Abstract
A recent study categorized the Madden-Julian Oscillation (MJO) during
the boreal winter season into four types including stand, jump, slow
eastward propagating, and fast eastward propagating MJO. This study
focuses on the stand and jump MJO events. Based on whether their
convection penetrates the Maritime Continent (MC), stand and jump MJO
are combined as non-penetrating (NP) MJO, while the rest two types are
combined as eastward-penetrating (EP) MJO. Results reveal the relative
roles of the westward-propagating wave (WPW), as well as the QBO and
ENSO, in limiting MJO propagation. Lack of the pre-moistening over the
southern sea surface of MC is responsible for NP MJO’s failure to
penetrate MC. The active convection of WPW hinders the NP MJO descending
branch over the Pacific and therfore leads to the insufficient
meridional advective moistening over the southern sea surface of the MC.
The overall drying over the MC for jump MJO also comes from the
intraseasonal dry amonalies induced by WPW over the western Pacific,
which are then advected to MC by the superposed seasonal mean northern
easterlies. The independent convection over the Pacific for jump MJO is
influenced by a combined effect of the QBO and ENSO. Under the influence
of the preferred QBO westerly (QBOW) phase, the tropopause instability
of NP MJO is decoupled with its convection. The decoupled tropopause
instability propagates eastward into the Pacific, and amplifies the
local WPW convection there for jump MJO. For stand MJO, however, the La
Nina-like seasonal mean cool sea surface temperature (SST) anmalies
confine WPW within the western Pacific. Therefore, the decoupled
tropopause instability of stand MJO is out phase of WPW over the central
Pacific, and fails to induce an independent convection as a result. The
similar diagnosis is further applied to the CESM2 and E3SM CMIP6
historical simulations to investigate the MJO diversity and propagation
in climate models. Four MJO types as in the observation are successfully
categorized in these two models. The accompanied westward propagating
waves over the central-western Pacific for NP MJO events are also
evident. However, models suffer from some biases in MJO propagation such
as too many NP MJO events than the observation.