Takao Yoshikane

Biennial and Lower-Frequency Variability Observed in the Early Summer Climate in the Western North Pacific

Early summer climate in the western North Pacific is largely represented by the baiu phenomenon. The meridional fluctuations of the baiu front on interannual time scales and the associated large-scale circulations are examined using the empirical orthogonal function (EOF) analysis and composite or correlation analyses based on the EOF time coefficients. The first EOF mode indicates a 5- or 6-yr low-frequency fluctuation (LF mode) appearing south of 358N. The development is concurrent with horseshoe sea surface temperature anomalies (SSTAs) in the entire tropical Pacific that are associated with the El Nino-Southern Oscillation (ENSO). SSTAs in the western North Pacific control the anomalous southward expansion of the baiu front through a modification of the convection at around 208-358N. The LF mode is negatively correlated with the south-southeast Asian summer monsoon. The second EOF mode is characterized by a meridional seesawlike fluctuation with a node at around 28 8N and a time scale of biennial oscillation (BO mode). The horseshoe SSTAs again control the anomalous meridional circulations, but with a different spatial phase through a convection off the Philippines. The spatial phase difference between the two horseshoe patterns is about 908 in both the zonal and meridional directions. The BO mode is negatively correlated with the tropical western North Pacific monsoon. SSTAs associated with the BO mode tend to be confined to the tropical western Pacific, while the signals of the LF mode extend rather broadly in the tropical Pacific-Indian Ocean sector, suggesting that the tropical BO is an aborted ENSO in the tropical central-western Pacific. The spatial phase of horseshoe SSTAs adjusts the interannual variability of the meridional fluctuation of the baiu front in the western North Pacific.

Formation Mechanism of the Simulated SPCZ and Baiu Front Using a Regional Climate Model

Abstract The formation mechanisms of both the South Pacific convergence zone (SPCZ) and the baiu front are investigated using a regional climate model. Some idealistic numerical experiments are carried out assuming zonally uniform and temporally constant atmospheric fields obtained from ECMWF analysis data as initial and lateral boundary conditions. A rainfall zone similar to the SPCZ is reproduced using a zonal mean atmospheric field of the Southern Hemisphere (SH) summer. The simulated SPCZ in the idealized model framework is highly sensitive to the variation of SST during 1997–98 in a manner similar to observation. The SPCZ is extremely weak in an experiment under the zonal mean field of the Northern Hemisphere (NH) early summer. Experiments with a different intensity of zonal wind speed and baroclinicity suggest that a mild zonal wind (weak baroclinicity) weakens the precipitation of the SPCZ and even occasionally suppresses precipitation when it is too weak. The heat contrast between the Australian c...

Roles of the Brazilian Plateau in the Formation of the SACZ

AbstractThe role of the Brazilian Plateau (BP) in maintaining the South Atlantic convergence zone (SACZ) has been examined by statistical analysis and numerical experiments. Statistical analysis using 27 years of data showed that the SACZ is most intense when it is over the BP. In this case, low-level cyclonic circulation appears over the southwestern part of the BP and forms westerly flow, which intensifies low-level convergence along the SACZ with northeasterly flow from the Amazon and northerly flow along the western edge of the South Atlantic subtropical high. A vorticity budget analysis indicates that precipitation over the BP that accompanies stretching maintains the cyclonic circulation.Sensitivity experiments using a regional atmospheric model for two different cases indicate that precipitation over the BP plays a dominant role as an atmospheric heat source in maintaining the cyclonic circulation and the SACZ. In model experiments in which rain was stopped around the BP but the topography was kept...