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

Abstract

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.

Tropical-Extratropical Interaction Associated with the 10-25-day Oscillation over the Western Pacific during the Northern Summer.

“…2c). This eastward shift from June-July through August is similar to the shift for MJO-scale eddies demonstrated by Kawamura and Murakami (1995).…”

“…Kawamura and Murakami (1995) investigated the role of the deformed mean tropical and extratropical westerlies on eddy growth for MJO-scale tropical disturbances. They showed that stretching and shearing deformation in the exit region of the monsoon westerlies (in the tropics) supplies barotropic energy to the MJO-scale eddies.…”

“…An advantage of the formulation is that the effects associated with eddy moisture transport and the release of latent heat by eddies are included (see e.g., Salustri and Stone 1983). Following Kawamura and Murakami (1995), the local barotropic interaction is given by…”

“…Few studies have assessed this interaction between submonthly scale eddies and the time-mean flow in the western Pacific during NH summer. Kawamura and Murakami (1995) examined the interaction between MJO-timescale eddies and the mean monsoon flow in the lower troposphere. They suggested that eddies are maintained mostly by barotropic instabilities in the tropics and by baroclinic instabilities in midlatitudes.…”

“…We will identify the seasonal dependence of the large-scale circulations associated with tropical convection over the South China Sea. Localized Eliassen-Palm (E-P) flux diagnostics (Trenberth 1986) and their application (Kawamura and Murakami 1995) are then used to assess eddy-mean flow interactions and to quantify the role of the mean flow in maintaining 10-25-day eddies. Eddy forcing of the mean flow is also investigated.…”