Extension of the bimodal intraseasonal oscillation index using JRA-55 reanalysis

Abstract

This study aims to extend the bimodal intraseasonal oscillation (ISO) index, developed by Kikuchi et al. (Clim Dyn 38:1989–2000, 2012), using JRA-55 reanalysis back in time to 1958. The bimodal ISO index is composed of two distinct ISO indices: the Madden–Julian oscillation (MJO) index and the boreal summer ISO (BSISO) index, each aiming to capture the ISO behavior during boreal winter and summer, respectively. These indices are derived by means of extended empirical orthogonal function (EEOF) analysis applied to outgoing longwave radiation (OLR) during boreal winter and summer, respectively. By combining the MJO and BSISO indices, the state of the ISO can be reasonably represented over the course of the year. First, the original index is updated using observed OLR until recently (–Jan. 2017) with a modification of the use of extended winter and summer months (Dec.–Apr./Jun.–Oct.) instead of normal winter and summer months (Dec.–Feb./Jun.–Aug.) in EEOF analysis. The updated version is quite consistent with the original counterpart, whereas it is arguably able to represent the ISO more faithfully throughout the year. In the same manner, a JRA-55-based index is constructed. Although JRA-55 OLR has systematic biases, the resulting JRA-55-based bimodal ISO index is in excellent agreement with the observation-based index. A comparison between JRA-55 and its subset, JRA-55C (satellite observations are not assimilated at all) suggests that the JRA-55-based index is reliable even in the pre-satellite era. Interannual variability and long-term trend of the ISO activity are also discussed. The newly developed, long-term JRA-55-based index will be useful for a number of applications.