Transcript Elements of the climate

Writing Assignment
2 page paper on recent news of climate change.
Reference. Grammar. Your analysis.
Due via email on Friday October 8 – redo this Friday
Elements of the
climate
EVSC 305
 ‘a great blush spreading from one pole to the
other’
ITCZ
ITCZ shift from January to
July
NASA satellite image of ITCZ
Ocean currents
Surface currents
World’s surface ocean
currents
Graphic of ocean surface currents.
Source: NASA.
Atmospheric circulation patterns
 Why do large-scale global atmospheric
circulation patterns shift from year to year?
 To answer this question: need to concentrate on
their average positions and paths they take
 They key to long-lasting weather patterns is found
in the middle levels of the atmsopher
Rossby waves / long waves /
planetary waves
Importance of a Rossby wave
Rossby waves
 What causes the number, amplitude and position of
Rossby waves to change?
 Northern Hemisphere vs Southern Hemisphere?
 What governs the changing speed of the winds
from year to year and within seasons?
 Answers: the oceans, the extent of snow and the
extent of ice in high latitudes
 Let’s remember the atmosphere. What drives the
system?
 Another point: periodic reversal of the winds; quasibiennial oscillation
Radiation balance
 Proportion of incoming solar radiation absorbed
the Earth depends on the absorption, reflection
and scattering properties of the atmosphere and
the surface
Clouds…
 Overall effect of clouds depends on their net effect on both
incoming and outgoing radiation
 Less thermal energy is radiated to space on cloudy day; i.e.
warmer
 Scale of the blanket-like warming effect depends:
 Thickness of the clouds
 Temperature of their tops
 High clouds: radiate less energy than low clouds
 Thick clouds: more efficient radiators than thin clouds
 Satellite measurements confirm: clouds have a net cooling
effect on global climate
Clouds…
 Blanketing effects of clouds reaches peak values over tropical
regions and decreases towards the poles
 Why?
 Clouds rise to a greater height in the tropics
 Cold tops of deep clouds radiate less energy than shallower,
warmer clouds
 These thick high clouds occur
 Tropical Pacific and Indian oceans around Indonesia and in the
Pacific north of the equator
 Monsoon region of Central Africa and the region of deep
convective activity over the northern third of South America
 mid-latitude storm tracks of the North Pacific and North Atlantic
Oceans
Clouds and albedo
 Clouds at high latitudes have less impact on the
outgoing thermal radiation as the underlying
surface is colder and thus emits less energy
whether or not there are clouds
 So: in the tropics, the net effect of clouds is
balanced out
 So: over the mid- and high-latitude oceans
polewards of 30N/30S, clouds have a cooling
effect
Albedo: amount of sunlight
reflected by a surface
Albedo and climate
change
Lowering Earth’s
albedo 
increase global
temperature
Earthshine
earthshine
Earthshine
Earthshine Project
 By using a combination of earthshine observations and satellite data on cloud
cover, the earthshine team has determined the following:
 = Earth’s average albedo is not constant from one year to the next; it also
changes over decadal timescales. The computer models currently used to
study the climate system do not show such large decadal-scale variability of
the albedo.
 = The annual average albedo declined very gradually from 1985 to 1995, and
then declined sharply in 1995 and 1996. These observed declines are broadly
consistent with previously known satellite measures of cloud amount.
 = The low albedo during 1997-2001 increased solar heating of the globe at a
rate more than twice that expected from a doubling of atmospheric carbon
dioxide. This “dimming” of Earth, as it would be seen from space, is perhaps
connected with the recent accelerated increase in mean global surface
temperatures.
 = 2001-2003 saw a reversal of the albedo to pre-1995 values; this “brightening”
of the Earth is most likely attributable to the effect of increased cloud cover
and thickness.
Hydrological cycle
 Review your notes from Ecology
 There is a continual recycling of water between the
oceans, the land, and the atmosphere
[hydrological cycle]
 Process of evaporating large amounts of water into
the atmosphere – and its subsequent precipitation
as rain or snow is a major factor in the energy
transport of the climate
 What is less understood? – how much water passes
into the atmosphere, the nature and form of the
clouds it forms, and how quickly it is precipitated
out again
Latest study…
 A new study led by the University of California at Irvine has determined that annual
fresh-water flow increased 18% from 1994 to 2006, suggesting an acceleration in the
global water cycle of evaporation and rainfall, which influences the intensity of
storms, floods and droughts. (October 6, 2010)
 Among the new study’s more dramatic calculations: River runoff into the seas has
been increasing by some 540 cubic kilometers per year, or about 1.5 percent
annually over the period analyzed (1994 to 2006). While that may not sound like
much, “over 20 or 30 years it would really add up,” notes study author James S.
Famiglietti, a hydrologist at the University of California, Irvine.
 Global annual precipitation also appears to be on the rise, but at only half the
increase seen in river runoff. If prolonged, this differential would suggest that major
terrestrial stores of water — such as ground aquifers and glaciers — are drying up (a
trend that other studies have been chronicling). This would also be expected to
eventually raise sea levels and generally dry temperate regions that depend on
rivers to slake their thirsts.
 Via Science News – October 5, 2010
The biosphere
 Totality of living matter – biosphere – has
capacity to influence the climate in a variety of
ways
 Photosynthesis: fundamental control over the level of
carbon dioxide in the atm.
 Production of methane by the anaerobic decay of
vegetation
 Potential of biosphere to absorb additional
carbon dioxide is an important factor
 Carbon cycle [remember ecology notes]
Carbon cycle
Carbon cycle
 Negative feedback
  productivity of biosphere rises with increasing
carbon dioxide levels  slows the build up of carbon
dioxide in the atmosphere
 Factor in delaying some of the consequences of
emissions of this greenhouse gas
 Particulates
 One example
 Production of Dimethylsulphide by phytoplankton
Dimethyl sulfide (DMS)
emissions
Start: Sustained abnormal
weather patterns
 Does a prolonged spell of extreme weather affect the underlying
components of the climate long enough to influence the weather in
subsequent seasons, and conceivably lead to more prolonged
changes?
 Extreme snow cover prolongs cold weather in the northern
hemisphere [high albedo  extensive snow cover has a cooling
effect. Thus more snow -> more snow]. Needs to be sustained
 Average snow cover in April in N H is ~ 31 million km2; declined by ~ 2
million km2 in 20th century
 Arctic pack ice: 7 – 15 million km2 (summer to spring). Variations in
snow cover: 5 – 45 million km2 (late summer to winter). Reverse is true
in S Hemisphere
 Decline in sea ice in the Arctic (especially in summer months]
 While snow and ice cover changes have a
substantial short-term impact on the energy
balance during the winter half of the year
 Longer-term variability of the global climate
appears to overwhelm temporary influence of
these variations
Ocean-atmosphere
interactions
 Review from Ecology
El Niño
El Niño
Great ocean conveyor
 From top to bottom of the mixed layer of oceans generated by
action of the winds – little temperature difference
 Thermocline: narrow zone over which there is a rapid drop in
temperature
 Thickness of the mixed layer, thermal mixing where the surface
waters are heated by the sun or altered by the passage of
warmer or colder air, and by advection (transfer of heat by the
flow of fluid) of warmer of colder water or the upwelling of cold
water
 Thermohaline circulation (THC)- also called the Global Ocean
Conveyor, moves water between the deep and surface ocean
worldwide.
THC process
 Changes in seawater density arising from variations in temperature
and salinity  THC process
 Note: density of seawater is not a simple function of temperature and
salinity.
 Need to consider actual values of density. While freshwater has a max
density at 4C, at normal levels of salinity (32.5 to 37.5%), density increases
with declining temperature to the freezing point around -2C.
 Changes in density with salinity are simpler: the saltier the water, the denser it
becomes
 Denser water  sinks to greater depths
 Temperature depends on where the surface waters come from and
how much heat the oceans pick up / release
 Salinity: balance between losses through evaporation – gains through
rainfall or freshwater run-off
THC
 Deep waters: water that sinks to middle levels of
major oceans; only northern fringes of Atlantic
Ocean
 Bottom waters: colder denser layer below deep
waters; limited regions near coast of Antarctica
in Weddell and Ross Seas
THC processes
 (1) where freshwater enters the oceans  it can float on
top of seawater preventing deep circulation
 (2) any surface warming from lower latitudes -> forms a
stable surface layer
 (3) stability of this surface layer (i.e. depth of thermocline)
– controlled by amount of surface mixing due to winds
 (4) deep mixing depends on formation of high salinity
water (freezing as salt is shed; or high evaporation)
  drive larger scale thermohaline circulation of the
oceans
Gulf stream
 The Atlantic is the
only ocean
where heat is
transported north
across the
equator. Here
warm surface
water from the
tropics reaches
further north than
in anywhere else
 the Gulf Stream
water releases
heat and
moisture to the
atmosphere,
making the
climate of the
region it passes
through warmer
and wetter than it
would otherwise
have been.
The Gulf Stream – satellite
image (NOAA)
Gulf Stream
 Although the Gulf Stream water is saltier than the deep water
below, it is much warmer, so its density is lower, and it remains on
the surface.
 On its journey north, the water releases heat to the atmosphere,
and cools gradually, until it is cold enough for its density to match
that of the deep layer. Sinking can begin.
 At this stage the surface water is still warmer than the deep water,
but it also saltier, so its density matches that of the deeper water,
allowing the two layers to mix. Should the surface water freshen for
some reasons, it would have to cool further before it can sink.
Sufficient freshwater input might reduce salinity to the extent that
the surface water could not possibly sink, even at sub-zero
temperatures.
?
 The possibility of abrupt changes in Gulf Stream heat
transport is one of the key uncertainties in predictions of
climate change for the coming centuries.
 Is THC in the lead – or doe the wind fields dominate the
ocean circulation?
 How winds and the THC combine to maintain the Gulf
Stream ties in neatly with the surface ocean currents
 One theory: Gulf Stream forced by torque exerted on the
ocean by the wind field.
 As long as the sun heats the Earth; Earth spins  Gulf Stream
 Primary mechanism of heat transport in the ocean is the windforcing of currents
Topics for you
 From the ‘Supporting Material’ links. Discuss one
of the bullets from the ‘Supporting Material’
 Due: Thursday.
topics
 Layal – Thursday (carbon dioxide in the oceans –
sink vs source; healthy components?)
 Lilliane – Tuesday (Marine algae and dimethyl
sulfide)
 Majd – Thursday (gulf stream – what is happening
and why?)