  The response of a spherical moist shallow water system to tropical imbalances
in the presence of inhomogeneous saturation fields is examined. While the
initial moist response is similar to the dry reference run, albeit with a
reduced equivalent depth, the long time solution depends quite strikingly on
the nature of the saturation field. For a saturation field that only depends on
latitude, specifically, one with a peak at the equator and falls off
meridionally in both hemispheres, height imbalances adjust to large-scale,
low-frequency westward propagating modes. When the background saturation
environment is also allowed to vary with longitude, in addition to a westward
quadrupole, there is a distinct eastward propagating response at long times.
The nature of this eastward propagating mode is well described by moist
potential vorticity conservation and it consists of wave packets that arc out
to midlatitudes and return to the tropics and are predominantly rotational in
character. In all moist cases, initially formed Kelvin waves decay, and this
appears to be tied to the off-equatorial organization of moisture anomalies by
rotational modes. Many of these basic features carry over to the response in
the presence of realistic saturation fields derived from reanalysis based
precipitable water. In boreal summer, long time eastward response is restricted
to northern hemisphere and takes the form of a wavetrain that passes over
Indian landmass into the subtropics, reaching across the Pacific to North
America. In boreal winter, the eastward mode consists of a subtropically
confined rotational quadrupole along with midlatitudinal disturbances. Thus, in
addition to circumnavigating westward Rossby waves, slow eastward propagating
modes appear to be a robust feature of the shallow water system with
interactive moisture in the presence of saturation fields that vary with
latitude and longitude.
