Is El Nino to blame for our dry winter?

Lewis Gay Water Levels 19-8-2010
Lewis Gay Dam, the Grey line near the top is the high water mark

It seems that this winter has been unusually warm and dry; some days have felt more like summer. Whilst this is very nice for the soul it does make you worry about the effects on water supplies this coming summer. Is this due to general global warming, a one off glitch or happening because of something else?
The data supplied by the Western Cape local government suggests that normally the dams are close to being full by now: as of the middle of this month the average for all water storage systems is listed as 84.13%. Doubting the official statistics I decided to go and have a look at the nearest dam I know about and so had a walk to the Lewis Gay Dam. I have to say that my anecdotal evidence is that the dam is several meters short of where it should be (see the photo, I remember seeing the dam only a meter or so short of the top of the wall last year) although this dam may not be as efficient at capturing water as the main ones in the mountains. Either way, with spring on the way we need a lot more water to get the dams full or we will face draconian water restrictions towards the end of the summer, that is unless there is a massive amount of rain in the late winter/early spring period.
It is possible that this is part of a continuing trend under the guise of global warning, and if anyone has the scientific data regarding a drop in local rain fall figures (maybe someone has been keeping their own records over the years?) then kindly let me know. Another possible suggestion was put forward by a neighbor of mine, that we have been in a period of weather affected by the El Nino phenomena.
El Nino is a weather pattern in the Pacific Ocean that has a global effect on world weather. The effect, which to give it its full name is the El Niño -Southern Oscillation (ENSO), is caused by a warming of the western Pacific Ocean, causing a high surface pressure. A resultant drop in the Pacific trade winds cause a further shift in normal weather patterns: technicalities aside, the basic effect of ENSO is that weather patterns are turned on their heads- normally dry areas become wet and vice versa- although effects are very localized.
The effect in the Western Cape has been to see a reduction in winter rainfall and a rise in temperatures, hence the long periods of warm sunny weather in the middle of winter.
Recent data compiled by the US government’s National Oceanic and Atmospheric Administration suggests that the period of El Nino has now come to and end is looking likely to be replaced by his little sister La Nina, which is a reversal of El Nino, namely a drop in the western Pacific Ocean temperature. This effect is likely to increase in intensity as we enter our spring, so we could be in for a wet September and October.
This is of course bad news if you want to be out and about; so if you have planned an outdoor event for this period make sure that you have an alternative plan ready just in case. On the other hand it would be great news for the water situation.
However, even if the dams are one hundred percent full, the chances are that due to an increase in demand, water restriction are going to be a reality every year from now on, and the best time to save water is when you have the most of it (I am planning a follow up article on water conservation so watch this space).
Of course, as with all extremes of nature La Nina is not going to be beneficial to everyone. One of the worst effects of La Nina is an increase in the severity of Hurricanes in the North Atlantic, so the Southern US and Caribbean is in for another rough autumn.
Only time will tell if the experts are right but I wouldn’t put the wet weather gear away for the summer just yet!

Sources of information for this post
City of Cape Town government web site
Rueters article “GLOBAL: Could be a busy season for disasters” 08 Jul 2010 10:20:15 GMT Source: IRIN
US National Oceanic and Atmospheric Administration

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