Seasonal Dependency of Bi-Decadal Precipitation Variability

Seasonal dependency of precipitation variability on a bi-decadal timescale and associated atmospheric circulation anomalies over the Pacific Ocean are investigated using NCEP/NCAR reanalysis data for a period from 1949-2001. We estimated bi-decadal variability of precipitation by calculating regression coefficients between the bidecadal filtered (pass-band of 10-30-year periods) reanalysis data and a representative time series of the Bi-Decadal Oscillation (BDO). As the representative BDO time series, we use wintertime North Pacific Index (NPI), which is a sea level pressure time series, averaged over the Aleutian Lows. The regression map reveals a precipitation band with positive regressions across the Tropical Pacific (EQ-20°N) from autumn to spring. The tropical precipitation band is consistent with gauge data and satellite estimates. In winter, more energetic precipitation changes also occur in the mid-latitude North Pacific with the positive regressions over the northern (50°-70°N) North Pacific and negative regressions in the central North Pacific (30°-50°N) as shown by Minobe and Nakanowari (2002, GRL) in addition to the equatorial precipitation change. In order to know the mechanisms of the bi-decadal precipitation changes, we perform a humidity-flux analysis using the reanalysis data. On the bi-decadal time scale, the precipitation anomalies can be approximately explained by the sum of horizontal humidity flux convergence and evaporation anomalies. For the tropical band, the humidity flux convergence has similar amplitudes to the evaporation in autumn and spring seasons, but dominate in winter season. In mid-latitudes, the wintertime precipitation anomalies are well explained by the humidity convergence. The humidity-flux convergences in mid-latitudes are due to anomalous geostrophic winds over climatological humidity gradients, and wind anomalies are essentially non-divergent. On the other hand, humidity-flux convergences in the tropics are resulted from surface wind convergence due to ageostrophic wind components. The difference of the convergence mechanism between mid-latitude and tropics results in that relatively small sea-level pressure anomalies in the tropics yield large humidity-flux convergences. The tropical pressure fluctuations are much smaller than those in mid-latitudes, but cannot be ignored for the bi-decadal precipitation changes.