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

Abstract. The sudden decrease in stratospheric water vapor at around the year 2000 to 2001 is relatively well accepted in spite of the difficulty to quantify the long-term variations. 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 from January 1997 to December 2002 at 400 K 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) where SMR becomes 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 the Tibetan high, shows eastward expansion since the year 2000. Associated changes are measured by three-dimensional bins, each having the dimension of 10° longitude by 10° latitude within the TTL. The probability distribution of LCPs shows an 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 patterns of the decrease over the equatorial western Pacific and the increase over the central Pacific that are almost symmetric with respect to the Equator. The SMR_min shows a 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 SMR_min, are calculated on each bin for both periods prior and posterior to the drop. These values are the spatial projection of [H₂O]_e on an individual bin. The results indicate that the drop is brought about by the decrease in water transport borne by the air parcels that have 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 would 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 oceanic bottom boundaries.