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Talking Points 9/6/2005

Talking Points 9/6/2005. Background.

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Talking Points 9/6/2005

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  1. Talking Points9/6/2005

  2. Background • In our continuing efforts to make sound water management decisions, the scientists and engineers at SFWMD have been investigating short-term and long-term behavior of climate in south Florida. We are well aware of the impact of El Nino and La Nina conditions in the Pacific Ocean on South Florida’s climate, particularly during the dry season months of November through May. In a landmark study, published in 2001, in the journal of Geophysical Research Letters, Paul Trimble, and his collaborators at National Oceanic and Atmospheric Administration (NOAA), and University of Miami, discovered a significant correlation between the climate of the continental US and a multi-decadal phenomenon associated with a natural, cyclic variation of North Atlantic Sea Surface Temperature. This climatic phenomenon is now commonly known as Atlantic Mutidecadal Oscillation, or AMO • AMO is a slow, but regular warming and cooling of the North Atlantic Ocean. This cycle of warming and cooling lasts for decades and hence the term Multidecadal in the AMO. The term “Oscillation” comes from the fact that climate swings one way and then the other. The cycles appear to last 60-80 years. • Using rainfall and river flow records, Paul and his colleagues showed that, during the “warm” periods of AMO, continental US receives less than normal rainfall and consequently less inflow into major reservoirs. However, effect of AMO in south Florida appears to be opposite in that, during the “warm cycles” the climate in south Florida appears to be much wetter with less frequent droughts. A good example of that was the period from 30s to 60s.

  3. Background (cont.) • In the 150-year record of measurements available to us, you can see about two cycles of warm and cold periods. Using proxy records, such as widths of old tree rings, scientists have reconstructed a longer period of climate variability and have confirmed presence of these AMO like cycles in a 450 year period. • Since mid 1990s, we appear to have returned to a “warmer cycle” of AMO, and we believe that, if this were the case, we can expect a multi-decade period of wetter climates in the region. • Because we have substantial evidence to this link our climate to AMO, and also the El Nino and La Nina phenomena, we have taken initial steps to incorporate this information into the operation of the water management system in south Florida. In particular, we have incorporated the seasonal and multi-seasonal climate outlooks based on AMO and ENSO into the operation of Lake Okeechobee. To our knowledge, we are the only water management agency in the country and probably the world, to incorporate these climate outlooks into rules of operations.

  4. Background (cont.) • Moving on to the situation this year, we appear to have a rare, but likely related, combination of several climatic and hydrologic factors: • We appear to be in the “warm cycle” of AMO - means multiple decades of possible wetter climate in south Florida. In particular, focusing on Lake Okeechobee, the expected inflow into Lake Okeechobee can be double that of a “cold” cycle • A very wet 12-month period (August 2004 to July 2005). National Climatic Data center records show that, at the beginning of this wet season, we had the wettest 12-month period in a 110-year record. Clearly this wet antecedent condition was a concern coming into this season • July Lake Okeechobee level was the third highest in a 74-year record. The concern this year was high lake levels that early in the season. In 2004, prior to the occurrence of storms in Florida, the lake was about 4 feet lower providing a larger storage to absorb inflows due to storms. This year that storage was not available early this wet season • Water Levels in Water Conservation Areas were also higher early in the wet season. In particular, the water level in 3A was the highest since they were built. Again, this type of antecedent condition does not provide much storage to absorb excess water during wet periods • Unusually high hurricane activity. NWS predicted a busy hurricane season, with up to 21 tropical storms including 9 to 11 hurricanes. Even the recent update by Prof. Gray shows an active period for the remainder of the season (8 storms, 6 hurricanes, 3 major). Other scientists have shown evidence that during AMO warm periods, there is a tendency to have increased number of hurricanes, particularly major ones (cat 3 or higher). Prof Gray, at Colorado State Universtiy predicted this for the current “warm” cycle

  5. Climate Variability • Climatic zones are defined by regions with similar means and variability of meteorological variables. • Over long periods (several decades) the climate of a particular region may change gradually or abruptly due to natural or anthropogenic reasons. • In recent years it has become recognized that South Florida’s climate is very much affected by natural periodic multidecadal variations in the North Atlantic Sea Surface temperature known as the Atlantic Multidecadal Oscillation (AMO). • Significant ties to large sections of the North America and Europe have also been found associated with the AMO

  6. Characteristics Of AMO • A particular phase of the AMO normally last for several decades and changes phase rather abruptly (1-5 years) • During the warm phase of the AMO south Florida’s climate becomes much wetter with less frequent droughts while during the cool phase is drier with more frequent droughts. • For example, Lake Okeechobee surface water inflow from its large tributary is nearly twice as large on average during the warm phase of the AMO as compared to the cool phase.

  7. Benefits and Limitations of the Application Climate Outlooks • Understanding the association of the south Florida climate to the AMO/ENSO variability has allowed the SFWMD to adopt operational rules for Lake Okeechobee that incorporate climate outlooks. • A unique combination of antecedent hydrologic conditions and continued high level of tropical activity has been the reason for increased concern for the natural ecological systems of south Florida and for flood protection for the remainder of this wet season. • The return to a period of increased tropical activity (number and strength of tropical storms) that began in 1995 to similar levels of the 1930s-1950’ was predicted several years in advance by Dr. William Gray.

  8. Maximum and Minimums number of Hurricanes

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