Masaru Inatsu

The Zonal Asymmetry of the Southern Hemisphere Winter Storm Track

Atmospheric general circulation model experiments have been performed to investigate how the significant zonal asymmetry in the Southern Hemisphere (SH) winter storm track is forced by sea surface temperature (SST) and orography. An experiment with zonally symmetric tropical SSTs expands the SH upper-tropospheric storm track poleward and eastward and destroys its spiral structure. Diagnosis suggests that these aspects of the observed storm track result from Rossby wave propagation from a wave source in the Indian Ocean region associated with the monsoon there. The lower-tropospheric storm track is not sensitive to this forcing. However, an experiment with zonally symmetric midlatitude SSTs exhibits a marked reduction in the magnitude of the maximum intensity of the lower-tropospheric storm track associated with reduced SST gradients in the western Indian Ocean. Experiments without the elevation of the South African Plateau or the Andes show reductions in the intensity of the major storm track downstream of them due to reduced cyclogenesis associated with the topography. These results suggest that the zonal asymmetry of the SH winter storm track is mainly established by stationary waves excited by zonal asymmetry in tropical SST in the upper troposphere and by local SST gradients in the lower troposphere, and that it is modified through cyclogenesis associated with the topography of South Africa and South America.

Influence of the Kuroshio in the East China Sea on the Early Summer (Baiu) Rain

AbstractInfluence of the Kuroshio in the East China Sea on the baiu rainband is examined using satellite observations, a reanalysis dataset, and a regional atmospheric model from 2003 to 2008. Satellite observations and reanalysis data reveal that precipitation over the Kuroshio is the highest in early summer (June), when the baiu rainband covers the East China Sea. The high rainfall is collocated with the warm sea surface temperature (SST) tongue of the Kuroshio. This locally enhanced precipitation is embedded in the large-scale baiu rainband, so that the amplitude of precipitation over the Kuroshio is twice as large as that in its surrounding area. The Kuroshio is also accompanied by high surface wind speed, energetic evaporation, and wind convergence. This wind convergence likely results from the SST influence on atmospheric pressure through not only temperature changes, but also humidity changes. Furthermore, the Kuroshio anchors the ascent motion and large diabatic heating with a peak in the midtropo...

Atmospheric Response to Zonal Variations in Midlatitude SST: Transient and Stationary Eddies and Their Feedback*

Midwinter storm track response to zonal variations in midlatitude sea surface temperatures (SSTs) has been investigated using an atmospheric general circulation model under aquaplanet and perpetual-January conditions. Zonal wavenumber-1 SST variations with a meridionally confined structure are placed at various latitudes. Having these SST variations centered at 308N leads to a zonally localized storm track, while the storm track becomes nearly zonally uniform when the same SST forcing is moved farther north at 408 and 508N. Large (small) baroclinic energy conversion north of the warm (cold) SST anomaly near the axis of the storm track (near 408N) is responsible for the large (small) storm growth. The equatorward transfer of eddy kinetic energy by the ageostrophic motion and the mechanical damping are important to diminish the storm track activity in the zonal direction. Significant stationary eddies form in the upper troposphere, with a ridge (trough) northeast of the warm (cold) SST anomaly at 308N. Heat and vorticity budget analyses indicate that zonally localized condensational heating in the storm track is the major cause for these stationary eddies, which in turn exert a positive feedback to maintain the localized storm track by strengthening the vertical shear near the surface. These results indicate an active role of synoptic eddies in inducing deep, tropospheric-scale response to midlatitude SST variations. Finally, the application of the model results to the real atmosphere is discussed.

Stationary Eddy Response to Surface Boundary Forcing: Idealized GCM Experiments*

Abstract A set of atmospheric general circulation model (AGCM) experiments under idealized conditions is performed to investigate atmospheric response to surface boundary forcing by extratropical land–sea contrast, large-scale orography, and tropical sea surface temperature (SST) distribution. Stationary eddies forced by the extratropical land–sea distribution are strongest in high latitudes, but their amplitudes are modest and comparable to internal chaotic variability. By contrast, the stationary eddy response to zonal variations in tropical SST is strong and robust in both the subtropics and midlatitudes. While these SST-forced stationary waves are trapped within the troposphere, those induced by orography show a strong vertical propagation into the stratosphere. Analysis of transient eddies indicates that orography is effective in generating a zonally localized storm track while extratropical land–sea contrast has little effect on the zonal variation of upper-level storm activity. A vorticity budget a...

Recent Changes in Explosively Developing Extratropical Cyclones over the Winter Northwestern Pacific

AbstractThis study investigated recent changes in the characteristics of explosively developing extratropical cyclones over the northwestern Pacific region in winter from 1979/80 to 2010/11 by using reanalysis data from the Japanese 25-yr Reanalysis/Japan Meteorological Agency Climate Data Assimilation System (JRA-25/JCDAS). The results showed that the frequency of explosive cyclones increased in the northwestern Pacific region east of Japan. This increase was accompanied by a decrease in the number of slowly developing cyclones, indicating an increase in the cyclone growth rate. Moreover, most of the increased explosive cyclones east of Japan originated southwest of Japan. A comparison of the dynamical features and energy budgets of two composite cyclones in the earlier and later halves of the study period suggested that the increase was due to an enhancement of the low-level baroclinicity to the east of Japan and an increase in humidity associated with sea surface temperature warming and enhanced evapor...

A Scale Interaction Study on East Asian Cyclogenesis Using a General Circulation Model Coupled with an Interactively Nested Regional Model

This study newly developed the interactively nested climate model (INCL) using a general circulation model (GCM) interactively nested with a regional atmospheric model (RAM). One interactive experiment with finer RAM topography and another with coarser topography, as well as offline versions of each experiment, were performed to investigate the effects of subsynoptic-scale eddies and subsynoptic-scale mountains in northeast Asia on the larger-scale climate, using the GCM with T42 atmosphere and the RAM with 40-km mesh size in the INCL system. The subsynoptic-scale eddy effect restrictively increased synoptic-scale eddy activity within the RAM domain. In contrast, subsynoptic-scale mountains had the effect of robust anticyclonic circulation around the Sea of Japan and effectively forced larger-scale circulation. The effect was positively fed back to the mean field and amplified the anticyclonic circulation accompanied by suppressed storm activity in northeast Asia. The results suggest that subsynoptic-scale mountains affect not only subsynoptic-scale eddies but also the global climate.

Formation of subtropical westerly jet core in an idealized GCM without mountains

In boreal winter, subtropical westerlies in the upper troposphere reach maxima in speed over the eastern edges of the Asian and North American continents. The zonal variations in the westerlies are generally attributed to large-scale orography and thermal forcing, but the latter mechanism remains largely unsubstantiated. Here we conduct general circulation model (GCM) experiments without orography to identify the most important thermal forcing for generating zonal asymmetries in subtropical westerlies. By changing sea surface temperature (SST) distribution in the GCM, we find that the tropical SST distribution plays a decisive role in producing a subtropical jet core to the north of the tropical warm water pool, while the effects of extratropical continent-ocean heating contrast on upper-level zonal wind speed distribution are secondary. The results from Aqua Planet runs further support this conclusion.

Diagnostics for Near-Surface Wind Response to the Gulf Stream in a Regional Atmospheric Model*

AbstractThe mechanisms acting on near-surface winds over the Gulf Stream are diagnosed using 5-yr outputs of a regional atmospheric model. The diagnostics for the surface-layer momentum vector, its curl, and its convergence are developed with a clear separation of pressure adjustment from downward momentum inputs from aloft in the surface-layer system. The results suggest that the downward momentum mixing mechanism plays a dominant role in contributing to the annual-mean climatological momentum curl, whereas the pressure adjustment mechanism plays a minor role. In contrast, the wind convergence is mainly due to the pressure adjustment mechanism. This can be explained by the orientation of background wind to the sea surface temperature front. The diagnostics also explain the relatively strong seasonal variation in surface-layer momentum convergence and the small seasonal variation in curl. Finally, the surface-layer response to other western boundary currents is examined using a reanalysis dataset.

Dynamics and Practical Predictability of Extratropical Wintertime Low-Frequency Variability in a Low-Dimensional System

AbstractDynamics and practical predictability of extratropical low-frequency variability (LFV) in Northern Hemisphere winter are examined in the framework of a two-dimensional (2D) stochastic differential equation (SDE) on the phase space spanned by two leading empirical orthogonal function modes of low-pass-filtered 500-hPa geopotential height variations. The drift vector and diffusion tensor of the 2D SDE with multiplicative noise are theoretically connected with deterministic and stochastic error growth, respectively; both are statistically estimated from a reanalysis dataset. Projected onto the 2D phase space is the practical predictability of the LFV estimated by the 10-day forecast spread based on the 1-month ensemble prediction operationally conducted by the Japan Meteorological Agency (JMA). It is shown that the forecast spread of the LFV prediction by the JMA model for a relatively shorter prediction period when the model bias does not hamper the forecast is primarily explained by the stochastic ...

Predictability of wintertime stratospheric circulation examined using a nonstationary fluctuation-dissipation relation

AbstractThe dynamics and predictability of stratospheric low-frequency variability in the Northern Hemisphere winter are examined using a two-dimensional (2D) phase space spanned by the leading empirical orthogonal functions of the 10-hPa geopotential height field. The 2D phase space represents the variation of the strength of the polar night jet and the amplitude of zonal wavenumber-1 eddy components. A linearized nonstationary fluctuation–dissipation relation (NFDR) is developed based on the deterministic drift vector and the stochastic diffusion tensor estimated from a reanalysis dataset. The authors find that the solution of the linearized NFDR with an optimal data sampling time interval for estimating the drift vector and the diffusion tensor provides a good representation in the phase space of the inhomogeneous distribution of the forecast spread of the operational ensemble forecast conducted by the Japan Meteorological Agency. In particular, the linearized NFDR captures the local maximum of the for...