This study investigates future changes in atmospheric circulation during the Baiu in Japan using 20-km-mesh Atmospheric General Circulation Model (AGCM) simulations for the present-day (1979–2003) and the future (2075–2099) climates under the Representative Concentration Pathways 8.5 scenario. The simulated future climates include the outputs obtained with one control sea surface temperature (SST) and three different SST patterns. The Baiu frontal zone defined as the meridional gradient of equivalent potential temperature gradually moves northward during June-July-August in the present-day climate. In the future climate simulations using the control SST, the Baiu frontal zone is projected to stay to the south of Japan in June. Thus, precipitation is projected to increase over this region, while decreasing in the western part of Japan. The future changes in precipitation activity and atmospheric circulations in June are consistent across all four SST patterns. However, precipitation and atmospheric circulation in July and August in the future climate simulation depends on the SST patterns as follows: in non-El Nino-like SST pattern, the Baiu terminates in late July, similar to that of the present-day climate; a result with an El-Nino like SST pattern shows that sufficient amount moisture is transported to the Japan islands and leads in a delay of the Baiu termination until August; and in the SST pattern with strong warming in the western North Pacific, a sufficient amount of moisture is transported to the south of Japan from June until August. The difference in SST pattern leads to a variation in sea level pressure in the western North Pacific, and affects a variation of the Northern Pacific subtropical high around the Japanese islands in July and August.
During the Tokyo Metropolitan Area Convection Study for Extreme Weather Resilient Cities (TOMACS), many isolated convective storms developed in the southern Kanto Plain on August 17, 2012. The aim of this study was to clarify the dynamics leading to the convection initiation of one of them using different remote sensing instruments.
Before the convection initiation, a southeasterly flow transported water vapor inland from Tokyo Bay and the well-mixed and a cumulus-cloud-topped convective boundary layer developed. A convergence line in the form of a sea breeze front (SBF) also moved inland from Tokyo Bay. A near-surface air parcel was lifted to its lifting condensation level (LCL) by an updraft in a convergence zone with a 3 km horizontal scale, which formed the west edge of the convergence line. The saturated air parcel at the LCL was then lifted to its level of free convection (LFC) by the updrafts associated with thermals below the cumulus cloud base. The first echo of hydrometeors was detected by a Ku-band radar about 6 minutes after the air parcel reached its LFC, then the convective cell developed rapidly. When an SBF arriving from Sagami Bay passed under the cell, the updraft over the nose of the SBF triggered a new precipitation cell, but no intensification of the preexisting cell was observed.
In Part Ι of this study, the development processes of Baiu frontal depressions (BFDs) have been examined through case-study numerical experiments. The numerical simulations revealed that latent heating is dominant for the development of BFDs in the western part of the Baiu frontal zone (W-BFDs), west of roughly 140°E, while both latent heating and baroclinicity are important for the development of BFDs in the eastern part of the zone (E-BFDs), east of roughly 140°E. In this study, idealized numerical simulations with zonally homogeneous basic fields are conducted to obtain a more generalized perspective of the development processes of BFDs.
The basic fields for the idealized simulations are made from the composites of the environments under which 28 W-BFDs and 43 E-BFDs developed. The idealized simulations successfully reproduce a realistic W-BFD and E-BFD. The W-BFD has a slightly westward-tilted vertical structure, which is modulated by latent heating at low levels of the atmosphere. In contrast, the E-BFD has a westward-tilted structure through the troposphere, which is similar to the well-known baroclinic wave structure. Results of available potential energy diagnosis for the effects of latent heating and baroclinicity on the BFD development are consistent with those in Part Ι. The W-BFD has a mechanism mainly driven by latent heating yielding strong convection, while the E-BFD develops through baroclinic instability in moist atmosphere.
The Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO) are the two dominant low-frequency modes in the North Pacific. This study focused on the simulation capability of the two leading low-frequency modes in current coupled models, based on 24 coupled model outputs from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Results showed that most of these models captured the two low-frequency modes, but the air-sea coupling relationship (covariability of the ocean low-frequency modes with the atmospheric forcing modes) captured by CMIP5 models had drastic differences. Four models (CCSM4, CESM-WACCM, MIROC5 and NorESM1-M) not only captured the spatial and temporal characteristics of PDO and NPGO modes but also simulated their air-sea coupling relationships. Therefore, we selected these four models to examine changes in PDO and NPGO modes under different global warming scenarios using RCP4.5 and RCP8.5 forcing (RCP: Representative Concentration Pathway). In future RCP scenarios, the spatial patterns of PDO and NPGO showed no obvious changes. However, the dominant periods of PDO and NPGO modes were shorter, which is consistent with faster oceanic Rossby waves induced by enhanced upper oceanic stratification in the warming scenarios.
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