The curious connection between ocean feedbacks and mean state moisture for MJO simulation

Ocean feedbacks to the MJO are assessed with a suite of four general circulation models (GCMs) using ocean coupled (C) and uncoupled, or atmosphere-only (A), simulations. Monthly mean sea surface temperatures (SSTs) from the CGCMs are prescribed to the respective AGCMs. Hence, each AGCM has the same SST mean state as its CGCM counterpart, but intraseasonal and higher-frequency SST variability is damped. As in previous studies, all four CGCMs produce a more realistic MJO that propagates farther into the western Pacific Ocean than in the AGCMs.

Despite the identical SST climatology for each simulation pair, mean state column water vapor (CWV) and its zonal and meridional gradients are consistently more favorable for MJO propagation in the CGCMs. Changes in mean surface latent heat (LH) fluxes are inconsistent among models, and are not correlated with CWV changes. Warm Pool equatorward wind anomalies in the CGCMs are both stronger and more frequent than poleward winds in the AGCMs—a finding that is consistent with the sharpened meridional moisture gradients in CGCMs.

One possible cause for the increased equatorward flow and more favorable meridional CWV gradients in CGCMs is the enhanced propagation of the MJO itself, since the west-of-convection equatorward flow would affect more Warm Pool longitudes, and maintain the meridionally more peaked CWV distribution. Such a "self-perpetuating MJO" paradigm, however, cannot explain the decline in AGCM MJO propagation since the AGCM has the same initial state as the CGCM. Rather, MJO skill among all four simulation pairs is significantly correlated with the near-surface vertical moisture gradient during warm SST and light (1-5 mm/day) rainrate conditions found east of MJO convection. From this perspective, ocean feedbacks directly affect low-level moistening east of MJO convection via surface fluxes, and indirectly affect mean state moisture via circulation responses to coupling. In this manner, coupled feedbacks promote MJO propagation through both ocean-atmosphere boundary layer processes, and through circulation-CWV interactions that regulate mid-level moistening by horizontal advection.