Cooler Patterns – For Now

In my last post back in September, I discussed the re-appearance of very unusual warmth in the North Pacific in 2018 and 2019, with the focus shifting from northwestern to northeastern parts of the basin during this period.  However, it became clear that the warmth was not as well-established in the sub-surface waters as in the earlier iteration of the so-called “Blob” pattern of 2013-2015; and consequently I suggested that it wouldn’t be surprising to see changing weather patterns eliminate much of the unusual warmth over the winter.

As it turns out, the weather patterns have indeed shifted towards a cooler phase in the northeastern North Pacific, and January was very chilly indeed in Alaska.  A persistent trough of low pressure centered over southeast Alaska produced a cold airflow over the North Pacific to the south of Alaska, and surface water temperatures dropped substantially in relation to normal.

Despite the cooling, however, the residual warmth is such that SSTs remain warmer than normal in most of the basin except near Alaska’s coast; the map below shows the latest weekly analysis.

With warmth still prevailing widely, the North Pacific Mode (NPM) remains positive, although with much less amplitude than just a few months ago.  As recently as November, the NPM index challenged the highs of the earlier Blob era, but with less sub-surface warmth to anchor the anomaly, the spikes of 2018 and 2019 have both proven to be rather short-lived.

The PDO (Pacific Decadal Oscillation) index also points to a contrast between a more robust warm pattern in 2014-2016 and a less pronounced warm episode in the last year; and in January the PDO index dropped to the most negative value since late 2013.  The PDO and NPM are independent modes of North Pacific SST variability, with unusual northeast Pacific warmth linked to the positive phases of both – see the spatial patterns below.

Here are updated time series charts of average North Pacific temperature anomalies north of 40°N, both at the surface and in the top 100m under the surface.  Both show a marked drop-off this winter, but both remain elevated compared to the long-term normal.

It will be interesting to see where the North Pacific goes from here.  The shift to colder conditions in Alaska this winter has been very dramatic in comparison to the pervasive warmth of recent years, but the vast majority of the Northern Hemisphere’s mid- to high-latitude land area has been extremely warm this winter; it seems it would be difficult to maintain a robust cooling trend over the North Pacific in this broader environment.

As if to support this notion, last month’s seasonal forecasts from the NMME models show a rather dramatic re-warming trend across the North Pacific in the coming months, with a wide area of +1-2°C SST anomalies predicted by summer (see below).  Interestingly this is expected to occur even as a La Niña episode develops in the eastern tropical Pacific, and it’s worth pointing out that the spatial orientation of the expected North Pacific warmth is consistent with a negative PDO phase even while the area-average is impressively warm.  A negative PDO tends to occur in tandem with La Niña, so this is self-consistent; but I am far from convinced that we will in fact see a La Niña this year.  Time will tell.