TRACKING OF TROPICAL INTRASEASONAL CONVECTIVE ANOMALIES

A new algorithm based upon a multiple object tracking method is developed to identify, tracks and classify Tropical intraseasonal oscillations (TISO) on the basis of their direction of propagation. Daily NOAA Outgoing Longwave Radiation anomalies (OLRA) from 1979-2013 are Lanczos bandpass filtered for the intraseasonal time scale (20-100 days) and spatially averaged with eight neighboring point to get large spatial scales (~10 km). Tracking of TISO is performed by using a two-stage Kalman filter predictor-corrector. Two dominant components of the TISO (Eastward propagating and Northward propagating) are classified, and it is found that TISO remains active throughout the year. Eastward propagation of the TISO occurs from November to April with phase speed of ~4 m/s and Northward propagation of the TISO occurs from May to October at ~2 m/s in both Indian and Pacific Ocean basins. I . M O T I V A T I O N The Tropical Intraseasonal Oscillation (TISO) refers to variability on the time scale of 20-100 days, intermediate between the time scales traditionally associated with weather and climate. Physical understanding of TISO is a very important and challenging aspect of making predictions beyond the limit of instantaneous weather. TISO can be classified into two dominant components on the basis seasonality: (a) Madden Julian Oscillations (MJO); and (b) Monsoon Intra-seasonal Oscillations (MISO). The prevailing view of the dynamics of the MJO is that it is governed by the coupling of a moist Kelvin-Rossby wave to convective heating by boundary layer moisture convergence [9] although there is debate [7]. In contrast, the dynamics of the MISO is governed primarily by barotropic cyclonic vorticity and easterly wind shear [5]. MJO and MISO occur in different seasons, at different latitudes and are governed by different mechanisms. MJO and MISO also have a different phase speed and direction of propagation. TISO has great importance because of its influence on the highly populated and largely agrarian economies of the tropics, where it regulates wet and dry spells of rainfall ([3]; [2]; [4]), which has a direct relationship with crop production. MJO also affects the tropical cyclone activity in all the ocean basins ([6]; [8]), El Nino Southern Oscillation (ENSO) as in [12] and extra-tropical weather and climate. Understanding of TISO is very crucial to realize the dream of seamless perdition. Most of the commonly used diagnostics to understand TISO consider dimensional reduction by seasonal and spatial averages or empirical orthogonal function decomposition. This approach may miss some information regarding direction of propagation, location and phase speed. As an alternative, we examine TISO by tracking each event and compositing events on the basis of direction of propagation. The characterstics of TISO such as prefered geographical loaction of propagation, phase speed, life span, regions of initiation and dissipation and their seasonal and intraannual variability are analyzed in this study. I I . D A T A A N D M E T H O D To identify and track TISOs, 34 years (1979-2013) of daily outgoing long-wave radiation (OLR) data from the National Oceanic and Atmospheric Administration (NOAA; 2.5° X 2.5° grid) is used. Anomalous OLR is considered a proxy for large-scale tropical convective anomalies [1] and [10], because negative OLR anomalies (OLRA) are well correlated with convective clouds. Daily OLR anomalies are obtained by removing the first four annual harmonics from the data at each gridpoint. The data is band pass filtered using a Lanczos filter in the 20-100 day band to obtain intra-