Examples of ENSO-Society Interactions

Impacts of ENSO on Costa Rica

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ENSO and Climate in Costa Rica

Costa Rica experiences very diverse sets of climatic regimes with influences from the Pacific and Atlantic basins and North and South America. It also possesses an excellent extensive network of meteorological stations run primarily by the Instituto Nacional Meteorologico and Instituto Costarricense de Electricidad. Opposing regimes exist on either coast separated by a Northwest–Southeast trending Cordillera. The mountains lie orthogonal to the northeast trade winds which blow from the North Atlantic anticyclone towards the the Intertropical Convergence Zone (ITCZ) in the Pacific. During much of the year this wind bring rainfall to the Caribbean coast while the Pacific flank remains dry. The northward migration of the ITCZ in the Eastern equatorial Pacific brings an associated broad band of precipitation during the boreal summer (May–October). This rainy season of the Pacific margin is punctuated by a marked reduction in rain during July and August, the Veranillos de San Juan or Canículas . The duration and intensity of this "mid-summer drought" vary tremendously from year to year. Simultaneously, rainfall along the Caribbean coast increases. Unusually for the tropics, this coast also experiences a winter rainfall maximum in association with cold fronts, emanating from North America.

The most marked change in precipitation regime in association with ENSO occurs during the Veranillos of July–August and the rains of September–October. Along the Pacific flank the mid-summer drought becomes more persistent in warm-phase years and almost non-existent in cold-phase years, in accordance with observed changes in the strength of the northeast trades. The effects are most pronounced in the northwest of the country, possibly reflecting the expansion of the cool water Costa Rica dome in the Gulf of Papagayo. The increased trade winds jet through the cordillera along the line of the Río San Juan, producing strong off-shore winds, upwelling and greater atmospheric stability. Excessive rains along the entire Pacific region of Central America have been linked to the occurrence of tropical cyclones in the Caribbean basin, which reverse the pressure gradient over the isthmus. ENSO is one factor controlling the annual number of North Atlantic tropical cyclones. Simultaneously, precipitation and runoff change in the opposing direction along the Caribbean, although this signal is not so consistent. The winter rainfalls along the Caribbean coast may also have a complex association to ENSO. Two different processes of frontogenesis over North America have been posited, yielding varying strengths and timings of fronts. There may be a greater tendency for stronger, later fronts with warm phases of ENSO. Because of the noted interplay between the Atlantic and Pacific, the effects of ENSO may be modified by sea surface temperatures in the Atlantic, giving rise to lower frequency changes in the association between ENSO, precipitation and streamflow.

Societal Impacts of ENSO in Costa Rica