Arctic Ocean Surface Warming Trends Over the 20th Century
Ocean temperature profiles have been analyzed for summertime sea surface temperature and upper ocean heat content variations over the 20th century, with a focus on the Arctic Ocean peripheral seas. We find that many areas cooled up to ~0.5 degC per decade during 1930-1965 as the Arctic Oscillation (AO) index generally fell, while these same areas warmed about the same amount during 1965-1995 as the AO index generally rose. Warming has continued since 1995 in many areas. The amount of upper ocean summertime warming during 1965-1995 is sufficient to reduce the following winter's ice growth by as much as 50 cm. Alternatively, this heat may return to the atmosphere before any ice forms, representing a delay in fall freeze-up of up to ~10 days. This returned heat might be carried by winds over nearby terrestrial tundra ecosystems such as Alaska's North Slope, contributing 15-20 W m-2 to the local heat budget.
The Recent Arctic Warm Period
Arctic winter and spring surface air temperature (SAT) anomalies and associated sea level pressure fields have a decidedly different spatial pattern at the beginning of the 21st century (2000-2007) than in the 20th century; we suggest calling this recent interval the Arctic warm period. The record minimum sea ice extent of summer 2007 is not a one year event, but is part of this extended period. Another example of the warm period is that the spring melt date as measured at the North Pole Environmental Observatory (2002-2007) is 7 days earlier than the records from the Russian North Pole stations (1937-1987), statistically different at 0.05. A main proximate cause of the warm period is an atmospheric dipole pressure anomaly pattern over the Arctic with anomalous winds blowing toward the central Arctic, suggesting a contribution from warm air advection to the regional loss of sea ice. Such persistence in the atmospheric flow pattern also suggests the importance of other climate memory processes such as ocean and sea ice advection contributing to the reduction of sea ice. The 20th century in contrast was dominated by the two main mid-latitude climate patterns, the Arctic Oscillation/Northern Annular Mode (AO/NAM) and the Pacific North American-like (PNA*) pattern. Positive phases of these patterns contribute to warm anomalies in the Arctic primarily over their respective eastern and western hemisphere land areas, as in 1989-1995 and 1977-1987. A hint of the importance of northward anomalous flow to unusual central Arctic warming comes from the late 1920s-1930s which also developed the dipole pressure pattern. The 21st century warm period is not inconsistent with IPCC AR4 model projections of SAT in amount and Arctic-wide extent.
Arctic Sea Ice Conditions Leading to Record Reduction in Summer 2007
Drastic reduction of Arctic sea ice has been occurring in the summer of 2007 as observed from data acquired by the QuikSCAT/SeaWinds satellite scatterometer (QSCAT). As of 24 August 2007, the total sea ice extent broke the record-low value set in September 2005; however, the difference of 0.02 million km2 was still within the satellite measurement uncertainty. On 28 August 2007, the total ice extent was lower than the September-2005 minimum by 0.22 million km2, which was larger than the uncertainty in QSCAT results. The rate of decrease of total ice extent from the summer solstice (21 June) to 9 August was the largest in 2007, which was 23% more rapid than the rate over the same time period in 2005. We will update the results as the latest data become available. Most importantly, we present the change in Arctic sea ice conditions causing such drastic ice reduction. The extent of Arctic perennial sea ice was significantly reduced between March 2005 and March 2007 by 1.08 million km2, a 23% loss between the two years, as observed by QSCAT. Consequently, the overall Arctic sea ice pack consisted more of the thinner seasonal ice, which would facilitate ice compression, export, and melt in summer. For a long-term perspective, the buoy-based Drift-Age Model (DM) has provided trends in Arctic sea ice age over the past five decades. Perennial-ice extent loss in March within the DM domain was noticeable after the 1960s, and the loss became more rapid in the 2000s when QSCAT observations were available to verify the DM results. The end-of-winter extent of perennial ice was the smallest on record in March 2007. QSCAT results also revealed mechanisms contributing to the perennial-ice extent loss. Dynamic and thermodynamic effects appear to be combining to expedite the loss of sea ice.
On the Relative Importance of Freshwater Fluxes and Variability From the Arctic Ocean into the North Atlantic
We use a high resolution coupled ice-ocean model of the Pan-Arctic region forced with realistic atmospheric data to investigate the variability of freshwater content within the Arctic Ocean as well as sea ice and liquid freshwater fluxes into the North Atlantic during 1979-2004. Modeled fluxes are validated against recently published estimates. Results are analyzed to compare the relative contribution of the total combined liquid and solid freshwater flux through the two main pathways: Fram-Denmark Strait (FDS) and the Canadian Arctic Archipelago- Davis-Hudson Strait (CAADHS). Our results suggest the relative importance of the freshwater flux through CAADHS into the Labrador Sea. This implies the need for ocean models to adequately represent mass and property fluxes through the narrow and shallow passages of the Canadian Archipelago and Davis and Hudson Straits. We argue that this requirement must be satisfied to advance studies of the Atlantic Meridional Overturning Circulation (MOC) and especially its variability. Given the recent record sea ice melt in the Arctic Ocean, it is critical that global ocean and climate models improve their skill in simulating and predicting effects of changing exports from the Arctic Ocean into the North Atlantic.
The Signature of the Albedo-Temperature Feedback in Recent Arctic Change
The Arctic climate of the recent few decades has been characterized by areas of relatively rapid warming. Since such patterns can arise from low-frequency natural variability, a key question is whether the recent warming has triggered and has been enhanced by the surface albedo-temperature feedback. This possibility is suggested by the recent decrease of terrestrial snow cover and the record retreat of sea ice over the Arctic Ocean. We demonstrate that the recent retreat of Arctic sea ice has created a footprint of enhanced warming during autumn and early winter in the periphery of the Arctic Ocean. In addition, the earlier springtime disappearance of snow cover from northern land areas has enhanced the springtime heating of the surface by approximately 1 Watt per square meter, consistent with the enhanced warming over northern land areas during spring. These patterns of albedo-enhanced warming will be shown to be consistent with the response of global climate models to enhanced greenhouse gas concentrations.
Interdependencies of Arctic land surface processes: A uniquely sensitive environment
The circumpolar arctic drainage basin is composed of several distinct ecoregions including steppe grassland and cropland, boreal forest and tundra. Land surface hydrology throughout this diverse region shares several unique features such as dramatic seasonal runoff differences controlled by snowmelt and ice break-up; the storage of significant portions of annual precipitation as snow and in lakes and wetlands; and the effects of ephemeral and permanently frozen soils. These arctic land processes are delicately balanced with the climate and are therefore important indicators of change. The litany of recently-detected changes in the Arctic includes changes in snow precipitation, trends and seasonal shifts in river discharge, increases and decreases in the extent of surface water, and warming soil temperatures. Although not unique to the arctic, increasing anthropogenic pressures represent an additional element of change in the form of resource extraction, fire threat and reservoir construction. The interdependence of the physical, biological and social systems mean that changes in primary indicators have large implications for land cover, animal populations and the regional carbon balance, all of which have the potential to feed back and induce further change. In fact, the complex relationships between the hydrological processes that make the Artic unique also render observed historical change difficult to interpret and predict, leading to conflicting explanations. For example, a decrease in snow accumulation may provide less insulation to the underlying soil resulting in greater frost development and increased spring runoff. Similarly, melting permafrost and ground ice may lead to ground subsidence and increased surface saturation and methane production, while more complete thaw may enhance drainage and result in drier soil conditions. The threshold nature of phase change around the freezing point makes the system especially sensitive to change. In addition, spatial and temporal variability in both water storage as soil moisture, surface water or snow, and the associated heat storage, leads to a phase shift between atmospheric forcings and land surface response. Continued efforts to link observed change with these interdependent processes rely on the interpretation of historic data, as well as maintaining or increasing current monitoring efforts.
An Intensified Arctic Water Cycle? Trend Analysis of the Arctic System Freshwater Cycle: Observations and Expectations
It is expected that a warming climate will be attended by an intensification of the global hydrological cycle. While there are signs of positive trends in several hydrological quantities emerging at the global scale, the scope, character, and quantitative significance of these changes are not well established. In particular, long-term increases in river discharge across Arctic Eurasia are assumed to represent such an intensification and have received considerable attention. Yet, no change in long-term annual precipitation across the region can be related with the discharge trend. Given linkages and feedbacks between the arctic and global climate systems, a more complete understanding of observed changes across northern high latitudes is needed. We present a working definition of an accelerated or intensified hydrological cycle and a synthesis of long-term (nominally 50 years) trends in observed freshwater stocks and fluxes across the arctic land-atmosphere-ocean system. Trend and significance measures from observed data are described alongside expectations of intensification based on GCM simulations of contemporary and future climate. Our domain of interest includes the terrestrial arctic drainage (including all of Alaska and drainage to Hudson Bay), the Arctic Ocean, and the atmosphere over the land and ocean domains. For the terrestrial Arctic, time series of spatial averages which are derived from station data and atmospheric reanalysis are available. Reconstructed data sets are used for quantities such as Arctic Ocean ice and liquid freshwater transports. Study goals include a comprehensive survey of past changes in freshwater across the pan-arctic and a set of benchmarks for expected changes based on an ensemble of GCM simulations, and identification of potential mechanistic linkages which may be examined with contemporary remote sensing data sets.
The Sensitivity of Northern Groundwaters to Climate Change: A Case Study in Northwest Alaska
Many cities in Alaska and the circumpolar north derive their municipal water supply from groundwater aquifers. Much of the groundwater recharge in the northern latitudes occurs during snowmelt. The last 30 years has seen an earlier snowmelt and later freeze-up in Alaskan locations from Anchorage to Barrow. A longer summer season, even with more annual precipitation will likely mean more water lost to evaporation and evapotranspiration, resulting in a net loss in groundwater recharge. The Anvil Mountain aquifer near Nome, Alaska exists in a fractured marble formation. Data at these wells has been collected since the fall of 2004 and continues to be recorded in real time. As a first step towards understanding the impacts of climate change on groundwater supplies, new data is being used to understand the sensitivity of the aquifer to climate. Preliminary analysis includes observed changes in well water surface elevations due to the onset of spring snowmelt, the magnitude of winter snow accumulation, low intensity and high intensity rainfall events, and aquifer head recession during the winter freeze period. Against a backdrop of observed climate change and the expectation of yet greater shifts in the climate and environment, particularly at high latitudes, sensitivity studies such as this are becoming both relevant and even critical to community planning and adaptation.