Subsurface Warmth Persists

Carol Janzen and Rick Thoman have graciously invited me to contribute here, and it is a pleasure for me to do so.  By way of introduction, I’m Richard James, a meteorologist with Prescient Weather and an Alaska weather enthusiast.  Last September I gave a talk about the North Pacific Mode (NPM, “the Blob”) and Alaska winter climate at the University of Alaska Fairbanks – see here for the presentation files.  There’s lots more to learn about North Pacific and Alaska climate, so I hope to share analysis and results here occasionally.

To begin, I thought it would be interesting to look at how the North Pacific temperature anomalies have changed in the last few months, both at the ocean surface and in the subsurface waters.  As Rick mentioned a few days ago, the NPM index has crashed since October, with the January index becoming significantly negative; so the classical positive NPM phase has disappeared and indeed reversed.  The chart below shows the evolution of the monthly NPM index since 2010; the recent change is dramatic, and it’s closely related to this winter’s strong El Niño episode.


The map below shows the spatial distribution of SST anomalies that is associated with the positive NPM phase; note the warm band stretching from the Sea of Okhotsk to the northeast Pacific, with the highest coefficients near 45°N/150°W (the Blob region).


Now take a look at the change in SST anomalies from October to January in the two maps below.  In October the NPM was strongly positive, but by January large parts of the North Pacific pattern were reversed, with slightly cooler than normal SSTs around 40-50°N and warmer conditions around 30°N/180°W.  The cooling south of the Aleutians was particularly dramatic.



How about temperatures under the surface?  For this we can look at data from NCEP’s Global Ocean Data Assimilation System.  The two maps below show that there was also notable and widespread cooling in the 0-100m (below surface) ocean layer, although there remains a large area of water more than 1°C warmer than normal off the coast of southeast Alaska and farther south.



Looking at the 100-200m layer, it’s remarkable to see that the region of warm water in the northeast Pacific actually increased slightly in size and intensity from October to January.  The anomaly is slightly east of the NPM center-of-action of 45°N/150°W, but nevertheless it’s clear that “the Blob” lives on under the surface.



The two images below show vertical cross-sections of temperature anomaly from 0-400m depth along 45°N for October and January.  It’s rather fascinating to see how shallow the warm layer of water was in October across the north-central North Pacific (south of the Aleutians), but the warm anomaly in the northeast Pacific is very much deeper.  Consequently the shallow warm anomaly was easily wiped out by surface cooling and mixing during the early winter, but the deep eastern anomaly remains largely intact.  In conclusion, although the positive NPM phase is gone for now, the major thermal anomaly associated with “the Blob” is still very much in play.






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