Why do super clusters and Madden Julian Oscillation exist over the equatorial region?

Abstract

Over the tropics, two types of precipitation systems (PSs), super clusters (SCs) and Madden Julian Oscillation (MJO), are frequently observed, having similar meridionally symmetric structures about the equator but different eastward-propagating (EP) speeds. To investigate the reasons why these PSs exist, the dependence of the longitudinal variations of surface sea temperature (SST) is examined using a NICAM on an aqua-planet. In a longitudinally uniform-SST case, only SC-like fast-EP PSs appear. When the longitudinal variation of the SST increases, meanwhile, a stationary Walker circulation (WC) emerges and MJO-like slowly EP PSs occur on the western part of a high-SST area. It is expected in the real atmosphere that two different types of EP PSs can exist in parallel due to complex surface conditions: 1) SCs as free PSs and 2) MJOs as forced PSs. Here, free (forced) PSs mean convection, which is uncontrolled (controlled) by the longitudinal variation of the SST. It is also obtained that an asymmetric WC is produced, even though the SST variation symmetric about 180° longitude is forced. Owing to the WC, combined with the Hadley circulation, the MJO is generated and decayed locally, and westward-propagating PSs are dominantly observed in the subsidence areas of the WC.