A Lagrangian description on the troposphere-to-stratosphere transport changes associated with the stratospheric water drop around the year 2000.

Stratospheric water vapor is known to have decreased suddenly at around the year 2000 to 2001 after a prolonged increase through the 1980s and 1990s. This stepwise change is studied by examining the entry value of water to the stratosphere ([H₂O]_e) and some Lagrangian diagnostics of dehydration taking place in the Tropical Tropopause Layer (TTL). The analysis is made using the backward kinematic trajectories initialized every ~ 10 days since January 1997 till December 2002 on 400K potential temperature surface in the tropics. The [H₂O]_e is estimated by the ensemble mean value of the water saturation mixing ratio (SMR) at the Lagrangian cold point (LCP) 10 where SMR takes minimum (SMR_min) in the TTL before reaching the 400 K surface. The drop in [H₂O]_e is identified to have occurred in September 2000. The horizontal projection of September trajectories, tightly trapped by anticyclonic circulation around Tibetan high, shows eastward expansion since the year 2000. Associated changes are measured by three-dimensional bins, each having the dimension of 10° longitude 15 by 10° latitude within the TTL. The probability distribution of LCPs shows appreciable change exhibiting a composite pattern of two components: (i) the dipole structure consisting of the decrease over the Bay of Bengal and Malay Peninsula and the increase over the northern subtropical western Pacific and (ii) the decrease over the equatorial western Pacific and the increase over the central Pacific almost symmetric with 20 respect to the equator. The SMRmin shows general decrease in the tropics with some enhancement in the central Pacific. The expectation values, defined by the multiple of the probability of LCP events and the ensemble mean values of SMRmin, are calculated on each bin for both periods prior and posterior to the drop. These values are the spatial projection of [H₂O]_e on individual bin. The results indicate that the drop is 25 brought about by the decrease of water transport borne by the air parcels having experienced the LCP over the Bay of Bengal and the western tropical Pacific. The former is related to the eastward expansion of the anticyclonic circulation around the weakened Tibetan high, while the latter will be linked to the eastward expansion of western tropical warm water to the central Pacific. This oceanic surface forcing may be responsible also for the modulation of dehydration efficiency in the successive northern winter. The drop in September 2000 and the sustained low values thereafter of [H₂O]_e are thus interpreted as being driven by the changes in thermal forcing from the continental and 5 oceanic bottom boundaries. [ABSTRACT FROM AUTHOR]