摘要：A global Hövmoller diagram of total column water vapor at 30N latitude reveals "ribbons" of locally enhanced humidity that evolve from the Eastern Pacific/Atlantic basins in the winter to the Western Pacific/Atlantic in the summer.  Onset of the East Asian Summer Monsoon (EASM) coincides with the time the Pacific ribbon first exceeds 40 mm, which occurs just before the ribbon makes landfall.  The ribbons are recognizable in the 39-year (1979-2017) climatology, demonstrating that they are potentially useful for monsoon onset prediction.  We propose a simple metric based on the seasonal evolution of the Pacific ribbon, namely that EASM onset occurs roughly a month after the water vapor ribbon crosses the dateline.  Our proposed onset metric is nearly exact following strong El Niño winters, while a wider range of lead-times occur following La Niña.  A set of simplified aquaplanet GCM simulations demonstrates a leading role for ocean heat transport in the longitudinal confinement of the ribbons, while topographic relief of continents plays a secondary role.  Finally, I will speculate as to the mechanism causing the ribbons and their seasonal evolution.
       
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