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[1] We examined possible responses of cyclone activities to the bimodal path states of the Kuroshio Current [i.e., large meander (LM) and non-LM (NLM)] by using the long-term reanalysis data and the 20th century hindcast experiment of a high-resolution atmosphere-ocean coupled model. Compared with a seasonal mean cyclone track frequency for the LM and NLM periods, a primary cyclone track shifts southward in association with the meander of Kuroshio Current. Composite analyses of the hindcast experiment showed remarkable atmospheric responses accompanying the Kuroshio LM. The Kuroshio LM causes a decrease in latent heat flux in the south of Japan and a southward shift of the near-surface baroclinic zone. Distinctive decreases in thermodynamic fluxes inhibit the rapid development of cyclones in the meander region, eventually inducing positive sea level pressure anomalies downstream from that region.

An ensemble of climate experiments has been performed using a T42 GCM version of the Japan Meteorological Agency global model to examine abnormal changes in the wintertime extratropical atmospheric response to anomalous sea surface temperature (SST) forcing in recent decades. Three independent 39-year integrations for the period 1955–1993 are forced by the same observed SST boundary condition. Although the tropical Pacific SST anomalies since the late 1980s indicate El Nino-like patterns, the Pacific/North American teleconnection (PNA) pattern is found to be hardly simulated, which stands in sharp contrast with that for the period 1977–1983. The SSTs in the vicinity of the maritime continent (the Indonesian region), including the tropical Indian Ocean, are very high during the period 1987–1993 and associated model atmosphere does undergo pronounced zonal heating over almost the entire tropics. Such tropical heating does not necessarily favor the barotropic Rossby wave dispersion into the extratropics emanating out of convective forcing since weakened Walker circulation is not capable of producing localized strong divergence anomalies, hence inhibiting generation of apparent vorticity sources in the subtropics. If the prevailing tendency of high SSTs in the vicinity of the maritime continent persists from now on, it is anticipated that a good correlation between El Nino-Southern Oscillation (ENSO) and PNA, which has been pointed out by previous numerous studies, becomes insignificant. This anomalous feature may be viewed as a signature of global warming.

[1] The lagged relationship between the El Nino-Southern Oscillation (ENSO) and Asian Summer Monsoon (ASM) variability is investigated using the Commonwealth Scientific and Industrial Research Organization (CSIRO) Mark3 coupled model. Composite analyses of 17 warm and 16 cold events from an extended unforced run reveal that the model captures the asymmetric structures of rainfall and SST anomalies over the tropical Indian Ocean during northern hemisphere spring which is one of the major precursory signals of anomalous ASM variability. As a remote forcing mechanism prior to the ASM onset, simulated anomalous convection over the northern Indian Ocean strongly influences the land surface hydrologic conditions over central and southwest Asia. Simulated land-surface processes result in anomalous temperatures over the land, which in turn produce a change in the land-ocean thermal contrast over the domain. The results are consistent with observed features, which show a delayed and indirect impact of ENSO on the ASM variability during the early summer monsoon period, but not during the rest of the monsoon season.