The effect of Atlantic and Pacific sea surface temperatures on the mid-summer drought of Costa Rica

P Waylen,M Quesada

doi:10.18172/cig.1123

Abstract

The mid-summer drought of Central America is an important feature of the regional precipitation regime within the Pacific drainage ranging from Mexico to Panama. The drought is less severe when the Atlantic is warmer. Cooler temperatures in the tropical Atlantic signify a strengthening of the North Atlantic anticyclone and the alisos. When combined with warmer than usual temperatures (lower atmospheric pressures) in the eastern equatorial pacific, the pressure gradient across the isthmous increases and the orographic effects of the Cordillera are emphasized. When the Atlantic is warmer, the resultant reduction in the strength of the alisos reduces rainfall over most of the country.

Highlights

The mid-summer drought of Central America, known locally as the Veranillos de SanJuan or Canicula, is an important feature of the regional precipitation regime within the Pacific drainage ranging from Mexico to Panama. Magaña et al (1999) indicate that this abatement of summer (July-August) precipitation is unique in tropical climatology
This paper investigates the nature and geographic extent of combined roles of sea surface temperature (SST) anomalies, in both the equatorial Pacific and North Atlantic Oceans, on the historic rainfall records of July and August at stations throughout Costa Rica
The two comparisons involving opposing conditions in both oceans should yield proportions which are the cross-products of the two “oceanic marginals” under the null hypothesis of independence of seasonal rainfall from the interactive effects of SST anomalies in the two oceans. Such comparisons are directly possible because the z-statistic used in the MannWhitney test is independent of sample size

Summary

Introduction

The mid-summer drought of Central America, known locally as the Veranillos de San. Juan or Canicula, is an important feature of the regional precipitation regime within the Pacific drainage ranging from Mexico to Panama. Magaña et al (1999) indicate that this abatement of summer (July-August) precipitation is unique in tropical climatology. Precipitation and flood risks during this period in Costa Rica have been shown to be associated with the El Niño-Southern Oscillation (ENSO) phenomenon (Waylen et al, 1996; Waylen and Laporte, 1999), that association is not as strong as has been found in many similar tropical regions (Ropelewski and Halpert, 1989). Enfield and Alfaro (1999) sought links between atmosphere/ocean conditions in the two ocean basins, suggesting that this may be a contributing factor to the unclear association between ENSO and precipitation in the region (Ropelewski and Halpert, 1989) They conclude that the greatest effect of SST can be identified in the dates of onset and conclusion of the rainy season in southern Central. For the sake of simplicity and to maximize the size of the available data sets, each set is subdivided into four categories dependent upon the SST anomalies (positive or negative) in the two oceans in the corresponding season

Analysis

Results

5.Conclusions