Abstract
This study investigates the influence of the anomalously warm Indian Ocean state on the unprecedentedly weak Indonesian Throughflow (ITF) and the unexpected evolution of El Niño-Southern Oscillation (ENSO) during 2014–2016. It uses 25-month-long coupled twin forecast experiments with modified Indian Ocean initial conditions sampling observed decadal variations. An unperturbed experiment initialized in Feb 2014 forecasts moderately warm ENSO conditions in year 1 and year 2 and an anomalously weak ITF throughout, which acts to keep tropical Pacific ocean heat content (OHC) anomalously high. Changing only the Indian Ocean to cooler 1997 conditions substantially alters the 2-year forecast of Tropical Pacific conditions. Differences include (i) increased probability of strong El Niño in 2014 and La Niña in 2015, (ii) significantly increased ITF transports and (iii), as a consequence, stronger Pacific ocean heat divergence and thus a reduction of Pacific OHC over the two years. The Indian Ocean’s impact in year 1 is via the atmospheric bridge arising from altered Indian Ocean Dipole conditions. Effects of altered ITF and associated ocean heat divergence (oceanic tunnel) become apparent by year 2, including modified ENSO probabilities and Tropical Pacific OHC. A mirrored twin experiment starting from unperturbed 1997 conditions and several sensitivity experiments corroborate these findings. This work demonstrates the importance of the Indian Ocean’s decadal variations on ENSO and highlights the previously underappreciated role of the oceanic tunnel. Results also indicate that, given the physical links between year-to-year ENSO variations, 2-year-long forecasts can provide additional guidance for interpretation of forecasted year-1 ENSO probabilities.
Highlights
In 2014 the scientific community and forecasters were expecting a major El Niño event, which was predicted by several seasonal forecasting systems (McPhaden 2015)
The South-Eastern Tropical Indian Ocean (SETIO) anomaly is neutral throughout the forecast, despite the remote forcing from the strong El Niño event in this experiment (Fig. 4d), which demonstrates the importance of the Indian Ocean conditions for the development of the 1997 Indian Ocean Dipole (IOD) event
In the previous section we found a profound impact of the Indian Ocean state on the Indo-Pacific atmosphere–ocean system, focussing on the influence on the ensemble mean forecasts
Summary
In 2014 the scientific community and forecasters were expecting a major El Niño event, which was predicted by several seasonal forecasting systems (McPhaden 2015). Physical indicators like equatorial warm water content suggested that tropical Pacific conditions in 2014 were favourable for El Niño that year (Levine and McPhaden 2016). Only moderately warm ENSO conditions materialized in 2014, but one year later in boreal winter 2015/16 one of the strongest El Niños on record occurred. Despite warm conditions in the tropical Pacific in 2014 and especially 2015, its basinwide ocean heat content (OHC) did not decrease during that period, which usually is the case during El Niño events (Mayer et al 2014; Cheng et al 2019). In contrast with the persistent La Niña conditions following the strong 1997/98 El Niño event, there were no strong La Niña events in 2016–2018, which possibly was a result of the weak OHC loss in 2015/16
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