Chapter 1 Minimum Flows and Levels - Southwest Florida Water ...

2.4.2.1 Multidecadal Periods of High and Low Flows
Citing Enfield et al. (2001), Basso and Schultz (2003) noted that the Atlantic
Multidecadal Oscillation (AMO) offered an apparent explanation for observed
rainfall deficits throughout central Florida. Although the District and others
(Hammett 1990, Hickey 1998) have discussed the lack of tropical storm activity
and deficit rainfall in recent decades, the mechanism or mechanisms that would
account for such differences were unknown. Based on an emerging body of
research, climatologists now believe that multidecadal periods of warming and
cooling of the North Atlantic Ocean's surface waters ultimately affect precipitation
patterns across much of the United States. What is particularly interesting is that
unlike most of the continental United States, there is for most of Florida a positive
(rather than negative) correlation between rainfall and prolonged periods of North
Atlantic Ocean sea surface warming (Enfield et al. 2001). While periods of
warmer ocean temperature generally resulted in less rainfall over most of the
United States, there are some areas, including peninsular Florida, where rainfall
increased.
Since river flows are largely rainfall dependent, variation in rainfall should result
in variations in river flows. To be consistent with Enfield et al.'s (2001)
conclusions regarding the AMO and rainfall and with Basso and Schultz (2003),
who examined long-term variations in rainfall in west-central Florida, Kelly (2004)
reasoned that in Florida, flows would be highest at streamflow gage sites when
sea surface temperatures in the North Atlantic are in a warm period (i.e.,
positively correlated). At the same time, most of the continental United States
would be expected to be in a period of lower flows. Conversely, the majority of
continental gage sites would be expected to exhibit higher flows during AMO cool
periods and much of peninsular Florida would be expected to be in a period of
low flows.
Based on these hypotheses, Kelly (2004) examined flow records for multidecadal
periods corresponding to warming and cooling phases of the AMO for numerous
gage sites within the District, the state, and the southeastern United States to
discern if increases and decreases in river flows were consistent with AMO
phases. He concluded that flow decreases and increases in the northern part of
the state and flow increases and decreases in peninsular Florida are consistent
with the AMO and the reported relationship with rainfall. When rivers in
peninsular Florida were in a multidecadal period of higher flows (1940 to 1969),
rivers in the north to northwestern part of the state were in a low-flow period.
Conversely rivers in peninsular Florida exhibited generally lower flows (1970 to
1999) when rivers in the northern portion of the state exhibited higher flows.
Examination of streams with a bi-modal flow pattern offered particularly strong
supporting evidence for a distinct difference in flows between northern and
southern rivers, since differences between pre- and post 1970 flows that
occurred during the spring were similar to differences noted for northern river
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