting that sea ice area could be a more reliable metric to forecast in this more seasonal, "New Arctic", sea ice regime.

sea ice↗
NASA NTRS2022

Impact of the Arctic Oscillation from March on summertime sea ice

Current understanding of the cold season Arctic oscillation (AO) impact on the summertime sea ice is revisited in this study by analyzing the role from each month. Earlier studies examined the prolonged AO impact using a smooth average over 1–2 seasons (e.g. December–March, December–April, March–May), ignoring large month-to-month AO variability. This study finds that the March AO is most influential on the summertime sea ice loss. First, the March AO is most highly negative-correlated with the AO in summer. Secondly, surface energy budget, sea level pressure, and low-tropospheric circulation exhibit that their time-lagged responses to the positive (negative) phase of the March AO grow with time, transitioning to the patterns associated with the negative (positive) phase of the AO that induces sea ice decrease (increase) in summer. Time evolution of the surface energy budget explains the growth of the sea ice concentration anomaly in summer, and a warming-to-cooling transition in October. The regional difference in sea ice anomaly distribution can be also explained by circulation and surface energy budget patterns. The sea ice concentration along the pan-Arctic including the Laptev, East Siberian, Chukchi, and Beaufort Sea decreases (increases) in summer in response to the positive (negative) phase of the March AO, while the sea ice to the northeast of Greenland increases (decreases). This sea ice response is better represented by the March AO than by the seasonally averaged winter AO, suggesting that the March AO can play more significant role. This study also finds that the sea ice decrease in response to the positive AO is distinctively smaller in the 20th century than in the 21st century, along with the opposite sea ice response over the Canada Basin due to circulation difference between the two periods.

Arctic Oscillation↗
DOE OSTI2020

Interseasonal Connections between the Timing of the Stratospheric Final Warming and Arctic Sea Ice

Connections across seasons in atmospheric circulation and sea ice have long been sought to advance seasonal prediction. This study presents a link between the springtime stratosphere and Arctic sea ice in summer through autumn. The polar stratospheric vortex dominates the winter stratosphere before breaking down each spring, which is called the stratospheric final warming, as solar radiation returns to the pole. Interannual variability of this breakdown is dynamically driven, leading to different springtime tropospheric and surface circulation patterns. To examine the different impacts of delayed and early final warmings, a multimodel composite was generated from selected CMIP5 models. Additionally, regressions were performed on JRA-55 against an index of springtime polar vortex strength. In both the multimodel composites and reanalysis regressions, significant anomalies in sea ice thickness persist several months following an anomalous timing of the final warming. A later final warming or stronger springtime polar stratospheric vortex leads to negative sea ice thickness anomalies in the East Siberian Sea and positive anomalies in the Beaufort Sea in comparison with an earlier final warming or weaker polar vortex. The spring polar stratospheric vortex is related to spring polar surface circulation patterns. The winds associated with this pattern induce anomalous sea ice motion, moving ice from the East Siberian Sea toward the Beaufort Sea. Reduced sea ice in the East Siberian Sea is linked to anomalous warmth over this region in autumn. Our results suggest that the timing of the stratospheric final warming exerts an influence on the tropospheric circulation and sea ice through autumn, which has implications for seasonal climate prediction.

Kelleher, Michael↗
DOE OSTI2022

Atmospheric particle abundance and sea salt aerosol observations in the springtime Arctic: a focus on blowing snow and leads

Abstract. Sea salt aerosols play an important role in the radiation budget and atmospheric composition over the Arctic, where the climate is rapidly changing. Previous observational studies have shown that Arctic sea ice leads are an important source of sea salt aerosols, and modeling efforts have also proposed blowing snow sublimation as a source. In this study, size-resolved atmospheric particle number concentrations and chemical composition were measured at the Arctic coastal tundra site of Utqiaġvik, Alaska, during spring (3 April–7 May 2016). Blowing snow conditions were observed during 25 % of the 5-week study period and were overpredicted by a commonly used blowing snow parameterization based solely on wind speed and temperature. Throughout the study, open leads were present locally. During periods when blowing snow was observed, significant increases in the number concentrations of 0.01–0.06 µm particles (factor of 6, on average) and 0.06–0.3 µm particles (67 %, on average) and a significant decrease (82 %, on average) in 1–4 µm particles were observed compared to low wind speed periods. These size distribution changes were likely caused by the generation of ultrafine particles from leads and/or blowing snow, with scavenging of supermicron particles by blowing snow. At elevated wind speeds, both submicron and supermicron sodium and chloride mass concentrations were enhanced, consistent with wind-dependent local sea salt aerosol production. At moderate wind speeds below the threshold for blowing snow as well as during observed blowing snow, individual sea spray aerosol particles were measured. These individual salt particles were enriched in calcium relative to sodium in seawater due to the binding of this divalent cation with organic matter in the sea surface microlayer and subsequent enrichment during seawater bubble bursting. The chemical composition of the surface snowpack also showed contributions from sea spray aerosol deposition. Overall, these results show the contribution of sea spray aerosol production from leads on both aerosols and the surface snowpack. Therefore, if blowing snow sublimation contributed to the observed sea salt aerosol, the snow being sublimated would have been impacted by sea spray aerosol deposition rather than upward brine migration through the snowpack. Sea spray aerosol production from leads is expected to increase, with thinning and fracturing of sea ice in the rapidly warming Arctic.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1992

Sea level variation

Published values for the long-term, global mean sea level rise determined from tide gauge records range from about one to three mm per year. The scatter of the estimates appears to arise largely from the use of data from gauges located at convergent tectonic plate boundaries where changes of land elevation give fictitious sea level trends, and the effects of large interdecadal and longer sea level variations on short (less than 50+ years) or sappy records. In addition, virtually all gauges undergo subsidence or uplift due to isostatic rebound from the last deglaciation at a rate comparable to or greater than the secular rise of sea level. Modeling rebound by the ICE-3G model of Tushingham and Peltier (1990) and avoiding tide gauge records in areas of converging tectonic plates produces a highly consistent set of long sea level records. A global set of 21 such stations in nine oceanic regions with an average record length of 76 years during the period 1880-1980 yields the global sea level rise value 1.8 mm/year +/- 0.1. Greenhouse warming scenarios commonly forecast an additional acceleration of global sea level in the next 5 or 6+ decades in the range 0.1-0.2 mm/yr2. Because of the large power at low frequencies in the sea level spectrum, very long tide gauge records (75 years minimum) have been examined for past apparent sea level acceleration. For the 80-year period 1905-1985, 23 essentially complete tide gauge records in 10 geographic groups are available for analysis. These yielded the apparent global acceleration -0.011 (+/- 0.012) mm/yr2. A larger, less uniform set of 37 records in the same 10 groups with 92 years average length covering the 141 years from 1850-1991 gave 0.001 (+/- 0.008) mm/yr2. Thus there is no evidence for an apparent acceleration in the past 100+ years that is significant either statistically, or in comparison to values associated with global warming. Unfortunately, the large interdecadal fluctuations of sea level severely affect estimates of global sea level acceleration for time spans of less than about 50 years. This means that tide gauges alone cannot serve as a reliable leading indicator of climate change in less than many decades. This time required can be significantly reduced if the interdecadal fluctuations of sea level can be understood in terms of their forcing mechanisms, and then removed from the tide gauge records.

Douglas, Bruce C.↗
NASA NTRS1994

Precise mean sea level measurements using the Global Positioning System

This paper describes the results of a sea level measurement test conducted off La Jolla, California, in November of 1991. The purpose of this test was to determine accurate sea level measurements using a Global Positioning System (GPS) equipped buoy. These measurements were intended to be used as the sea level component for calibration of the ERS 1 satellite altimeter. Measurements were collected on November 25 and 28 when the ERS 1 satellite overflew the calibration area. Two different types of buoys were used. A waverider design was used on November 25 and a spar design on November 28. This provided the opportunity to examine how dynamic effects of the measurement platform might affect the sea level accuracy. The two buoys were deployed at locations approximately 1.2 km apart and about 15 km west of a reference GPS receiver located on the rooftop of the Institute of Geophysics and Planetary Physics at the Scripps Institute of Oceanography. GPS solutions were computed for 45 minutes on each day and used to produce two sea level time series. An estimate of the mean sea level at both locations was computed by subtracting tide gage data collected at the Scripps Pier from the GPS-determined sea level measurements and then filtering out the high-frequency components due to waves and buoy dynamics. In both cases the GPS estimate differed from Rapp's mean altimetric surface by 0.06 m. Thus, the gradient in the GPS measurements matched the gradient in Rapp's surface. These results suggest that accurate sea level can be determined using GPS on widely differing platforms as long as care is taken to determine the height of the GPS antenna phase center above water level. Application areas include measurement of absolute sea level, of temporal variations in sea level, and of sea level gradients (dominantly the geoid). Specific applications would include ocean altimeter calibration, monitoring of sea level in remote regions, and regional experiments requiring spatial and temporal resolution higher than that available from altimeter data.

Kelecy, Thomas M.↗
NASA NTRS1998

Variability of Arctic Sea Ice as Viewed from Space

Over the past 20 years, satellite passive-microwave radiometry has provided a marvelous means for obtaining information about the variability of the Arctic sea ice cover and particularly about sea ice concentrations (% areal coverages) and from them ice extents and the lengths of the sea ice season. This ability derives from the sharp contrast between the microwave emissions of sea ice versus liquid water and allows routine monitoring of the vast Arctic sea ice cover, which typically varies in extent from a minimum of about 8,000,000 sq km in September to a maximum of about 15,000,000 sq km in March, the latter value being over 1.5 times the area of either the United States or Canada. The vast Arctic ice cover has many impacts, including hindering heat, mass, and y momentum exchanges between the oceans and the atmosphere, reducing the amount of solar radiation absorbed at the Earth's surface, affecting freshwater transports and ocean circulation, and serving as a vital surface for many species of polar animals. These direct impacts also lead to indirect impacts, including effects on local and perhaps global atmospheric temperatures, effects that are being examined in general circulation modeling studies, where preliminary results indicate that changes on the order of a few percent sea ice concentration can lead to temperature changes of 1 K or greater even in local areas outside of the sea ice region. Satellite passive-microwave data for November 1978 through December 1996 reveal marked regional and interannual variabilities in both the ice extents and the lengths of the sea ice season, as well as some statistically significant trends. For the north polar ice cover as a whole, maximum ice extents varied over a range of 14,700,000 - 15,900,000 km(2), while individual regions showed much greater percentage variations, e.g., with the Greenland Sea experiencing a range of 740,000 - 1,1110,000 km(2) in its yearly maximum ice coverage. Although variations from year to year and region to region are large, overall the Arctic ice extents did show a statistically significant, 2.8%/ decade negative trend over the 18.2-year period. Ice season lengths, which vary from only a few weeks near the ice margins to the full year in the large region of perennial ice coverage, also experienced interannual variability, and mapping their trends allows detailed geographic information on exactly where the ice season lengthened and where it shortened. Over the 18 years, ice season lengthening occurred predominantly in the western hemisphere and was strongest in the western Labrador Sea, while ice season shortening occurred predominantly in the eastern hemisphere and was strongest in the eastern Barents Sea. Much information about other important Arctic sea ice variables has also been obtained from satellite data, including information about melt ponding, temperature, snow cover, and ice velocities. For instance, maps of ice velocities have now been made from satellite scatterometry data, including information about melt ponding, temperature, snow cover, and ice velocities.

Parkinson, Claire L.↗
NASA NTRS2001

Calibration of Sea Ice Motion from QuikSCAT with those from SSM/I and Buoy

QuikSCAT backscatter and DMSP SSM/I radiance data are used to derive sea ice motion for both the Arctic and Antarctic region using wavelet analysis method. This technique provides improved spatial coverage over the existing array of Arctic Ocean buoys and better temporal resolution over techniques utilizing satellite data from Synthetic Aperture Radar (SAR). Sea ice motion of the Arctic for the period from October 1999 to March 2000 derived from QuikSCAT and SSM/I data agrees well with that derived from ocean buoys quantitatively. Thus the ice tracking results from QuikSCAT and SSM/I are complement to each other, Then, three sea-ice drift daily results from QuikSCAT, SSM/I, and buoy data can be merged to generate composite maps with more complete coverage of sea ice motion than those from single data source. A series of composite sea ice motion maps for December 1999 show that the major circulation patterns of sea ice motion are changing and shifting significantly within every four days and they are dominated by wind forcing. Sea-ice drift in the summer can not be derived from NSCAT and SSM/I data. In later summer of 1999 (in September), however, QuikSCAT data can provide good sea ice motion information in the Arctic. QuiksCAT can also provide at least partial sea ice motion information until June 15 in early summer 1999. For the Antarctic, case study shows that sea ice motion derived from QuikSCAT data is predominantly forced by and is consistent with wind field derived from QuikSCAT around the polar region. These calibrated/validated results indicate that QuikSCAT, SSM/I, and buoy merged daily ice motion are suitably accurate to identify and closely locate sea ice processes, and to improve our current knowledge of sea ice drift and related processes through the data assimilation of ocean-ice numerical model.

Liu, Antony K.↗
NASA NTRS2001

Variability of Antarctic Sea Ice 1979-1998

The principal characteristics of the variability of Antarctic sea ice cover as previously described from satellite passive-microwave observations are also evident in a systematically-calibrated and analyzed data set for 20.2 years (1979-1998). The total Antarctic sea ice extent (concentration > 15 %) increased by 13,440 +/- 4180 sq km/year (+1.18 +/- 0.37%/decade). The area of sea ice within the extent boundary increased by 16,960 +/- 3,840 sq km/year (+1.96 +/- 0.44%/decade). Regionally, the trends in extent are positive in the Weddell Sea (1.5 +/- 0.9%/decade), Pacific Ocean (2.4 +/- 1.4%/decade), and Ross (6.9 +/- 1.1 %/decade) sectors, slightly negative in the Indian Ocean (-1.5 +/- 1.8%/decade, and strongly negative in the Bellingshausen-Amundsen Seas sector (-9.5 +/- 1.5%/decade). For the entire ice pack, small ice increases occur in all seasons with the largest increase during autumn. On a regional basis, the trends differ season to season. During summer and fall, the trends are positive or near zero in all sectors except the Bellingshausen-Amundsen Seas sector. During winter and spring, the trends are negative or near zero in all sectors except the Ross Sea, which has positive trends in all seasons. Components of interannual variability with periods of about 3 to 5 years are regionally large, but tend to counterbalance each other in the total ice pack. The interannual variability of the annual mean sea-ice extent is only 1.6% overall, compared to 5% to 9% in each of five regional sectors. Analysis of the relation between regional sea ice extents and spatially-averaged surface temperatures over the ice pack gives an overall sensitivity between winter ice cover and temperature of -0.7% change in sea ice extent per K. For summer, some regional ice extents vary positively with temperature and others negatively. The observed increase in Antarctic sea ice cover is counter to the observed decreases in the Arctic. It is also qualitatively consistent with the counterintuitive prediction of a global atmospheric-ocean model of increasing sea ice around Antarctica with climate warming due to the stabilizing effects of increased snowfall on the Southern Ocean.

Zwally, H. Jay↗