The Role of Tropical Forests in theThe Role of Tropical Forests in theRegional and Global Hydroclimate Regional and Global Hydroclimate

Roni AvissarRoni AvissarW.H. Gardner Professor and ChairW.H. Gardner Professor and Chair

Department of Civil & Environmental EngineeringDepartment of Civil & Environmental EngineeringDuke UniversityDuke University

andandR. Ramos da Silva, D. Werth, S. Badya RoyR. Ramos da Silva, D. Werth, S. Badya Roy

Duke UniversityDuke UniversityMay 10, 2005May 10, 2005

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Our overall objective is to understand the impact of land-cover change on the local, regional, and global hydroclimate.

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Low Pressure Low PressureHigh Pressure

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w (m/s) at 3 p.m., 500 m Clouds at 3 p.m. (from GOES)

Impact of surface heat flux heterogeneity

We use the data collected during the joint “Wet Atmospheric Mesoscale Campaign – TRMM validation (WetAMC – TRMM),” in January – February 1999.

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RAMS (400 m) S-POL (4 km)

Average Precipitation Rate (mm/hr)

a & e – February 4, 1999b & f – February 6, 1999c & g – February 14, 1999d & h – February 23, 1999

S-POL

RH test Soil Moisture test Grid Size testControl

36% increase in the daily mean accumulation!

Current deforestation pattern10.76 mm (daily mean accumulation)

No deforestation7.94 mm (daily mean accumulation)

Impact of Deforestation on Precipitation

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Impact of Deforestation on Precipitation

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Soares Filho & Nepstad 2004

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Cerrado

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Amazon Basin Landscape Scenarios

Final

A typical Global Climate Model (GCM) grid

36% increase in the daily mean accumulation!

Current deforestation pattern10.76 mm (daily mean accumulation)

No deforestation7.94 mm (daily mean accumulation)

Current deforestation pattern6.75 mm (daily mean accumulation)

A Global Climate Model (GCM) assumesflat terrain and homogeneous land cover

With the GCM, 15% decreasein the daily mean accumulation!

RAMSRAMS

OBSERVATIONSOBSERVATIONS

Amazonian Deforestation

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Deforestation (%)

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Five ensembles of six, 12-year realizations were performed with the NASA-GISS GCM (with imposed monthly mean sea-surface temperatures):

1. Control – land-cover in early 70’s, before intensive deforestation started (Matthews, 1983);

2. Amazonian deforestation;3. Central African deforestation;4. South-Eastern Asian deforestation;5. All three tropical regions deforested

simultaneously (i.e., “total” deforestation). Control case is compared to Cases (2) – (5). Each grid

point is tested for monthly, ensemble mean, statistically significant difference in precipitation.

Amazonia vs Control

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Africa vs Control

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Total vs Control

Impact of Deforestation on Precipitation

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The Luni-Solar Oscillation (LSO) affects tides, which affect SSTs and, as a result, the amount of water evaporated from the ocean that is then advected into the basin, where it precipitates.

The atmospheric boundary layer is significantly affected by landscape The atmospheric boundary layer is significantly affected by landscape structure. Frequently, landscape heterogeneity created by land use generates structure. Frequently, landscape heterogeneity created by land use generates horizontal pressure gradients strong enough to create and sustain organized horizontal pressure gradients strong enough to create and sustain organized mesoscale circulations, which can trigger thunderstorm activity. In tropical mesoscale circulations, which can trigger thunderstorm activity. In tropical regions, this happens in all seasons;regions, this happens in all seasons; Tropical deforestation significantly affects the regional hydroclimatology of Tropical deforestation significantly affects the regional hydroclimatology of these regions and, to a lesser but still quite significant degree, the mid-latitude these regions and, to a lesser but still quite significant degree, the mid-latitude hydroclimatology (e.g., the US Midwest and the Arabian Peninsula). Among hydroclimatology (e.g., the US Midwest and the Arabian Peninsula). Among tropical regions, the Amazon Basin seems to have the largest impact, probably tropical regions, the Amazon Basin seems to have the largest impact, probably due to the fact that its dry season lasts longer and is dryer than in Central due to the fact that its dry season lasts longer and is dryer than in Central Africa and South-East Asia;Africa and South-East Asia; Simultaneous deforestation of Amazonia, Central Africa and South-East Asia Simultaneous deforestation of Amazonia, Central Africa and South-East Asia has a cumulative impact in a few regions (notably, in the Arabian Peninsula) has a cumulative impact in a few regions (notably, in the Arabian Peninsula) and synergistic impact in others (including in the US by shifting the main and synergistic impact in others (including in the US by shifting the main impact to California);impact to California); Convective activity is one of the key factors that triggers teleconnections Convective activity is one of the key factors that triggers teleconnections between tropical regions and higher latitudes but our GCMs do not account for between tropical regions and higher latitudes but our GCMs do not account for (mesoscale) landscape-generated convection. Clearly, this limits our capability (mesoscale) landscape-generated convection. Clearly, this limits our capability to estimate the real magnitude of teleconnections between tropical rainforests to estimate the real magnitude of teleconnections between tropical rainforests and the rest of the world;and the rest of the world; Inter-decadal variability is associated with the LSO, which is a deterministic, Inter-decadal variability is associated with the LSO, which is a deterministic, very predictable signal. Thus, it improves the predictability of the Amazonian very predictable signal. Thus, it improves the predictability of the Amazonian hydroclimate system and it should be included in new earth system models.hydroclimate system and it should be included in new earth system models.

ConclusionsConclusions