Southern Hemisphere Teleconnection Patterns for Boreal Winters of El Nino years: Ray Tracing Analysis

During boreal winter, the occurence of an El Nino/Southern Oscillation (ENSO) event reflects as enhanced precipitation in various regions of the Americas. Anomalous convective activity in the central-eastern Pacific forces quasi-stationary Rossby waves following paths to North and South America. The so-called Pacific-North American (PNA) pattern forces a southward shift in the mean position of the subtropical jet, ascending motion and enhanced precipitation over California and around the Gulf of Mexico. Over South America, a weaker quasi-stationary wave train is also triggered, resulting in enhanced precipitation around southern Brazil, northern Argentina and Uruguay. In central America and northeast Brazil, the effects of enhanced subsidence associated with ENSO, inhibit the development of deep convection and produce precipitation deficit and drought. Not all El Nino events reflect as enhanced precipitation. Changes in the phases and amplitude of the quasi-stationary Rossby Waves over the subtropical Americas result in inter El Nino variability, in particular over South America. To help us in the discussion of the observational and numerical results, we used a simple and inexpensive method for studying the propagation of forced planetary waves in different basic states called the ray tracing technique. This method has been used in many previous studies with great success. The agreement between the rays and the numerical solutions of linearized barotropic models shows that the essential characteristics of barotropic Rossby wave propagation is retained by the rays which make this technique an important tool for such analysis. Ray tracing analyses show that the phase and amplitude of the quasi-stationary waves are basically affected by the structure of the mean flow. Applying this technique for four different ENSO events (1982-83, 1986-87, 1991-92 and 1997-98) during boreal winters. It was found that depending on the warm episode year, the zonal wavenumber 3 or 4 will determine the ray path in the Southern Hemisphere. For instance, the propagation of the packet during the El NInos of 82/83 and 97/98 are similar and dominated by zonal wavenumber 3. However, the trajectories are slightly different. A better understanding of the Southern Hemisphere teleconnection patterns during ENSO events can be very useful to improve the skill of the seasonal precipitation predictions over the South and North America.