Role of internal oceanic variability and atmospheric forcing in the generation of South Indian Ocean Dipole
Anjana S*, Abhisek Chatterjee, Prerna Singh
Indian National Centre for Ocean Information Services
Indian National Centre for Ocean Information Services INCOIS, Pragathi nagar Hyderabad, Telangana, India Pincode: 500090 Mobile: 9746841478 E-mail: email@example.com
Variability of ocean surface temperature in interannual/decadal timescales in the south tropical Indian ocean plays a significant modulator of the regional climate. The south Indian ocean dipole/ subtropical Indian Ocean dipole (SIOD) mode is one of the primary contributors to the climate variability in this region in interannual timescale and is associated with large-scale sea surface temperature anomaly and wind anomaly in the southern Indian Ocean. The positive phase of SIOD is characterized by a warm SST anomaly in the western Indian Ocean southeast of Madagascar and a cold SST anomaly in the southeast Indian Ocean off the northwestern coast of Australia. The impact of SIOD is widespread and plays a significant role in modulating regional climate. Its positive phase contributes to the enhanced rainfall in southeastern Africa and dry year in southwestern Australia.
In this study, we show that, in addition to its interannual variability, SIOD also exhibits a strong quasi-decadal variability of ~6-10 years. However, the positive phase of this quasi-decadal mode of SIOD, unlike its interannual pattern, show anomalous warming in the centre of the south tropical IO and a strong negative off the west coast of Australia. This shift in the warm pole is primarily linked to the quasi-decadal variability of the Mascarene high and the subtropical westerlies. A high-resolution global model simulation shows strong a correlation between SIOD index and D20 at this quasi-decadal band indicating that this observed variability is primarily linked to the oceanic dynamical processes. The further model analysis also suggests that the oceanic internal variability, driven by the shear between South Equatorial Current and South Indian Counter Current, also significantly contributes to these low-frequency climatic variations. These findings highlight the importance of internal instability in regional climate and, as a result, interannual to decadal forecast skills of the models.