Yoshiki Fukutomi

Interannual Variability of Summer Water Balance Components in Three Major River Basins of Northern Eurasia

Abstract This study investigated water balance components in the three major river basins of Siberia (the Lena, Yenisey, and Ob) based on the National Centers for Environmental Prediction (NCEP)–Department of Energy (DOE) Atmospheric Model Intercomparison Project II (AMIP-II) reanalysis. The primary focus is the nature of the interannual signatures of summer precipitation, moisture convergence, and runoff in individual basins, and their linkage to the large-scale water transport and circulation fields over northern Eurasia from 1979 to 1995. The temporal characteristics of the hydrological cycle and the associated large-scale circulation structure are discussed. Some interesting features are found in the interannual variability of basin-scale hydrometeorological elements. In the Lena and Ob basins, the temporal signatures of both precipitation and moisture convergence indicate a cycle of approximately 6 to 8 yr. The mid- and late-summer runoff variation is significantly correlated with these two elements....

Southerly Surges on Submonthly Time Scales over the Eastern Indian Ocean during the Southern Hemisphere Winter

Abstract Meridional wind surges from the extratropics into the Tropics strongly regulate tropical convective activity. This paper confirms that extratropical forcing manifested as a meridional surge does modulate the tropical atmosphere over the eastern Indian Ocean, and it describes the tropical–extratropical connection in the region. Surges in the lower atmosphere on submonthly (6–25 days) time scales over the eastern Indian Ocean were examined in tandem with associated tropical convection and large-scale atmospheric fields during the Southern Hemisphere (SH) winter (June–August). Data used in this study are NCEP-2 reanalyses and daily NOAA/Climate Diagnostics Center (CDC) outgoing longwave radiation (OLR) data for 23 yr, from 1979 to 2001. A low-level surge index was calculated using the 850-hPa meridional wind component (υ) averaged over a region where sub–monthly scale υ variance shows a local maximum (17.5°–2.5°S, 87.5°–97.5°E). The surge index defines 62 different surge events. Composites of variou...

Tropical synoptic-scale wave disturbances over the western Pacific simulated by a global cloud-system resolving model

Lower-tropospheric tropical synoptic-scale disturbances (TSDs) are associated with severe weather systems in the Asian Monsoon region. Therefore, exact prediction of the development and behavior of TSDs using atmospheric general circulation models is expected to improve weather forecasting for this region. Recent state-of-the art global cloud-system resolving modeling approaches using a Nonhydrostatic Icosahedral Atmospheric Model (NICAM) may improve representation of TSDs. This study evaluates TSDs over the western Pacific in output from an Atmospheric Model Intercomparison Project (AMIP)-like control experiment using NICAM. Data analysis compared the simulated and observed fields. NICAM successfully simulates the average activity, three-dimensional structures, and characteristics of the TSDs during the Northern summer. The variance statistics and spectral analysis showed that the average activity of the simulated TSDs over the western Pacific during Northern summer broadly captures that of observations. The composite analysis revealed that the structures of the simulated TSDs resemble the observed TSDs to a large degree. The simulated TSDs exhibited a typical southeast- to northwest-oriented wave-train pattern that propagates northwestward from near the equator around 150 ∘ E toward the southern coast of China. However, the location of the simulated wave train and wave activity center was displaced northward by approximately a few degrees of latitude from that in the observation. This displacement can be attributed to the structure and strength of the background basic flow in the simulated fields. Better representation of the background basic states is required for more successful simulation of TSDs.

Extratropical forcing of tropical wave disturbances along the Indian Ocean ITCZ

The role of extratropical waves in the excitation of tropical waves on submonthly timescales is explored along the Indian Ocean Intertropical Convergence Zone (ITCZ) during the austral summer using Japanese Reanalysis products and NOAA outgoing longwave radiation data. The analysis period is DecemberŰFebruary of 1979/1980 to 2008/2009. The submonthly tropical waves are regarded as a type of Rossby wave propagating along the mean monsoon westerly flow. They play an important role in modulating the Indian Ocean ITCZ convection. The linkage between the tropical and extratropical waves, which is responsible for the formation and strengthening of tropical waves, is examined. Composite analysis results linked to the tropical wave train development show that the midlatitude Rossby wave train progresses eastward and northeastward from the South Atlantic into the subtropical Indian Ocean. As a trough and ridge that form part of the midlatitude wave train approach the southern Africa-southwest Indian Ocean (SWIO) region, a southwest-northeast-oriented wave train is subsequently established, originating from this feature, and is strengthened across the tropical Indian Ocean. The midlatitude wave propagation toward the subtropics induces the growth of the trough and ridge over the SWIO. Wave activity flux diagnostics indicate that the amplified trough and ridge over the SWIO act as an energy source for northeastward amplification of the tropical waves through the wave energy dispersion process. The results suggest that the propagation of the midlatitude wave toward the SWIO is the fundamental mechanism behind the development of the tropical waves along the Indian Ocean ITCZ.