1998 Year of the Ocean Impacts of Global Climate Change 



cold episode; El Nino will be used to specify a warm episode, and La Nina will be used to 

 specify a cold episode. 



The shift in the distribution of winds, surface temperatures, and tropical convection leads 

 to changes in the atmospheric circulation with the possibility of regional droughts, floods, and 

 temperature changes in areas well beyond the tropical Pacific. The typical dependence for El 

 Nino is shown in Figure 3 for Northern Hemisphere winter and summer. In northern middle 

 latitudes, the strongest relationship occurs in the Northern Hemisphere winter when the 

 atmospheric circulation is strongest. The figure shows that El Nifio tends to cause warmer than 

 normal winter temperatures in the U.S. Pacific northwest and higher than normal winter 

 precipitation along the U.S. Gulf Coast. La Nina generally impacts the same areas as El Nifio but 

 with opposite effects. The ENSO cycle is rooted in the instability of the coupled atmosphere- 

 ocean system. The instability produces repetition of an irregular, quasi-periodic cycle which 

 varies between three and seven years. 



The ENSO research effort includes studies on the evolution of sea surface temperatures 

 (SST) as part of the oceanic response to atmospheric forcing and meteorological studies on 

 regional and large-scale air-sea interactions. Thus, monitoring and prediction of sea surface 

 temperatures is an important part of monitoring and predicting ENSO. SST anomalies for 1950 

 to present are shown for a region with strong ENSO variability (10°N-10°S, 150°W-90°W) in 

 Figure 4. The anomalous SSTs shown here are computed as the difference between measured 

 monthly SSTs and the normal expected monthly SSTs for the period 1950-79. The figure shows 

 positive and negative SST anomalies. Although the distinction between normal. El Nino, and La 

 Nina is not rigorously defined, SSTs which persist for at least six months above roughly 0.75°C 

 can be considered to indicate El Nino, while those that persist below -0.75°C can be considered 

 to indicate La Nina. The figure also shows an overall warming of the tropical ocean by 0.5°C in 

 the decades of the 1980s and 90s with stronger El Niiio episodes occurring in the latter part of the 

 record. The strongest complete El Nino occurred in 1982-83. However, if current predictions are 

 validated, the present El Nino episode may become even larger and persist into 1998, which has 

 been designated the Year of the Ocean. 



ENSO (El Nino and La Nina) episodes cause changes in the normal global atmospheric 

 circulation. The changes lead to changes in precipitation and temperature which strongly depend 

 on season and location as shown in Figure 3. Areas that are strongly impacted during Northern 

 Hemisphere fall and winter are the south of Africa, Australia, South America, and the U.S. The 

 occurrence of El Nino or La Nina does not guarantee a specific precipitation or temperature 

 response, but only increases the likelihood that a deviation from normal will occur. 



Although crop yields depend on many factors, rainfall during part of the growth cycle is 

 often critical. Despite the uncertainties, links between both El Nino and La Niiia and crop yields 

 have been established in a number of regions. As an example, winter crop yields in Texas, 

 Oklahoma, Kansas, and Colorado show that the presence of El Nino, with its likelihood of 

 increased rainfall, can increase yields by 1 5 percent, while La Nina, with its likelihood of 

 decreased rainfall, can decrease yields by 1 5 percent. 



G-9 



