An Overview of the Climate System

An Overview of the Climate System

Chris Lennard

Climate Systems Climate Systems
Analysis Group Analysis Group

Contents of this module Contents of this module
Energy Energy
The Sun The Sun
Energy imbalance Energy imbalance
Continents Continents
Turning earth Turning earth
Coliolis Coliolis
Large scale circulations Large scale circulations
Radiation budget Radiation budget
Greenhouse effect Greenhouse effect

Variability
Seasonal
Inter-annual (ENSO, SAM, NAO, Volcanic)
Decadal


Contents of this module Why do we have weather at all?
Energy
The Sun
Energy imbalance
Continents
Turning earth
Coliolis
Large scale circulations
Radiation budget
Greenhouse effect

Variability
Seasonal
Decadal

Climate Change

Weather and Climate

Scales of our decision space

Why do we have weather at all? Why do we have weather at all?
An active sun....

Incoming solar radiation;
Differential heating of the globe


Why do we have weather at all? Why do we have weather at all?

Incoming solar radiation; 1. Differential heating of the
Differential heating of the globe globe results in energy transfer
which together with
2. the spin of the earth and
3. position of the continents
gives rise to our weather
systems as we know them.


Why do we have weather at all? Large Scale Circulations

1. Differential heating of the  Tropics characterized by
globe results in energy transfer rising air and convection
which together with
2. the spin of the earth and
3. position of the continents  Sub-tropics characterized by
gives rise to our weather descending air (dry)
systems as we know them. If energy were not redistributed in this way
the Equator would be about 14 degrees Mid-latitudes are high energy
hotter and the Poles about 25 degrees zones (frontal systems)
colder!!


Looking more closely....radiation Looking more closely....radiation
Incoming radiation directly from the sun is 1370 W/m2 Interacts with the earth’s atmosphere
Averaged over the whole earth is 342 W/m2
Different gases and particles absorb, reflect and re-
radiate radiation at different wave lengths

High clouds and aerosols reflect short wave
radiation back to space cooling the earth
342 W/m2 1370 W/m2
Low clouds, water vapor and other green house
gases absorb and re-radiate infrared radiation near


Looking more closely....radiation The greenhouse effect....


The greenhouse effect.... The greenhouse effect....
The Greenhouse effect keeps the earth warmer than it The Greenhouse effect keeps the earth warmer than it
would be if it did not have an atmosphere. would be if it did not have an atmosphere.

The surface of the Earth's surface receives nearly twice as
much energy from the atmosphere as it does from the
Sun.


The greenhouse effect.... The greenhouse effect....
The Greenhouse effect keeps the earth warmer than it The Greenhouse effect keeps the earth warmer than it
would be if it did not have an atmosphere. would be if it did not have an atmosphere.

The surface of the Earth's surface receives nearly twice as The surface of the Earth's surface receives nearly twice as
much energy from the atmosphere as it does from the much energy from the atmosphere as it does from the
Sun. Sun.

In the absence of an atmosphere the Earth would average In the absence of an atmosphere the Earth would average
about 30 degrees Celsius lower than it does at present. about 30 degrees Celsius lower than it does at present.

Life (as we now know it) could not exist!


So we have this situation... Notice the Variation in the system
Variability in the climate system occurs at a number of scales in
Time (minutes to millennia) & Space (meters to 1000's km)

The earth
system's

Natural
Variability

Notice:
1. Scales
2. Variability


Natural Variability Natural Variability
Start with one we all know – Seasonality Seasonal cycle
Seasonal
Spring Summer
cycle is the
largest
single
source of
variability
(besides
Autumn Winter diurnal) and
a dominant
driver of
human
activities
and
adaptations http://geography.uoregon.edu/envchange/clim_animations/


Seasonal cycle Seasonal cycle - Monsoons
Results from the oscillating tilt of the earth It is most often applied to the seasonal reversals of the wind direction

Latitude of most intense heating moves north and south

Tropical variability tied to Inter-tropical Convergence Zone
(ITCZ) which moves north and south – bi-modal seasons

Sub-tropical variability linked to the descending high pressure
cell variations

Mid-latitude variability linked to the north-south shift of mid-
latitude frontal systems


Seasonal cycle – Monsoons Modifying the seasons: Intra- and Inter- Seasonal variation
Longer time period (3 – 10+ years) variability often linked to slower changing
ocean oscillations
Monsoons include  El-Ninõ Southern Oscillation (ENSO)
almost all of the West
phenomena associated African  Southern African Mode (SAM)
with the annual weather Monsoon
cycle within the tropical  Indian Ocean Dipole (IOD)
and subtropical
continents of Asia,  North Atlantic Oscillation (NAO)
Australia and Africa and
the adjacent seas and
 Volcanic eruptions
oceans. East  Solar cycle
Asian
Monsoon
 Decadal and longer........


Intra Seasonal variation - El Nino and La Nina (3-6 years) Intra Seasonal variation - El Nino and La Nina (3-6 years)
Nino
3 region

SST
anomalies


Intra Seasonal variation - El Nino and La Nina (3-6 years) Intra Seasonal variation - El Nino and La Nina (3-6 years)
Nino 3 region SST anomalies


Intra Seasonal variation - El Nino and La Nina (3-6 years) Intra Seasonal variation – Southern Annular Mode (weeks - years)

Difference in the zonal mean sea-level
pressure between 40oS and 65oS.
Annular pattern with a large low
pressure anomaly centred on the South
Pole and a ring of high pressure
anomalies at mid-latitudes.
This positive phase → stronger
westerlies around 55oS when SAM
index is high.


Intra Seasonal variation – Southern Annular Mode (weeks - years) Intra Seasonal variation – North Atlantic Oscillation
Due to the southward shift of the storm track, a high SAM index is associated

The NAO index: the anomaly in pressure difference between the polar low
with and the subtropical high in the boreal winter season (Lisbon and Iceland).
 Anomalously dry conditions over southern South America, New Zealand

and Tasmania A positive NAO means a more pronounced low over Iceland and high over the
 Wet conditions over much of Australia and South Africa.
Azores. The larger gradient leads to more and stronger storms on a more
 Associated with warming trends over Antarctic peninsula, Argentina,
northerly track and to warm and wet winters in Northern Europe.
Tasmania and the south of New Zealand in summer and autumn.

The SAM has shown a significant upward trend over the past 50 years,
particularly in austral summer.


Inter Annual variation – Observed variability with multiple year Inter Annual variation – Indian Ocean Dipole (4-6 years)
cycles Positive phase
Warmer average sea-surface

temperatures and greater precipitation
in the western Indian Ocean region,
with a corresponding cooling of waters
in the eastern Indian Ocean.
Tends to cause droughts in adjacent

land areas of Indonesia and Australia
and heavy rainfall over east Africa.
Negative phase
Opposite conditions with warmer water

and greater precipitation in the eastern
Indian Ocean, and cooler and drier
conditions in effected African regions.
Affects the strength of monsoons over

the Indian subcontinent - often negates
ENSO effect so ‘-’ phase IOD and El
Nino = no drought.


Inter Annual variation – Solar cycle (sun spots) Inter Annual variation – Solar cycle (~ 11-12 years)


Inter Annual variation – Solar cycle (sun spots) Longer time scales: Decadal and Inter decadal variation

These cycles affect/influence the shorter time scale cycles

More difficult to observe and characterize as a results of poorer
observational records the further we go back in time

How they influence the shorter time scale cycles is often not well understood

Termed - “Low frequency variability”


Pacific Decadal Oscillation (15-30 years) Atlantic multi-decadal oscillation (70 year cycle)
During a "warm", or "positive",  Principle expression in the sea surface temperature (SST) field in the
phase, the west Pacific becomes cool North Atlantic.
and part of the eastern ocean warms  Effects temperatures and rainfall over much of the Northern Hemisphere
(North America, Europe, North Eastern Brazil, African Sahel).
During a "cool" or "negative" phase,  Associated with changes in the frequency of North American droughts
the opposite pattern occurs. and is reflected in the frequency of severe Atlantic hurricanes.
 It alternately obscures and exaggerates the global increase in
Modulates ENSO....or does ENSO

temperatures due to human-induced global warming.
modulate it? Uncertain...


Scales of Natural Variability How does Climate Change fit in...?
e
< decad
years to
cycles -
Shorter

Shorter cycles - years to < decade Shorter cycles - years to < decade

eks)
al to we
Short cycles (diurnal to weeks) Short cycles (diurn to weeks)
cycles (diurnal
Short
Amplitude

Amplitude
longer)
eca des and
Long cycles (d

Long cycles (decades and longer) Long cycles (decades and longer)

Time Time


How does Climate Change fit in...? Some evidence...temperature

Enhanced
Back to

Radiative Greenhouse
Forcing

Effect


Some evidence...Rainfall, more difficult What about the Future?

Temperature


What about the Future? What about the Future?
Rainfall Sea Level

High degree of uncertainty
PPT increases very likely in high latitudes
PPT decreases very likely in most subtropical land regions


In summary....... Question.......
There is a lot of natural variability in the earth-atmosphere-ocean system

These occur on many time scales and they modulate each other Are you, in your particular sector, able to adapt to and/or cope with

So climate change is constant..... and complex. natural variability inherent in the climate system?

We do not understand the mechanisms of many of the natural oscillations

Challenges:

Are there cycles we have not discovered yet?

How do we filter the effects of natural cycles in our weather from those
effects caused by greenhouse gas emissions?

How do these cycles change through an enhanced greenhouse effect?


Weather Weather
Climate and weather Climate and weather
The expression of climate variability is in the weather …....to this!

It is important to understand the difference between weather and climate:
Climate is what we expect, weather is what we get!

So we move from this....
(average in space and time)


Weather Weather
Remembering the large scale set up... The effects of our weather...
We live in this large
scale climate and we
experience it's effects
through our

WEATHER!

 Tropics characterized by
rising air and convection
(thunderstorms)

 Sub-tropics characterized by
descending air (dry)
Mid-latitudes are high energy
zones (frontal systems)


Weather Weather
The effects of our weather... The effects of our weather...


Weather Weather
The effects of our weather... Aaaaaaahhhhhhh.........


In summary How does this affect me?
Energy from the Sun drives the system

Large scale circulations set up by:
Energy imbalance
Continents
Turning earth

Radiation balance - Greenhouse effect

Variability in the climate system
Seasonal
Decadal
Which of these do/can we currently adapt to?

Climate Change
Past Evidence
Future possibilities

Living in the climate system - Weather
Climate vs weather


How does this affect me? How does this affect me?
We operate in decision spaces at different scales of variability We operate in decision spaces at different scales of variability
Fill in the table and discuss what modes of variability you are exposed to in your sector
and what decisions you are called to make based on these
Met services Water adaptation Biodiversity
Weather Intermediate Climate
Short term (0-7days) Medium Term (6-9mths) Long Term (10-50yrs)

Agriculture met services Irrigation engineers Real Time → Week Seasonal Forecasts Decadal Changes

Type of
Decision
Soil engineer Remote sensing
Coastal management Operational
(Days to weeks)
Agricultural expertise
Water conservation Tactical
(weeks to months)

Climatologist
Strategic
(Years to decades)
GUI development for CC info


Very long cycles : Thousands of years (Milankovitch cycles) Very long cycles : Thousands of years (Milankovitch cycles)
Eccentricity
100 000 years
Currently the difference between closest approach to
the Sun (perihelion) and furthest distance (aphelion) is
http://www.sciencecourseware.org/eec/GlobalWarming/Tutorials/Milankovitch/ only 3.4% (5.1 million km). This difference amounts to
about a 6.8% increase in incoming solar radiation
(insolation). Perihelion presently occurs around
January 3, while aphelion is around July 4. When the
orbit is at its most highly elliptical, the amount of solar
radiation at perihelion is about 23% greater than at
aphelion.


Very long cycles : Thousands of years (Milankovitch cycles) Very long cycles : Thousands of years (Milankovitch cycles)
Obliquity When the obliquity increases, the amplitude Precession When the axis is aligned so it points toward
41 000 years of the seasonal cycle in insolation 26 000 years the Sun during perihelion, one polar
(23.44o) increases, with summers in both hemisphere will have a greater difference
hemispheres receiving more energy from the between the seasons while the other
Sun, and the winters less. hemisphere will have milder seasons. The
hemisphere which is in summer at perihelion
will receive much of the corresponding
Lower obliquity favours ice ages both increase in solar radiation, but that same
because of the mean energy from the sun is hemisphere will be in winter at aphelion and
have a colder winter. The other hemisphere
reduced in high latitudes (polar regions) as will have a relatively warmer winter and cooler
well as the additional reduction in summer summer.
insolation.


Natural Variability
Very long cycles : Thousands of years (Milankovitch cycles)

Ice Ice Ice Ice Ice
age age age age age

At present, only precession is in the glacial mode, with tilt and eccentricity not
favourable to glaciation
Climate Systems
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An Overview of the Climate System

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