Forecast Study of the Cold December of 2005 in Japan: Role of Rossby Waves and Tropical Convection

Abstract

Japan experienced unusually heavy snowfall and low temperatures in December 2005 owing to the cold air advected from Siberia. As a result of this strong and sustained cold surge, record-breaking snowfalls occurred repeatedly along the coast of the Sea of Japan. To determine the cause and to examine the accuracy of numerical forecasts of such unusual weather as well as to investigate the impact of initial and boundary conditions on the forecasts, we conducted one-month ensemble forecasts for December 2005 by changing lower boundary conditions, such as sea-surface temperature (SST) and sea-ice coverage (SIC). Forecasts were initiated every day between November 1 and December 1 to investigate their dependency on initial atmospheric conditions. Our results showed that SST and SIC appeared to have little impact on the forecasts of a cold December in Japan. Instead, high sensitivity to initial atmospheric conditions was found. Almost all forecasts initiated before November 16, 2005 failed to reproduce the cold December; however, those initiated after that date successfully predicted it.
By comparing the differences between successful and unsuccessful members for predicting the cold December, we identified the processes producing the cold December of 2005. In the successful forecasts, we found that blocking developed over the North Atlantic in mid-November 2005. When it decayed, the emitted Rossby waves propagated along the subpolar jet across Siberia, inducing a cold surge over Japan. Moreover, the Rossby waves propagating over the Indian sector along the subtropical jet gave rise to intense convection over the Bay of Bengal and the South China Sea, which led to the deepening of a low-pressure trough over Japan. The unusual weather conditions in December have been due to the simultaneous occurrence of these two processes.