WESTWARD PROPAGATING LOW-FREQUENCY OSCILLATION AND ITS TELECONNECTION IN THE EASTERN HEMISPHERE

By use of daily OLR data of eight years (1975—1977,1979—1983),the propagation features of 30—60day low-frequency oscillation (LFO) and its teleconnections are studied.The results are as follows:(1)The LFO is quite active in the regions of the South China Sea,mainland of China and subtrop-ical western-North-Pacific.(2)The zonal propagation direction of LFO is eastward along the equator and gradually changes towestward north of 10°N and south of 10°S.The westward propagation of LFO dominates in the areaof 15°N-30°N,Eastern Hemisphere.(3)In the region of east Asia (120°E),the main meridional directions are northward in tropics andsouthward in high latitudes.These two opposite propagating LFO are merged in the vicinity of subtropics.Sometimes,the northward propagating LFO can penetrate through the subtropics to high latitudes and viceversa.On the average,the northward propagation dominates in summer time.(4)The EOF analysis of the summer data shows that there are two main eiginvector centers of OLR-LFO,one is located over the Bay of Bengal and the other over the tropical western-North-Pacific.Thesign of these two centers are just opposite to each other.It should be noted that on the normal,thesetwo oscillation centers mentioned above coincide with the two strong centers of atmospheric 12eat source insummer.It means that the activities of LFO in the Indian monsoon system and the East Asian monsoonsystem are reverse.For the first component of eiginvector,a belt of LFO with the same sign stretcheswith a SW-NE direction from the tropical center in the western-North-Pacific northwestward,passing bythe point at 15°N,180°E and reaches southwestern states of the United States.To the north and southof this belt,there are other two belts with opposite sign.Again further north and south of them,there areother two belts with the same sign as the first one.Furthermore,to the NW (near Taiwan) and SE (10°S,160°W) of the tropical East Asian center,there is,respectively,another center with opposite sign.Analmost straight line can go through all three centers.The main characteristics of the second,third andfourth components of eiginvector are the same as that of the first one.It indicates that the teleconnectioncentered around the tropical East Asian center of LFO is characterized by a SW-NE oriented wave frontand the energy transport of oscillation from SE to NW.That is to say,the oscillations in the tropicalwestern-North-Pacific may be the source of those in China during summer.We call this teleconnection pat-tern the WPC (western Pacific-China) pattern so as to distinguish from the PNA pattern.

WESTWARD PROPAGATING LOW-FREQUENCY OSCILLATION AND ITS TELECONNECTION IN THE EASTERN HEMISPHERE

By use of daily OLR data of eight years (1975-1977,1979-1983),the propagation features of 30-60 day low-frequency oscillation (LFO) and its teleconnections are studied.The results are as follows:(1) The LFO is quite active in the regions of the South China Sea,mainland of China and subtropical western-North-Pacific.(2) The zonal propagation direction of LFO is eastward along the equator and gradually changes to westward north of 10°N and south of 10°S.The westward propagation of LFO dominates in the area of 15°N-30°N,Eastern Hemisphere.(3) In the region of east Asia (120°E),the main meridional directions are northward in tropics and southward in high latitudes.These two opposite propagating LFO are merged in the vicinity of subtropics.Sometimes,the northward propagating LFO can penetrate through the subtropics to high latitudes and vice versa.On the average,the northward propagation dominates in summer time.(4) The EOF analysis of the summer data shows that there are two main eiginvector centers of OLR-LFO,one is located over the Bay of Bengal and the other over the tropical western-North-Pacific.The sign of these two centers are just opposite to each other.It should be noted that on the normal,these two oscillation centers mentioned above coincide with the two strong centers of atmospheric 12eat source in summer.It means that the activities of LFO in the Indian monsoon system and the East Asian monsoon system are reverse.For the first component of eiginvector,a belt of LFO with the same sign stretches with a SW-NE direction from the tropical center in the western-North-Pacific northwestward,passing by the point at 15°N,180°E and reaches southwestern states of the United States.To the north and south of this belt,there are other two belts with opposite sign.Again further north and south of them,there are other two belts with the same sign as the first one.Furthermore,to the NW (near Taiwan) and SE (10°S,160°W) of the tropical East Asian center,there is,respectively,another center with opposite sign.An almost straight line can go through all three centers.The main characteristics of the second,third and fourth components of eiginvector are the same as that of the first one.It indicates that the teleconnection centered around the tropical East Asian center of LFO is characterized by a SW-NE oriented wave front and the energy transport of oscillation from SE to NW.That is to say,the oscillations in the tropical western-North-Pacific may be the source of those in China during summer.We call this teleconnection pattern the WPC (western Pacific-China) pattern so as to distinguish from the PNA pattern.