In this study, the impacts of warm-to-cold El Niño-Southern Oscillation (ENSO) phase transitions on the summer rainfall over the northeastern Indian monsoon (NEIM) region were investigated. Extremely rapid warm-to-cold ENSO transitions were identified in 1983, 1988, and 1998. In these rapid ENSO transition years, during August, the NEIM rainfall tended to be higher than that in other years, leading to large river water discharge into Bangladesh in August and September through the Ganges and Brahmaputra. In summer (June, July, and August), the western North Pacific monsoon (WNPM) was significantly suppressed in association with the east-west sea surface temperature (SST) anomaly contrast between an above-normal SST over the Indian Ocean and a negative SST anomaly that developed over the central equatorial Pacific. The suppression of WNPM was highly likely to be a key factor linking rapid ENSO transition to the heavy NEIM rainfall in August. WNPM suppression caused a prominent lower tropospheric anticyclonic anomaly, which extended westward over the Indian Ocean and Indian subcontinent. Associated with this circulation anomaly, a lower tropospheric westerly anomaly developed over the Bengal Plain in August, which activated the NEIM rainfall. Thus, especially in August, WNPM suppression significantly correlated with heavy rainfall over a large area over the northeastern Indian subcontinent. In all three rapid warm-to-cold ENSO transition years, WNPM was extremely suppressed in summer. However, in the August of 1983, owing to atmospheric variations at intraseasonal time scales, WNPM suppression was damped, and both the lower tropospheric circulation anomaly and the NEIM rainfall activation were not clear. On the other hand, in 1988 and 1998, WNPM suppression enhanced the NEIM rainfall in August and increased flood risk in Bangladesh.
By using a linearized Boussinesq model on the tangent plane in the mid-latitudes, how the effect of the horizontal component of the angular velocity of the Earth's rotation (fH-effect) modifies the characteristics of inertia-gravity waves is examined. The fH-effect widens the range of the intrinsic wave frequency. A physical interpretation of this modification is made in terms of restoring forces. There are cases in which the rotational direction of the hodograph with time is anticlockwise (clockwise) even in the Northern (Southern) Hemisphere, unlike the case of fH = 0. Considering the form stress over the potential temperature surface, the sign of the vertical group velocity can be the same as that of the vertical phase speed, unlike the case of fH = 0, because the fH-effect can reverse the direction of the form stress through the vertical force balance. The minimum frequency increases with the buoyancy frequency (N) for fH ≠ 0, when the latitude and the direction of the wavevector are fixed. This fact indicates that waves trapped in a weakly stratified layer (WSL) exist, where N is low. Using an idealized vertical profile of N in the form of a square well, the trapped wave solution is derived. The solution is composed of two plane waves in the WSL, while it decays exponentially outside. Using operational radiosonde data in Japan, it is shown that there is a persistent WSL slightly below the tropopause where the climatological minimum value of N (Nmin) is about a half times lower than the typical tropospheric value (~0.01 s−1). The Nmin value is not sufficiently small to form trapped waves having wavelengths in a realistic range within a few days, because the condition of Nmin < 0.001 s−1 is necessary. Thus, such trapped waves are rarely observed in the WSL slightly below the tropopause.
Procedures for retrieving two indices indicating the degree of inhomogeneity of water vapor using the carrier phase of a global positioning system (GPS) have been introduced. One index describes the spatial concentration of water vapor, while the other indicates higher-order water vapor inhomogeneity. Horizontal scales of the two indices are approximately considered to be 60 km and 2-3 km, respectively. The characteristics of the water vapor field over Japan in August 2011 were studied using the spatiotemporal variation in these two indices along with GPS-derived precipitable water vapor (PWV). The monthly averaged indices indicate distinct diurnal variation in the mountainous region of central Honshu and coincidence with the diurnal variation in precipitation frequencies in the area. The relations between these indices and precipitation were examined statistically. The results indicate that the inhomogeneity indices are more strongly correlated with strong rainfall than PWV. PWV seemed to relate to precipitation of less than 10 mm h−1, but did not exhibit much of a relation with precipitation greater than 10 mm h−1. These relations hold true for both present and imminent precipitation. The spatiotemporal variations in the indices of a thunderstorm on August 11, 2011, were also examined. Both water vapor concentration (WVC) and higher-order inhomogeneity indicated an increase ahead of the initiation of convective precipitation. The results suggest that the two GPS-derived indices of water vapor inhomogeneity reflect local variations in the water vapor associated with the convection phenomena and can potentially be used for monitoring thunderstorms.
The optical and chemical properties of atmospheric aerosols were determined from the ground-based measurements at Amami Oshima in April 2001 during the Asian Atmospheric Particle Environmental Change Studies (APEX) campaign and at Fukue Island in March 2001. At Amami Oshima from April 10 to 16, an aerosol event was observed in which the volume scattering coefficient and sulfate concentration of fine particles increased conspicuously. At the former term of the aerosol event, the single scattering albedo reached 0.98. At the latter term of the event, on the other hand, it was 0.80-0.90 and the concentrations of elemental carbon, aluminum, and zinc increased by a factor of several to ten times compared with the rest of the observation terms. Using chemical and backward trajectory analyses, it was established that the transparent aerosols, rich in sulfate, were converted from sulfur dioxide gas emitted by the Miyake Island volcano at the former term of the aerosol event, while the turbid and absorptive aerosols were anthropogenic aerosols appearing together with Asian yellow dust from continental China at the latter term of the event. The measurements at Fukue Island showed that the volume scattering coefficients as well as the concentrations of sulfate, elemental carbon, aluminum, and zinc were higher than those at Amami Oshima, while the single scattering albedo was relatively low. This study elucidates that in spring, large amounts of anthropogenic particles are frequently transported together with Asian yellow dust from inland continental China to the Pacific Ocean, and that the single scattering albedo and the ratio of organic to elemental carbons are approximately 0.80-0.85 and 1, respectively, over the north-western Pacific Ocean.