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Nicholas Tyrrell added From_one_year_to_the__.tex
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From one year to the next, the global temperature fluctuates up and down around
an average temperature. If we compare the fluctuations of land surface
temperatures to the fluctuations of the ocean surface temperatures, we see that
the land temperature fluctuates more than the ocean. Why? We might expect
that the oceans are slower to respond to changes than the land. This is
certainly part of the story but we can do an experiment to test how important
this is. \\
In our experiment, we take a global climate model and we specify the ocean
surface temperatures; we make them vary up and down each year and see how the
land responds. The results show that the land varies \textit{more} than the
ocean temperatures. The ocean variability - which we have input into the model -
is being amplified over the land. Interesting! But lets pull it apart a bit
more.
We know that the \href{http://www.bom.gov.au/climate/enso/}{El Ni{\~n}o-Southern
Oscillation} is the main source of year to year temperature variability in the
oceans. It occurs in the tropical Pacific but is strong enough to have a global
influence.\\
We can take our climate model again but instead of assigning the
whole ocean temperature we just put in a "fake" El-Ni{\~n}o/La-Ni{\~n}a.
We make the tropical Pacific ocean temperatures go up and down - over
a period of four years - and let the rest of the ocean, and the land, respond.\\
In this experiment, the global land surface temperature still
responds with amplified variability relative to the ocean, so if the ocean surface temperature increases or decreases by $1^{\circ}$ the land temperature will increase or decrease by almost $1.5^{\circ}$. Essentially, we can
control the global land temperature by changing the temperature of the tropical
Pacific ocean. \\
You might ask; what if you changed the temperature of the Southern Ocean or the
North Atlantic, would you get the same land response?\\
The answer is, no.\\
We've found that the tropical oceans have the greatest influence on land temperatures. The reason is
that warmer tropical oceans result in lots of atmospheric convection from the
surface up to about 10-15km above the ocean: the troposphere. Changes in colder
ocean temperatures will not cause the kind of convection that penetrates up into
the troposphere.\\
If you add what we call a temperature perturbation to the troposphere, the tropical atmosphere will efficiently transport that perturbation around the globe, so for a big enough perturbation the Once the warmer temperatures reach the troposphere, they spread
out and encircle the earth. The ocean temperature changes are amplified because
the land is more responsive to these tropospheric temperature changes.
What does this mean for us?\\
Right now, in the Pacific ocean, there is a very
large El Ni{\~n}o brewing. It has the potential to be one of the largest on
record and will probably keep getting warmer until December or January. This
means that we expect the global land surface to have an amplified response to
the ocean temperatures and 2015 and 2016 to be very warm years!