Search Results

You are looking at 1 - 10 of 21 items for

  • Author or Editor: Takio Murakami x
  • Refine by Access: All Content x
Clear All Modify Search
Takio Murakami

Abstract

The problem of finding the equatorial stratospheric response to prescribed tropospheric heat sources is treated by using a linearized primitive system on a Mercator map projection. The perturbation geopotential is assumed to be symmetric about the equator. Eastward propagating heat sources are prescribed at 20° latitude with zonal wavenumbers 1 and 2 and a period of about 16 days. They excite a substantial response near the equator with maximum zonal wind perturbation in the vicinity of 18 km, which perhaps corresponds to the lower regime of the observed Wallace-Kousky waves. However, no significant response occurs above 20 km in the equatorial stratosphere, although the zonal wind perturbation exhibits a weak maximum (<1 m sec−1) near 26 km. Presumably, diabatic heat sources at different latitudes (rather than 20°) are responsible for exciting the upper regime of the Wallace-Kousky waves.

Full access
Takio Murakami

Abstract

Based on monthly mean satellite cloudiness data over the tropical belt between 25°S and 25°N from February 1965 to July 1973, covering 102 months, the intensity of interannual changes in perturbation cloudiness, as measured from the standard deviation, was found to be a maximum over the equatorial western Pacific. The equatorial region between 15°S and 15°N is characterized by large interannual changes in cloudiness perturbations with wavenumbers 1 and 2, which occur without apparent connection with the changes in zonal mean cloudiness. Of particular interest is the tendency for the zonal mean cloudiness to increase or decrease almost simultaneously at all latitudes between 25°5 and 25°N.

Cloudiness anomalies over the western North Pacific are negatively correlated with the anomaly cloudiness over the eastern North Pacific. Similar inverse relationship is found between cloudiness anomalies over the western and eastern South Pacific. It appears that interannual cloudiness changes are coherent over a considerable geographical extent and strongly indicate a form of atmospheric teleconnection between the western North Pacific and other parts of the Pacific region. Through computations of time lag correlation, it was found that above (below) normal cloudiness in the western North Pacific near 140°E, 20°N takes place about 5 months prior to the anomaly cloudiness maximum (minimum) over the western South Pacific near Australia.

Full access
Takio Murakami

Abstract

Empirical orthogonal function analysis was applied to outgoing longwave radiation (OLR) data over three limited regions during the three winters of 1974–75, 1975–76 and 1976–77. Regional composite maps were constructed by truncating the eigenvector series of 4–6 day OLR anomalies for each winter at 70% representation. Based on these composite charts, an attempt was made to investigate the phase relationships between changes in truncated 4–6 day filtered OLR data at a selected reference point, and those in other areas within the same region.

Over region 1(25°N‐O°, 95–160°E), the winters of 1974–75 and 1976–77 have similar features in 4–6 day OLR disturbances, with systematic westward phase propagation in equatorial latitudes over the western North Pacific (∼8 m s−1) and the South China Sea (∼5 m s−1). In contrast, the 1975–76 winter is of a different character with eastward (westward) phase propagation to the west (cast) of about 130°E at equatorial latitudes. These large year-to-year changes in phase propagation appear to be related to inter-annual differences in monsoonal surge activity near the East China Sea-Japan region.

Over region 2(5°N–20°S, 90–155°E), OLR perturbations are most active over the Arafura Sea at around 10°S, 130°E. The existence of cross-equatorial, southward propagation of OLR perturbations along about 120–130°E is a common feature in all three winters. Intensification of negative OLR perturbations (increase in convection or rainfall) over the Arafura Sea region takes place approximately 3 days after the occurrence of large negative OLR anomalies near the Philippines.

Over region 3 (10°N–15°8, 50–115°E), the equatorial latitudes in the Southern Hemisphere are characterized by westward phase propagation in each of the three winters. OLR perturbations intensify as they approach 70°E and weaken after passing through 60°E. They also tend to move northward along the 60–70°E meridians from about 15°S to 10°N. These longitudes appear to correspond to a region of maximum penetration of Southern Hemisphere tropical effects and, perhaps, interhemispheric interactions in 4–6 day filtered OLR perturbations.

Full access
Takio Murakami

Abstract

The energy equation was applied to specific limited to investigate the effect of barotropic and baroclinic processes upon area-averaged eddy kinetic energy during the northern winter. At 200 mb, area-averaged eddy kinetic energy was sometimes well above its 90-day winter mean over the extratropical North Pacific, tropical central North Pacific and South China Sea. Almost simultaneously, it was below its seasonal mean over the Timor Sea. Such occasions exhibited certain definite regional energetic characteristics. Upper zonal mean flows over the extratropical North Pacific were barotropically stable and received large amounts of energy from eddy motions via barotropic interactions. In the tropical central North Pacific, eddy motions at 200 and 300 mb were maintained almost equally by barotropic instability and lateral coupling. This region imported substantial amounts of energy across the northern boundary from the extratropical North Pacific. Over the South China Sea (Timor Sea), the conversion of eddy available potential to kinetic energy was more (less) pronounced than usual.

Full access
Takio Murakami

Abstract

A scale analysis of large-scale tropical disturbances in a conditionally unstable atmosphere is made by using a Mercator map projection at the equator. The release of latent heat in convection is included through the CISK process. Provided the north-south scale of the disturbances considered is of the order of 1000 km, we obtain a balance system in which the balance equation is used to estimate the geopotential height as a function of the prescribed streamfunction, and the equation of potential vorticity includes the condensation effect. This balance system is valid not only in tropical but also extratropical regions and, if there is no condensation, becomes identically equal to the system obtained by Charney.

The problem of finding the instability of large-scale tropical disturbances is treated by using a three-level linearized balance system applied over a region extending from the equator to 30N. The frequency equation is solved numerically to find unstable waves. It appears that the balance system is capable of simulating many of the observed structural features of tropical disturbances with a remarkable degree of reality. When the basic zonal flow is set to zero we obtain a type of unstable wave, which, by being prominent in the upper troposphere with a period of about 4 days and a zonal wavelength of approximately 10,000 km, is similar to the Yanai-Maruyama wave. In the case of an easterly zonal current with westerly vertical shear, some unstable waves greatly resemble, in their structural features, the lower tropospheric equatorial waves observed in the easterly trade wind regime. Their wavelength range is 3000–6000 km and that of their period 3–5 days; their maximum wind amplitude is located either at the equator or in the vicinity of 10N. When the observed wind is assigned as the basic zonal flow, some tropical disturbances with wavelengths of 5000 km and periods of 20–27 days become unstable by receiving most of their kinetic energy from the zonal current.

Full access
Takio Murakami

Abstract

Empirical orthogonal function analysis was applied to outgoing longwave radiation (OLR) data obtained from NOAA polar orbiting satellites during the winter months in 1974–77, over the monsoon region extending from 50°E to the date-line and from 30°N to 20°S. Spectral analyses of the amplitude functions for the ten largest eigenvectors exhibit marked peaks in a period range of about 15 to 30 days. Standard deviations of 15°30 day filtered OLR data am extremely large over the Arafura-Indonesian Seas region with a maximum at 10°S and 130°E.

A compositing technique was applied to 15–30 day filtered OLR data to investigate the relationships between long-period changes at a reference point (10°S, 130°E) and those over other regions. Composite maps, which were constructed by considering only the first ten eigenvectors, indicate distinct southward propagation of OLR perturbations along 100–115°E from about 25°N to 20°S. These longitudes appear to correspond to a region of maximum penetration of Northern Hemisphere midlatitude effects and, perhaps, interhemispheric interactions in 15–30 day filtered OLR fields. Intensification of negative OLR perturbations over the South China Sea-Bay of Bengal region, which is presumably associated with active winter monsoonal surges, tends to occur about 5–7 days prior to a decrease in OLR values (increase in convection and/or rainfall) over the Arafura-Indonesian Seas region.

Full access
Takio Murakami

Abstract

The time-longitude cross section of 30–60 day filtered equatorial outgoing longwave radiation indicates many occasions or irregular (or even westward) movement during the five winters of 1979–84. Such occasions are defined as “NE” phase, while periods of regular eastward movement are designated as “E” phase. Global-scale behavior of the 30–60 day filtered velocity potential and streamfunction fields differ significantly from the E to NE phase.

East (west) of an intense convective cell during the E phase are well-organized twin upper-tropospheric cyclonic (anticyclonic) systems straddling the equator with anomalous westerlies (easterlies) between them. A convective cell surrounded by four tropical disturbances at 200 mb constitutes a single equatorial eastward propagating low-frequency mode, which becomes prominent only during the E phase. When equatorial convection reaches the Indonesian region, a well-defined teleconnection pattern develops over the eastern Pacific and the North American continent. This teleconnection pattern is in phase (barotropic) in the vertical. The 850 mb streamfunction fields during the E phase appear to be dominated by three types of 30–60 day northerly surges in the Northern Hemisphere. The first type is directed southward along the western periphery of the Tibetan Plateau, eventually reaching the Arabian Sea and the equatorial Indian Ocean where convection becomes more pronounced than usual. The second type is associated with a strong pressure gradient between an anomalous anticyclone over Siberia and an intensified cyclone near Japan. These northerly surges, which become strongest about 15 days after the first type of northerly surge, do not appear to contribute much to the enhancement of equatorial convection. The third type is of subtropical North Pacific origin and acts as an effective regulator for equatorial convection over the western Pacific east of New Guinea.

At 850 mb, no significant meridional surges occur during the NE phase. The immediate consequences are weak convective activity and an ill-defined equatorial mode in the NE phase compared with the E phase. Poleward of about 25°N, 30–60 day perturbations during the NE phase are as pronounced as in the E phase. The predominance of a well-organized wave train over the Eurasian continent is of interest. Also prominent is the teleconnection pattern over the eastern Pacific and the North American continent. Since the equatorial convective activity is depressed well below normal during the NE phase, the contribution of convection toward the evolution of these teleconnection patterns is negligible.

Full access
Takio Murakami

Abstract

The problem of finding the equatorial tropospheric response to a prescribed heat source is treated by using a linearized primitive system on a Mercator map projection. The diabatic heat source is assumed to be westward propagating in a sinusoidal form with zonal wavelength 7000 km and phase speed 4 m sec−1, the corresponding period being approximately 20 days. The structural features of the induced waves are quite sensitive to the vertical shear of the basic zonal current. The case with westerly vertical shear reveals that the axis of geopotential perturbation tilts eastward with increasing height in the lower troposphere, and the wave activity in the upper troposphere is far more pronounced than that in the lower troposphere. The amplitudes of perturbation geopotential and horizontal wind attain their maxima near 13 km and about 1–2° north of the maximum heating prescribed at 8.3° latitude. In the case with easterly vertical shear, a westward tilt of the axis of geopotential perturbation occurs in the lower troposphere, and at 13 km where the atmospheric response is most active the wind field is characterized by the predominance of its zonal component equatorward of the latitude of the maximum heating. Along that level, the geopotential perturbation becomes maximum right at the equator and is almost in phase with the perturbation zonal wind.

Full access
Takio Murakami

Abstract

Some of the characteristic features of the summer monsoon circulation are investigated using 200 mb wind data operationally determined by the National Meteorological Center during three summers (1970–72). Three-year summer mean 200 mb divergence was large near India, Japan and the Philippines, three of the most actively convective regions during the summer monsoon. A pronounced 200 mb anticyclone lay centered over north India, while the 200 mb velocity potential outflow center was located near the Philippines with a prominent north–south overturning, vertical circulation along 125°E. The intensity and location of the 200 mb anticyclone changed substantially from one monsoon season to the other. Little change was observed in the location of the primary 200 mb velocity potential outflow center.

The energy equation was applied to specific limited regions to investigate the effect of boundary fluxes and barotropic processes upon area-averaged eddy kinetic energy during summer. Area-averaged 200 mb zonal mean flows are barotropically unstable over the subtropical North Pacific, the South China Sea-Indonesia region and the tropical South Indian Ocean. Over Indonesia and the South Indian Ocean, the upper zonal mean flows were substantially more barotropically unstable in 1972 than in 1970 or 1971. This is contrasted with exceptionally stable 200 mb zonal mean flows in summer 1972 near India where the monsoon rains were abnormally weak.

Full access
Takio Murakami

Abstract

In the present study a steady heat source is prescribed to be maximum at 28N and 100E, while a transient heat source specified at 20N oscillates in time with a period of 16 days and zonal wavenumbers 1 to 3. The basic equations for excited wave perturbations are the primitive equations in spherical coordinates with a log-pressure system in the vertical. We apply them over a region extending from 48S to 64N. The basic nonlinear equations are separated into two parts, one for 16-day time mean motions and another for transient wave perturbations. The numerical integration is done as a marching problem with respect to time for the period from 0 to 48 days, at which time the 16-day mean motions are almost independent of time (steady) and transient waves change nearly periodically in time.

The steady wave perturbations are characterized by a pronounced anticyclonic circulation centering at about 15 km near 80E and 32N, accompanied by strong easterlies prevailing to its south. They transport wave energy southward across the equator, but carry westerly momentum from the winter to the summer hemispheres. The transient wave perturbations exhibit a great similarity in their structural features to the 16-day filtered wind fluctuations observed in the monsoon region.

Full access