Takio Murakami

Relationship among Seasonal Cycles, Low-Frequency Oscillations, and Transient Disturbances as Revealed from Outgoing Longwave Radiation Data

Applying harmonic analyses to outgoing longwave radiation (OLR) data at a 2.5° longitude-latitude resolution over a tropical belt between 45°N and 45°S, the harmonics (m = 1–15) were computed for each year in 1975–1977 and 1970–1983. The sum of the first three harmonics (m = 1–3), which is referred to as Y(t), corresponds to the seasonal cycles. The sum of m = 4–15 represents low-frequency oscillations, denoted as L(t), with an approximate period range of 24 to 91 days.

Tropical 45 Day Oscillations during the 1979 Northern Hemisphere Summer

Abstract Based on FGGE 1,cvel IIIb data, the structural features of 45 day perturbations over a tropical belt (15°N-15°S) during the 1979 summer are detailed. At the equator, 45 day perturbations which are primarily associated with the zonal wind components of wavenumber 1, propagate eastward (8° of longitude per day) and upward (0.7 km per day), probably indicating downward energy flux. In the Southern Hemisphere tropics (0°–15°S), the 45 day zonal mean wind perturbations propagate downward with an approximate phase speed of 0.8 km per day. In the Northern Hemisphere tropics, they are largely of standing character with the maximum amplitude (3 m s−1) near 200 mb at 15°N. There exists a strong association between monsoon activity over South Asia and changes in the intensity of the equatorial Walker circulation. When active monsoons occur over South Asia, the Walker circulation becomes stronger than usual with prominent 850 mb easterlies (200 mb easterlies) over the eastern Pacific east of the date line an...

Empirical Orthogonal Function Analysis of Satellite-Observed Outgoing Longwave Radiation During Summer

Abstract Some of the characteristic features of the Asiatic summer monsoon were investigated using outgoing longwave radiation data obtained from NOAA polar orbiting satellites during the summer mouths in 1974–77. Monthly mean outgoing longwave radiation patterns clearly reflect pronounced monsoon activity over the India-Southeast Asian and Philippines-western North Pacific regions. Between these two regions, values of outgoing longwave radiation are relatively high and indicate less pronounced monsoon activity over the South China Sea along about 110°E. Empirical orthogonal function analysis was applied to identify the major modes of intraseasonal oscillations present in outgoing longwave radiation data during the summer. Spectral analyses of the amplitude functions for the four largest eigenvectors exhibit marked peaks in a period range of about 20–30 days. The first four eigenvector patterns were examined to investigate the zonal and meridional phase propagation of 20–30 day perturbations. Zonal phase ...

Eastward Propagation of 30–60 Day Perturbations as Revealed from Outgoing Longwave Radiation Data

Abstract Some of the characteristic features of low-frequency oscillations were investigated by applying an extended empirical orthogonal function (EEOF) analysis to 30–60 day filtered outgoing longwave radiation (OLR) data during the eight years of.1975–77 and 1979–83. The first two EEOF eigenvectors indicate a systematic eastward propagation of low-frequency modes between about 60°E and the date line (Indian Ocean–western Pacific), where they are most predominant. They weaken considerably by the time they reach the eastern Pacific east of the date line, and their phase propagation becomes ill-organized over the equatorial South America–Atlantic Ocean–Africa region. This appears to be due to an out-of-phase relationship between zonal (Fourier) wavenumbers 1 and 2 over the equatorial western hemisphere. In contrast, there exists a general in-phase relationship between these two Fourier components, and, thus, the consolidated perturbations of wavenumbers 1 and 2 become very pronounced with a distinct eastw...

Transition from the Southern to Northern Hemisphere Summer Monsoon

Abstract During the period from 15 February to 20 June 1979, the northward migration of the zones of low outgoing longwave radiation was most clearly defined over the convectively active continental regions of equatorial Africa, the maritime continent, and equatorial South America. Regionally, the withdrawal of the Southern Hemisphere summer monsoon over Indonesia and Australia was followed by the first establishment of the summer monsoon over the Malaysian Peninsula during the midtransition around 15 April, which is about two months earlier than the monsoon onset over central India. In investigating the wind changes during the transition period, smoothed time-series data were constructed by adding three components of winds; viz., the annual mean winds, which are approximately symmetric about the equator; and the yearly and half-yearly wind harmonics that are asymmetric about the equator with a distinct seasonal character. The 200 mb wind patterns during the midtransition exhibit new symmetric character w...

Atmospheric Circulation During December 1970 Through February 1971

Abstract National Meteorological Center wind data from December 1970 to February 1971 were used to compute the 90-day winter mean eddy kinetic energy, and the energy exchange between waves and between waves and zonal mean flows at 200 mb, over mid-latitude (33.0–48.1°N), subtropical (10.0–28.7°N) and tropical (5.0°N–14.8°S) belts. In the mid-latitude (subtropical) belt, waves of wavenumbers 1–10 lose (gain) energy to (from) zonal mean flows with large contributions from wavenumbers 2–3 and 5–7. Energy exchanges over the tropical belt are quite different from those in the mid-latitude and subtropical belts with conspicuously small wave-zonal flow interaction and somewhat larger wave-wave nonlinear interactions. Wavenumber 8 receives substantial energy via wave-wave interaction with other waves in both the sub-tropical and tropical belts. Sometimes the 200 mb eddy kinetic energy was well above its 90-day winter mean at all latitudinal belts. Such occasions were characterized by an extremely strong jet strea...

Westerly Bursts during the 1982/83 ENSO

Abstract Some of the characteristic features of the 1982/83 ENSO are described in terms of 3-month running mean anomalous winds (u″, v″) at 850 mb, outgoing longwave radiation (OLR″), and sea surface temperature (SST″) during the 6 yr period 1980–85 over the Indian and Pacific means. Climatological information on “westerly bursts” is obtained from band-pass filtered winds, which are denoted as us, and vs (1–6 day), um and vm (7–20 day), and ul and vl (30–60 day). Atmospheric interannual u″ and OLR″ modes are characterized by typical spatial scales of zonal wavenumbers 1 and 2, and very slow eastward phase propagations of about 0.3 m s−1 all the way from the western Indian Ocean to the eastern Pacific. Oceanic interannual SST″ modes also exhibit a slow eastward movement across the Pacific. Westerly ul bursts are sandwiched between twin cyclonic disturbances which are nearly symmetric with respect to the equator. The east-west extent of ul westerly bursts along the equator is about 10 000 km, while the corr...

Intraseasonal Atmospheric Teleconnection Patterns during the Northern Hemisphere Winter

Abstract At times the 30–60 day filtered outgoing longwave radiation (OLR) perturbations exhibited a systematic eastward propagation across the equatorial Indian Ocean–western Pacific during the five summers (1 May–30 September) of 1979–83. Such occasions are defined as “E” phase, while periods of irregular movement are designated as “NE” phase. Global-scale behavior of the 30–60 day filtered streamfunction and velocity potential fields differs significantly from E to NE phase. During E phase at 200 mb, a series of time-clustered, space-overlapping disturbances develop over the northern as well as the southern subtropics. Although individual disturbances are nearly stationary, a wave packet clearly propagates eastward with an approximate phase speed of 8° longitude per day and a space scale of wavenumber 1. Sandwiched between the Southern and Northern hemisphere wave packets are relatively weak equatorial zonal wind perturbations which also move eastward. At 850 mb, E phase behavior is characterized by st...

Annual cycle of equatorial East-West circulation over the Indian and Pacific Oceans

Abstract Along the equator, annual mean 200-mb zonal wind is approximately in phase with annual mean outgoing longwave radiation (OLR); namely, easterlies are strongest above the convective center over the maritime continent, while westerlies reach their maximum just above the dry zone over the equatorial Pacific. This is much different from what is anticipated by theories that predict that the phase of the upper-tropospheric zonal wind is in quadrature with that of the prescribed heating. The present study provides evidence that the midlatitude-equatorial coupling is primarily responsible for the maintenance of the annual mean total 200-mb zonal winds along the equator, whereas convection contributes a great deal to the annual mean upper-level equatorial divergent winds. Annual cycles occurring over the extratropics act as a transient eddy forcing of the equatorial annual mean 200-mb zonal wind through three-dimensional convergence of localized Eliassen-Palm (E-P) fluxes. They are acting to accelerate th...

Interannual Cloudiness Changes

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...