This is probably the most important and interesting example of the ocean-atmosphere interrelationship. It is the El Nino and La Nina events which occur ever 2 - 7 years across the pacific ocean.
A NORMAL YEAR:
The Atacama Desert in Peru experiences warm, dry conditions, whereas South East Asia experiences heavy convectional rainfall.
DURING AN EL NINO EVENT:
Heavy rainfall off the coast of Peru results in flooding and mudslides. Rainfall is reduced in South-East Asia, New Zealand and Australia leading to drought conditions.
A Normal Year...
Under normal atmospheric conditions, pressure rises over the eastern Pacific Ocean and falls over the western Pacific Ocean. The descending air over the eastern Pacific gives the clear, dry conditions that created the Atacama Desert in Peru. Warm, moist ascending air over the western Pacific gives that region its heavy convectional rainfall.
This circulation is named after Walker, who first described it. His basic overview is the upper air moves from west to east and the surface air from east to west as the trade winds.
The trade winds push surface water westwards so that sea-level in the Philippines is normal 60cm higher than Panama and Colombia. Water in the western Pacific is the warmest in the world at ~28.C.
In contrast, as warm water is pushed away from South America, it is replaced by an upwelling of cooler, nutrient-rich water. This water often lowers temperatures but provides a plentiful supply of plankton, which forms the basis of Peru's fishing industry.
El Nino Event...
El Nino means 'little Christ Child' in Spanish. It was named this as the event occurs just after Christmas. It often lasts for 12 - 18 months.
In contrast to normal conditions, there is a reversal in the equatorial Pacific region.
Descending air, now over South-east Asia, gives that region much drier conditions than normal and has been known to cause drought on extreme occasions. The air over the eastern Pacific is now rising, creating much wetter conditions in places such as Peru, that normal experience the reverse.
The change in direction of the trade winds now means that surface water tends to be pushed eastwards so that sea-level in South-east Asia falls, while it rises in Peru. This also creates warmer waters in Peru, which all-but stops the fishing industry.
La Nina Event...
Often described an extreme version of a normal year, a La Nina event follows an El Nino.
The cycle goes back to normal but at a much exaggerated intensity until it settles out. Very strong trade winds pick up more moisture of the ocean, making storm clouds intensify over the western Pacific. This can lead to very heavy rainfall and tropical cyclones.Warm water is pushed west and can can increase sea-levels by up to 1 metre higher.
In the eastern Pacific, a strong up-welling of nutriets is very good for fishing but higher than average temperatures can lead to drought and an increase in wild-fires.
One example of this event is the severe storms in Australia in 2011. Please research yourself as there is too much information for one slide.
Climatologists have noticed a corrolation between climate and large volcanic eruptions.
For example, one of the coldest years on record came the year following the Tambora volcanic eruption in 1815. The following summer became known as 'The Year Without a Summer' all over the northern hemisphere due to a drop in temperature of up to 0.7.C
At first, scientists believed it was the dust emited by the eruptions that was responsible for cooling the earth by partially blocking the solar radiation heading for earth from the sun.
However, measurements indicated that most of the dust throw into the atmosphere returned to the earth within 6 months.
Volcanic eruptions continued...
Recent data suggests that large explosive volcanic eruptions eject large quantities of sulphur dioxide gas, which remains in the atmosphere for as long as three years.
Atmospheric chemists have determined that the ejected sulphur dioxide reacts with water vapour to form a dense optically bright haze layer that reduces the transmission of solar radiation.
The following slide will inform you on two case studies linked to this topic...
Volcanic eruptions continued...
In very recent history, two significant climate-modifying eruptions have taken place.
El Chichon in Mexico erupted in April of 1982 and Mount Pinatubo erupted in June, 1991. Of these two volcanic events, Mount Pinatubo had a greater effect on the earth's climate.
About 20 million tons of sulphur dioxide was ejected into the atmosphere. Researchers believe that Pinatubo eruption caused a 0.8 degree drop in global temperatures the following two years.
Data confirmed this connection.
Volcanic eruptions continued...
When a volcano erupts...
1. Ash and dust blocks the sun's incoming solar radiation.
2. Sulphur dioxide reacts with water vapour to produce sulphate aerosols. These act ti reflect the sun's radiation back into space.
3. The sulphate aerosols also absorb some of the infrared radiation that otherwise would be reflected back to earth as part of the natural green house effect.
A Little Ice Age occurred berween the 15th and 19th centuries between 1607 and 1804.
The River Thames froze regularly and people were able to hold fairs on the ice. Evidence for this colder period is found around the world.
There were colder winters in Europe and North America. Settlements in the Alps were destroyed by advancing glaciers during the mid-seventeenth century.
Average temperatures in many regions were at least 1 degree lower than those of today.
Little Ice Age...
Historical records and events that occurred during the Little Ice Age are listed below:
- Fishermen reported large amounts of sea ice floating in the North Atlantic
- British people saw Eskimos paddling canoes off the coast of England
- Alpine glaciers grew larger. In some cases, ice engulfed entire mountain villages.
- Wet weather caused disease that affected people, animals and crops including the bubonic plague. This disease killed more than a third of Europeans.
- Farms and villages in Northern Europe were deserted because the farmers couldn't grow crops in the cooler climate. During the harshest winters, bread had to be made from the bark of trees because grains would no longer grow.
- Limited crops and unhealthy livestock caused famine in areas of northern and eastern Europe. Unlike today, there was no way to transport food around the world to areas where crops and failed and the people were hungry.
Prior to the Little Ice Age...
... the temperatures were higher than average. The peak of this warm period was from 1000 to 1300, the High Middle Ages. It is estimated that temperatures were 1-2 degrees above the norm. In the high North, it was even up to 4 degrees warmer.
The regualr voyages of the Vikings between Iceland and Greenland were rarely hindered by ice and many burial places of the Vikings in Greenland still lie in the permafrost.
Global retreat of glaciers occurred in the period about 900 to 1300 speaks for the existance of the Medieval Warm Period. An interesting details is that many glaciers pulling back since 1850 reveal plant remnants from the Middle Ages, which is clear proof that the extent of glacial retreat was higher than today.
On top of this, the tree line in the Alps climbed to 2000 metres, higher than the current levels. Winery was possible in Germany at Rhine. In many parts of the UK, arable land reached heights that were never reached again since.
The amount of energy emitted by the sun varies as a result of sunspots. These are dark spots on the sun's surface caused by intense magnetic storms. The effect of sunspots is to blast more solar radiation towards the Earth, this making it warmer.
A suggested cause of the Little Ice Age, and the preceding medieval warm period is variations in solar output. During the period 1645 - 1715, the the middle of the Little Ice Age, there was a period of low solar activity known as the Maunder Minimum. During this time, there were no recorded sunspots.
There is well-known 11 year sunspot cycle as well as longer cycles. The total variation in solar radiation caused by sunspots is about 0.1%. Sunspots have been recorded for 2,000 years and there is a good record of the 400 years. Some scientists believe that around 20% of the 20th century warming may be attributed to solar output variation.