Equatorial disturbances, monsoons, and 1982–1983 ENSO

Summary

Interannual modes are described in terms of three-month running mean anomaly winds (u″,v″), outgoing longwave radiation (OLR″), and sea surface temperature (T ^″_* ). Normal atmospheric monsoon circulations are defined by long-term average winds (u _n,v _n) computed every month from January to December. Daily winds are grouped into three frequency bands, i.e., 30–60 day filtered winds (u _L,v _L); 7–20 day filtered winds (u _M,v _M); and 2–6 day filtered winds (u _S,v _S). Three-month running mean anomaly kinetic energy (signified asK ^″_L , K ^″_M , andK ^″_S , respectively) is then introduced as a measure of interannual variation of equatorial disturbance activity. Interestingly, all of theseK ^″_L , K ^″_M , andK ^″_S perturbations propagate slowly eastward with same phase speed (0.3 ms⁻¹) as ENSO modes. Associated with this eastward propagation is a positive (negative) correlation between interannual disturbance activity (K ^″_L , K ^″_M , K ^″_S ) and interannualu″ (OLR″) modes. Namely, (K ^″_L , K ^″_M , K ^″_S ) becomes more pronounced than usual nearly simultaneously with the arrival of westerlyu″ and negativeOLR″ (above normal convection) perturbutions. In these disturbed areas with (K ^″_L , K ^″_M , K ^″_S >0), upper ocean mixing tends to increase, resulting in decreased sea surface temperature, i.e.T ^″_* ≦0. Thus, groups (not individual) of equatorial disturbances appear to play an important role in determiningT ^″_* variations on interannual time scales. HighestT ^″_* occurs about 3 months prior to the lowestOLR″ (convection) due primarily to radiational effects. This favors the eastward propagation of ENSO modes. The interannualT ^″_* variations are also controlled by the prevailing monsoonal zonal windsu _n, as well as the zonal advection of sea surface temperature on interannual time scales. Over the central Pacific, all of the above mentioned physical processes contribute to the intensification of eastward propagating ENSO modes. Over the Indian Ocean, on the other hand, some of the physical processes become insignificant, or even compensated for by other processes. This results in less pronounced ENSO modes over the Indian Ocean.