Download Arctic Climate and Zonation

Arctic Climate and Zonation
Presentation based on:
Bliss, L.C. Arctic Ecosystems of North America. pp.
568-587, in Wiegolawski, F.E. Polar and Alpine
Tundra. Elsevier: Amsterdam, 1997.
Chernov, Yu.I. and N.V. Matveyeva. Arctic Ecosystems in
Russia. pp. 367-374, in Wiegolawski, F.E. Polar
and Alpine Tundra. Elsevier: Amsterdam, 1997.
Corinne Munger
September 20, 2005
Arctic Ecosystems
The Arctic Climate:
Chilly
But there’s more…
… the arctic has diverse local climates due to
diversity in topography, land-sea relationships,
and latitude.
Air, water, ice, vegetation,
. and land surfaces
all play a role in Arctic climatology.
•Major components of arctic climate presented by Bliss on the North
American arctic and Chernov and Matveyeva on the Russian arctic:
•Solar Radiation and Energy Balance
•Large-scale Circulation patterns
•Temperature
•Microenvironments
•Snow cover
•Precipitation and moisture
•Regional climate characteristics of Greenland, Canada, and Alaska
•Recent climate trends
•Various approaches to arctic zonation, which are based on climatic and
biotic factors.
Solar radiation and energy balance:
•Radiation balance describes the partitioning of radiant energy into
incoming and absorbed, long-wave incoming and outgoing, and
reflected radiation.
•The difference between the total radiation reaching the ground and
the amount reradiatated back into space is the amount that can be
used for different processes taking place in the ground cover. This
value decreases at higher latitudes.
•Most annual solar radiation in the Arctic is reflected from the
ground surface, but albedo varies widely: 20% in summer, 80% in
snowy tundra, and 85-95% on fresh snow.
•Arctic has reduced solar radiation due to high albedo from
snow cover until mid-late June and to high cloud cover in
summer. In general, the North American Arctic’s net
radiation is 45-50% of global (shortwave) incoming radiation
and it, like total radiation, decreases poleward. Net radiation
is negative over permanent pack ice (-100 to -200MJ/m2/yr)
•Annual positive energy balance results from energy transfer
from lower latitudes via cyclonic air masses and the
continuous daylight during summer.
•Land surfaces snow-free:
o 3-4 months in Low Arctic
o 1.5-2.5 months in High Arctic
•The mean value of the radiation balance for the Eurasian Arctic is
15 kcal/cm2/yr, which is about 21% of total radiation.
•Only 16% of total radiation is retained in polar desert, due to
higher albedo in polar landscapes, which are under snow cover for
the major part of the year.
•Highest absorption of solar radiation is in July, when albedo is at
a minimum and solar radiation is still high (radiation balance is 810 kcal/cm2).
•June has highest total radiation (up to 14 kcal/cm2), but a lower
radiation balance (5-6kcal/cm2) because snow melt is still in
progress.
•25-30% of total solar radiation in Eurasian tundra regions
(70kcal/cm2/yr) is received in June.
•At Barrow and Truelove Lowland, Canada, net radiation
does not become positive until snow melts in June.
•For most arctic sites about 50% of annual solar radiation is
received and dissipated before snowmelt.
MJ m -2 day -1
Net
Radiation
Heating
Soil/ Snow
(longwave)
Heating
Air
(shortwave)
Melting/
Evaporation
Typical
Dates
The width and direction of the arrows indicated magnitude and direction of energy flux. The numbers at the base
of each arrow give typical rates of MJ m -2 day -1(1 J= 0.239 cal). Evaporation rates are for open water.
•Period when temperatures are conducive to life in tundra is
very short, which contributes to the unique dynamics of
seasonal developments of living organisms and short
phenological periods.
•Time of peak total radiation is different from the time of peak
radiation balance. When solar radiation can be effectively
used by organisms, its levels are already in decline.
Large scale circulation patterns:
•North American high-pressure system occurs over the
continental land masses where radiative cooling is intense.
•When outward flow of cold, dry air (anticyclone) is strong in
winter, cyclonic (low-pressure) systems with warmer and
moister air remain in temperate latitudes and generally do not
enter Arctic.
•In spring/summer when land surfaces warm and snow melts
the reduced latitudinal thermal gradient allows cyclonic
systems to penetrate arctic, often with increased precipitation.
Role of uplands and mountains:
•Mountains block pressure systems- Low-pressure (cyclonic)
storms from south are diverted (eg. In Alaska and Greenland)
and Arctic high-pressure systems from north are diverted (eg.
Rocky mountains)
Temperature:
•While annual heat supply depends on radiation balance
at ground surface, the main factor that affects organisms
is summer temperature.
•Winter temperatures of a given location are strongly
dependent on degree of continentality (remoteness from a
warm ocean not covered by ice), therefore mean annual
temperature obscures actual conditions for life (ie. summer
temperatures)
•For example, in the polar desert in the far north of
Novaya Zemlya, the mean annual temperature is 7.6˚C, about the same as in the typical tundra (the
second most northerly subzone of the tundra zone).
Mean July temperature, however, is 4˚C cooler in
Novaya Zemlya. Its warmer mean annual temperature
is a result of its proximity to warm oceanic currents
which bring less extreme winters.
•Length of summer period (growing season) is one of the
most decisive factors for the development of organic life in
the arctic.
•In Eurasian arctic, the date when mean air
temperature increases to above 0˚C is around June 10
in southern tundra but not until beginning of July in
polar desert.
•The end of the growing season arrives in the
beginning of September in the far north and around
September 20 in southern parts of the tundra.
•This means the frost-free or growing period is
around 2 months or less in the polar desert and 3.5
months in the southern parts of the tundra.
•The growing period in northeastern Europe (the western
Eurasian Arctic) is strongly influenced by the unfrozen
Atlantic Ocean. The growing season here is 4.5-5 months
(from about May 20 to mid-October)
•This again shows the effects of a continental vs. maritime
climate. Proximity to frozen ocean causes even colder
winters.
•Lack of heat is main factor causing extremeness of living
conditions in the Arctic. This factor is very difficult to
overcome with adaptations because heat is the main condition
needed for all biological processes. Therefore, the arctic
allows for limited species and types of adaptations.
•There are many organisms that are able to survive in these
conditions, however there are probably no unique and
specifically adapted forms which are sharply distinguished
from animals and plants inhabiting other zones.
•During winter extremes in the arctic, almost no
organisms are subjected to direct effect of cold;
most are either under the snow or have migrated
south.
•Taiga plants may in fact be more adapted to
winter cold; while plants in the tundra are
insulated by snow all winter, trees and shrubs of
the taiga are uncovered.
•The continuous summer daylight might also be
more important to taiga organisms, where the
climax of plant growth and annual cycles of
animals (especially processes related to
reproduction) occur during the brightest time of
the year. In most of the tundra, plant growth
peaks after nights have begun to darken again.
• Air temperature within vegetation
differs from air temperature above it
because of the heat exchange between
air, plant surfaces, and soil surfaces.
•Vertical profile of air temperature
through canopy depends of solar
angle and density of plant canopy.
•Plants often have adaptations to help
retain heat (eg. cushion growth forms,
transparent hairs on willow catkins
that trap longwave (infra-red)
radiation (enough to affect measures
of NDVI?), heliotropic flowers).
•Warmer microclimates within canopy permit higher rates of
metabolic activity for animals and plants than would
otherwise occur.
•Differences in soil temperature are often more important
than differences in air temperature in influencing plant
growth
Microenvironments:
•Environmental conditions 1-2 m above and below the soil
surface.
•Characterized by the radiation, temperature, wind, and
moisture regimes which are controlled by regional climate
as modified by local topography, plant structure, and cover.
•These local habitats are provide the conditions to which
arctic organisms must be adapted.
Snow Cover:
•Except for tall shrubs growing along rivers, streams, and
steep slopes in the Low Arctic and plants on wind-blown
ridges that are temporarily blown free of snow, plants in
the Arctic are covered by snow all winter beginning in late
August or September, when frontal storms can penetrate
the dominant high-pressure systems.
•Snow is a good insulator so soil temperatures change little
while surface temperatures fluctuate greatly.
Precipitation and moisture:
Levels of precipitation vary with:
o Latitude and longitude
o Geographical conditions
o Distance from the sea
•Most North American arctic areas receive limited
precipitation except for the arctic maritime climates of
Baffin Island and southern Greenland.
•For the central Siberian region, the distribution of annual
precipitation by latitude is 150-200mm in the polar desert,
200-250 mm in arctic tundra, and 300-350 mm in typical,
southern, and forest tundra.
•In the North American Arctic around 30-50% of annual
precipitation occurs between June and August, mostly falling as
rain. Cold summers tend to have less rain than warm summers
(warm air can hold more water). Summer rain tends to be
frequent, light rains as opposed to infrequent, heavy rain so
surface soils usually remain relatively wet (soil moisture rarely
drops below field capacity)
•Simliarly, in the Eurasian arctic approximately 1/3 of annual
precipitation falls in the form of rain in July-August.
Orographic precipitation (condensation of water vapor as warm air
rises to cooler elevations) also influences rainfall distribution (eg.
Byrranga mountains in central part of Taymyr receive 450-500mm
annually, while surrounding plains receive 300-350mm).
Despite limited precipitation, arctic landscapes are generally
wet due to high percentage of cloud cover, high relative
humidity, and accompanying reduced evapotranspiration rates
that result in reduced rates of soil drying.
• “Radiation aridity index” measures ratio between radiation
balance and total yearly precipitation in terms of latent heat
of evaporation (relationship between amount of heat and
atmospheric humidity)
• Areas with an index of 1 are most favorable to life (typical
of landscapes in temperate zone)
• Tundra index values range from 0.4 to 0.2. The values of
0.2 or less in the polar desert indicate that the temperature
in these areas in not high enough to evaporate annual
precipitation.
Regional climatic characteristics of
North American Arctic:
Greenland
o 2,189,000 km2
o Group of islands buried in ice 66-80 N, 80%
covered by ice cap and isolated peripheral glaciers.
o Variable climates due to:
•Its great length that lies across the path of the
westerlies.
•The splitting of the Gulf Stream along the
southern coasts.
•The massive ice cap.
•Central and southern Greenland are strongly influenced by
cyclonic storms that frequently move up the west coast and
pass over the ice cap, providing snow from these lowpressure systems which maintains the ice cap.
•Cyclonic activity increases in fall (increasing snow fall on
ice cap) and then there is a shift from cyclonic to
anticyclonic air masses in winter.
•Coastal Greenland is influenced by interactions of the polar
air masses and the mild maritime air masses of the ocean,
which especially affects meteorological conditions of the
fjords.
•Northern Greenland is a “polar desert” characterized by very
low mean annual temperature, strong winds, and low
precipitation, but high rates of evaporation.
From: Walter, Heinrich. 1973. Vegetation of the Earth and Ecological
systems of the Geo-biosphere. Second Edition. Springer-Verlag, New
York, pg. 25.
Canadian Arctic
•2,500,000 km 2, with Canadian Arctic Archipelago
contributing 1,442,000 km 2 of the total. Extending from 55˚
N to 83˚39’
•Eastern Arctic is characterized by mountains and glaciers
(Ellesmere, Devon, and Baffin)
•Mainland is of low relief, heavily glaciated with many
lakes, “Barrens”
•Northwest arctic islands are of low relief and surrounded by
ice nearly every summer.
Canadian mainland
•Continental climate compared to more maritime
archipelago
•Temperatures not as low as locations further north with
longer periods of darkness and temperature inversions
•Compared to locations further north, greater mean annual
temperature range (44˚C) due to warmer summers
Alaska:
•Less complex arctic climatology than Canada or Greenland
•simpler patterns of air-mass movement
•general low relief except in Brooks Range
•much smaller region
•Brooks Range provides sharp delineation of climatic and
vegetation boundary between arctic and boreal forest biomes
•Cyclonic systems dominate over Gulf of Alaska and Bering Sea,
but seldom reach Arctic coast
•Anticyclonic systems predominate along the Arctic coast and
south towards the Brooks Range.
•Earlier springs in Alaska- At a given latitude in April or
May temperatures are often 15˚C higher in Alaska than in
Canadian Arctic
•Due to easy penetration of mild Pacific air masses
•Winter snow cover (average 30-60 cm) usually melts in
early to mid-June, a period of high sun angle and high
levels of sunshine.
•Summer and autumn are typically mild (8˚ to 12˚ C), but
cooler along coasts (5˚ to 8˚C)
•Inland thunderstorms occur in July and August (as they do
in the Canadian Arctic mainland) but are rare in Canadian
Arctic Archipelago.
Recent climate trends:
•Climate warming resulting from “greenhouse effect” as CO2
absorbs long wave terrestrial reradiation.
•CO2 concentration has increased from an estimated 265 ppm
in 1850 to about 350 ppm in 1990.
•Increases due primarily to burning of fossil fuels. Global
CO2 budget also affected by land-use practices (harvest of
forests, conversion of forest to agriculture, afforestion,
regrowth of harvested forests, and build up of soil organic
matter).
•Annual increase in atmospheric CO2 is now 1-1.5 ppm, with
an accelerating rate- by 2030 concentrations could reach 600
ppm
•Models suggest that a doubling of CO2 in the atmosphere will
result in a global warming of 2-4˚C
•In the Arctic a 3-10˚C temperature rise is predicted as a product
of greenhouse gases, 0.5-1˚ C in summer and 8-10˚C in winter
•Global warming in progress…
•Circumpolar basis: several years in 1980s and 1990s have
been the warmest in recorded history.
•Earlier melt of snow in North American arctic tundra.
•Rise in permafrost temperatures of 2˚C or more in last few
decades.
Zonation of Arctic:
Forest-tundra bioclimatic boundary:
Arctic defined as region north of treeline.
Which environmental parameters correspond with
treeline?
•Temperature (MJT<10 C)- this line is north of actual tree-line in
North America (Koppen 1918)
•Equation considering mean temp of warmest month and mean
temp of coldest month (Nordenskhold and Mecking 1928).
•Mean annual temperature, length of growing season, and
potential evapotranspiration used to demonstrate relationship with
broad vegetation zones (Hare 1950, 1954).
Relative position (>50% frequency) of Arctic front (leading
edege of Arctic air masses)
•Tundra exists in areas that are covered by Arctic air
masses year- round, whereas boreal forest areas are
covered by arctic air masses in winter (high pressure
systems) and Pacific air masses (low pressure systems) in
summer.
•The location of the arctic front over northern tree line in
summer results from
1.Differences in albedo.
2.Differences in aerodynamic roughness between forest
and tundra vegetation.
3.Differences in evaporation potential between forest and
tundra.
•Broad vegetation units are responding to and being controlled
by a complex of factors and interactions rather than a response to
a single environmental factor.
Zonation of Russian Arctic:
Zonation of Eurasian tundra based on Mean July isotherms
•Northern border of tundra zone (southern border of polar desert)
coincides with +1.5˚C isotherm
•Southern border of tundra zone (northern border of tundra-forest
zone) coincides with +10˚C isotherm.
•Some authors do not recognize polar desert as a separate zone
and instead consider it a subzone of the tundra zone, while
Chernov and Matveyeva consider it a unique zone based on its
scarcity of life, its many special features on the landscape, and
community structure, species composition, and food relations
between animals and plants.
•Debate about forest-tundra zone- Unique zone or transition belt
between tundra and taiga?
CAVM approach to zonation:
Table 1. Vegetation properties in each bioclimate subzone
Subzone
A
B
Mean
July
Temp1
(ÞC)
Summer
warmth
index2
(ÞC)
0-3
<6
3-5
6-9
Vertical structure
of plant cover3
Horizontal
structure
of plant cover3
Mostly barren. In favorable
microsites, 1 lichen or moss
layer <2 cm tall, very scattered
vascular plants hardly
exceeding the moss layer
2 layers, moss layer 1-3 cm
thick and herbaceous layer, 510 cm tall, prostrate dwarf
shrubs <5 cm tall
2 layers, moss layer 3-5 cm
thick and herbaceous layer 5-10
cm tall, prostrate and hemiprostrate dwarf shrubs <15 cm
tall
2 layers, moss layer 5-10 cm
thick and herbaceous and
dwarf-shrub layer 10-40 cm tall
<5% cover of
vascular plants,
up to 40% cover
by mosses and
lichens
5-25% cover of
vascular plants,
up to 60% cover
of cryptogams
5-50% cover of
vascular plants,
open patchy
vegetation
Major plant
growth forms4
b, g, r, cf, of,
ol, c
npds, dpds, b,
r, ns, cf, of, ol
Dominant
vegetation
unit (see
Detailed
Vegetation
Descriptions
for species)
Number
of
vascular
plant
species
in local
floras7
Total
phytomass5
(t ha-1)
Net annual
production6
(t ha-1 yr-1)
B1, G1
<3
<0.3
<50
P1, G1
5-20
0.2-1.9
50-100
npds, dpds, b,
ns, cf, of, ol,
ehds*
G2, P2
10-30
1.7-2.9
75-150
* in acidic
areas
50-80% cover of
ns, nb, npds,
D
7-9
dpds, deds,
G3, S1
30-60
2.7-3.9
125-250
12-20
vascular plants,
neds, cf, of, ol,
interrupted
b
closed vegetation
2-3 layers, moss layer 5-10 cm
80-100% cover
dls, ts*, ns,
E
9-12
20-35
thick, herbaceous/
deds, neds, sb,
200 to
of vascular
nb, rl, ol
dwarf-shrub layer 20-50 cm tall, plants, closed
G4, S1, S2
50-100
3.3-4.3
500
sometimes with low-shrub layer canopy
*in Beringia
to 80 cm
1
based on Edlund (1996) and Matveyeva (1998).
2
Sum of mean monthly temperatures greater than 0ūC, modified from Young (1971).
3
Chernov and Matveyeva (1997).
4
b - barren; c - cryptogam; cf - cushion or rosette forb; deds - deciduous erect dwarf shrub; dls - deciduous low shrub; dpds - deciduous prostrate dwarf shrub; g - grass; ehds - evergreen hemiprostrate dwarf shrub;
nb - nonsphagnoid bryophyte; neds - nondeciduous erect dwarf shrub; npds - nondeciduous prostrate dwarf shrub; ns - nontussock sedge; of - other forb; ol - other lichen; r - rush; rl - reindeer lichen; sb - sphagnoid
bryophyte; ts - tussock sedge. Underlined codes are dominant.
5
Based on Bazilevich, Tishkov and Vilcheck (1997), aboveground + belowground, live + dead.
6
Based on Bazilevich, Tishkov and Vilcheck (1997), aboveground + belowground.
7
Number of vascular species in local floras based mainly on Young (1971).
C
5-7
9-12
Table 2. Other bioclimate zonation approaches
Russia
Polunin
Matveyeva
Yurtsev
CAVM Alexandrova
(1951)
(1998)
(1994)
(1980)
subzone
Northern polar
A
desert
High Arctic Polar desert High Arctic
tundra
Southern polar
desert
Arctic tundra
Arctic
tundra:
B
Northern
northern
Arctic tundra
Middle
variant
Arctic
Arctic
Middle
tundra:
C
Arctic tundra
southern
Southern
Typical
variant
Arctic tundra
tundra
Northern sub- Northern
D
Arctic tundra hypo-Arctic
tundra
Middle subLow Arctic
Arctic tundra
Southern
Southern sub- Southern
tundra
E
Arctic tundra hypo-Arctic
tundra
North America
Fennoscandia
Edlund (1990)
Edlund & Alt Bliss (1997) Daniels et al. Walker et al. Tuhkanen
(1986)
(2002)
(2000)
(1989)
Inner polar
Arctic herb Cushion forb
Herbaceous
and cryptogam High Arctic
Outer polar
Northern
Arctic dwarf
shrub
Herb-prostrate
shrub
transition
Prostrate shrub
HemiMiddle Arctic prostrate
dwarf shrub dwarf shrub
Dwarf and
prostrate shrub
Low erect
shrub
Prostrate
dwarf shrub
Southern
Arctic dwarf
shrub
Erect dwarf
shrub
Arctic shrub
Low shrub
Low Arctic
Northern
Arctic
Middle
Arctic
Southern
Arctic
Elvebakk
(1999)
Arctic polar
desert
Northern
Arctic tundra
Middle
Arctic tundra
Southern
Arctic tundra
Arctic
shrub-tundra
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