Physical processes involved in the 1988 drought and 1993 floods in North America

Trenberth, K. E. and Guillemot, C. J., 1996

J. Climate, 9 1288-1298.


An analysis of the spring-summer 1988 drought and 1993 floods over North America reveals a reversal in sign of anomalies in several fields. Large sea surface temperature anomalies of opposite sign existed in the tropical Pacific with strong La Ni\~na conditions in 1988 and a mature El Ni\~no in 1993. The distribution of tropical convection in the convergence zones and associated latent heating of the atmosphere were correspondingly altered implying a large-scale switch in the anomalous tropical heating and forcing of extratropical quasistationary waves in the atmosphere, influencing the subtropical jet stream over the North Pacific and across North America. In 1988 the jet stream and the closely related storm track of high frequency disturbances in the upper troposphere were displaced into Canada, well north of the normal location. In 1993 a broader jet stream and the storm track were displaced well south of normal to a more spring-like location across the U.S. An analysis of the moisture budgets reveals a stronger river of moisture flowing across the Gulf of Mexico into the eastern U.S. in 1993. Also, in the lower atmosphere, the storm track in 1993 was more active and its lower latitude allowed the cyclonic disturbances to tap into the moisture source, transport moisture into the upper Mississippi River basin and precipitate it out. It is deduced that local evaporation enhanced the precipitation and helped perpetuate and prolong the conditions. In contrast, in 1988, disturbances were weaker and displaced far enough north to avoid most of the moisture source, and the drought was perpetuated by the dry conditions. Consequently these effects should be viewed as feedbacks which amplify and prolong the response, while the tropical Pacific sea surface temperatures are a more fundamental (but not sole) cause of the patterns.

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