Despite Warm Subsurface Why is El Nino Slow to Develop?

Heavy Rains To Hit U.S. Corn Belt This Week
06/18/2018, 4:22 am EST
The Great Rainfall Regime of Late Calendar Spring 2018 for The Midwest U.S. to Virginia
06/25/2018, 4:08 am EST
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Discussion: Interestingly, the eastern equatorial Pacific Ocean surface has not warmed significantly (Fig. 1) despite subsurface robust warming. Why?

South of the eastern Pacific equator sea surface temperature anomalies (SSTA) are somewhat cooler than normal while just north of the equator SSTA is warm (Fig. 2). Surface pressure is higher over the cooler waters and attendant wind surges across the equator toward the warmer water where surface pressure is lower. In the equatorial East Pacific this process is preventing trade winds from shutting down to allow subsurface warmth to up-well to the ocean surface and initiate El Nino. At play here is a robust positive phase of the Pacific meridional mode (+PMM) which identifies oceanic cross-equatorial energy/wind flow.

Just-the-opposite condition is present in the tropical Atlantic where wind is extending across the equatorial region from the cool waters north of the equator to the warmer waters south of the equator. A robust negative phase of the Atlantic meridional mode (-AMM) is present.

The subsurface warmth in the central/east equatorial Pacific is impressive (Fig. 3). Trade winds are just now easing likely due to recent passing of the transient Madden Julian oscillation (MJO). If trade winds ease and stay inactive for a few weeks El Nino will evolve FAST! If trade winds return due to the +PMM condition El Nino develops more slowly.

Speaking of the transient MJO (as mentioned last week), this feature is largely to blame for the excessive rainfall in Texas and the Corn Belt this week.

As for the ENSO forecast, NCEP CFS V2 is likely the best model indicating neutral ENSO at initialization and El Nino ahead for late summer/early autumn (Fig. 4).

Fig. 1: Despite a very warm subsurface the Nino SSTA regions are neutral while Nino12 (northwest coast S. America) is much cooler.

Fig. 2: Robust positive phase of the Pacific Meridional Mode/negative phase of the Atlantic Meridional Mode has been propelling low level wind flow preventing up-welling of warm water to initiate El Nino in the East Pacific.

Fig. 3: The subsurface warming of the equatorial eastern Pacific is strengthening and is impressive! Normally, this type of warm Kelvin Wave signature brings on an El Nino. Interestingly, the subsurface West Pacific is also warming. This Pacific-wide subsurface warmth is RARE and suggests that ENSO 2018 may bring a few (not seen before) surprises.

Fig. 4: The latest NCEP CFS V2 ENSO forecast using Nino34 SSTA indicates El Nino remains in the forecast although latest ensemble members (blue) are less aggressive.