Distribution of net radiation

The net allwave radiation (or more simply net radiation) is the sum of the shortwave and longwave fluxes (Equation 5.1). Fields of net radiation based on ISCCP-D for the four mid-season months are provided in Figure 5.6. Values across the Arctic are negative from October through March. During polar darkness, the fields are of course essentially identical to those for the net longwave flux. For April, the ISCCP-D fields indicate that net radiation is slightly positive over the Arctic Ocean. The...

Abbreviations

AAAS American Association for the Advancement of Science ACIA Arctic Climate Impact Assessment ACSYS Arctic Climate System Study ADEOS Advanced Earth Observing Satellite AGCM Atmospheric Global Climate Model (or General Circulation Model) AMIP Atmospheric Model Intercomparison Project AMS American Meteorological Society AOGCM Atmosphere-Ocean Global Climate Model (or General Circulation Model) AOMIP Arctic Ocean Model Intercomparison Project ARCMIP Arctic Regional Climate Model Intercomparison...

Ecosystem models

As discussed by Kittel etal. (2000), terrestrial ecosystems in the Arctic and boreal forest regions are expected to be highly sensitive to climate change and may play a strong role in biospheric feedbacks to global climate. This sensitivity arises from complex interactions among ecosystem structure and function, soil and permafrost processes, and regional climate. Biophysical and biogeochemical dynamics of these landscapes in turn impact the global climate system through control over...

Lowlevel temperature inversions

A prominent feature of the Arctic environment is the frequent occurrence of low-level temperature inversions (i.e., temperature increases with height). This was first demonstrated by Brooks (1931) from kite ascents over Siberia. More detailed studies from kite and captive balloon ascents made by Sverdrup (1933) during the Maud expedition provided some of the first detailed information on inversion structure. Wexler (1936) was the first to address physical controls behind the formation of Arctic...

North America and Beringia

During the last glacial advance (known as the Wisconsin in North America), the major Laurentide Ice Sheet covered most of Canada. It merged in the west with the Cordilleran Ice Sheet and reached into the northern United States. The glacial history of the early Wisconsin is little known as paleoclimate information over land was largely destroyed by the extensive late Wisconsin ice. More problematic is the ice extent in the Canadian Arctic Archipelago. England (1999) and O'Cofaigh et al.(1999)...

Distribution of downwelling and net longwave fluxes

Again there are insufficient direct measurements of the downwelling longwave flux to compile maps for the Arctic. While a number of empirical formulae have been derived that employ the surface air temperature we rely on the ISCCP-D fields. Figure 5.4 and Figure 5.4 Mean monthly downwelling longwave radiation at the surface (W m-2) for the four mid-season months based on ISCCP-D data (courtesy of J. Key, NOAA, Madison, WI). Figure 5.4 Mean monthly downwelling longwave radiation at the surface (W...

Sea ice deformation

Ice thickness and concentration are strongly determined by differential ice velocity. Consider two neighboring plates (floes) of ice. If the velocity difference between the neighboring plates is such that they tend to move apart (diverge), a lead is created. During winter, new ice may form in the lead. If the motion changes, such as by a change in the winds so that the neighboring plates move toward each other (converge), the lead closes. Any new ice that was formed in the lead must rearrange...

Cyclone activity

Figure 4.10 depicts by season the frequency of extratropical cyclones north of 60 N over the period 1970-99. Figure 4.11 gives total counts of cyclogenesis (cyclone formation) events by season over the same period. Results are based on an algorithm applied to six-hourly NCEP NCAR SLP fields. Cyclones are identified using a series of search patterns, testing whether a grid-point SLP value is surrounded by grid-point values at least 1 hPa higher than the central point being tested. Cyclones are...

Frozen ground

Perennially frozen ground, known as permafrost, underlies nearly all of the Arctic land area. Permafrost is said to be present whenever ground temperatures are below freezing through two summer seasons. Ground ice need not be present, although in sediments ice may be present either in segregated form throughout, or as lenses and wedges (French, 1996). The upper part of the ground in permafrost regions, termed the active layer, thaws seasonally. The active layer depth (the maximum thaw depth)...

Ozone characteristics

Ozone measurements can provide three types of information. The first is the total ozone in an atmospheric column. This is measured with the Dobson spectrophotometer, which compares the solar radiation at a wavelength where ozone absorption occurs with that in another wavelength where such effects are absent. Second is the spatial pattern of total ozone. This is determined by satellite sounders such as NASA's Total Ozone Mapping Spectrometer (TOMS) aboard Nimbus-7, Meteor-3 and ADEOS (Advanced...

The basic Arctic heat budget 321 Overview

With the basic role of the Arctic in the global climate system established, focus turns to the energetics of the Arctic itself. Following the framework of the classic paper by Nakamura and Oort (1988), we consider the heat budget of a simplified northern polar cap, taken as the region from 70 to 90 N. Additional information can be found in Piexoto and Oort (1992). Following discussion of the budget equation, we examine the mean annual cycle of the budget terms, drawing strongly from NCEP NCAR...

Radiative processes

During polar night, the radiation budget is almost entirely determined by longwave fluxes (starlight and moonlight provide a negligible shortwave flux). Referring back to Equation (5.1), infrared radiation is emitted by the surface as a function of its temperature and emissivity, scaled by the Stefan-Boltzmann constant. Part of the emitted energy from the surface escapes to space through the atmospheric windows, primarily in the 8-12 m region. The remainder is absorbed in the lower atmosphere...

River discharge and runoff 641 River discharge data

The most comprehensive network of discharge gauging stations in the terrestrial Arctic drainage is represented by R-ArcticNET, which was compiled from original national sources by investigators at the University of New Hampshire, Durham. Version 2.0 of R-ArcticNET holds data from 3754 sites. Figure 6.12 gives the location of the subset of gauges for basins of at least 104 km2. Also shown is a digital river network at 30 min x 30 min resolution (longitude x latitude) (known as STN-30p) developed...

Cyclonic and anticyclonic regimes

Proshutinsky and Johnson 1997 document two regimes of wind-forced circulation of the Arctic Ocean. They simulated ocean currents, sea level height and ice drift on the Arctic Ocean from 1946 to 1993 using a two-dimensional, wind-forced model that includes coupling between the ocean and ice. Based on the modeled sea level and ice motion, the wind-driven circulation in the central Arctic alternates between cyclonic and anticyclonic regimes, with each regime persisting from 5 to 7 years....

Precipitation over Greenland

Direct observations of Greenland precipitation are particularly scanty. Stations with long records are limited to the coasts. As mentioned, in recent years, data over the ice sheet have been acquired from automatic weather stations. But quite a few measurements have been made of annual snow accumulation over the ice sheet. Bender ( ) provides a synthesis of observed annual accumulation (snow water equivalent) over the ice sheet, modified along the coast with a simple model of orographic...

Frontal activity

Early studies of frontal activity in the Arctic, such as those of Reed and Kunkel (1960) and Barry (1967), were based on manual analysis. While extremely time consuming, manually depicted fronts always contain an element of subjectivity. With the advent of fast computers, thinking has turned to the application of automated methods. Hewson (1998) provides a comprehensive review. Of the various methods that can be found in the literature, one that seems to work fairly well is a thermal front...

Net precipitation from the aerological method

In hydrologic analysis, net precipitation, or P ET, is itself a valuable term, and can be readily obtained in the absence of direct surface measurements of the two variables. Consider the moisture budget of an atmospheric column, extending from the surface to a height above which moisture content is negligible (about 300 hPa). The budget can be expressed as where the moisture content of the atmosphere, W, is expressed as precipitable water (the equivalent water depth of the vapor in the...

Topography and permanent ice masses

The physiography of the Arctic lands is summarized in Plate 1. Much of the Arctic land area is low lying, especially western and central Siberia, the western part of the Canadian Arctic Archipelago and coastal Alaska. Mountains and high plateaus are prominent in the eastern Canadian Arctic, coastal regions bordering the Greenland Ice Sheet, interior Alaska, northeastern Siberia and Scandinavia. The highest mountain north of the Arctic Circle is Gunnbjorns Fjaeld (86.92 N, 29.87 W), which rises...

Sea ice formation growth and melt 711 The existence of the sea ice cover

To build on concepts introduced in Chapter 2, it is useful to draw from the review of Maykut (1985) and compare the processes of ice formation in a freshwater body with those that occur in the Arctic Ocean. Figure 7.1 gives the temperature versus density relationship for freshwater. For most substances, decreased temperature results in higher density. But freshwater is a very unusual substance. Down to a fixed threshold temperature, cooling results in increased density. Below this temperature...

Sea ice zones

The sea ice cover can be divided into different ice zones. Again following Maykut (1985), the perennial ice zone (PIZ) is where ice is present throughout the year. The PIZ can be broadly considered as the area north of the mean September ice margin shown in Figure 2.4. During winter, the PIZ typically consists of 10-15 FYI formed in leads, and the remainder MYI. In summer, most of the FYI melts. The Arctic's perennial ice zone contains about two-thirds of all the MYI in the world's oceans....

Ice cores

Snow that falls on an ice sheet is compressed by subsequent snowfall, becomes firn, with a density of 550 kg m-3 around 10 m depth, and then ice (density of at least 840 kg m-3). This occurs at depths of about 60 m in Greenland. Seasonality of snow accumulation leads to annual layers of ice that can be identified back to around 40 ka in central Greenland. Researchers drill vertically through the ice to recover cores comprising layer upon layer of fossil snow. Among the most valuable records...

The modern era

World War II demonstrated the strategic importance of the Arctic seas, Greenland and Alaska. Germany established a number of clandestine weather stations in Spitsbergen and East Greenland during 1940-5 (Blyth, 1951 Selinger and Glen, 1983), while the USA established air bases in West Greenland. Strategic concerns were reinforced with the advent of the Cold War. Between 1947 and 1950, the Canadian and US governments established five weather stations in the Canadian Arctic Archipelago and the...

Moisture budget

As part of their summary of high-latitude NAO impacts, Dickson etal.(2000) compared patterns of the vertically integrated moisture flux across 70 N for winters associated with NAO extremes. The data set spanned the years 1974-91. Their analysis (Figure 11.15) shows that the poleward meridional moisture flux for longitudes along the Nordic seas is much higher under the positive NAO phase. The positive NAO is also associated with a stronger equatorward moisture flux between about 70 and 140 W...

The Arctic and the global heat budget 311 The radiation balance

Considered as a whole and for long-term annual means, and assuming a stationary climate, the Earth is in a state of radiative equilibrium. The ultimate energy source to the Earth is solar radiation. About 99.9 of the radiation emitted by the Sun is in wavelengths of 0.15 to 4 m with a peak intensity near 0.5 m. About 50 of the total emitted energy is within the visible spectrum (approximately 0.4-0.7 m). The net incoming (i.e., the available) solar radiation to the Earth system, defined as that...

Basic climatic elements 231 Snow cover

Most of the Arctic land and sea ice surface has a snow cover for at least 6-8 months of the year. For a number of reasons snow cover is a key climatic variable. These include (1) its high albedo (reflectivity in solar wavelengths), typically 0.80 to 0.90 for new snow (2) the insulating effect it has on the underlying tundra or sea ice and (3) its Figure 2.15 Average number of weeks of snow cover over the Northern Hemisphere, based on the NSIDC blended weekly product for 1972-2001 (courtesy of...

The albedo of snow

As outlined by Nolin and Liang (2000), snow can be considered as a layered particulate medium composed of ice spheres in air. Scattering by the spheres is primarily through refraction. Using the refractive indices of ice and an optically equivalent ice sphere radius, Mie theory (appropriate when the effective particle radius is much larger than the wavelength of the interacting radiation) can be used to calculate single-particle scattering and absorption (i.e., the scattering and absorption of...

Climate model projections

The recently completed Arctic Climate Impact Assessment (ACIA), an international activity with about 300 participating scientists, was aimed at evaluating and synthesizing Arctic climate variability and change (ACIA, 2005). One of the components of the ACIA was to provide scenarios of future Arctic climate. The ACIA made use of five different AOGCMs. These are (1) CGCM2 (Canadian Climate Center, Canada) (2) CSM (National Center for Atmospheric Research, USA) (3) ECHAM4 OPYC3 (Max Plank...

The Greenland Ice Sheet 811 General features

The Greenland Ice Sheet (Figure 8.1) is by far the largest terrestrial ice mass in the Arctic and is the second largest in the world (following the Antarctic Ice Sheet). The ice sheet covers an area of 1.71 x 106 km2. Other glaciers and ice caps on Greenland cover a further 0.049 x 106 km2 (Table 2.2). The ice sheet reaches a maximum elevation of 3208 m at Summit (72.6 N, 37.5 W). A secondary elevation maximum on the southern part of the ice sheet rises to about 2800 m. The surface slope over...

Polar Lows 461 Definition

Polar Lows are intense maritime mesocyclones of typically 100-500 km in diameter. They may intensify rapidly and surface wind speeds can sometimes reach hurricane force (Businger and Reed, 1989). They tend to be short lived, generally lasting only 3-36 hours. Polar Lows are the most intense category of the family of mesoscale cyclonic vortices found poleward of the main polar front, which are known generically as polar mesoscale cyclones. Polar Lows, which can present significant hazards to...

Numerical weather prediction models

Almost any study of observed variability in the Arctic atmospheric circulation makes use of output from the data assimilation cycles of NWP models. From the preceding discussion, NWP output is also commonly used as lateral forcing for regional climate models or to provide wind forcing for sea ice and coupled ice-ocean models. Output from GCMs is also commonly verified against output from NWP models. We hence 91 96 101 106 111 116 121 126 131 136 141 146 151 Figure 9.13 Time series of 24-48 hour...

The heat budget equation

The heat budget of the northern polar cap can be approximated as A E At Frad + Fwall + Fsfc (3.3) Equation (3.3) states that the time change in the storage of moist static energy (defined in the introduction) in the Arctic atmosphere AE At is represented by the sum of the Figure 3.5 Schematic of the energy balance for the north polar cap (from Nakamura and Oort, 1988, by permission of AGU). net radiation at the top of the atmosphere (Frad), the net poleward energy flux across a hypothetical...

Partitioning of net radiation 581 Characteristics over sea ice

Figure 5.10, also from Persson et al. (2002), illustrates annual cycles of the turbulent energy fluxes and conduction at the SHEBA site in comparison with other estimates. Recall that our convention is that non-radiative fluxes are positive when directed away from the surface and negative when directed toward the surface. The SHEBA results are representative of reasonably thick ice. They show the sensible heat flux as directed toward the surface in winter and variously toward or away from the...

Clouds

Cloud cover has first order impacts on the Arctic surface radiation balance (see Chapter 5). Cloud microphysical and radiative properties are hence a vibrant area of research. While much is being learned from modeling and special observation programs (e.g., SHEBA and the Department of Energy Atmospheric Radiation Monitoring program), a continuing problem is the general lack of accurate data on even cloud amounts over the Arctic. Curry et al. (1996) provide a comprehensive review of the problem....

Nonradiative terms

The net radiation at the surface must be balanced by non-radiative energy transfers. Namely, where S and L are the sensible and latent heat fluxes respectively, M is melt and C is conduction. The sign convention used here is that non-radiative fluxes are positive when directed away from the surface and negative when directed toward the surface. The net radiation is positive when directed downward and negative when directed upward. With this convention, when both sides of Equation (5.2) are...

The sea ice cover

The surface of the Arctic Ocean is characterized by its floating cover of sea ice. The processes of sea ice growth, melt, circulation and variability are discussed at length in Chapter 7. As will become clear in the next few chapters, sea ice is intimately coupled with the atmospheric energy budget, the atmospheric circulation, the surface energy Figure 2.2 Definition of Arctic seas, based on Russian sources. 1. Greenland Sea, 2. Labrador Sea, 3. Baffin Bay, 4. Canadian Arctic Archipelago, 5....

Precipitation accumulation and sublimation

Some aspects of precipitation and accumulation over the ice sheet were outlined in Chapter 6. Accumulation basically represents the net effects of direct precipitation, its redistribution on the surface via wind scour and drifting, and mass losses due to melt and evapo-sublimation. Accumulation is typically assessed via snow pits or ice cores. Based on coastal station observations of precipitation, adjusted for wind speed, and accumulation data from recent ice cores, the annual precipitation...

Late Holocene cooling and the LIA to present

As just discussed, the HTM in some regions occurred quite early, while for others it ended as late as 3 ka. The HTM was nevertheless followed by a period of cooling. For example, reconstructions by Koc etal. (1993) indicate that by about 5 ka, warm Atlantic waters had retreated to the central GIN seas, attended by strengthening of the cold East Greenland Current. By 3 ka, the sea ice cover had expanded along eastern Greenland. A variety of sources points to a subsequent period of warming...

Precipitation frequency and phase

Present weather codes that represent part of the synoptic reports in COADS were used by Serreze et al. (1997b) to examine the characteristics of precipitation frequency and phase (solid, liquid, mixed) over the Arctic Ocean. The techniques follow those Figure 6.6 Modeled annual precipitation averaged for 1985 though 1999 (mm). The contour interval is 200 mm, but 100 mm if smaller than 400 mm, and 300 mm if larger than 1000 mm (from Bromwich et al., 2001b, by permission of AGU). Figure 6.6...

Poleward energy transports

The implication of Figures 3.1 and 3.2 is that there must be poleward transports of energy by the atmosphere and oceans that warm the polar regions and cool the equatorial regions. The uneven solar heating results in a mean poleward decline in tro-pospheric temperatures and hence the height of pressure surfaces, inducing a pressure gradient. The atmosphere works to reduce these gradients. In lower latitudes, where the Coriolis parameter is fairly small, the poleward atmospheric energy transport...

Features of the Quaternary 1031 General timeline

How many major ice advances and retreats occurred over the Quaternary is unclear and there are discrepancies to be resolved between continental and deep-sea chronologies. Ice-rafted detritus (IRD) in marine sediments dated to around 2.6 Ma marks the initiation of glaciation at sea level in the circum-North Atlantic region (Ehlers and Gibbard, 2003). This implies that the base of the Pleistocene epoch (and the Quaternary period) might be better placed earlier than the international stratigraphic...

Vortex statistics

Climatologies of the stratospheric polar vortex in both hemispheres have been prepared based on areal extent (Baldwin and Holton, 1988) and on elliptical diagnostics (Waugh, 1997 Waugh and Randel, 1999). Fitting an ellipse to a specified contour allows one to summarize various measures of the vortex. These include the equivalent latitude of the vortex edge (defining its area), the offset of the vortex from the pole, the longitude Figure 4.3 Schematic of the configuration of the winter...

Examples Svalbard and Barrow

Following from the above discussion, a good example of a maritime Arctic climate is Isfjord Radio (Svalbard) (Figure 8.6). Note first the sharply higher winter air temperatures as compared to Resolute Bay, the polar desert site (Figure 8.5). The maximum summer temperature, however, is similar at about 5 C. The sites also differ stongly in terms of precipitation. Compared to the summer maximum at Resolute Bay, Isfjord exhibits a distinct autumn winter precipitation maximum and late spring early...

Distribution of global radiation

Satellite retrievals offer systematic assessments of global radiation and other surface radiative fluxes with complete Arctic coverage. There are two primary data sets - APP-x (25-km grids, covering 1982-99) and ISCCP-D (280-km grids, covering 1985-93). Both were introduced in Chapter 2. The APP-x product is based on data from AVHRR. Briefly, surface (skin) temperature is calculated with a split-window infrared algorithm. Surface albedo retrieval for clear and cloudy skies employs corrections...

Regional climate models

As of this writing, the Arctic Regional Climate Model Intercomparison Project (ARCMIP) was gaining momentum. This effort, a coordinated set of limited-area simulations of present-day Arctic climate, focuses on the SHEBA year (October 1997-October 1998). This year was chosen due to the availability of high-quality data from the field camp in the Beaufort and Chukchi seas, aircraft observations, as well as data from the Barrow Arctic Radiation Monitoring site and remote sensing. Use is also being...

Precipitation recycling

The amount of precipitation falling over a region can be divided into (1) precipitation associated with water vapor transported into the region (advected precipitation) (2) precipitation associated with water that evaporates from the surface of the region and falls within the same region (locally derived precipitation). The precipitation recycling ratio is defined as Pl P, where Pl is the precipitation of local origin and P is the total precipitation. The recycling ratio can be thought of as...

Historical exploration

In the sixteenth century, the Arctic came to be seen by the nations of northern Europe as a potential route to China. Three possible routes were considered - directly across the Arctic Ocean, the Northwest Passage (from Davis Strait, through the channels of the Canadian Arctic Archipelago, and then along the coast of Alaska) and the Northeast Passage (along the Eurasian coast, also known as the Northern Sea Route) (Figure 1.1). Although the motives of the explorers and their backers were...

The terrestrial cryosphere precipitation and river discharge

Data through the mid 1990s indicate generally negative mass balances for small Arctic glaciers (Dyurgerov and Meier, 1997), parallelling a global tendency. Few of the Arctic records extend before 1960. Overall, these changes are consistent with the warming of recent decades. While mass balances for some glaciers, such as in the montane parts of Scandinavia and Iceland, had been positive due to increased winter precipitation, there are recent indications of mass loss. Based on data from 1979 to...

Winter

Figure 4.12 Frontal frequencies (top, fronts per day) and mean 850-hPa temperature gradients (bottom, K (100 km)-1) for winter and summer. White areas in the frontal frequency maps represent regions with known problems related to extreme topography and surface heating. Areas where the mean temperature gradient exceeds 0.5 K (100 km)-1 are shaded (adapted from Serreze et al., 2001, by permission of AMS). Figure 4.12 Frontal frequencies (top, fronts per day) and mean 850-hPa temperature gradients...

The downward solar radiation flux 521 The clearsky flux

The total downward flux of solar radiation Fsw represents a combination of direct beam and diffuse beam components, which together are often termed global radiation. Diffuse radiation is largely isotropic (i.e., the flux is roughly the same no matter what direction it is coming from), although the intensity is higher underneath the portion of the celestial dome nearest the Sun. The fundamental control on the global radiation flux reaching the surface is the TOA (or extraterrestrial) flux. The...

Physical oceanography

Figure 2.7 shows typical vertical profiles of temperature and salinity for the Beaufort Sea (north of Alaska, see Figure 2.2) and for near the North Pole, collected during the August-September 1993 cruise of the USS Pargo. While the two profiles show some obvious differences, which will be addressed shortly, several common features stand out. Note first the existence of a low-salinity surface layer. In these two examples, surface salinities are about 28 psu (Beaufort Sea) and 31 psu (near the...

The Great Salinity Anomaly

As also noted by Holland et al. (2001), model simulations of the variability and sensitivity of the THC appear to depend strongly on model design and complexity. Whether the THC is as sensitive to disruption as some models have indicated is still very much in debate. However, there is observational evidence, the best example being the Great Salinity Anomaly (GSA) of 1968-82. During the late 1960s to early 1970s, the upper 100 m of the waters in the Greenland, Iceland and Labrador Seas underwent...

Land surface models

Numerous LSMs of differing complexity and architecture are in existence. A major research avenue is model intercomparison. This has been conducted under the Project for Intercomparison of Land Surface Parameterization Schemes (PILPS), within the GEWEX Global Land Atmosphere System Study. PILPS 2e (Bowling et al., 2003) is a component of this effort aimed at evaluating the performance of uncoupled (stand alone) LSMs in northern high latitudes. The project was motivated by the need to improve...