ENSO Explained

The El Niño Southern Oscillation (ENSO) is a natural phenomenon that occurs in the Pacific Ocean around the equator every 2-7 years. Weather systems on the West Coast of the Americas, the East Coast of Australia and East Asia can all be affected by this phenomenon.

The ENSO oscillates between two phases: El Niño and La Niña (meaning ‘the boy’ and ‘the girl’ respectively in Spanish. The boy can also be a specific reference to the Christ child. I am unsure why they were named this however!). These phases are the two extremes of this oscillation. To explain them in more detail I first need to explain the ‘normal’ state of the Tropical Pacific Ocean. The eastern Pacific normally has atmospheric higher pressures than the west, this causes easterly winds (from the Americas to Australia) blowing along the equator. These winds take with them warm moist air which rises up and turns into rain. This elevated air then travels back to the east creating the ‘Walker circulation’. A diagram showing this circulation is shown below. The winds blowing westwards move the warm sea surface water. This then causes the deeper (and colder) water to rise up to the surface in the east producing a temperature gradient along the equator.

Original found here.

Original found here.

The Walker circulation is responsible for countries like Indonesia experiencing warm downpours of rain, especially in the northern hemisphere winter months. However, when ENSO is in its El Niño phase the areas of high and low pressure reverse, therefore changing the wind direction. This drives the warm sea surface waters back to the east, taking with it the rain.

The La Niña phase is when the ‘normal’ conditions are more extreme, i.e. the easterly winds are even stronger and the pressure difference is even greater. This means even more warm water is moved towards Australia and East Asia. The diagram below shows the sea surface temperature anomalies for El Niño (left) and La Niña (right) events. El Niño has higher than average temperatures because the warm water has been moved back towards the east whereas La Niña shows the surface is much colder than average as all the warm water has been shifted even more westward.

Original can be found here.

Original can be found here.

ENSO affects the weather systems of the world in different ways, many of which are summarised in the diagram below. Unlike the North Atlantic Oscillation which was the subject of my last blog (please click here to see) this oscillation does not directly influence Europe’s weather. El Niño years tend to give a warmer global temperature average especially in the winter, whereas La Niña years will give cooler than average winters. It is also thought that during La Niña events more tornadoes are experienced, although no mechanism for this has so far been provided .

Original image from here.

Original image from here.

The video below is a basic but well explained summary of how ENSO affects the weather in Australia and is worth a watch.

The intensity of the El Niño / La Niña phase can be quantified by an index where positive and negative values represent La Niña and El Niño respectively. A time series for the last ~130 years is shown below. In the future, as a result of global warming, it is predicted that there may be more El Niño events, as in recent decades, however more observational data is needed to improve the confidence of these results.

Original from here.

Original from here.

The final video below is an overall summary of ENSO, and helps to visualise the oscillation more clearly. It’s a little over 4 minutes long but you miss nothing by skipping the first 25 seconds.

Thanks for reading!

The North Atlantic Oscillation

Hello everyone, sorry it’s been a while since my last post. This post is about the North Atlantic Oscillation – an atmospheric phenomenon that heavily influences the northern hemisphere’s (NH) climate, especially in the winter.

Firstly, I think it good to get some grounding on the NH jet stream. The video below from the Met Office introduces it pretty well and shows how the stream can fluctuate.

The North Atlantic Oscillation (NAO) can have a large impact on the strength of the jet stream. When the jet stream is weakened its direction can change more often (which is why UK weather can be pretty variable!). The best way to describe the NAO is as a particular state of the atmosphere which can change between so called ‘positive’ and ‘negative’ phases, like a seesaw. These phases are identified by calculating pressure differences between the Azores and Iceland. The Azores has a typically high pressure and Iceland has a low pressure, however this difference can vary in magnitude. The diagram below shows that in a positive phase there is a large difference between high and low pressures, and in a negative phase there is a small difference. If you look closely on the diagram you can see the outline of Europe and Africa to the right of the NAO.

The NAO positive and negative phases. Soucred from here.

The NAO positive and negative phases. Sourced from here.

Depending on if the NAO is in positive or negative phase then the jet stream is affected differently. The diagram below shows the position of the jet stream when the NAO is positive and negative. In the NAO positive phase the jet stream is coming to the UK from the south west, bringing with it warm, wet air. Conveniently for the UK, in the winter/spring time the jet stream is also pointing in this direction, bringing us wet weather which is relatively warm compared to other places in the world at this latitude. In the negative phase, the jet stream is coming from the north, bringing cooler, much drier air. This may sound like the wrong way around (warm air being experienced in the winter) but this is one of the reasons the UK experiences a temperate climate.

How the Jet stream can change with the phases of the NAO. Sourced from here.

How the Jet stream can change with the phases of the NAO. Modified from original source here.

In the summer of 2007 and 2012 Britain experienced severe flooding and as the NAO was uncharacteristically in its positive phase, and so the jet stream brought with it much more moist air, which turned to rain. The picture below just shows the effect of the NAO in a more simplistic way.

Sourced from here.

Sourced from here.

Of course there are many other events that can affect the UK’s weather (the Gulf Stream for example) and there are other types of oscillations that occur elsewhere in the world but this particular one can strongly influence how nice the UK’s summers will be!

The Medieval Warming Period

In my last post (4.5 Billion Years of the Earth’s Temperature) I mention the Medieval Warming Period. This was when Europe experienced higher atmospheric temperatures than today. It occurred between 1000-1400 AD, and it was recently established that the NAO was one of the major driving forces behind this event. It is thought that the NAO was stuck in a very strongly positive phase, blowing vast amounts of warm air over Europe for an extended period of time. For more information on the medieval warming period follow this link.

A Changing Climate

As the global climate changes and the world becomes warmer, it is predicted that this will have an effect on the NAO. The oscillation between positive and negative phases is thought to increase in frequency so we will experience a more variable climate within our normal seasons. This is due to changes in the Earth’s circulation cells which I hope to explain in more detail in another post!

I hope you found this post interesting, as always please post any comments or questions below!

An Update To The Milankovitch Cycles

My second post (The Milankovitch Cycles) was recently linked to another blog post. This post, which is a bit more ‘sciencey’ than mine, gives a new approach to the Milankovitch Cycles. It states that the variation in energy on the Earth’s surface because of these cycles is not enough to explain the amount of global temperature variation. It then explains that a recent publication by Abe-Ouchi, A. et  al. (2013) can explain these discrepancies using a climate model. Results show that ice-sheets formed on the Canadian Shield are key to the variation in the Earth’s climate. To read the blog post in full please click the link below:


4.5 Billion Years of the Earth’s Temperature

This blog post will give a brief overview of how the Earth’s temperature has changed up to the present day. We only have actual temperature measurements going back a couple of hundred years however there are several other methods we can use to give reliable estimates of the Earth’s temperature in the past. ‘Proxy’ measurements include rock sediment sampling, tree rings and ice cores. The video below gives a good introduction to how the British Antarctic Survey use ice cores to generate accurate atmospheric gas and temperature records going back 800,000 years! The diagram below shows an overview of the Earth’s temperature from 500 million years ago to the present and may help with picturing the changes in temperature when reading this post.

 Very Early Earth’s History (4.5 billion – 3.8 billion years ago)

The Earth was formed roughly 4.5 billion years ago. Until 3.8 billion years ago it was a completely inhospitable environment with the surface being mainly molten lava. The Earth eventually cooled enough for its crust to form. Land masses could then exist and, when it was cold enough to rain, the oceans formed.  Around this time the atmosphere was predominantly consisted of methane (CH4) and ammonia (NH3), two extremely important greenhouse gases, thus their radiative forcing kept the Earth’s atmosphere warm and toasty!

The Oxygen Explosion (2.5 billion – 500 million years ago)

Oxygen (O2) in the atmosphere was almost non-existent until ~2.5 billion years ago. The evolution of cyanobacteria, which produced oxygen as a bi-product of photosynthesis, meant that Olevels dramatically increased. This rapid change in atmospheric composition caused widespread extinctions of most of the previous anaerobic bacteria. This ‘new’ atmosphere made the Earth much colder as there were no longer bacteria emitting radiative forcing-methane and carbon dioxide into the atmosphere. It is thought that the average temperature at the equator was roughly the same as current Antarctic conditions!

History of the Earth's Temperature. originally sourced from here.

History of the Earth’s Temperature. originally sourced from here.

500 – 250 million years ago

During this period the Earth’s atmosphere became more stable, eventually cooling to similar temperatures to today’s average (see first section on plot above where the temp change is ~0 ΔT).

Animal Evolution (250 65 million years ago)

During this time the evolution of aerobically respiring animals occurred, i.e. DINOSAURS! This meant the concentration of CO2 increased and global temperatures increased again. We know that there was a sudden decrease in temperatures around 65 million years ago which resulted in the extinction of the dinosaurs. The most widely accepted reason for this is a massive comet hitting the Earth sending huge amounts of matter (read: aerosols) into the atmosphere. This caused a global decrease in temperature due to an increased albedo effect (for more information about this and the contribution of aerosols to this effect please read my previous blog: An introduction to aerosols).

Thermal Maximum (55 million years ago)~55 million years ago, records show a massive warming of between 5-8 ⁰C in just 20,000 years (It is thought that during this time it was so warm palm trees could have grown in the poles!). The direct cause is still disputed amongst scientists, however it is generally agreed that a sudden release of carbon into the atmosphere caused the warming. This was probably in the form of methane from either the ocean bed or from within ice structures called clathrates. It was after this period that mammals started to evolve.


Ice Age (35 million years ago)

The thermal maximum continued to around 35 million years ago when the Earth cooled into the Ice Age. The theory behind this change in temperature is that a type of fern named Azolla became extinct. The Azolla then sank to the bottom of the ocean, taking with it much of the carbon absorbed as carbon dioxide, therefore removing it from the atmosphere. With the carbon dioxide not present to act as a greenhouse gas, global temperatures decreased again. Unlike the last period of cooling, this time the Earth had fully formed continents, including mountain ranges, and land mass at the South Pole (Antarctica). This new land coverage helped amplify the cooling via circulation.

An ice age is defined as when a planet’s poles are covered with ice, so technically we are still in one! Within an ice age there are periods of glacials and inter-glacials. Glacials are episodes of colder temperatures whereas inter-glacials are warmer time phases. Both will last several thousands of years. These changes in climate can be explained with the Milankovitch cycles (please read post #2 – The Milankovitch Cycles – for more information). NB: You can see on the plot above sections labelled ‘the mini ice age’ and ‘the medieval warming’ period. I plan to do future blogs on these events as this post is getting far too long!

Recent Warming (1880 – present day)

The warming we have seen in recent years has been like nothing experienced before in the Earth’s history. The last 100 years of warming has cancelled out the previous 6000 years of cooling that occurred before. The video below (sourced from NASA) shows just how dramatic the rate of global warming is over this time period.

Thanks for reading to the end of this post, it ended up a bit too long! Next time I want to introduce an event called the Northern Atlantic Oscillation: the phenomenon that is thought to have caused the medieval warming period.

A brief introduction to aerosols..

Let’s skip the chit chat and go straight into post number three… Spray-cans will come to mind for many people when they first think of aerosols, but this term has a much more general meaning and plays an important role in climate science. Anything that has a suspension of liquid or solid in a gas is deemed an aerosol. In fact, a spray-can may actually contain hundreds of different individual aerosols.

A simple demonstration that shows the significance of aerosols in the atmosphere is the ‘cloud in a bottle’ trick. A trickle of water is added to an empty bottle and the cap put on. By just shaking and squeezing the bottle (simulating pressure changes that occur every day in the Earth’s atmosphere) no cloud is formed. However, if smoke is added, from a blown out match for example, and then the bottle squeezed, a small vapour cloud becomes visible – try it yourself or see the video of it below!

These tiny particles – aerosols – are necessary for clouds to form anywhere in the world. The photograph below shows how clouds have formed from aerosols released by ships.

Cloud formation using aerosols emitted from ships

Cloud formation using aerosols emitted from ships

Aerosol sources can be both natural and anthropogenic. The photo above is an example of industrial aerosol emissions but other major sources can include deserts (in the form of dust and sand) and from fires / biomass burning, which can be both natural and manmade, even tiny ice crystals can be classed as aerosols. One of the global climate models, Global Earth Observing System Model – Version 5 (GEOS-5) has simulated the emissions and transportation of all major aerosols from September 1st 2006 to March 17th 2007 and the video can be seen below. For a description of the colours see the figure description.

Summary copied directly from you tube link: “Dust (red) blows over the Saharan desert and interacts with two Atlantic tropical cyclones. Sea salt (blue) churned up from the ocean by surface winds is most prevalent along mid-latitude storm tracks and fronts in the Southern Ocean and within tropical cyclones. Organic and black carbon (green) burst from extensive biomass burning in South America and Africa. Sulphate (white) arises from three primary sources: fossil fuel emissions over Asia, Europe, and the United States; a persistently active volcano in Mozambique, Africa; and a large eruption from the Karthala Volcano on Grande Comore Island, Comoros, in January 2007.

Aerosols have both positive and negative radiative forcing effects (for more information on radiative forcing please read my first post! – An Introduction to Global Warming). The diagram below shoes the major contributors to global warming and whether they have a positive effect (a warming – coloured red) or negative effect (a cooling – coloured blue) on the Earth’s climate. The highlighted box shows the contribution from aersols – a very negative effect! However the black lines show how much of this estimation is uncertain. There is a large amount of research currently trying to reduce these uncertainty bars so we can be more sure of how much an overall effect aerosols have on our climate.

Global average radiative forcing estimates and ranges. Oringially produced by the IPCC.

Global average radiative forcing estimates and ranges. Originally produced by the IPCC.

There are two main ways aerosols effect our climate. The first is a ‘direct‘ way. I.e through re-radiating energy (warming the Earth) or reflecting incomming sun’s rays back out to space (cooling the Earth). Again, my first post explains this in more detail. The other way aerosols effect our climate is called the ‘indirect effect‘. As aerosols are needed as ‘seeds’ for clouds to form, an increase in aerosols can cause an increase in cloud number thus increasing the Albedo effect. When aerosols help form clouds they are referred to as ‘cloud condensation nuclei’ (CCN). The more CCN present the whiter the clouds can appear which further increases the albedo effect.

As you read this you may be thinking, the more aerosols present the better in the atmosphere as they have an overall cooling effect of climate however not all effects of aerosols are good. Aerosols can have a huge impact of air quality and atmospheric pollution. The recent smog events in Singapore (shown in the picture below) are thought to have occurred because of aerosols released from illegal biomass burning from Indonesia and travelling up to Singapore where they formed these smog clouds which can have severe health effects.

Comparison of Singapore skyline from Feb 2012 and June 2013. Image found on BBC News website.

Comparison of Singapore skyline from Feb 2012 and June 2013. Image originally from BBC News website.

Any questions or comments please write them below, or perhaps even a request for future posts?? Next time I plan to give a very brief overview of the history of the Earth’s temperature has changed over 65 millions years. They’ll be videos in that one as well!

Thanks for reading.

The Milankovitch Cycles

Hello everyone, welcome to my second blog post. I wanted to start off with a diagram that I should have included in my last post. It shows an overview of the world’s energy budget, which includes several terms I described before, including the albedo effect and the greenhouse effect!

Original image found here: http://www.nasa.gov/audience/foreducators/topnav/materials/listbytype/Earths_Energy_Budget.html

Original image found on the NASA website here.

In this post I want to explain how the Earth’s temperature is affected over longer timescales as a result of changes in its orbit. These changes can have a significant impact on the Earth’s climate and temperature. The variations in the Earth’s orbit were first discovered by a mathematician named Milankovitch in the early 20th century, whose theories were confirmed in the 1970s. However, to understand why these cycles have such an effect on temperature I should first review the Earth’s annual cycle and why it has seasonality.

The Sun emits energy which travels to the Earth as rays. The amount of heat generated in the atmosphere depends on the angle these rays hit the Earth (their angle of incidence). The figure below shows an example of how the Sun’s rays reach the Earth at two areas. The rays around the equator have a much higher angle of incidence (~90⁰ in relation to the Earth’s surface) and so they cover a relatively small area. In energy terms there is a higher Watts per metre squared value (Wm-2). The rays that reach much higher latitudes have a lower incidence angle and therefore diffuse themselves over a larger area (lower Wm-2). This combined with the Earth being tilted on an axis explains our seasons (NB. Not represented in the figure). In northern hemisphere summer time (NHST) the North Pole is tilted towards the Sun and so the northern hemisphere experiences incoming rays at a high angle of incidence. The opposite occurs northern hemisphere winter time (NHWT).

Sourced from here.

Sourced from here.

As I stated before, the Earth’s orbit around the Sun is not fixed. Milankovitch discovered that it varies in three main ways, of which all have an impact on its climate. The three Milankovitch cycles are:


The Earth’s orbit around the Sun is not spherical but in fact elliptical. When the distance between the Earth and Sun is greatest this is called ‘aphelion’ (see figure below) and when it’s at its shortest, ‘perihelion’. The elliptical orbit can vary greatly. The two extremes of the Earth’s eccentricity can be seen on the diagram summarising the Milakovitch cycles below. The variation between summer and winter temperatures is at its most extreme when the difference between aphelion and perihelion is largest. The time it takes for the Earth’s orbit to change from its most elliptical shape to its most spherical and back again is ~95,000-100,000 years. This period of time is known as its cycle.

Aphelion & Perihelion Sourced from here.

Aphelion & Perihelion
Sourced from here.

Obliquity / Axis Tilt

The Earth’s axis is currently on a tilt of 23.45⁰ however this can vary from 22.1-24.3⁰ over a ~41,000 year cycle. When the angle is decreased the difference between summer and winter climates is less extreme.


As well as the Earth’s axis being tilted it also wobbles about this axis. Think of a spinning top just before is comes to a rest. Currently the North Pole is pointing towards the North Star, however in ~12,000 years’ time it will be pointing in a completely different direction which will make NHWT in June and summer time will be in December! A full cycle takes 19,000-24,000 years to complete. As the precession is effectively swapping our seasons this has a large impact on the temperature of the Earth, as well as its climate.

Original image from the UCAR website found here.

Original image from the UCAR website found here.

For those of you who read my first blog post you will recognise the next figure but hopefully now the cycles in the Earth’s temperature will make more sense to you. You can definitely see the impact of the Milankovitch cycles within the plot. These sorts of temperature plots have sometimes been used by climate sceptics to show the degree of natural variability in the Earth’s Climate and how current climate change is minimal compared to the past. This is extremely difficult to justify, especially when comparing two extremely different timescales. As you can see from the plot below we are actually in one of the Earth’s warmer periods on this timescale, so if anything we should be expecting Earth to be moving into a cooler period in the future. However the rate of global warming we have experienced in the last 150 years has not occurred for several million years. As I said in my last blog post I will do a future post about the history of the Earth’s temperature in more detail, however next time it’ll be more chemistry with a brief introduction into aerosols!

Record of the Earth's temperature and Carbon Dioxide concentrations taken from ice core data in Antarctica.

Record of the Earth’s temperature and Carbon Dioxide concentrations taken from ice core data in Antarctica.

A brief Introduction to Global Warming

Hello and welcome to my first blog post! I can’t really start without an explanation of how global warming occurs, as this is one of the most fundamental reasons why climate science exists! So here we go…

The Earth’s atmosphere allows life as we know it to exist. Without it, global temperatures would be around 33-35 oC colder. The gases that make up our atmosphere can absorb some of the sun’s energy leading to an increase in temperature. Energy from the Sun reaches the Earth in the form of light and heat. The Earth has the ability to absorb all of this radiation but some is reflected back into space either by the Earth’s surface or by its atmosphere. This process is called the Albedo Effect and without it the Earth would be much warmer. The Earth re-radiates energy at a slightly longer wavelength that can be absorbed by some atmospheric gases. These gases then re-radiate this as thermal energy.

A measure of how strong an effect this re-radiated energy has on the Earth’s temperature is called a gas’ radiative forcing. Radiative forcing is the difference between radiant energy received by the earth and energy radiated back into space, and its units are in watts per meter squared (Wm-2). The gases that contribute most to the greenhouse effect, in terms of their radiative forcing, their lifetimes and also their abundance in the atmosphere include carbon dioxide (CO2), water vapour (H2O) and methane (Ch4). The relationship between temperature and concentrations of these gases is strikingly positively correlated, as the graph below shows. In another blog I plan to explain more about how the Earth’s temperature has changed throughout geological history.

Record of the Earth's temperature and Carbon Dioxide concentrations taken from ice core data in Antarctica.

Record of the Earth’s temperature and carbon dioxide concentrations taken from ice core data in Antarctica.

Almost all atmospheric gases have both natural and anthropogenic (man-made) sources. In fact, for most of the major greenhouse gases natural sources outweigh man-made. Since the Industrial Revolution, however, anthropogenic emissions have been rapidly increasing. The plots below, published by the Intergovernmental Panel on Climate Change (IPCC), show just how high the atmospheric concentrations have risen. It is this increase, or offset from the pre-industrial balance between sources and sinks, that is causing temperatures to rise. For more information on the IPCC and its soon to be released 5th assessment report click here.  Or if you want a more detailed summary of all the aspects of climate change this ‘summary for policy makers’ is really interesting.

Timeseries of Methane (CH4), Carbon Dioxide (CO2) and Nitrous Oxide (N2O) emissionsfor the last 10,000 years.

Time series of Methane (CH4), Carbon Dioxide (CO2) and Nitrous Oxide (N2O) emissions for the last 10,000 years.

Of course natural variability also affects the earth’s climate and can contribute to global warming. Examples of natural variability include variations in the Earth’s orbits (i.e. Milankovitch cycles – more to come in another blog), solar radiation variability, lunar tides and interactions between the oceans and the atmosphere. The world’s scientific community agree that the Earth is currently warming however there is a degree of discussion about how much human impact is contributing to global warming. The IPCC has stated that ‘the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations’. Very likely being classed as a >90% probability.

Thanks for reading what I hope is the first of many blog posts.  Next up I will discuss changes to climate that occur on longer times scales, i.e changes to the Earth’s orbit about that sun.