Hiroaki Hatsushika

Stratospheric drain over Indonesia and dehydration within the tropical tropopause layer diagnosed by air parcel trajectories

[1] The structures of temperature and velocity fields in the tropical tropopause layer (TTL) in boreal winter are investigated using an atmospheric general circulation model (AGCM). The model reveals strong upward motions in the lower part of the TTL over the maritime continent and the western tropical Pacific, corresponding to the ‘‘stratospheric fountain’’ region, and downward motions in the upper part of the TTL over Indonesia, representing the stratospheric drain. In the TTL, strong easterlies prevail, and the cold ascent region tilts eastward. A down-slope flow over the upward-bulging isentropic surface produces the downward p velocity over Indonesia. In addition, reduction of longwave heating over deep convection suppresses the upward motion. The model simulates the observed stratospheric drain signature well, without convective overshootings. A trajectory analysis using the AGCM-simulated three-dimensional wind and temperature is performed to clarify the entry process of air parcels from the tropical troposphere to the stratosphere and to investigate the dehydration process during passage through the TTL. Tropospheric air parcels are advected upward to the bottom of the TTL mainly from the stratospheric fountain region. A pair of anticyclonic circulations in the tropical western Pacific entrains air parcels, which then pass through the equatorial cold region several times during the slow ascent in the TTL. This slow spirally ascending motion brings about low humidity in the stratosphere, despite the local downward motion over Indonesia. In addition, transient disturbances, particularly low-frequency disturbances, produce intermittent upward motions over the fountain region, resulting in effective dehydration of the air. The spiral ascent and transient mechanisms are key factors in the dehydration process in the TTL. The interannual variation in the water vapor

Interannual variations of temperature and vertical motion at the tropical tropopause associated with ENSO

Interannual variations of tropopause temperature and vertical p-velocity around the tropical tropopause are investigated. Observations show the tropopause temperature (altitude) is higher (lower) over the east side of maritime continent and lower (higher) over the equatorial eastern Pacific during El Nino compared with La Nina. As for the QBO, the tropopause temperature (altitude) is higher (lower) in all longitudes in the westerly phase. A multiple regression analysis shows the combined effect of ENSO and QBO can explain 25–50 % of the interannual variation. An AGCM simulation agrees with the observation for the ENSO signal. The simulation supports the existence of the stratospheric drain over the maritime continent in the tropopause region and indicates a zonal shift of the upward (downward) motion below (above) the tropopause with ENSO.

Measurement of halogenated dicarboxylic acids in the Arctic aerosols at polar sunrise

Halogenated dicarboxylic acids, such as bromomalonic (Br-C3), chlorosuccinic (Cl-C4) and bromosuccinic (Br-C4) acids, have been measured, for the first time, in the arctic aerosols during the polar sunrise experiment ALERT2000 (February to May). They were detected in the light spring, but not in the dark winter. Concentration ranges of halogenated diacids in the spring were 0.11–0.68 ng m−3 for Br-C3 diacid, 0.04–0.10 ng m−3 for Cl-C4 diacid and 0.12–0.20 ng m−3 for Br-C4 diacid. Those of Br-C3 diacid increased from late April to early May, whereas Cl-C4 diacid decreased. In contrast, Br-C4 diacid showed maximum concentrations in the middle of the experiment. A strong negative correlation (R = −0.98) was obtained between Br-C3 and Cl-C4 diacids. Concentrations of methanesulfonic acid (MSA) also increased from late April to early May whereas those of Cl− ion decreased. A strong positive correlation was found between Cl-C4 diacid and Cl− ion (R = 0.99) and between Br-C3 diacid and MSA (R = 0.96). These results suggest that Br-C3 diacid is primarily derived from marine biogenic source, whereas Cl-C4 diacid is secondarily formed by heterogeneous reaction involving halogen chemistry on sea salt. Satellite images of sea ice concentrations and backward air mass trajectories suggest that the aerosols containing halogenated diacids were transported over the sampling sites from the Arctic Ocean covered with sea ice.