Saturday, November 21, 2015

Cold During Positive PDO

Last week I noted that strong El Niño conditions in the tropical Pacific Ocean do not preclude unusual cold in Fairbanks.  However, it's a little more surprising that below-normal temperatures have occurred statewide recently in conjunction with a positive PDO phase, and the recent cold in Anchorage stands out as an unusual event in this regard.  A daily record low temperature was tied on the 18th (-8°F), and record low maximum temperatures were tied or broken on 3 of 4 days from the 15th to the 18th.  If it weren't for the November 1989 cold spell, Wednesday's high temperature of +3°F would have been the earliest on record for such a cold day in Anchorage.

Looking at weekly numbers, the week ending November 19 was 16.3°F below normal in Anchorage, which has only been observed a handful of times in the past 20 years in November (most recently in 2011).  In view of the fact that the PDO was significantly positive in October, and a positive PDO favors unusual warmth, this is surprising.

The chart below shows the mean temperature anomaly in Anchorage for all weeks ending in November since 1952, with the horizontal position of the points corresponding to the October mean PDO index in the same year.  The recent cold week is highlighted; in the past, the October PDO index was always below +0.40 when a week this cold was observed in November.  The closest analog was in 2003, when the October PDO index was +1.1 and the week ending November 20 was 14.2°F colder than normal.

Here's a comparison between recent weekly SST anomaly maps, showing that the positive PDO pattern has largely disappeared in the past month, so the November PDO index will be lower than October, but almost certainly still positive.  North Pacific waters remain warmer than normal overall.

In the interior, the first -40 temperatures of the season were observed with the recent cold spell.  These included:

-43°F   Kanuti Lake in the Kanuti NWR southeast of Allakaket
-42°F   Clear Creek RAWS northwest of Hughes
-42°F   Chicken COOP
-40°F   Huslia


  1. As I vaguely recall, a CCW Low (upper/lower/?) moved across the State ~E>W and the cold was advected in from the NW...I assume Siberia.

    Not much a positive PDO can do in the short term if the upper level dynamics support a NW flow of cold air. Plus it cleared off so radiational cooling took effect.

    Clouds are my friend in winter.


  2. Gary, yes indeed the flow pattern brought in the cold air and the ocean temperatures had little to do with it. The interesting aspect is that the large-scale ocean anomaly tends to make such a flow pattern less likely; a positive PDO shifts the typical flow pattern in the direction of importing warm, not cold. At least that's how I interpret the effect.

    1. Maybe on a greater scale some Rossby Wave forcing via ENSO or PNA phases moved and flattened the Jet south of Alaska for a period, allowing clearing and cold air to advect SE across Alaska?