The role of the Indian Ocean sector for prediction of the coupled Indo-Pacific system: Impact of atmospheric coupling

Indian Ocean (IO) dynamics impact ENSO predictability by influencing wind and precipitation anomalies in the Pacific. To test if the upstream influence of the IO improves ENSO validation statistics, a combination of forced ocean, atmosphere, and coupled models are utilized. In one experiment, the full tropical Indo-Pacific region atmosphere is forced by observed interannual SST anomalies. In the other, the IO is forced by climatological SST. Differences between these two forced atmospheric model experiments spotlight a much richer wind response pattern in the Pacific than previous studies that used idealized forcing and simple linear atmospheric models. Weak westerlies are found near the equator similar to earlier literature. However, at initialization strong easterlies between 30 degrees S-10 degrees S and 0 degrees N-25 degrees N and equatorial convergence of the meridional winds across the entire Pacific are unique findings from this paper. The large-scale equatorial divergence west of the dateline and northeasterly-to-northwesterly cross-equatorial flow converging on the equator east of the dateline in the Pacific are generated from interannual IO SST coupling. In addition, off-equatorial downwelling curl impacts large-scale oceanic waves (i.e., Rossby waves reflect as western boundary Kelvin waves). After 3 months, these downwelling equatorial Kelvin waves propagate across the Pacific and strengthen the NINO3 SST. Eventually Bjerknes feedbacks take hold in the eastern Pacific which allows this warm anomaly to grow. Coupled forecasts for NINO3 SST anomalies for 1993-2014 demonstrate that including interannual IO forcing significantly improves predictions for 3-9 month lead times. Plain Language Summary It is important goal to help to extend our ability to predict large-scale global weather patterns. One such well known disruption to global weather pattern is El Nino. Past researchers have explored if different regions around the world impact El Nino. Here, we test the impact of the Indian Ocean sector against real observations by differencing model experiments that include and exclude forcing from this region. These results show that the impact of the Indian Ocean is significant for El Nino in the Pacific. Large-scale ocean waves are generated off the equator in the Pacific by this Indian Ocean forcing. These waves travel from east to west, eventually hitting the western boundary of the Pacific, reflecting, and traveling along the equator to impact the eastern tropical Pacific, the home of El Nino. Normally this region is cooler than the surrounding area because cool, deep water is brought to the surface. However, these waves act to cut off cool water effectively warming the eastern Pacific and enhancing El Nino. This process takes several months so we show that forcing in the Indian Ocean sector improves forecasts of El Nino from 3-9 months for the tropical Pacific.