The Indo-Western Pacific climate variability and the impacts on Indian summer monsoon: Two decades of advancement in India

Interannual climate variations over the Indo-Pacific region are mostly dominated by El Niño Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), subtropical Northwest Pacific circulation and Indo-Pacific Ocean Capacitor (IPOC) Mode. The present manuscript reviews and summarizes the work mostly carried out in our lab on the weather and climate variability with a special emphasis on Indian Ocean, western north Pacific and Indian Summer Monsoon Rainfall (ISMR). Here we have provided a synthesis on the Indian Ocean modes, physical processes, biases in models, interaction with Pacific, and their impact on ISMR and also unraveled some important mechanisms. A new mode of variability in the Tropical Indian Ocean (TIO) subsurface temperature was recently reported and which is found to have strong impact on the regional climate variability. We have also demonstrated the formation mechanism using in situ observation, reanalysis, ocean models and coupled ocean atmosphere models. Intense basin wide TIO warming during El Niño and IOD co-occurrence years and the dynamic processes behind maintaining this warming are explored. We have shown for the first time that a pronounced up-westward propagation of subsurface warming along the sloping mean thermocline and westward propagation of Barrier Layers corroborated by downwelling Rossby waves excited by equatorial zonal wind anomalies during El Niño-IOD co-occurring years. A robust positive feedback between SST warming and Barrier Layer development lead to changes in air-sea interactions over the western TIO, which is a region crucial for ISMR prediction and development of MJO. Several science questions leading to the understanding of ENSO and non-ENSO teleconnections to ISMR with more emphasis on decay phase of El Niño and the Indo-western Pacific Ocean capacitor (IPOC) mode and Pacific Japan (PJ) pattern are addressed. We have also explored the influence of the Pacific-Japan pattern (PJ) on ISMR and proposed the possible physical linkages through coupled (ocean-atmospheric) and uncoupled (atmospheric) pathways and showed that in response to the PJ pattern the rainfall over the southern India enhances due to northwestward propagating Rossby waves corroborated by low level convergence in the southern flank of westward extended western north Pacific (WNP) anticyclone. On the other hand, it is found that westward propagating atmospheric cold Rossby wave as a response to suppressed convection over the western north Pacific region induced moisture divergence associated with IPOC mode and downward motion play important role in maintaining negative rainfall anomalies over the monsoon trough region. The coupled interaction between Indian Ocean basin mode, asymmetric mode in the TIO, northwest Pacific circulation etc. paved way for renewed understanding of monsoon teleconnections. We have carried out several ocean model and coupled model simulations and sensitivity experiments to understand the processes responsible for the Indian Ocean variability, the ISMR variability and the teleconnections. The efforts were also made to understand the ocean biases in the coupled model used for monsoon forecast in India. While examining ocean state in coupled models, we noted that they exhibit too much surface cooling and subsurface warming over the TIO. Our work for the first time revealed that the enhanced vertical mixing by strong vertical shear of horizontal currents is primarily responsible for TIO subsurface warming and feeds back to surface cooling in coupled models. The misrepresentation of ENSO-monsoon teleconnection and ENSO-Indian Ocean teleconnection in the monsoon forecast model is also addressed in detail with a view to improve the forecasting system. We have shown that most of the models failed to represent ENSO-Monsoon teleconnections, mainly due to unrealistic westward extension of SST warming/cooling associated with El Niño/La Niña, changes in WNP circulation and TIO SST response. These contributions towards climate variability would be useful to further development in coupled models and improvisations. Developing a predictive understanding of regional climate including the new modes of variability is a grand challenge for research community and our two decades of contributions were nailing towards these aspects.