Given the importance of moisture to the heating gradients which drive the monsoon, an assessment has been made of the processes which supply moisture to the monsoon environment and their variability on interannual timescales.

Anomalies present in the monsoon hydrologic cycle during ENSO events are substantially different from those outside of ENSO events. Distinct modes of variability are suggested.

• The ENSO mode involves a modulation of the strength of the trade winds and their moisture content. Anomalies are strongest in the Pacific Ocean and are weak in most other regions.
• The non-ENSO mode(s) involves large anomalies in the Indian Ocean basin and over northern Africa corresponding to the transverse monsoon (Webster, 1994). The Indian Ocean is warmer than normal preceding and during strong years associated with this mode. The monsoon gyre is more intense during these years and moisture transports from northern Africa are enhanced. Anomalies in the Pacific during these years are small.

Causal relationships among the observed anomalies remain to be identified. Is the non-ENSO pattern of anomalies simply forced by internal monsoon variability?

Will the 40-year ECMWF reanalysis project show a similar non-ENSO mode?

How well are the ENSO and non-ENSO modes of variability represented by forecast models?