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¡¡¡¡Directions:

¡¡¡¡In the following article, some sentences have been removed. For Questions 41¡«45, choose the most suitable one from the list A¡«G to fit into each of the numbered blanks. There are two extra choices which do not fit in any of the gaps. Mark your answers on ANSWER SHEET 1. (10 points)

¡¡¡¡Just under a year ago, a sharp drop in equatorial Pacific sea-surface temperature indicated the end of the 1997¡«1998 El Nino. Called by someone ¡°the climate event of the century¡±, it was by several measures the strongest on record.

¡¡¡¡41) . This is more than simply an academic question: the 1997¡«1998 El Nino severely disrupted global weather patterns and Pacific marine ecosystems, and by one estimate caused $33 billion in damage and cost 23 000 lives worldwide.

¡¡¡¡42) . Clearly we have much to learn from this experience.

¡¡¡¡43) . Now El Nino more generally refers to a warming of the tropical Pacific basin that occurs roughly every three to seven years in association with a weakening of the trade winds. The opposite side of El Nino, La Nina, is characterized by strongerª²thanª²normal trade winds and unusually cold sea-surface temperatures in the tropical Pacific. Both El Nino and La Nina are accompanied by swings in atmospheric pressure between the eastern and western Pacific. These swings are known as the Southern Oscillation. These Phenomena are collectively referred to as ENSO or El Nino/Southern Oscillation.

¡¡¡¡The general mechanisms underlying the ENSO involve large-scale ocean-atmosphere interactions and equatorial ocean dynamics. But each El Nino and La Nina is unique in the combination of its strength, duration and pattern of development. Irregularity in the ENSO cycle can be seen both in the record dating back to the middle of the 19th century, and in other supporting data, such as lake sediments, coral growth rings and tree rings, going back hundreds or even thousands of years. 44) .

¡¡¡¡Nonetheless, the 1997¡«1998 El Nino was an unusual one. It developed so rapidly that every month between June and December 1997 set a new monthly record high for seaª²surface temperatures in the eastern equatorial Pacific. Anomalies (that is, deviations from normal) in December 1997 were the highest ever recorded along the Equator in the eastern Pacific. Moreover, before 1997¡«1998, the previous recordª²setting El Nino occurred in 1982¡«1983. 45) .

¡¡¡¡Several factors may have contributed to the strength of the 1997¡«1998 El Nino. One is chaos, which some theories invoke to account for the irregularity of the ENSO cycle. Nonlinear resonances involving ENSO and the seasonal cycle have received special attention, but other chaotic interactions may affect ENSO as well. In 1997¡«1998, events possibly acted together to produce an extraordinarily strong El Nino simply due to the underlying tendency towards chaos in the climate system.

¡¡¡¡£ÛA£Ý So in principle, it should not be surprising that an unusually strong El Nino occurs every so often.

¡¡¡¡£ÛB£Ý Identifying why it was so strong challenges our understanding of the physical mechanisms responsible for El Nino.

¡¡¡¡£ÛC£Ý From that perspective, the strength of the 1997¡«1998 El Nino may be but one manifestation of a linkage between interannual and decadal climate variations in the Pacific.

¡¡¡¡£ÛD£Ý These two ¡°super El Ninos¡± were separated by only 15 years, compared with a typical 30¡«40 year gap between such events earlier in the 20th century.

¡¡¡¡£ÛE£Ý El Nino, Spanish for ¡°the child¡± (and specifically the Christ child), is the name Peruvian fisherman gave to coastal sea-temperature warmings that first appeared around Christmas time.

¡¡¡¡£ÛF£Ý There were warnings of a coming El Nino before it occurred. But although many computer forecast models predicted that 1997 would be warm in the tropical Pacific up to three season in advance, none predicted the rapid development or ultimate intensity of the event before it began.

¡¡¡¡£ÛG£Ý In association with the Pacific Decadal Oscillation, seaª²surface temperatures have generally been higher in the tropical Pacific from the Mid-1970s. Since then, there have been more El Ninos than La Ninas.