An Overview of the Climate System

Climate Systems Analysis Group

An Overview of the Climate System

Chris Lennard



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



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



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



Incoming solar radiation;Differential heating of the globe



1. Differential heating of the 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.



If energy were not redistributed in this way the Equator would be about 14 degrees hotter and the Poles about 25 degrees colder!!

1. Differential heating of the 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.


Large Scale Circulations

Tropics characterized by rising air and convection

Sub-tropics characterized by descending air (dry)

Mid-latitudes are high energy zones (frontal systems)


Looking more closely....radiation

1370 W/m2

Incoming radiation directly from the sun is 1370 W/m2

342 W/m2

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

Low clouds, water vapor and other green house gases absorb and re-radiate infrared radiation near

Looking more closely....radiationInteracts with the earth’s atmosphere


Looking more closely....radiation


The greenhouse effect....


The greenhouse effect....The Greenhouse effect keeps the earth warmer than it

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.





Sun.

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





Sun.

In the absence of an atmosphere the Earth would average 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:1. Scales2. Variability


Notice the Variation in the system

The earth system's

Natural Variability

Variability in the climate system occurs at a number of scales in Time (minutes to millennia) & Space (meters to 1000's km)


Start with one we all know – Seasonality

Spring

Autumn

Summer

Winter

Natural VariabilitySeasonal

cycle is the largest single

source of variability (besides

diurnal) and a dominant

driver of human

activities and

adaptationsClimate Systems Analysis Group

http://geography.uoregon.edu/envchange/clim_animations/

Natural VariabilitySeasonal cycle


Results from the oscillating tilt of the earth

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

Natural VariabilitySeasonal cycle


Natural VariabilitySeasonal cycle - MonsoonsIt is most often applied to the seasonal reversals of the wind direction


Natural VariabilitySeasonal cycle – Monsoons

Monsoons include almost all of the phenomena associated with the annual weather cycle within the tropical and subtropical continents of Asia, Australia and Africa and the adjacent seas and oceans.

West African Monsoon

East Asian Monsoon


Modifying the seasons: Intra- and Inter- Seasonal variationNatural Variability

Longer time period (3 – 10+ years) variability often linked to slower changing ocean oscillations

El-Ninõ Southern Oscillation (ENSO)

Southern African Mode (SAM)

Indian Ocean Dipole (IOD)

North Atlantic Oscillation (NAO)

Volcanic eruptions

Solar cycle

Decadal and longer........


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



Nino3 region

SST anomalies



Nino 3 region SST anomalies






Natural Variability

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 – Southern Annular Mode (weeks - years)Natural Variability

Due to the southward shift of the storm track, a high SAM index is associated with

Anomalously dry conditions over southern South America, New Zealand and Tasmania

Wet conditions over much of Australia and South Africa. Associated with warming trends over Antarctic peninsula, Argentina,

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.


Intra Seasonal variation – North Atlantic OscillationNatural Variability

The NAO index: the anomaly in pressure difference between the polar low and the subtropical high in the boreal winter season (Lisbon and Iceland).

A positive NAO means a more pronounced low over Iceland and high over the Azores. The larger gradient leads to more and stronger storms on a more northerly track and to warm and wet winters in Northern Europe.


Inter Annual variation – Observed variability with multiple year cycles

Natural Variability


Inter Annual variation – Indian Ocean Dipole (4-6 years)Natural Variability

Positive phaseWarmer 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 phaseOpposite 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)Natural Variability


Natural VariabilityInter Annual variation – Solar cycle (~ 11-12 years)


Natural VariabilityInter Annual variation – Solar cycle (sun spots)


Natural VariabilityLonger 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”


Natural VariabilityPacific Decadal Oscillation (15-30 years)During a "warm", or "positive", phase, the west Pacific becomes cool and part of the eastern ocean warms

During a "cool" or "negative" phase, the opposite pattern occurs.

Modulates ENSO....or does ENSO modulate it? Uncertain...


Natural VariabilityAtlantic multi-decadal oscillation (70 year cycle)

Principle expression in the sea surface temperature (SST) field in the North Atlantic.

Effects temperatures and rainfall over much of the Northern Hemisphere (North America, Europe, North Eastern Brazil, African Sahel).

Associated with changes in the frequency of North American droughts and is reflected in the frequency of severe Atlantic hurricanes.

It alternately obscures and exaggerates the global increase in temperatures due to human-induced global warming.


Scales of Natural Variability

Time

Am

plitu

de

Long cycles (decades and longer)

Shorter cycles - years to < decade

Short cycles (diurnal to weeks)


How does Climate Change fit in...?

Time

Am

plitu

de








How does Climate Change fit in...?

Back to Radiative

Forcing

Enhanced

Greenhouse

Effect


Some evidence...temperature


Some evidence...Rainfall, more difficult


What about the Future?

Temperature

What about the Future?


Rainfall

High degree of uncertaintyPPT increases very likely in high latitudesPPT decreases very likely in most subtropical land regions


What about the Future?Sea Level


Natural Variability

There is a lot of natural variability in the earth-atmosphere-ocean system

These occur on many time scales and they modulate each other

So climate change is constant..... and complex.

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?

In summary.......


Natural Variability

Are you, in your particular sector, able to adapt to and/or cope with natural variability inherent in the climate system?

Question.......


Weather

The expression of climate variability is in the weather

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)

Climate and weather


Weather

…....to this!Climate and weather


Tropics characterized by rising air and convection (thunderstorms)

Sub-tropics characterized by descending air (dry)

Mid-latitudes are high energy zones (frontal systems)

Remembering the large scale set up...Weather

We live in this large scale climate and we experience it's effects through our

WEATHER!


WeatherThe effects of our weather...








WeatherAaaaaaahhhhhhh.........


In summaryEnergy 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 Inter-annual (ENSO, SAM, NAO, Volcanic) Decadal

Which of these do/can we currently adapt to?

Climate ChangePast 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

Met services

Agriculture met services

Agricultural expertise

Irrigation engineers

Climatologist

Remote sensingSoil engineerCoastal management

GUI development for CC info

Biodiversity

Water conservation

Water adaptation


How does this affect me?

WeatherShort term (0-7days)Real Time → Week

IntermediateMedium Term (6-9mths)Seasonal Forecasts

ClimateLong Term (10-50yrs)Decadal Changes

Type of Decision

Operational (Days to weeks)

Tactical (weeks to months)

Strategic (Years to decades)

We operate in decision spaces at different scales of variabilityFill 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


Natural VariabilityVery long cycles : Thousands of years (Milankovitch cycles)

http://www.sciencecourseware.org/eec/GlobalWarming/Tutorials/Milankovitch/



Eccentricity 100 000 years

Currently the difference between closest approach to the Sun (perihelion) and furthest distance (aphelion) is 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.



Obliquity 41 000 years(23.44o)

When the obliquity increases, the amplitude of the seasonal cycle in insolation i n c r e a s e s , w i t h s u m m e r s i n b o t h hemispheres receiving more energy from the Sun, and the winters less.

Lower obliquity favours ice ages both because of the mean energy from the sun is reduced in high latitudes (polar regions) as well as the additional reduction in summer insolation.



Precession 26 000 years

When the axis is aligned so it points toward the Sun during perihelion, one polar hemisphere will have a greater difference between the seasons while the other hemisphere will have milder seasons. The hemisphere which is in summer at perihelion will receive much of the corresponding increase in solar radiation, but that same hemisphere will be in winter at aphelion and have a colder winter. The other hemisphere will have a relatively warmer winter and cooler summer.



At present, only precession is in the glacial mode, with tilt and eccentricity not favourable to glaciation

Ice age

Ice age

Ice age

Ice age

Ice age

An Overview of the Climate System

Education

climate scales

itthe greenhouse effect

earth warmer

earth and3

energy transferwhich

earths surface

southtropical variability

radiationincoming radiation