In my last update way back in February, I highlighted a long-range NMME forecast suggesting that extraordinary warmth would develop by summer across much of the North Pacific Ocean. Here’s the sea surface temperature (SST) forecast that caught my attention at the time:
The forecast turned out to be fairly good, with the verification map (below) showing some distinct similarities to the forecast – particularly the warmth east of Japan and in the Bering Sea, and also the warm band stretching southwest-northeast just to the south of Hawaii. However, there wasn’t much of a warm anomaly near 50°N and in fact the area in which the SST anomaly exceeded +1°C was fairly small – and nowhere near as large as the expansive warm anomaly in the forecast.
However, as summer began to wane, the warming trend picked up quite dramatically and unusual warmth became much more widespread across the North Pacific. Looking at the SST anomaly map for September-October, we’d be less inclined to say that the aggressive NMME forecast was “overdone”; indeed, it seems to have been highly prescient.
But what of the North Pacific “blob”? In previous posts I’ve discussed the North Pacific Mode, which was proposed by Hartmann (2015) as a pattern of North Pacific SST variability that was strongly expressed during the unusual “blob” winter of 2013-2014. The map below shows the NPM’s positive phase, based on NOAA ERSSTv5 monthly data. The variance associated with the mode is concentrated between 40 and 50°N and mostly east of the Date Line.
The juxtaposition of very unusual warmth with the NPM region in the past couple of months has of course produced a strongly positive NPM index. The figure below shows monthly NPM index values (ERSSTv5 data) from the past several years, indicating that the NPM mode has recently returned to the upper end of the range that was observed from late 2013 through late 2015 (leading into the strong El Niño winter of 2015-16).
So it seems that “the blob” has returned. Or has it? An important point to be made is that the warmth we’ve seen lately in the North Pacific is much more widespread than the warmth of 2013-2015, which was largely (although not entirely) focused in the northeastern North Pacific (hence “the blob”).
The difference in the spatial distribution of warm anomalies is quite evident when we compare the subsurface temperature anomalies at 0-100m depth from October 2015 and October 2018; see the figures below. Based on NOAA’s GODAS data from last month, the most anomalous warmth is now actually in the northwestern, rather than the northeastern, North Pacific, and this contrasts strongly with the situation at the end of the 2013-2015 “blob” regime.
We see the same thing in the cross-section along 45°N from the same two months; in 2015 the deep warmth was all in the east, but now the deep warmth is all west of the Date Line; and this is consistent with what the NMME forecast was suggesting many months ago.
So to summarize, we’ve seen a dramatic return to the positive NPM phase in recent months, but so far the pattern of unusual warmth is not particularly “blob”-like, and in fact the subsurface data points to the northwestern North Pacific as the focus of the current “marine heatwave”.
And one more piece of analysis to conclude and to illustrate just how unusual the current situation is: based on NOAA’s GODAS data, the upper 100m of the North Pacific Ocean north of 40°N is now warmer (relative to normal) than at any time in the modern data record (1980-present).
Hartmann, D. L. (2015), Pacific sea surface temperature and the winter of 2014. Geophys. Res. Lett., 42, 1894–1902. doi: 10.1002/2015GL063083.