Tropical-wide teleconnection and oscillation. I: Teleconnection indices and type I/type II states

Abstract

The tropical oscillation of the ENSO-monsoon (El Niño Southern Oscillation-Asian monsoons) system is studied based on 34-year simulations of an atmospheric general-circulation model with specified sea surface temperature (SST), together with some observed data for the same period. In particular, two indices are studied: the first is the generalized index of Indian monsoon rainfall (IMR), and the other is an index by which the interaction/non-interaction between the ENSO and the Indian monsoon system is assessed. In order to examine the validity of the first index, the Indian subcontinent and the near-equatorial western Pacific were selected as two key locations. They correspond, approximately, to the dominant regions in the first empirical orthogonal function of precipitation, for boreal summer and winter, respectively. These two regions form the western rim of a ‘horseshoe’ teleconnection pattern in their respective seasons. the time coefficients of empirical orthogonal function mode 1 in the two seasons are taken as the key indices which are referred to as the Tropical-wide Oscillation Index (TOI). Having defined the spatial patterns in both seasons, the lead-lag teleconnection structure associated with the ENSO-monsoon system is studied; the result is that the TOI for boreal summer is more useful than the TOI for boreal winter for identifying the lead-lag nature of the ENSO-monsoon. the new index is then related to traditional indices like the IMR and the Southern Oscillation Index, but proves to be a more comprehensive index for lead-lag correlations with key variables, while the latter, the TOI for winter, has no precursory signal for the Indian monsoon for the following summer. The second index is the Walker circulation Index (WAI), which represents the eastward/westward shift of the updraught region of the Walker circulation. It is proposed that the WAI can usefully distinguish between years when the ENSO-monsoon oscillation operates (type I) and years when it does not (type II). If the SST in the key region has a climatologically close-to-normal condition, the state of the coupled equatorial system is in type II. On the other hand, if the deviation from climatology is large, the tropical-wide oscillation will continue to be active; these years are type I. Elimination of the type II years from the data significantly enhances the time-lagged correlations between the TOI and some key variables, such as SST. As a result, the biennial oscillation becomes clearer.