Tuesday, November 21, 2017

Florida's Fragile Oasis

Monday, October 15, 2012

by Brian Kahn — NOAA Climate Program Office / International Research Institute for Climate and Society

This is an adapted version of a feature story that appeared on NOAA Climate.Gov.

Northern Desert Belt Map

Northern Desert Belt Map

Surrounded by ocean, Florida is an oasis in the Northern Hemisphere “desert belt.” Located between the tropics and the mid-latitudes, the sub-tropical desert belt occurs w here, on average, air is sinking from high altitudes back toward the surface. As air sinks, it tends to warm, and moisture evaporates. (Credit: NOAA)

Surrounded by ocean moisture, Florida is an oasis sitting right in the middle of the desert belt, which traverses the subtropical latitudes north and south of the equator. But even oases sometimes go dry.

In the 1980s and 1990s, a dramatic population boom and a reliance solely on groundwater tested the limits of this oasis. Learning from those lessons, Tampa Bay Water, a regional water supply authority, has spent the past 20 years diversifying their water supply to include surface and desalinated seawater. Without further innovative thinking and planning, the limits of the oasis might again be tested by further population growth and climate change.

Vasu Misra, a climate researcher from Florida State University, is working with a group of climate researchers and water managers—including Alison Adams from Tampa Bay Water—to unravel some of the climate-related challenges facing them. Funded by the NOAA Climate Program Office’s Modeling, Analysis, Predictions, and Projections (MAPP) program, Misra is investigating local-scale climate impacts, including the variability and predictability of tropical storms in the Western Atlantic that can deliver a large percentage of precipitation to areas of Florida.

In addition to this research, Misra is also helping water managers by downscaling climate models. This effort is funded in part by the NOAA Climate Program Office’s Sectoral Applications Research Program. “For people who make decisions on regional or local scales, the global climate models have insufficient resolution to help them with [these] decisions,” Misra said. Downscaling is climate modelers’ shorthand for improving that resolution by using coarse global climate model outputs as inputs into more detailed regionalscale models. Misra equates it with looking at a map at the state level and then zooming in to the street level view of a particular city.

Climate Model Resolutions

Climate Model Resolutions

The grid lines on these maps show the scale at which a global model (left, about 100 miles) or a regional model (right, about 6 miles) is capable of simulating climate; within a grid box, the model calculates just one value per climate variable, such as surface temperature or precipitation change. (Credit: NOAA)
Most global models predict wet winters in the southeast U.S. However, according to Misra, models are split over the summer and annual average, with roughly half predicting an increase in precipitation during the summer months and the yearly average, and half predicting a decrease.

Summer is the rainy season in Tampa with nearly 28 inches of rain falling from June through September. That’s roughly 60% of the annual average, so it’s a crucial time for people like Adams to capture water and prepare for the dry season.

Misra’s research has further broken down how that summer rain currently falls. Nearly 20-40% of the rainfall from June to August is due to late afternoon thunderstorms, he says. The rest of the rain comes in random showers, storms, and the occasional hurricane, which are much more difficult to capture in future climate projections. In other words, honing in on the predictability of those afternoon storms can provide a key benefit to Tampa Bay Water.

Florida Precipitation Model

Florida Precipitation Model

Example of a regional model’s prediction of the percent decrease in June-August rainfall in Florida by the middle of this century compared to the middle of the past century, based on a greenhouse gas emission scenario know n to climate modelers as the “A2” storyline. The A2 path represents neither the largest nor the smallest increase in greenhouse gas emissions that are possible in the future. (Credit: NOAA)
The downscaled models Misra is working with project a local trend in rain similar to what most models are projecting on a global scale: though rain will fall less frequently, when it does, it will be in stronger bursts.

“Understanding the change in summer rainfall patterns, with significant amounts of rain coming at infrequent intervals,” Adams said, “that’s a really different surface water management philosophy than a little bit of rain every day. We have some offstream storage, but we don’t have enough if there’s a shift in how summer rainfall arrives.”

Misra’s work is still experimental, but it already presents Tampa Bay Water with options to consider, including increasing storage. “It becomes a management objective to take advantage of these high-capacity events. You want the opportunity to have the reservoir full at the end of the rainy season,” Adams said.

Read more about lessons learned about applying both present and future climate forecasts to water management and planning in Florida’s Fragile Oasis in NOAA’s ClimateWatch Magazine.



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