  The intraseasonal Madden-Julian oscillation (MJO) involves a slow
eastward-propagating signal in the tropical atmosphere which significantly
influences climate yet is not well understood despite significant theoretical
and observational progress.
  We study the atmosphere's response to nonlinear "Wind Induced Surface Heat
Exchange" (WISHE) forcing in the tropics using a simple shallow water
atmospheric model. The model produces an interestingly rich interannual
behavior including a slow, eastward propagating equatorial westerly multiscale
signal, not consistent with any free linear waves, and with MJO-like
characteristics. It is shown that the slow signal is due to a Kelvin wave
forced by WISHE due to the meridional wind induced by a Yanai wave group. The
forced Kelvin wave has a velocity similar to the group velocity of the Yanai
waves, allowing the two to interact nonlinearly via the WISHE term while slowly
propagating eastward. These results may have implications for observed tropical
WISHE-related atmospheric intraseasonal phenomena.
