Book 2.indb - US Climate Change Science Program

Abrupt Climate ChangeFigure 4.15. Left: various observed (OBS) quantities with an apparent association with the AMO.Right: Simulated responses of various quantities to AMO-like fluctuations in the Atlantic Ocean froma hybrid coupled model (adapted from Zhang and Delworth, 2006). Dashed green lines are unfilteredvalues, while the red and blue color-shaded values denote low-pass filtered values. Blue shaded regionsindicate values below their long-term mean, while red shading denotes values above their long-termmean. The vertical blue lines denote transitions between warm and cold phases of the AMO. Time incalendar years is along the bottom axis. (a), (e) AMO Index, a measure of SST over the North Atlantic.Positive values denote an unusually warm North Atlantic. (b), (f) Normalized summer rainfall anomaliesover the Sahel (20ºW.–40ºE.,10–20ºN.). (c), (g) Normalized summer rainfall over west-central India(65–80ºE.,15–25ºN.). (d) Number of major Atlantic Hurricanes from the NOAA HURDAT data set.The brown lines denote the vertical shear of the zonal (westerly) wind (multiplied by –1) derived fromthe ERA-40 reanalysis, i.e., the difference in the zonal wind between 850 and 200 hectopascals (hPa)over the south-central part of the main development region (MDR) for tropical storms (10–14ºN.,70–20ºW.). (h) Vertical shear of the simulated zonal wind (multiplied by –1), calculated as in (d).155