The Pacific/North American (PNA) and Western Pacific (WP) teleconnection patterns during the winter are considered to be associated with El Niño events. Although each PNA or WP index is only weakly correlated with the El Niño/ Southern Oscillation (ENSO) cycle, when they are combined together a high correlation is obtained. This result suggests that both atmospheric circulation patterns, PNA and WP, are not directly connected to the ENSO cycle, but their overall occurrence frequencies are controlled by El Niño events. In order to examine the condition that determines the type of atmospheric circulation that appears concurrently during an El Niño winter, a composite analysis is conducted by classifying the El Niño winters according to the relative amplitudes of PNA and WP indices. The result suggests that different types of responses are not strongly dependent on the differences in sea surface temperature (SST) distribution among El Niño events, but rather they depend on the circulation condition over the Eurasian continent during the early winter.
The physical connections between dry/moist (conditional) symmetric instability (SI/CSI) off equatorial diabatic thermal forcing, and the onset of the Asian monsoon are investigated using the Goddard Laboratory for Atmospheres general circulation model (GLA GCM). The objective of this study is two-fold: to elucidate the causal relationship between SI/CSI and monsoon onset, and to investigate the threshold behavior of asymmetric monsoonal thermal forcing under condition of SI/CSI. This work corroborates the authors' earlier linear instability analysis results, which show, that SI/CSI in the boreal summer monsoon basic states may be a plausible explanation for the abrupt monsoon transition. Monsoon transitions in the model, as depicted by the abrupt meridional movement of the axis of maximum vertical motion from equator to northern latitudes, occur during 16-20 May for the East Asian Monsoon (EAM) and 1-5 June for the South Asian Monsoon (SAM) regions. The necessary stability criterion for dry (moist) SI over the EAM and SAM regions reveals a sudden cross equatorial advection of negative dry potential vorticity (DPV) and moist potential vorticity (MPV) into the summer hemisphere five to ten days preceding the model monsoon transition. This causes dry and moist SI. Maximum shift of the zero line of DPV and MPV (dry and moist symmetrically unstable regions) happens subsequent to monsoon transition. Simplified analysis of the potential vorticity (PV) budget equation reveals that the lower tropospheric negative PV advection into the summer hemisphere is largely governed by the dominance of vertical differential diabatic heating over horizontal differential diabatic heating. The diabatic heating also shows an abrupt increase from 2-3K day-1 before the transition, to 12-14K day-1 at the time of monsoon transition. The genesis of pre-monsoon weak heat source arises primarily due to unstable SI and CSI of the pre-monsoon basic states, which consequently produce moderately large scale lower (upper) tropospheric convergence (divergence) patterns slightly poleward of the zero line of DPV and MPV. Lower tropospheric conditionally unstable tropical atmosphere, in the presence of off equatorial large scale lower (upper) tropospheric convergence (divergence), is conducive to exciting CISK-like processes, which may eventually release large amounts of latent heat and develop a strong heat source at the time of monsoon transition. We have noted that a fully established model meridional circulation originates only when the diabatic forcing magnitude exceeds some threshold value of around 5K day-1 at the time of monsoon transition. The model transition is more pronounced over the EAM region than over the SAM region. The linear steady-state dynamical response of a zonally symmetric atmosphere as a consequence of varying the location and magnitude of an idealized asymmetric thermal forcing reveals that the most intense meridional circulation (maximum efficiency of vertical motion) is accomplished when the thermal forcing is located around 10°N. The interrelationship between the location of zero DPV/MPV contour, lower tropospheric maximum convergence versus maximum vertical velocity of the monsoonal circulation in the summer hemisphere, clearly suggests that SI (CSI) of zonal monsoon flows is a causal mechanism for the onset of monsoon transition.
In this study, we examine the nonlinear effect in the evolution of the Rossby wave propagating in the jet stream using a nonlinear barotropic model. We focus primarily on the formation of isolated eddy from the Rossby wave. The Rossby wave is induced by localized vorticity forcing in a zonally-varying basic flow. For weak, steady forcing we obtain the Rossby wavetrain trapped in the jet stream, as in the linear model. For strong, steady forcing it is occasionally seen that an isolated eddy with negative vorticity is formed from some domain in the Rossby wavetrain, when the zonal asymmetry of the basic flow is large. For strong forcing of a finite period, a strong isolated eddy with negative vorticity can be formed from some domain when the zonal asymmetry of basic flow is large. This eddy breaks down to finally emanate Rossby wave downward.
The general aspects of land and sea breezes in western Seto Inland Sea and the surrounding areas are revealed from an analysis of the wind data obtained from the AMeDAS. The study area consists of Kyushu, the western part of Chugoku, and the western end of Shikoku surrounded by the Pacific Ocean, the East China Sea and the Sea of Japan, and the study area contains inland seas both wide and narrow. The day of the land and sea breeze in the study area is specified from the day when the land and sea breeze appears on a coastal area in the middle of the study area. The appearance of unexpected wind directions in the sea breeze situation in that area and the presence or not of the sea breeze in a nearby coastal area of smaller water body are taken into account in the classification of types of the land and sea breeze; this classification results in eight types. The type of sea breeze in both coastal areas is characterized by the developed sea breezes all over the area, except for the northeastern half of the coast facing the Sea of Japan in Chugoku and the eastern end of the Seto Inland Sea. Meeting of the sea breezes in the central region from either side of Chugoku and Kyushu is clearly seen in the afternoon. The other types are discussed and characteristics are compared with this fundamental type. The location of a significant land breeze in northwestern Shikoku is noted in relation to the well-known local gale “Hijikawa-arashi.” The results from the previous study of the relationship between the occurrence of the land and sea breezes and the general meteorological conditions in Osaka Bay and the surrounding areas are shown to hold also for the present area. By coupling the features of the sea breeze in the present study area and those in Osaka Bay and the surrounding areas, the typical features of the sea breeze for a wider area of western Japan are revealed. The interrelation of the behavior of the land and sea breezes in these two areas is also discussed.
The intraseasonal variations in the subtropical jet over the Eurasian continent and the western North Pacific in the northern summer are investigated through a statistical study using the ECMWF objective analysis data for 14 years, from 1980 to 1993. The quasi-stationary Rossby waves are evident in the climatological subtropical jet. They are characterized by well-confined zonal wavenumbers of about 5-7 and less confined spectral peaks at 30- to 45-day and 14-day periods. They are strongly phase-locked in the zonal direction. Through a case study for a typical year (1983), evident standing wave-like disturbances with a 32-day period are found in the subtropical jet. Their propagation routes correspond to the well-defined waveguide for the quasi-stationary Rossby wave formed along the subtropical jet. However, they are not completely confined in the waveguide, but protrude slightly from the waveguide with its southern rim crossing their critical latitude, which lies along the center of the Tibetan High. The vertical structure of the disturbances shows a slight but systematic westward phase tilt, indicating a northward heat flux. In ray-path analysis for barotropic quasi-stationary Rossby waves, temporally and meridionally smoothed basic flows are used in order to take into account temporal and zonal extensions of wave packets. This method is successfully used to diagnose the propagation routes of the observed disturbances.
Precipitation system size, shape, location, lifetime, and motion in 16 mature and early weakening stages of typhoons around Japan was studied, using conventional radar network data. Based on these findings, precipitation systems in typhoons apart from mid-latitude frontal zones were classified into inner and outer rain shields, inner and outer rainbands, and eyewalls. Delta-shaped precipitation systems, called delta rain shields, were found to characterize typhoons approaching mid-latitude frontal zones.
Hydrogen peroxide (H2O2) concentrations in an ice core obtained at site S25, near the coastal region on Mizuho Plateau, East Antarctica, have been determined. The time series of H2O2 concentrations showed not only seasonal variations, but also about 11-year variations corresponding to the solar cycle. There is a possibility that atmospheric H2O2 concentrations might have been associated with the solar cycle.
The influence of a simulated equatorial quasi-biennial oscillation (QBO) on the Northern Hemisphere winter was studied by analyzing output data from a general circulation model (GCM). The differences between the 7-50hPa equatorial QBO phases of the January-March (JFM) averaged zonal wind composite exhibit a weaker polar-night jet during the easterly phase of the equatorial QBO and a stronger polar-night jet in the westerly phase. The Eliassen-Palm flux of planetary waves is stronger and more convergent in the high latitude stratosphere for the easterly phase, while it exhibits similar characteristics in the mid latitude stratosphere during the westerly phase. These results are nearly consistent with observed results. Zonal wind deceleration anomalies are located slightly north of the zonal wind anomalies. It is likely that the JFM time-mean zonal wind anomalies develop from the small zonal wind anomalies during the early winter and propagate northward with time. The anomaly pattern in the troposphere coupled with that in the stratosphere reveals a tripole structure among equatorial, middle, and high latitudes, similar to the North Atlantic oscillation (NAO) pattern.
Ueno and Ohata (1996) pointed out the importance of the correction of precipitation measurements on the Tibetan Plateau. The present author offers some comments to evaluate more quantitatively their results, which are summarized as follows: (1) the validity of the correction of precipitation should be checked for the individual cases, along with the total amount; (2) the diameter of the gauge should be investigated for any systematic bias of the measured precipitation; (3) the increment obtained through the correction procedure should be quantitatively compared with the standard error of the corresponding regression analysis; and (4) the effect of the correction should be looked at from various viewpoints, e. g., quantitative comparisons of the corrected precipitation with precipitation estimates from space, as well as with the surface energy budget on the Tibetan Plateau.