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The relationship between intraseasonal disturbance activity and interannual variation of the tropical atmosphere
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|Title:||The relationship between intraseasonal disturbance activity and interannual variation of the tropical atmosphere|
|Keywords:||Atmosphere -- Research|
|Abstract:||The relationship between intraseasonal disturbance activity (cluster of disturbances) 'and interannual variation of tropical atmosphere is investigated using five years (1979-83) of outgoing longwave radiation (OLR) and wind data. The space and time scale of the disturbance cluster, as defined by spectral amplitude, are much larger than those of individual disturbances. The nonlinear energy cascade between intraseasonal activity and interannual modes, in terms of a frequency modulation process, increases before and after ENSO, and decreases during ENSO. Two constructed period bands of disturbance activity, i.e., 2-15 day and 30-60 day, exhibit different spatial and temporal variations on interannual time scale. The 2-15 day activity is primarily associated with interannual variations, while the 30-60 day activity is strongest over the equatorial Indian Ocean and western Pacific prior to the 1982/83 ENSO onset and below normal during the episode. There is evidence that the 30-60 day oscillation is one of the triggering mechanisms for the 1982/83 ENSO event. The results of empirical orthogonal function analysis show that ENSO, as the most dominant mode of interannual variations, is essentially one realization of a bimodal climate system. The occurrence of ENSO is the phase shift from non-ENSO steady state to ENSO steady state. The transition period is relatively short with enhanced intraseasonal disturbance, and ENSO state lasts much shorter than non-ENSO state does. The largest and most easily detectable climate signal (interannual variability) over the central and eastern Pacific is traceable to the western Pacific-Indian Ocean region (a warm pool), and is phase locked to seasonality. It is suggested that an interaction among intraseasonal activity, annual cycle, and interannual modes over the warm pool region may be instrumental in leading to ENSO onset. The phase changes for the 1982/83 ENSO event are from the pre-ENSO in Jan. 1982, through the mid-ENSO in Jan. 1983, and back to post-ENSO of Dec. 1983. Associated with this cycle is an eastward migration of interannual component of OLR and zonal winds along the equator from the Indian Ocean, across the maritime continent, to the eastern Pacific. Both the pre-ENSO and post-ENSO are possible representatives of anti-ENSO, an extreme of non-ENSO steady state. The pre-ENSO (or post-ENSO) and mid-ENSO (mature ENSO) are phase locked with the annual cycle. The pre-ENSO (or post-ENSO) synoptics are the manifestation of an anomalous enhancement of the characteristics of the normal winter circulation, whereas the mid-ENSO patterns reflect an unusual weakening of the normal winter circulation. Thus, the mid-ENSO exhibits an approximate reversal of the synoptics from the pre-ENSO (or post-ENSO) phase. Over the North and South Pacific between about the date line and 120oW, an important area for the 1982/83 ENSO, the main characteristics of the pre-ENSO (or post-ENSO) and mid-ENSO phases are summarized as follows: Pre-ENSO (or post-ENSO) is characterized by (1) an intensified indirect N-S vertical overturning with below normal equatorial convection in contrast to above normal rainfall over the extratropics, (2) an intensification of upper oceanic troughs over both the North and south Pacific with prominent equatorial westerlies between them, and (3) substantial intraseasonal disturbance activity at 200 rob. Mid-ENSO exhibits: (1) a direct N-S vertical overturning (anomaly) accompanied by equatorial convection and extratropical dry spells, (2) an unusual weakening of upper oceanic troughs (or anomalous upper anticyclones) with equatorial easterlies inhibiting intraseasonal disturbance activity, and (3) enhanced midlatitude westerlies poleward of twin anomalous anticyclones, facilitating above normal baroclinic disturbance activity. The Australasian sector (100°-160°E) is another key area for a well-defined reversal of circulation characteristics. Here, circulation changes are out of phase with those over the eastern North and South Pacific. Less organized, circulation reversals also take place over the Afghanistan-Indian Ocean region (40°-80°E) and the Central America-South America region (80°-40°W).|
Thesis (Ph. D.)--University of Hawaii at Manoa, 1988.
Bibliography: leaves 147-154.
xx, 154 leaves, bound ill. 29 cm
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|Appears in Collections:||Ph.D. - Meteorology|
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