Variability and trends of the NHSM precipitation and circulation. Time series of (A) NH summer (MJJAS) mean monsoon precipitation rate (millimeters
per day) averaged over the entire NH monsoon rainfall domain, (B) vertical shear of zonal wind (U850–U200) averaged over (0–20°N, 120°W–120°E), which
significantly correlates with the NHSM’s precipitation index (r = 0.85). (C) MJJAS 925-hPa zonal wind (U925) averaged over (0–20°N, 120°W–140°E). (D) The
200-hPa zonal wind (U200) averaged over (0–20°N, 120°W–120°E). The correlation coefficient between NHSM vertical wind shear (VWS) index and U925
index (U200 index) is 0.75 (−0.79). The dashed lines in each panel denote the linear trends, and their statistical significances are shown in Table 1.
the EOF PC1 of the ENSO-year mean precipitation and Niño-3.4 index (NHSM precipitation index; Fig. 2A) is 0.86 (–0