On-Line Resource 2
Title: Projected 21st century trends in hydroclimatology of the Tahoe basin
Journal: Climatic Change
Authors: Robert Coats1, Mariza Costa-Cabral, John Riverson, John Reuter, Goloka Sahoo, Geoffrey Schladow and Brent Wolfe
(1) University of California Tahoe Environmental Researh Center, Davis, CA ()
Modeling wind speeds
The GFDL model produced daily wind data for the A2 and B1 scenarios for 1961-2100. The 1961-1998 period used historic GHC wind speeds, and the 1999-2100 period used the wind speeds from the two scenarios. The downscaling of wind speeds used the high-resolution regional climate reanalysis (CARD10) of the meteorology over Califorina, Nevada and surrounding regions for 1948-2005, on a 10-km grid (Kanamitsue and Kanamaru, 2007; Kanamaru and Kanamitsu, 2007; Dettinger, this issue). For the historic period we had two downscaled data sets, labeled “A2” and “B1”. These were used separately in correcting for bias in the 21st century modeled winds, although the two data sets are not significantly different (by a paired t-test) at either daily or annual average time scales.
Empirical and modeling studies on the effects of climate change on wind can produce seemingly conflicting results. Gastineau and Soden (2011), using satellite microwave measurements and a suite of GCMs, found downward trends (1987-2008) in frequency of the strongest wind events over the tropical oceans, a finding consistent with the expected decline in atmospheric circulation as the climate warms. Young et al. (2011), however, found upward trends (1991-2008) in the 90th and 99th percentile wind speed over oceans world-wide. And in the mid-latitudes of the northern hemisphere, surface wind speeds have declined 5-15 percent (1979-2008), a change that is in part attributable to increasing surface roughness that has accompanied land use changes (Vautard et al., 2010). The wind record at the South Lake Tahoe Airport is too short to be useful for trend analysis.
Figure 5 shows the trends in average annual wind for the GFDL A2 and B1 scenarios. The downward trend for the A2 is significant (P < .0003, two-tailed, by the Mann-Kendall trend test), but the trend in average annual wind speed for the B1 case is not significant.
Analysis of the sensitivity of deep mixing of the Lake to wind (Sahoo et al., 2012, this issue) found that if the 21st century winds turn out to be 10-15 percent higher than the GFDL-modeled winds, then the lake will mix deeply and often enough to prevent bottom anoxia. It is important, however, to consider both the trends in average and extreme winds, since both may play a role in the mixing of the Lake. The seasonal distribution is also critical, since summer winds may deepen the warm epilimnion and thus increase stability, and winter winds are responsible for deep mixing and breaking down the thermal stratification. Figure 6 shows the significant trends in average monthly winds, and Figure 7 shows the trends in maximum monthly wind. Note that winds tend to increase during the summer, and decrease in fall and winter. These trends will both contribute to increased thermal stability of the lake (Sahoo et al., 2010).
References
Gastineau G, Soden BJ (2011) Evidence for a weakening of tropical surface wind extremes in response to atmospheric warming. Geophysical Research Letters 38 (9) doi:10.1029/2011gl047138
Kanamitsu M, Kanamaru H (2007) 57-Year California Reanalysis Downscaling at 10km (CaRD10) Part I. System Detail and Validation with Observations. J. Climate 20:5527-5552.
Kanamaru H, Kanamitsu M (2007) 57-Year California Reanalysis Downscaling at 10km (CaRD10) Part II. Comparison with North American Regional Reanalysis. J. Climate 20:5553-5571
Sahoo GB, Schladow SG, Reuter JE (2010) Effect of sediment and nutrient loading on Lake Tahoe optical conditions and restoration opportunities using a newly developed lake clarity model. Water Resour. Res. 46
Vautard R, Cattiaux, J, Yiou, P, Thepaut, J-N, Ciais P (2010) Northern hemisphere atmospheric stilling partly attributed to an increase in surface roughness. Nature Geoscience Letters doi: 10.1038/NGEO979
Wood AW, Maurer EP, Kumar A, Lettenmaier DP (2002) Long-range experimental hydrologic forecasting for the eastern United States. J. Geophys. Res. 107:4429 doi: 4410.1029/2001JD000659
Young IR, Zieger S, Babanin AV (2011) Global Trends in Wind Speed and Wave Height. Science 332:451-455
Fig, 1a-1b 21st Century average annual wind speed, bias-corrected for a) A2 and b) B1 Scenarios. Only the A2 trend is significant (P < 0.003)
Fig. 2 Trends in average monthly wind speed.
Fig. 3 Trends in the maximum of daily average wind speed, by month, for the GFDL A2 and B1 scenarios