“Climate is what we expect, weather is what we get” – Mark Twain
So I’ve written about the Shipping News and the databuoys and waves and wind etc. I thought I might do some more on the weather subject and move up to a bigger scale, particularly as it affects Ireland and the North Atlantic.
Last winter in Ireland especially, saw constant discussion about the weather. In the media the discussion always went the same way:
“If climate change is real, why are we having the coldest winter (or wettest summer, etc) in years?”.
To which the meteorologists response is that you can’t mistake weather for climate.
Weather is a localized effect, whereas climate is a pattern. You can have climate at various scales from localised micro-climates (e.g. because of some effect like mountains near sea) up to global climate.
We all know what Irish climate is like. Moderate winters, cool and wet summers. But why?
Almost anyone you talk to in Ireland will recall one bit of science from school: Ireland doesn’t freeze in the winter because of the Gulf Current.
The Gulf Stream Current is part is a larger North Atlantic system called the North Atlantic Gyre which includes the Equatorial Current and is one of the five main oceanic gyres. (See pic).
A Gyre is a circulating current. It is driven by the Earth’s Coriolis Effect, which means the rotating earth has a friction effect on the atmosphere.
The Equatorial Current feeds across from Africa to the Caribbean, driving the Gulf Stream. The Gulf Stream in turn flows North East. part turns into the North Atlantic Drift and runs further North east toward the Arctic, while part splits and flows back southward, passing Portugal and cooling, and feeding in the turn the Equatorial Current again, thus setting up a circulating current.
The gyres themselves operate independently, but they are a surface effect usually going no deeper than about one kilometer.
At an even bigger scale however they fit in a larger pattern often called the global ocean conveyor belt (Thermohaline circulation). This moves heat around the globe with for example the warmer water from the Gulf Stream getting colder as it goes further north, and becoming more dense and sinking, while southward flowing water warms and becomes less dense and rises. (Cold water also carries more nutrients and drives the global food ecosystem). The global conveyor belt connects all the oceans with a current that flows around the world rising and falling, taking about 1500 years.
The Oceanic Gyres are only part of the story though.
The most basic fact about weather: It’s all driven by the Sun. The Sun inputs the energy that drives the whole system. That energy varies slightly, (for example with Sunpots) and the amount of energy hitting the Earth varies according to local effects like cloud cover, ice, water or land. This is all aggregated over the whole world/system. However even more important is that the amount of energy hitting the Earth at the equator is significantly greater than that at the Poles. This discrepancy is the generator for the changing weather. The mechanism by which the weather propagates is the Coriolis Effect.