District-level forecast of northeast monsoon rainfall over Tamilnadu using Indian Ocean dipole mode index

The northeast monsoon season (October-December) contributes a substantial percentage of annual rainfall over Tamilnadu. The present paper describes a method for prediction of northeast monsoon rainfall (NEMR) over Tamilnadu on smaller spatial scale, i.e., district-level with sufficient lead time. Tamilnadu has been divided into ten homogeneous clusters of districts and the predictions are made for each cluster with lead times of two and one months using Indian Ocean dipole mode (IODM) index. A stronger western pole of IODM during August-September is associated with enhanced northeast monsoon activity over most of the districts of Tamilnadu. The predictions on the basis of regressions developed from NEMR and IODM index data have been validated for six years from 1997-2002. For many districts the mean errors between actual (realized) and predicted rainfall are within ±10%. Hence, using IODM index, it is possible to predict the NEMR activity over most of the districts of Tamilnadu with a lead time of two months, with only exception of NEMR over Kanyakumari which is not significantly correlated to IODM phenomenon.

Relationship between Indian Ocean dipole mode and summer monsoon

Utilizing the Indian Ocean Dipole Mode (IODM) and Indian Summer Monsoon Rainfall (ISMR) data for the period 1960-2002 the relationships between the IODM and monsoon onset over Kerala and rainfall distribution over the country have been studied. It has been found that stronger/weaker western pole during April-May is associated with delayed/early monsoon onset over Kerala. Stronger eastern pole during March-April seems to be associated with enhanced seasonal (June-September) rainfall over peninsular India. The IODM index of July-August can provide good indications of summer monsoon activity over peninsular India during the withdrawal phase of the monsoon, i.e., during September.

Role of the eastern boundary-generated waves on the termination of 1997 Indian Ocean Dipole event

The termination of Indian Ocean Dipole (IOD) events is examined in terms of equatorial wave dynamics. In situ and satellite observations combined with an output from a linear wave model are used in this study. Our emphasis is on the 1997 IOD event but our results apply to other positive IOD events as well. We find that the termination of anomalously cold sea surface temperature (SST) in the eastern pole of the dipole is associated with a warming tendency caused by the net surface heat fluxes. However, net surface heat fluxes alone cannot explain the total change in the SST. We show that during the peak phase of an IOD event, the weakening of zonal heat advection caused by eastern boundary-generated Rossby waves combined with the reduction of vertical entrainment and diffusion creates favorable conditions for surface heat fluxes to warm the SST in the eastern basin.

Mechanisms of asymmetry in sea surface temperature anomalies associated with the Indian Ocean Dipole revealed by closed heat budget

The Indian Ocean Dipole (IOD) is an interannual climate mode of the tropical Indian Ocean. Although it is known that negative sea surface temperature (SST) anomalies in the eastern pole during the positive IOD are stronger than positive SST anomalies during the negative IOD, no consensus has been reached on the relative importance of various mechanisms that contribute to this asymmetry. Based on a closed mixed layer heat budget analysis using a regional ocean model, here we show for the first time that the vertical mixing plays an important role in causing such asymmetry in SST anomalies in addition to the contributions from the nonlinear advection and the thermocline feedback proposed by previous studies. A decomposition of the vertical mixing term indicates that nonlinearity in the anomalous vertical temperature gradient associated with subsurface temperature anomalies and anomalous vertical mixing coefficients is the main driver of such asymmetry. Such variations in subsurface temperature are induced by the anomalous southeasterly trade winds along the Indonesian coast that modulate the thermocline depth through coastal upwelling/downwelling. Thus, the thermocline feedback contributes to the SST asymmetry not through the vertical advection as previously suggested, but via the vertical mixing.

Observed of Equatorial Currents in the Indian Ocean during Two Contrasting IOD Events: 2006 and 2010

The evolution of Indian Ocean Dipole (IOD) events in 2006 and 2010 is investigated using observational data products that are made to understand several processes in the positive (negative) phase of IOD events. Two Acoustic Doppler Current Profiler (ADCP) moorings mounted at 90°E and 80.5°E along the equator were used to evaluate the zonal current variation during two contrasting Indian Ocean Dipole (IO) events. Westward anomalies of the zonal current were observed at 0°, 80.5°E during the peak phase of the positive IOD event from October to December 2006. Meanwhile, the observed zonal currents at 0°, 90°E only showed the short-term westward anomalies during October 2006. On the other hand, during the negative IOD event in 2010, the observed zonal current at both mooring locations indicated strong intraseasonal variations of the eastward anomalies from August to December 2010. Strong easterly (westerly) anomalies of the surface zonal winds were observed during the peak phase of the positive (negative) IOD event in 2006 (2010). These easterly (westerly) anomalies forced upwelling (downwelling) equatorial Kelvin waves indicated by the negative (positive) sea surface height anomalies. Strengthening (weakening) of upwelling (downwelling) along the equatorial Indian Ocean would be a significant factor for further understanding of IOD evolution.