Trenberth, K. E. and Guillemot, C. J., 1996: Physical processes
involved in the 1988 drought and 1993 floods in North America.
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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