The Blob in Hibernation

It’s been about five months now since the North Pacific Mode, a useful measure of the “Blob” spatial pattern, turned negative.  Cooler than normal sea surface temperatures across the northern North Pacific (but south of the Bering Sea) kept the NPM in the negative phase all winter, and the December-February average of the NPM index was the lowest since 2000-2001.


Here’s a recent weekly SST anomaly map derived from NOAA’s OISST data (1971-2000 climatology); the region of below-normal SSTs in the northeastern Pacific Ocean is now quite extensive and basically resembles the opposite of the notorious Blob pattern.


So is the Blob dead?  From a surface perspective, the answer is definitely yes; but subsurface data indicate that warm conditions are still lingering below 100m depth in the northeast Pacific.


Here’s the latest vertical cross-section along 45°N, showing that the cool water in the northeast extends down to about 100m depth, and below that it’s still warm.  Compared to the situation in early November (also shown below), the warm anomaly at depth is a little weaker and has been suppressed a little deeper, but other than that it’s largely unchanged.  So we might say the Blob has gone into hibernation; it remains to be seen if it will emerge again in the coming months or years.



To see the big picture of how the Blob has evolved over time, we can look at a time-depth cross-section extending back to 2013, when the Blob first made a big splash, so to speak.  The chart below shows the evolution of temperature anomalies at 45°N 140°W, which is near the epicenter of Blob warmth.


The initial warm feature was formed near the surface in the summer of 2013, and then the anomaly intensified and deepened in the subsequent winter.  The warm conditions became more and more entrenched at depth through 2014 and 2015, and even the North Pacific winter storms associated with the intense 2015-2016 El Niño did not permanently eliminate the warm anomaly.  This winter, however, has seen a much more dramatic change and a return to significantly cool anomalies in the upper 100m.

It’s very interesting to observe that a broadly similar sequence of events was observed in the same part of the North Pacific in the years surrounding the strong El Niño of 1997-98.  The time-depth section below, which spans an equivalent period of time, shows persistent (though much less intense) warm anomalies in the 2.5 years prior to the El Niño winter, and we also see that significantly cool anomalies did not show up until the autumn of the year after the El Niño (i.e. autumn 1998).


So if history repeats itself, can we expect the cool North Pacific conditions to stick around for a while?  The figure below shows the next few years in the history, revealing that the cool anomaly gradually deepened over the several years following the 1997-98 El Niño.  A word of caution is in order, however: after the El Niño of 1997-98, La Niña conditions persisted more or less continuously for 3 years, and that seems unlikely to occur this time around; there are already some indications of a return to El Niño in the next several months.



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