05/29/2023, 2:51 pm EDT

Weekly ENSO Diagnostics Report: El Nino trying but failing to reach onset conditions.

Highlight: El Nino trying but failing to reach onset conditions. Fig. 1: The unusual evolution of subsurface equatorial Pacific warming on both sides of the Pacific. Discussion: The equatorial subsurface Pacific Ocean has above normal upper ocean heat on both sides of the Dateline which is very unusual (Fig. 1). Usually, a La Nina or El Nino regime causes one side or the other to have warmer (cooler) subsurface temperature anomalies. As the 1997 and 2015 El Nino episodes began, all the subsurface warming consolidated in the East Pacific tropics where El Nino was generated. During May 2023, the warming in the East Pacific subsurface is quite strong near the northwest coast of South America, weaker toward the Dateline, and moderately strong in the West Pacific. The trend is toward less warming in the West Pacific and increased warming east of the Dateline but support for developing El Nino is marginal. The southern oscillation index was a very El Nino-like -1.3 for May (Fig. 2) thanks to a mid-to-late month surge of MJO-inspired tropical convection across the central and eastern equatorial Pacific. The MJO influence is fading, and SOI has returned to near neutral the past 2 days. The Nino SSTA regions are varying with robust warmth continuing off the northwest coast of South America while the central and east-central equatorial Pacific are struggling to reach the El Nino threshold (Fig. 3). At the surface, the cool phase of the Pacific decadal oscillation (-PDO) continues which funnels cool waters from off the West Coast of the U.S. southwestward into the tropical waters south of Hawaii and preventing the westward expansion of warm waters near the equator to cause El Nino onset (Fig. 4). This process strengthened last week (Fig. 5). Fig. 2: Southern oscillation index collapses to a very El Nino-like -1.3 during May. Fig. 3: The Nino SSTA regions weekly SSTA during the past 12 weeks indicates Nino34 SSTA is not quite into El Nino territory. Fig. 4-5: The daily SSTA analysis for the Northeast Pacific basin and 7-day change.