Moist Shallow Water Response to Tropical Forcing: Initial Value Problems

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.