Strong Positive Pacific Meridional Mode Causes Central Tropical Pacific Warming

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One of the strongest positive phases of the Pacific Meridional Mode on record has contributed to the Hawaii hurricanes this season and may trigger an El Nino Modoki.

The Pacific Meridional Mode (PMM) is defined as the leading mode on non-ENSO coupled ocean/atmosphere variability in the Pacific basin. While ENSO describes west-to-east ocean/atmosphere coupling in the equatorial Pacific region the PMM is a cross-equatorial transport of energy. +PMM identifies a transport of oceanic/atmospheric energy from the southern hemisphere tropics to the northern hemisphere tropics usually most evident in the central tropical Pacific. +PMM shuts down trade winds allowing this region to warm. +PMM can also initiate an El Nino episode usually biased toward the central Pacific (Modoki). In 2018 a +PMM regime has been present and ranks among the strongest 3 or 4 +PMM regimes in the 1950-2018 climatology for the months of January to July. The +PMM is a likely contributor to the warm ocean surface extending to Hawaii (Fig. 1) where 2 unusually intense hurricanes have already occurred this season. The +PMM tendency to warm the equatorial Pacific near the Dateline might explain while El Nino has been slow to develop in the conventional eastern equatorial Pacific region while warming near the Dateline seems to be increasing (Modoki).

Fig. 1: The south-to-north energy transport into the central tropical Pacific Ocean caused by the positive phase of the Pacific Meridional Mode is demonstrated in the current Pacific Ocean SSTA analysis.