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 The Local Impacts of ENSO across the Northeastern Caribbean


           El Niño/La Niña Southern Oscillation (ENSO) is a periodic oceanic atmospheric phenomenon that occurs across the tropical Pacific and is recognized to have worldwide impacts. While developing countries bordering the Pacific Ocean tend to be the most affected by ENSO, major floods and droughts around the world have been associated with ENSO, including the flooding of the Nile river basin across northern Africa as well as shifts in the monsoonal pattern across India.

          The oceanic components of ENSO, referred to as El Niño (the warm phase) and La Niña (the cool phase), are measured by the warming and cooling of the surface waters across the eastern tropical Pacific respectively. The atmospheric component, referred to as the Southern Oscillation, is characterized by the surface pressure differences across the western tropical Pacific Ocean. The combination of these components, ENSO, is depicted by calculating anomalies in the sea surface temperatures across the Niño 3.4 region of the tropical Pacific compared to their 1971-2000 mean. After 5 consecutive months of an anomaly of 0.5C or greater, either an El Niño or La Niña event is declared.

          Because meteorologists are not able to predict when an ENSO event is expected to develop or dissipate, knowing the local impacts of previous ENSO events across a particular region can have a valuable benefit to the area of interest. With this in mind, in order to get an idea of the local impacts across the northeastern Caribbean, specifically across Puerto Rico and the U.S. Virgin Islands, 16 high quality observation sites were selected to be analyzed, with a minimum of 2 sites from each of the different climatic zones across Puerto Rico (as defined by the National Climatic Data Center (NCDC)) and 2 from the U.S. Virgin Islands. This data was then compared to the different phases of ENSO as measured in the ENSO 3.4 region and quantified by the Climate Prediction Center (CPC).


Stations used for comparison with ENSO 3.4 values


Map of Puerto Rico and the U.S. Virgin Islands identifying the stations which were used in this study. Legend: North Coast (purple), South Coast (red), North Slopes (blue), South Slopes (orange), East Interior (green), West Interior (yellow), U.S. Virgin Islands (pink)




          After analyzing 55 years worth of ENSO, temperature and precipitation data for a total of 109 different seasons (55 dry seasons and 54 wet seasons), several interesting results were noted.
           During the dry season (which was defined as the months December through April), significantly more precipitation accumulated during El Niño years versus La Niña years (approximately 13% more precipitation fell during El Niño years). During the wet season (which was defined to be the months of May through November), significantly more precipitation fell during La Niña years versus El Niño years (approximately 14% more precipitation fell during La Niña years). In both cases, the years which were defined to be ENSO neutral (years where at least 70% of the months of a season were not defined as either El Niño or La Niña years), observed precipitation values that fell in between those of the El Niño and La Niña averaged values (see plots below).


Observed precipitation during the dry season according to phase of ENSO

The effects of ENSO on precipitation across Puerto Rico and the US Virgin Islands during the dry season (December through April).

Wet Season precipitation across the northeastern Caribbean according to phase of ENSO.

The effects of ENSO on precipitation across Puerto Rico and the US Virgin Islands during the wet season (May through November).


          While the signal in precipitation values was significant, equally interesting was the noted signal in temperatures (see plots below). Regardless of the season, temperatures were approximately 0.5F degrees above normal during El Niño events and temperatures were approximately 0.5F degrees below normal during La Niña events. While this 1F degree difference between the different phase of ENSO may seem insignificant, in the tropics, this 1F degree swing represents a significant deviation from normal and also represents a much larger portion of the diurnal temperature range than for a typical station at mid-latitudes.


Dry Season Temperature Effects across the northeastern Caribbean

The effects of ENSO on temperatures across Puerto Rico and the US Virgin Islands during the Dry Season (December through April).

Wet Season Temperature effects across the northeastern Caribbean

The effects of ENSO on temperatures across Puerto Rico and the US Virgin Islands during the Wet Season (May through November).