Fig. 8: Large-scale environment changes from the mid-RU to mid-RD periods.

Shadings denote differences in (a) SST (K), (b) 850 hPa relative vorticity (10⁻⁵ s⁻¹), (c) vertical wind shear (m s⁻¹), (d) sign-reversed 500 hPa geopotential height (m), (e) sign-reversed 500-hPa pressure velocity (10⁻² Pa s⁻¹), (f) sign-reversed shear vorticity (10⁻⁶ s⁻¹), and (g) 600 hPa relative humidity (%). In (b), (d), and (f), red and blue shadings signify cyclonic and anticyclonic circulation differences, respectively, in the mid-RD period relative to the mid-RU period. In (e), red and blue colors indicate enhanced and suppressed convection, respectively, in the mid-RD period compared to the mid-RU period. Vectors show (b) 850 hPa and (c) 200 hPa wind differences between the two periods. Only statistically significant differences, according to a two-tailed Student’s t-test at the 95% confidence level, are shaded. Black contours in (a) and (e–g) indicate the mid-RU climatology of corresponding variables. Black and red contours are the zero isolines in the mid-RU and mid-RD periods, respectively, for (b) 850-hPa zonal wind (m s⁻¹) which roughly represent MT, (c) 200 hPa zonal wind (m s⁻¹) to indicate TUTT, and (d) 500-hPa eddy geopotential height (m), defined as deviation from the regional average of 0–40°N and 180°W–180°E⁵⁸, to delineate the WNPSH boundary. The blue box in each panel defines the Philippine Sea (15–27.5°N, 105–170°E), where WNP TC genesis significantly decreased from the mid-RU to mid-RD periods. Values over land are masked for the purpose of visualization. Only the TC peak season from July to September is analyzed.