Climate Change and Eastern Caribbean Rainfall

This year’s Climate and Society class is out in the field (or lab or office) completing a summer internship or thesis. They’ll be documenting their experiences one blog post at a time. Read on to see what they’re up to.

By Dale Destin, Climate and Society 2014

The climate of the eastern Caribbean stretches from the Virgin Islands to Trinidad. And like other tropical regions, it is highly dependent on rainfall for freshwater resources. Yet climate change could throw the current precipitation cycle out of whack

Generally, wet places are projected to get wetter and the dry places drier. NCAR scientist Kevin Trenberth refers to this phenomenon as, “the rich get richer and the poor get poorer.” That raises big questions for the eastern Caribbean.

Company Line

Thus far, the “company line” is that climate change will cause annual eastern Carribean rainfall to decline. Successive Intergovernmental Panel on Climate Change (IPCC) reports, including the most recent published earlier this year, show the region is likely to get drier.

However, there’s a catch: the models the IPCC uses to do future projections do a very poor job in hindcasting eastern Caribbean rainfall. In fact, they show there should be a decrease in rainfall but observations show the exact opposite over the past 43 years.

Research Results

Analyses of rainfall data for 1970-2012 for the eastern Caribbean show an uptick in precipitation. Although, not significant, the data show annual rainfall is increasing by around 40 millimeters per decade. The most marked increase in rainfall is in April-June, which is traditionally a part of the region’s dry season.

The data show that while the annual number of days with one or more millimeters is decreasing, days with rainfall with 10 millimeters or more are increasing. This implies that the increased rainfall is coming as a result of heavier downpours as opposed to more rainy days. In other words, we are getting more rain in a shorter bursts. This is captured by the simple daily intensity index. 

Drivers of Eastern Caribbean Rainfall

Large scale patterns of sea surface temperatures (SSTs) and atmospheric pressure are the main drivers of eastern Caribbean rainfall.

Research has shown that a cool central-eastern tropical Pacific, commonly associated with La Niña, enhances eastern Caribbean rainfall and vice versa. Low pressure or high SSTs over the Atlantic usually result in more rainfall for the eastern Caribbean and vice versa for high pressure and low SSTs. Over the 43 years of data, it appears that the increasing rainfall trend is largely due to a warming Atlantic waters, particularly in the tropical North Atlantic.   

Conclusion

Eastern Caribbean rainfall is rising in contrast with previous research. It’s noted that analysis of subsets of the 1970-2012 dataset could result in the opposite conclusion, as others have. However, the shorter a dataset, the more susceptible the trend is to decadal variability and vice versa. Forty-three years is not a very long time but it is just about the longest available for the entire region.

Clearly the “company line” needs revisiting. The eastern Caribbean has been getting wetter particularly during the dry season. This is good news, especially for agriculture and tourism, the region’s main industry.  Given the trend, we also need to revisit strategies to cope with climate change. Plans need to be designed to deal with a wetter future as opposed to an unlikely drier one.