The effects of ENSO‐types and SAM on the large‐scale southern blockings

Abstract

ABSTRACTIn this observational study, the effects of the two types (Canonical and Modoki) of El Niño – Southern Oscillation (ENSO) and Southern Annular Mode (SAM) on the large‐scale southern blockings are investigated using the NCEP Climate Forecast System Reanalysis (CFSR) dataset. The automated 2D blocking index used here removes those structures that do not represent a typical blocking pattern. The daily SAM and monthly ENSO indices are defined when their time coefficient of the empirical orthogonal function is above or below 0.5 and 1.0 standard deviation, respectively. This study indicates that the El Niño Modoki is the most favourable ENSO mechanism for blocking events during the southern cool season. In particular, when El Niño Modoki occurs with positive SAM and/or weak SAM it acts to increase the number of days of blocking over the Southwest Pacific, or when it occurs with the negative SAM acts to maintain the observed number of days of blocking over the Southeast Pacific. Also, increases in the number of days of blocking are found in the warm season over the Southeast Pacific when the canonical El Niño occurs with negative SAM. On the other hand, the long‐term change in the annual number of days of blocking shows a strong (moderate) downward linear trend over the Southwest (Southeast) Pacific, as well as the two types of La Niña, their temporal distribution with a positive SAM appears as the major cause.