Why La Nina Was Weak? No -PDO!

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All La Nina's are not the same but this one was really different and unusually weak. Normally, waters in the southeast North Pacific are cooler than normal during a La Nina episode characteristic of cool phase Pacific decadal oscillation. But not in 2017. Surface water southwest of California remains unusually warm. This latest "warm blob" prevented, in-part La Nina from realizing more intensity.

La Nina 2017-18 is weak barely ranking in the top 20 according to multivariate ENSO index (MEI) since 1950 possibly making the current cold ENSO the weakest on record. La Nina is currently in the mature phase and despite the likely peak intensity only the eastern half of the equatorial eastern Pacific is La Nina cool while SSTA near and east of the Dateline never made it to the La Nina threshold (Fig. 1). Why was La Nina 2017-18 weak?

Fig. 1:  Global SSTA analysis for December depicting aspects of the ENSO system and why La Nina 2017-18 is weak.

During La Nina, especially the mature phase a cool gyre of ocean currents off the west coast of both North and South America flow equatorward. The up-welling character of these ocean currents off each continental coast lifts cool subsurface water to the surface and coupled with trade winds the cool waters surge toward the equatorial region helping to cool the ocean surface to sustain and intensify La Nina.

In 2017 the cool waters off the west coast of North America did not develop. Instead a large area of anomalous warmth developed southwest of California and remains in-place in early 2018. The warm water in this zone prevented any cooling inflow from the north and prevented the equatorial zone near and east of the Dateline not to reach La Nina thresholds.

Fig. 2:  Global SSTA analysis from the last mature La Nina occurring in December 2010 indicates the classic La Nina “horseshoe” of cool anomalies looping from the polar oceans into the equatorial region.

Typically, a mature phase La Nina looks like the last cold ENSO event observed during 2010-11. In December 2010 La Nina was in the mature phase (Fig. 2) with the classic cool gyre in both the southeast North Pacific and northeast South Pacific. The cool SSTA pattern indicated is sometimes referred to as the “horseshoe pattern” typical of a classic La Nina.

The usual cool northeast Pacific during classic La Nina is also well represented by the cool phase of the Pacific decadal oscillation. During the 2-year La Nina in 2010-11 note how the PDO index ran almost exactly parallel to ENSO (Fig. 3). The PDO tends to run about 3-4 months ahead of ENSO during this paralleling cycle also common when El Nino is present and PDO flips to the warm phase.

Fig. 3:  A moderate to strong La Nina AND Pacific decadal oscillation occurred with the 2010-11 La Nina episode.

But notice in late 2017 as weak La Nina matures the PDO warms to weak warm phase (Fig. 4) and this warmer shift likely foreshadows La Nina’s demise by late winter/early spring.

Fig. 4:  In 2017 La Nina has been weaker and PDO never flipped to the cool phase typical of La Nina episodes.

Fig. 5:  The 90-day percent of normal precipitation pattern across the U.S. is indicated.

Fig. 6:  The 90-day upper air features forcing the U.S. climate pattern are indicated.

Pointing out these regional and unusual SSTA regions is important. The 90-day U.S. precipitation anomalies (Fig. 5) indicate a very dry scenario for most of the country. The tendency amongst the scientific community is to attribute these climate regimes to ENSO and in this case La Nina.

Climate Impact company disagrees. The tendency for upper level high pressure ridge areas to emerge over or down-wind large areas of warm SSTA helps to explain the semi-permanent high pressure ridge over the Southwest U.S. and North Atlantic (Fig. 6). In-between the polar vortex found a location to emerge. The combination of dry high pressure in the Southwest and mostly dry cold flow out of Canada forced by the polar vortex has rendered much of the U.S. drier than normal the past 90 days. La Nina’s contribution is likely suppressed due to these other factors.

Why these “warm blobs” of regional SSTA occur is not well understood. Another exists now east of Australia and has contributed to the record heat observed in eastern Australia earlier this month by enhancing subtropical ridging aloft. A 3-year long super warm zone of water in the northeast Pacific which actually was named the “warm blob” by climate scientists from 2013-15 and leading to enhanced drought in California was the first of these warm blobs to occur.