Impact of the TAO/TRITON Array on Reanalyses and Predictions of the 2015 El Niño

Starting in the early 1990's, the Tropical Atmosphere Ocean (TAO)/TRIangle Trans Ocean buoy Network (TRITON) array has been the pervasive source for observing large‐scale equatorial wave propagation which is key for El Nino/Southern Oscillation (ENSO) predictions. However, removal of western TRITON moorings, the plan to reorganize the array (i.e., TPOS 2020), and availability of other sources of in situ data (e.g., Argo) have highlighted the need to rigorously assess the impact of TAO/TRITON data on ENSO predictions. Therefore, we evaluate TAO/TRITON array using data denial assimilation experiments and assess the impact on coupled atmosphere/ocean predictions of the big 2015 El Niño. Validation of the CONTROL (assimilates all available data) and NOTAO (withholds TAO/TRITON data) reanalyses shows that assimilating TAO/TRITON data generally improves comparisons versus gridded and pointwise in situ observations. This is especially true across the entire basin above and in the eastern half of the Pacific just below the thermocline for temperature. Even with relatively few observations, salinity is generally improved except near 120°W near the surface. To evaluate the impact of TAO/TRITON data on ENSO initialization, seasonal forecasts were initialized from the CONTROL and NOTAO experiments. For the 9‐month forecasts which were initialized in January, July, and October 2015, both the amplitude and the accuracy of the ensembles initialized with TAO/TRITON data were closer to observations. Through the analysis of Kelvin and Rossby waves, we show that the impact of TAO/TRITON is to generally shoal the mixed layer depth, leading to amplification of the El Niño downwelling signal, and improving the amplitude of the ENSO signal. Plain Language Summary The Tropical Atmosphere Ocean (TAO)/TRIangle Trans Ocean buoy Network (TRITON) mooring array straddles the equator in the tropical Pacific and is designed to monitor the large‐scale, basin‐wide ocean waves that comprise the changes in the El Niño/Southern Oscillation (ENSO) strength. We isolate the impact of these moorings by contrasting two simulations. In one, we use the TAO/TRITON data (i.e., the CONTROL experiment) and in the other we withhold these data (NOTAO). Using these data in the CONTROL generally improves the temperature and salinity model reconstruction especially in the eastern tropical Pacific, where variations in upwelling of the cold tongue define the ENSO cycle. Next, seasonal ENSO forecasts initialize