Short-Term Planetary-Scale Interactions over the Tropics and Midlatitudes. Part II: Winter-MONEX Period

Abstract

Short-term teleconnections over the Pacific sector of the tropics and the midlatitudes in relation to monsoonal surges during the winter of 1978?79 are investigated using 850 and 200 mb wind data from FGGE/Winter-MONEX. Results show that the intensification of the 200 mb jet streak over Japan is a precursor of cold surges and subsequent downstream development over the central Pacific. A large part of the transient variation of the jet at the entrance region near northeastern China is controlled by the upper level ageostrophic flow transverse to the jet while elsewhere the contribution by momentum flux convergence is more important. It is found that the interaction between cold surges and tropical convection over the maritime continent of Borneo and Indonesia basically agrees with the model of Chang and Lau (1980) and Part I of this paper, except the intensity of the interaction was much weaker during Winter-MONEX. A new feature observed during the Winter-MONEX period is that the main convective heat source was found over the equatorial central Pacific and was enhanced 3?4 days after the surge onset. The enhancement coincided with the occurrence of a subtropical jet stream over Hawaii and a pronounced upper level trough-ridge system extending from the equatorial central Pacific to the west coast of North America. It is also hypothesized that the abnormally weak surges during Winter-MONEX may be a result of the reduced positive feedback between the tropical convection over the maritime continent and midlatitude disturbances near the east coast of China due to the remoteness of the primary tropical heat source from the surge area. The apparent downstream teleconnection observed during the post-surge periods suggests enhanced midlatitude-tropical coupling over the central Pacific which may have strong influence on synoptic developments over the west coast of the United States. Notwithstanding possible uncertainties for our results in the tropics, the study suggests that midlatitude systems are capable of forcing tropical convection which in turn may give rise to midlatitude remote responses. Comparison of this study with Part I and other related work, also reveals some interesting aspects of the interannual variability of short-term planetary-scale interactions.