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02/18/2019, 4:45 pm EST

Weekly ENSO Diagnostics

 Will a Westerly Wind Burst Return El Nino Ocean Warming? If so, coupling with subsurface warmth could ignite El Nino in March. Discussion: A westerly wind burst (WWB) has emerged the past 7-10 days near the Dateline (Fig. 1). At least 6 WWB have occurred since last September each having promise of pushing oceanic warmth into the far eastern equatorial Pacific Ocean to ignite El Nino. However, in each case the WWB event has weakened prior to reaching 120W longitude and El Nino onset failed. The current WWB is likely the strongest as the daily southern oscillation index (SOI) has dipped to -3.59 indicating significant lowering of surface pressure in the eastern Pacific tropics. The recent tendency of SOI to strengthen in the negative phase suggests that the atmosphere is finally starting to react to El Nino warming of the central/eastern tropical Pacific basin (Fig. 2). Last week the Nino34 SSTA warmed to above the El Nino threshold of +0.5C (to +0.6C). The warmer trend follows 5 consecutive weeks of Nino34 SSTA below the El Nino threshold (Fig. 3). NOAA definition of El Nino onset is a minimum of 3 consecutive months of exceedance by Nino34 SSTA of the threshold. So we’re not there yet (despite NOAA announcement of an El Nino Advisory last week). The subsurface equatorial central and eastern Pacific basin is robustly warm and that warmth is shifting eastward (Fig. 4). An effective WWB will direct ENSO phase toward El Nino rapidly in March if the WWB holds together.   Fig. 1: Arc of westerly wind bursts across the equatorial Pacific since last September show that intensity is lost in the East Pacific. Fig. 2: Tracking the southern oscillation index trend of the past 90 days. Fig. 3: The Nino SSTA observations from the past 12 weeks. Three consecutive months of Nino34 SSTA exceeding +0.5C is needed for El Nino onset. Fig. 4-5: Subsurface equatorial Pacific basin temperature anomalies indicate increasing warming shifting east!