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El Nino events are characterized by anomalously warm tropical Pacific surface waters and concurrent ocean heat discharge, a precursor of subsequent cold La Nina conditions. Here we show that El Nino 2015/2016 departed from this norm: despite extreme peak surface temperatures, tropical Pacific (30 degrees N-30 degrees S) upper ocean heat content increased by 9.6 +/- 1.7 ZJ (1 ZJ = 10(21) J), in stark contrast to the previous strong El Nino in 1997/1998 (-11.5 +/- 2.9 ZJ). Unprecedented reduction of Indonesian Throughflow volume and heat transport played a key role in the anomalous 2015/2016 event. We argue that this anomaly is linked with the previously documented intensified warming and associated rising sea levels in the Indian Ocean during the last decade. Additionally, increased absorption of solar radiation acted to dampen Pacific ocean heat content discharge. These results explain the weak and short-lived La Nina conditions in 2016/2017 and indicate the need for realistic representation of Indo-Pacific energy transfers for skillful seasonal-to-decadal predictions.

Plain Language Summary El Nino events are characterized by anomalously warm surface waters in the equatorial Pacific and concurrent heat loss of the upper ocean, a precursor for the swing toward anomalously cool conditions (La Nina). Here we show that El Nino 2015/2016 departed from this norm: despite extreme peak surface temperatures, the tropical Pacific upper ocean gained heat, which is in stark contrast to previous El Ninos like the strong event in 1997/1998. We find that unprecedented reduction of oceanic transports by the Indonesian Throughflow played a key role in the anomalous 2015/2016 event. Transports through this passage usually export warm water from the Pacific to the Indian Ocean, but their reduction was strong enough to outweigh Pacific surface heat loss typical for El Nino. Enhanced warming and sea level rise in the Indian Ocean over the past decade reduced the sea level difference between western Pacific and Indian Oceans, preparing the ground for the extreme throughflow reduction in 2015/2016. These results explain the weak and short-lived La Nina conditions in 2016/2017 and demonstrate how decadal changes in the Indian Ocean influenced the 2015/2016 El Nino. We conclude that realistic representation of Indo-Pacific energy transfers is needed in numerical models for skillful seasonal-to-decadal predictions.