Aspects of the 40–50 Day Oscillation during the Northern Summer as Inferred from Outgoing Longwave Radiation

Section: Introductionsupporting

confidence: 58%

Section: Introductionmentioning

confidence: 99%

Seasonal Transition of the Lower-Level Circulation Systems around the Baiu Front in China in 1979 and Its Relation to the Northern Summer Monsoon

Kato

1989

“…Based on OLR data during the eight years of 1975-77 and 1979-83, Murakami et al (1986 found that (1) 45-day transient waves are not of purely propagating character, but rather are interrupted by many occasions of near stagnation or even westward movement, and (2) they tend to dramatically intensify when passing through the eastern Indian and western Pacific Oceans, a feature similar to standing waves with monsoonal effects acting as the major regulator (or forcing) of 45-day transients. An approximate out-of-phase relationship between 1 On leave from the National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Ibaraki, Japan c 1995, Meteorological Society of Japan intraseasonal convective activities over the tropical Indian and western Pacific Oceans was further elaborated on by many authors, including Chan (1986), andRui (1990a). One of the interesting findings by Zhu and Wang (1993) is that the summertime convective seesaw results from a time-lag development of two systems: a western system evolving and moving eastward and/or northward over the Indian Ocean, and an eastern system that exhibits a westward and/or northward progression across the western Pacific.…”

mentioning

confidence: 98%

Interaction Between the Mean Summer Monsoon Flow and 45-Day Transient Perturbations

Kawamura

Murakami

1995

The influence of the low-level mean summer monsoon flow on 45-day transient eddies through barotropic interaction is diagnosed by a scalar product between the horizontal shear vector and Eliassen-Palm (E-P) flux, while the dry (moist) baroclinic interaction is measured by a vector product between the vertical wind shear and the eddy sensible (latent) heat transport, which represents the vertical component of E-P fluxes. Activity of 45-day transient eddies was monitored by the evolution of kinetic energy during the 9-year period of [1985][1986][1987][1988][1989][1990][1991][1992][1993].The 45-day wave activity is relatively weak over the SEAM (Southeast Asian monsoon) domain. Absence of geographically fixed forcing makes the WNPM (western North Pacific monsoon) more violent, with the peak 45-day wave activity occurring in late August, the time of frequent development of vigorous tropical cyclones. Near the WNPM updraft center (15N, 140-150E), the E-P wave energy fluxes are oriented in a direction which causes weakening of the sheared mean zonal and meridional winds, implying barotropic amplification of 45-day waves. The weak mean temperature gradient relegates the dry baroclinic instability to that of secondary importance over the WNPM domain. This is contrasted with the significant moist baroclinic process due to an approximate perpendicular relationship between the eddy moisture transport and the strong vertical shear of the monsoon flow.Curiously, 45-day waves are quite active near Japan even during August's dry, hot climate. Neither barotropic nor dry (moist) baroclinic instability can account for late-summer disturbance activity. Evidence is presented that 45-day convective oscillations occuring over the WNPM domain induce a strong 45-day response near Japan and further eastward, possibly indicating an intraseasonal Rossby wave dispersion along the westerly jet stream. Near the Aleutian Islands, a pronounced poleward eddy sensible heat flux is directed down the mean temperature gradient, resulting in dry baroclinic amplification of mid-latitude 45-day waves via action of the Coriolis torque. An import of moisture from the WNPM source region brings about a moist baroclinic instability of extratropical 45-day waves poleward of the Pacific High.

“…This result implies that meridional wind is important to strengthen the convergence in the boundary layer, which acts to intensify convective heating through the updraft at the top of the boundary layer. Furthermore, we can recognize that the phase line of each variable slightly tilts westward away from the equator as observed (e.g., Lau and Chan, 1986a;Knutson and Weickmann, 1987;Hendon and Liebmann, 1990;Wang and Rui, 1990b). As discussed above, the effect of surface friction plays an important role for the instability of FK by yielding the phase lag between vertical velocity at the top of the boundary layer and that aloft.…”

Section: Structure Of the Frictionally-controlled Modementioning

confidence: 99%

Kelvin Wave-CISK Controlled by Surface Friction

Ohuchi

Yamasaki

1997

As a step toward an understanding of the mechanism of super cloud cluster (SCC), the effect of surface friction on the instability of Kelvin wave is discussed, together with the condition of the vertical profile of parameterized convective heating. In this paper (Part I), linear theory is developed with an equatorial beta-plane model in which the amount of convective heating is assumed to be proportional to the vertical velocity at the top of the boundary layer. Results from the eigenvalue problem suggest that the effect of surface friction is, at least under such parameterization, indispensable for essential understanding of SCC; this study is along the same line as Wang (1988) with respect to the importance of surface friction.Issue is focused on three unstable modes. One is a stationary mode excited with much heating at lower levels, and identified with conditional instability of the first kind (CIFK). The other two are Kelvin modes that contrast with each other; one is destabilized by enough heating at upper levels, even without surface friction (referred to as NFK: Non-Frictional Kelvin mode), and the other is destabilized by moderate heating at lower levels under the control of surface friction (FK: Frictional Kelvin mode). NFK has phase velocities of more than 10ms-1, and its growth rates are on the order of 10-3s-1 at small wavelengths. On the contrary, FK is characterized by smaller phase velocities of less than 10ms-l, and by only slight dependence of its growth rates on wavelength. In addition, the convergence in the boundary layer exhibits a phase shift slightly eastward relative to the convergence aloft; this is the key feature that induces effective feedback between convection and the wave.In view of the large phase velocity and the small preferred scale, NFK may be interpreted as a Kelvin mode of the type that has been treated in many previous studies on SCC. Difficulties in explaining the observed phase velocity and preferred scale of SCC are inherent in NFK. By contrast, FK can give a realistic phase velocity and preferred scale, and clear up major difficulties in the previous studies. Inclusion of surface friction is expected not only to serve as the missing link of the past linear theories of SCC, but also as a basis of future non-linear experiments with explicit moisture processes.