Abstract
The Madden Julian Oscillation (MJO) is a prominent mode of tropical
variability and a key driver of intraseasonal predictability. Accurate
prediction of the MJO is challenging due to complex interactions with
the background state and sensitivity to initial conditions and
unresolved processes. Using NOAA’s Unified Forecast System, we explore
the impact of subtle differences in initial conditions on MJO forecasts
over the Indo-Pacific. Model runs initialized with two independent
reanalyses show significant and systematic differences in
MJO-circulation amplitude that persists over a 15-day forecast period.
Additional analysis attributes this to differences in their initial
atmospheric static stability. The reanalysis that is initially less
stable is associated with enhanced large-scale vertical motion and
divergent winds throughout the forecast. Notably, a convection-based MJO
index shows less sensitivity to initial conditions, suggesting that
variations in convection and diabatic heating are not the main drivers
for the predicted differences in MJO circulation amplitude.