Abstract Author: Charles Jones
Abstract Title: A probabilistic view of the actvity of the Madden-Julian Oscillation
Abstract: The Madden–Julian oscillation (MJO) is the most prominent mode of tropical intraseasonal variability with significant influences on monsoons systems and interactions with El Nino/Southern oscillation (ENSO). The MJO also modulates rainfall and extreme events in many locations around the world. Since the MJO involves intense tropical convective heating anomalies, tropical-extratropical interactions are significant during its life cycle. Therefore, some studies have detected noticeable impacts on the skills of weather forecasts on medium-to-extended ranges. Currently, numerical weather prediction and climate models are unable to fully simulate the eastward propagation, zonal scale, periodicity and phase speeds of the MJO.
Observational studies based on NCEP/NCAR reanalysis suggest increases in MJO activity with a positive linear trend in the number of MJO events since the mid 1970’s. While reanalysis products are consistently derived based on comprehensive data assimilation systems, the number of observational samplings (e.g., radiosonde, satellite-derived winds etc) has also increased over time. This presentation will show results from stochastic models which are able to simulate the statistical properties of the MJO. Daily averages of zonal winds at 200-hPa and 850-hPa from NCEP/NCAR reanalysis are used (1948-2007). The MJO is represented with combined empirical orthogonal functions (EOF) of U200 and U850 anomalies. MJO occurrences are identified based on the amplitudes of the first two principal components. The first stochastic model simulates MJO variability using a nine-state homogeneous Markov model. In the second model, transition probabilities are allowed to vary over time (non-homogeneous) and are derived as functions of sea surface temperature anomalies in the tropics. The presentation will contrast ensemble simulations of both models and investigate the likelihood that the increase in MJO activity is associated with tropical sea surface warming.