Figure 14. Prediction skill of the MJO’s RMM indices in
S-SHiELD with (solid) and without (dotted) the interactive MLO for 92
40-day predictions initialized during the 2011-2012 DYNAMO period. Top:
Correlation coefficient; bottom: RMSE.
The behavior of the MJO in GFDL’s CMIP6-generation climate models (Zhao
et al. 2018) suggests that the two keys for a good MJO simulation are an
appropriate convection scheme and some form of interactive ocean, a
result found also by DeMott et al. (2019) and others. A second set of
S-SHiELD experiments was performed using specified climatological SSTs
plus frozen anomalies. These simulations without the interactive MLO had
much smaller RMM correlations, with predictions no longer useful after
day 20, and larger errors. The effect of the interactive ocean is made
clear in Figure 15, in which S-SHiELD with the MLO correctly predicted
the formation of all three strong MJO events during this period 10–15
days in advance, and correctly propagated all events through the
Maritime Continent (near 120 E longitude), although the propagation
speed is slower than observed and there is some disruption near the
Maritime Continent. However, S-SHiELD with prescribed SSTs has
difficulty propagating the MJO through the Maritime Continent and for
the November event creates no MJO whatsoever. The November event proves
particularly challenging for S-SHiELD without the MLO as it performs
poorly at a range of lead times (Supplemental Figure S2) but poses no
problem for S-SHiELD with the MLO. It is clear that the simple,
inexpensive MLO used in S-SHiELD is sufficient to significantly extend
the predictability of the MJO.
DeMott et al. (2019) did not describe any deficiencies of the MJO from
models using a 1D column ocean instead of a 3D dynamical ocean, which
suggests a limited role for direct feedbacks between ocean circulations
and the MJO. However they did not rule out indirect effects of the MJO
on ocean circulation that could impact other S2S-timescale phenomena or
MJO teleconnections. Other investigators have found that the MJO does
alter ocean circulations on intraseasonal timescales, notably the result
of Moum et al. (2014) found during DYNAMO. It remains to be seen whether
these ocean dynamical effects of the MJO are of sufficient impact to
affect S2S prediction skill. One advantage of the MLO is that we can
nudge to climatological SSTs and so do not have climate drifts that
challenge fully coupled models.