Statistical Analysis and Numerical Simulations of the Intertropoical Convergence Zone during Normal and ENSO Years.

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Title: Statistical Analysis and Numerical Simulations of the Intertropoical Convergence Zone during Normal and ENSO Years.
Author: Roswintiarti, Orbita
Advisors: Carolyn R. Miller, Chair
Edwin R. Gerler, Member
Brad S. Mehlenbacher, Member
Abstract: The purpose of the research has been to provide a better understanding of the climatology, variability, mechanisms, physical processes, and predictability of the Intertropical Convergence Zone (ITCZ), one of the most prominent and important features of the tropical atmosphere. The effort is focused on six different tasks based on statistical and numerical methods. Analysis of the mean and meridional profile of the global ITCZ suggests that each tropical ocean domain has different structure and latitude preference of convection due to different interactions between the oceans and the atmosphere. Moreover, the power spectrum density analysis shows that regions of the dominant interannual variation of deep convection occur over Indonesia and the central Pacific. With respect to the annual cycle, large variations in convection are mainly over the monsoon and continental regions. At the semiannual time scale, convection is greatest over the Indian subcontinent and Australia. The relationships between sea surface temperature, large-scale atmospheric circulation, and convective activity in tropical oceans are examined in the second study by the regression analysis. The results show that the relationship between sea surface temperature and convection is strongly influenced by the large-scale circulation, particularly in the Indian and western Pacific oceans. However, the relationship between large-scale circulation and convection is less dependent on SST in all the ocean domains. In the third study, the relationship and long-term predictability between sea surface temperature anomalies over the Pacific Ocean during El Nino/La Nina events and convective anomalies over Indonesia are examined using the Empirical Orthogonal Function (EOF) and Canonical Correlation Analysis (CCA) techniques. The results show that the models are potentially useful in predicting convective anomalies over Indonesia during boreal fall and winter months up to six months ahead. On the other hand, the models have lower skills in spring and summer months caused by the reduction of the east-west pressure gradients of the Southern Oscillation and the so-called spring barrier. The numerical simulations presented in the last three studies are performed to study the dynamics and the physics of the ITCZ system. The behavior of the northeast monsoon over the Indian Ocean and Indonesia during a normal and during an El Nino/Southern Oscillation (ENSO) year is compared using the Naval Research Laboratory/North Carolina State University (NRL/NCSU) model and the Fifth Generation of the Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (PSU-NCAR MM5). During the active monsoon in 1997, both models generally simulate the mean circulations up to 24 hours, but show rapid decline in the forecasts beyond that when the winds become stronger. In contrast, when the synoptic-scale dynamic forcing was strong during the 1998 ENSO event, the simulated mean flows are in good agreement with the analyses up to 48 hours. It is also found that the rates and distributions of oceanic and land mass rainfall are more realistically simulated by the NRL/NCSU model than by the PSU-NCAR MM5 for these two cases. The model errors can be attributed among others to the inaccuracies in the PBL parameterization and uncertainties in the initial conditions. In the fifth study, the transport of air parcels during the 1997 forest fires in Kalimantan, Indonesia is investigated using simulated wind fields from the PSU-NCAR MM5. The interactions between the synoptic conditions and the regional winds are found to be very important on the long-range mean transport. Drought conditions and the persistence of strong easterly winds caused air masses from east and south Kalimantan to travel a distance of 1500 km in only four days. Finally, in the sixth study the PBL structure simulated from the PSU-NCAR MM5 and its role in the transport of air masses during the Indian Ocean Experiment 99 (INDOEX 99) are examined. Results indicate that during undisturbed conditions, the PBL over the Indian Ocean reaches its maximum height in the night hours and minimum height in the early morning hours. Near the coast, localized low-level circulations are simulated in which the air parcels are trapped. An elevated land plume is also simulated over the ocean.
Date: 1999-11-19
Degree: PhD
Discipline: Marine, Earth and Atmospheric Sciences

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