Several times each boreal summer, a disturbance forms in the western Indian Ocean just north of the equator and commences an eastward progression growing in scale and intensity. On reaching the eastern basin the convection bifurcates into two poleward arms, one of which propagates over the Bay of Bengal and commences an active phase of the monsoon. The passage of the disturbance described thus takes about 12-15 days. After its demise, the monsoon enters a break phase over South Asia lasting another 10-20 days before the next sequence begins. Monsoon intraseasonal oscillations (MISOs) dominate the summer rainfall of South Asia. To a degree far greater than the absolute magnitude of the mean summer rainfall, the timing of the members of the seasonal suite of MISOs determines the success or failure of the summer agriculture for a large proportion of the population of the planet.

One of the great mysteries of a MISO is why it forms in the western Indian Ocean? Noting that this part of the ocean has the largest cross-equatorial surface pressure gradient (CEPG) anywhere on the planet we pose the hypothesis that symmetric instability is a necessary condition for the initial formation. Within this scheme, potential vorticity is advected across the equator. Secondary circulations counter this instability through the generation of cyclonic vorticity. If the CEPG is weak (early in the season) or the near-equatorial environment suppressed, the secondary circulation will be shallow. However, if the boundary layer is sufficiently preconditioned and the XEPG sufficiently strong, the generation of cyclonic vorticity allows the development of deep penetrative convection. The argument is extended to show why the first MISO (which, coincidently, is the onset of the monsoon) is the strongest MISO of the season. We also note that subsequent MISOs will only form when the two necessary conditions (the CEPG and a preconditioned boundary layer) are both set. Noting that the XEPG remains strong for the entirety of the summer, it is the second condition that sets the time scale of the MISO.