Reasons For the Monsoon Fizzle

In communicating with the SMN best we can tell it was the 700mb height pattern across N.A. and the Atlantic that was the problem: a block in Canada, a trough to the south in the eastern US and deep trough off Europe. The US trough directed dry northerly flow into Mexico for most of the month with easterly waves being sheared by the two troughs to the north as they tried to make it westward. El Nino probably contributed to the pattern as Great Lakes troughs are more common in August with El Ninos) and the QBO changing to a strong East Phase is also associated with lower rainfall in Mexico (and India).The cross equatorial flow index (SLP gradient Easter Island to Costa Rica) stayed high all month suggesting good flow from the south into the ITCZ. Problem is there were no wild swings in the SLP gradients to promote wind bursts to push the ITCZ north into Mexico. June had some strong pulses heavy rains and good TC genesis east of 110W, but not July which saw storms form well offshore and west of 110W. The very high index (>10mb)is counter to any farther development of El Nino at this time. The Aussies have noted the same in their daily discussions pointing out that the SOI is very high and counter to strengthening of the event.

Art Douglas