Unexpected: Cooling of Both the Tropical East Pacific/North Atlantic

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Despite approaching mid-summer, forecast confidence of the 2019 North Atlantic tropical season is not particularly confident and with good reason. Expected is the inhibiting influence of upper westerly shear across the tropical North Atlantic caused by El Nino holding number of hurricanes down BUT potentially offset by a forecast of a warmer than normal tropical North Atlantic. As it turns out, on July 6 the North Atlantic tropics have unexpectedly cooled and the El Nino is fading (and NOAA is more concrened about La Nina develping).

Unexpected Cooler East Pacific/North Atlantic Tropics Early July

Discussion: Outlooks for the 2019 North Atlantic tropical cyclone season indicate near or slightly below activity due to the inhibiting upper shear across the tropical North Atlantic caused by El Nino although expected warmer than normal conditions in the North Atlantic could negate that suppressing influence.

Let’s take a look at the current SSTA diagnostics. The Nino34 SSTA region in the east-central equatorial Pacific Ocean is near normal (+0.17C) and characterized by still El Nino-like warming near the Dateline while the eastern equatorial Pacific has cooled significantly (Fig. 1). If this trend continues the expected upper westerly shear in the tropical North Atlantic basin may be weaker (or fade completely) causing increased risk of North Atlantic hurricanes.

While the equatorial East Pacific SSTA pattern is trending toward a more supportive role of North Atlantic hurricanes for the 2019 season the tropical North Atlantic has also cooler, which is unexpected (Fig. 2). The tropical North Atlantic index is now cooler than normal (-0.14C). The cooler waters inhibit hurricane development.

Interesting is how the tropical SSTA patterns in the East Pacific and North Atlantic have shifted to the opposite regimes expected early in the 2019 North Atlantic tropical cyclone season. Note the latest NCEP CFS V2 Nino34 SSTA forecast which drops El Nino and introduces La Nina for autumn 2019 (Fig. 3).

As related to tropical cyclones…the ENSO trend is disturbing. The support for above normal hurricanes probably mid-to-late season is increasing.

Fig. 1: The Nino34 SSTA zone where ENSO phase is evaluated is now neutral. Expected is a season-suppressing El Nino induced tropical shear across the North Atlantic but that projection appears to be failing.

Fig. 2: The tropical North Atlantic index region which also coincides with the main development region for hurricanes is cooler than normal. A cool tropical North Atlantic lowers risk of hurricanes especially the usually powerful Cape Verde hurricanes.

Fig. 3: The latest NCEP CFS V2 ENSO phase forecast indicates increasing concern for La Nina.