North Pacific Forecast

The past six months have been rather quiet for “blob watchers”, as the North Pacific temperature patterns have been mixed and variable.  The North Pacific Mode has been oscillating between negative and positive phases with some regularity, and the PDO did the same thing for the second half of 2017 before turning modestly positive so far this year.  It’s a bit unusual to have a positive PDO phase during a La Niña episode, but by most metrics this winter’s La Niña has been quite weak.

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In view of the uneventful conditions of late, it’s intriguing to see that the latest long-range computer model forecasts are showing a major change in the coming months.  The map below shows this month’s NMME forecast of summer (June-August) SST anomalies; this is based on a grand average of ensemble forecasts from 7 different models (see here for more information).  The North Pacific is extraordinarily warm in this forecast, with a large area north of 30°N expected to be more than 1.0°C above the 1981-2010 normal.  While the warm anomaly is focused in the northwestern North Pacific, rather than in the “blob” region of the northeast, the NPM phase would still become strongly positive if this forecast is correct.

NMME_ensemble_tmpsfc_season4

To put the forecast in perspective, I worked my way through the history of NMME forecasts, both retrospective and real-time, initialized at the beginning of every month since 1982, and I pulled out the SST forecasts for 30-65°N in the North Pacific.  The chart below shows the ensemble mean forecasts for 4-6 months in the future, revealing that the current forecast is the warmest relative to normal of any of the 434 monthly forecasts; the NMME models have never before predicted such anomalous warmth for the North Pacific at this lead time.

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Below is a version of the chart with the verification from NOAA’s ERSSTv5 data.  There is a modest amount of skill, with an overall anomaly correlation coefficient of +0.76, but obviously part of this is from the trend; the correlation coefficient after de-trending is +0.51.

nmme_northpac_sst_with_verif

Perhaps the most interesting aspect of the current forecast is the rather close agreement between the models, with the predicted North Pacific area-average SSTs ranging from 0.7°C to 1.3C above normal for summer.  For reference, the January 2018 area-average SST was 0.5°C above the 1981-2010 normal according to ERSSTv5.  Here’s a thumbnail view of all 7 of the component forecasts (original figures are at the NMME website).

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So what could explain the very anomalous warming portrayed by the models?  Based on a closer look at the aggressively warm CFSv2 forecast, it’s clear that the model expects persistent high pressure over the northwestern and north-central North Pacific in coming months, and this would lead to increased solar heating and reduced wind-driven cooling; it’s the same process that led to the formation of the original “blob” anomaly in 2013-2014.  Indeed, the process is already under way, as strong warming has occurred in the past month in the top 100m of the northwestern North Pacific; compare the two cross-sections below (mid-January on top, mid-February on bottom).

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In summary, a rather remarkable set of long-range forecasts from the NMME models suggest that the North Pacific could be in for a very warm summer; the NPM phase appears rather likely to become strongly positive again, and we may soon be discussing a new manifestation of the “blob” pattern.  It will be interesting to see how it plays out.

 

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