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.

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.

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.

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.

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. 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. 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.

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:

http://earth-pages.co.uk/2013/08/23/new-approach-to-the-milankovitch-mystery/

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 O₂ levels 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.

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.

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 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 20^th 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.

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:

Eccentricity

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.

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.

Precession

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.

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.