15 September, 2011

2011 Pakastani Floods

2010 Pakastan Floods (source: Wikimedia Commons)
For the second year in a row heavy monsoonal rains have resulted in devastating floods in the Sindh region of Pakistan. Today I want to look at the efforts of scientists to understand the causes of these floods because if we can understand the causes we may be able to predict heavy rainfall events 5 to 10 days ahead then the authorities can prepare by lowering dam levels and taking other precautions to lessen the impacts
We have known for since the work of Sir Gilbert Walker in the early 1900’s that there is a strong link between the El Niño climate phenomenon and the Asian monsoon. During a La Nina event, which is an anti-El Niño , water temperatures in the western Pacific Ocean are relatively warm and this feeds warm, moist air into the Asian Monsoon resulting in heavy rains in India and Pakistan.  The index we use to gauge this weather pattern is the ENSO index and it currently shows a falling trend just like that in Aug 2010.

El Niño  is a Pacific Ocean phenomenon, and while it is the major seasonal influence on the Monsoon we are beginning to appreciate the significance of a similar seasonal phenomenon that operates in the Indian Ocean called the Indian Ocean Dipole. A positive dipole indicates warm water in the western Indian Ocean relative to the east, and these warmer waters also feed the monsoons in India and Pakistan. It is quite rare to get a La Nina and a positive Dipole event at the same time but this is what happened in 2010 and seems to be happening this year.

Both La Nina and the Dipole are seasonal influences, but there is another influence that operates on a shorter timescale called the Madden-Julian Oscillation. This is a 20-40 day cycle of alternating high then low rainfall that starts in the Western Indian Ocean and progresses eastwards to the Pacific.

So knowing all of this, are floods like those we have seen in Pakistan predictable?  In February this year Peter Webster and colleagues from the School of Atmospheric Science at Georgia Tech published a paper in the journal Geophysical Research Letters. They took data on these phenomena and plugged them into a weather forecasting model developed by the European Centre for Medium Range Forecasts. They used this model to make predictions of July-August rainfall in Pakistan for the years 2007 to 2010 and found they could reliably predict the high rainfall events 6 to 8 days ahead of time. This is just one study over a relatively short period of time, but if their technique proves reliable and, importantly if it can be used in conjunction with hydrological models which predict the behaviour or rivers after the rain falls, then this could be really useful ‘advanced warning’ that would allow the authorities to manage flood events.

One last point to note is that there has been a significant increased frequency of these major flooding events in the last 30 years in a way that is consistent with what we would expect as a result of general global warming.