A combination of ambiguous sea surface temperature anomalies
(SSTAs) and wind patterns over the Pacific mean that the Pacific Decadal
Oscillation (PDO) could be rendered for a little while as too ambiguous to use
in seasonal forecasting as we move into the fall. Click on any image to enlarge it.
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Graphic showing SST anomalies (shaded) and surface wind
patterns (arrows) during the positive phase of the PDO (left image) and the
negative phase (right image).
Source: University of Washington
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The Pacific Decadal Oscillation comes in two phases: a
“warm” phase (positive phase) and a “cool” phase (negative phase). The state of
the PDO is identified primarily by the alignment of sea surface temperature
anomalies over the Pacific basin. When SSTAs are notably below normal east of
Japan into the waters south of Alaska, the PDO is said to be in the positive
phase. In contrast, when anomalies are above normal in those same areas, the
PDO is negative. This seems upside-down, so it’s helpful to also look at the
anomalies immediately offshore the western coast of North America. Indeed, in a
positive PDO those coastal waters exhibit positive SSTAs, while a negative PDO
typically brings colder waters.
Let’s see how the PDO looks currently.
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SST anomalies over the globe, as of June 17, 2019.
Source: NOAA
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A look at the Pacific basin as of the last graphic of SST
anomalies doesn’t provide much of a clear-cut direction of where the PDO
currently stands. While there are cooler than normal sea surface temperatures
wrapping into the waters offshore of the western United States – typically
indicative of a negative PDO – there is a mass of colder than normal waters
south of the Aleutian Islands and extending west towards Japan – typically indicative
of a positive PDO. Additionally, there’s a swath of above-normal SST anomalies
extending from Hawaii into the Gulf of Alaska, which doesn’t fit comfortably
into either composite of the two PDO phases, making the picture quite muddy.
However, also shown on that two-panel image above are
surface wind patterns exhibited during the different PDO phases. In a positive
PDO, surface winds over the north Pacific flow from west to east, while in a
negative PDO these surface winds flow from east to west. Let’s take a look at
recent surface wind patterns in the Pacific to try and decipher the PDO.
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Surface winds over the Pacific basin, from April 1st through June 16th.
Source: ESRL
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Over the months of April, May, and the first half of June,
surface winds were seen flowing from west to east over the waters south of the
Aleutian Islands, a prominent mark of the PDO being in the positive phase.
Contrasting with this, however, is a channel of northerly winds just offshore
western North America, which the composite graphic at the top of this post
indicates is associated with a negative PDO. Much like the SSTA comparison,
surface wind patterns seem to be giving us conflicting signals that make it
difficult to draw out which phase the PDO is actually in.
There’s one more method I want to look at to determine the
state of the PDO.
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Seasonal correlation of 500-millibar geopotential heights with the PDO in the months of April through June.
Source: ESRL
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Shown above is an image that provides valuable insight on to
how the PDO affects the atmospheric pattern. It is a seasonal correlation
image, and although that sounds daunting, its interpretation is rather
straightforward. Suppose, for a moment, we assume there is a positive PDO in
place. The graphic above takes that information and asserts, based on history,
that the positive PDO will result generally in positive 500-millibar height
anomalies over the western swath of North America – in other words, a ridge.
Why? Because for all the warm colors in this graph, 500-millibar heights are
positively correlated to the PDO’s state. This means that in a negative PDO,
all areas under warmer colors would see lower 500-millibar heights. Similarly,
colder colors on this chart mean that if the PDO is in a positive (negative)
phase, 500-millibar heights will be lower (higher) in areas with colder-color
shading, indicative of troughs (ridges).
Let’s see if we can use this image to determine where the
PDO is now.
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500-millibar geopotential height anomalies from April 1st through mid-June.
Source: ESRL
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Unfortunately, even this method doesn’t provide much more
clarity on the state of the PDO. During the April-through-mid-June time period,
we have seen below-normal geopotential heights (stormy weather) south of the
Aleutian Islands into Japan. Using the image immediately prior to the one
directly above, colder colors are draped over that same area, implying the PDO
state is opposite the 500-millibar height anomalies over the northern Pacific.
This would tell us that a positive PDO is present. However, moving into the
Gulf of Alaska, persistent ridging has taken place over this time period, in an
area that also has a negative correlation with the PDO. Therefore, that ridging
off the western coast of North America implies a negative PDO. These are the
same takeaways we found by analyzing SSTAs and surface wind patterns, and don’t
really improve our understanding of what phase the PDO is actually in.
One could make an argument that it’s easier to just go by what
the NOAA’s PDO index itself actually says, which indicates we are in a
marginally-positive phase. In some situations, that’s certainly fine to do, but
as was shown extensively in this article, the atmosphere is not totally
reflective of a positive PDO state, meaning it would be misleading to just use
the index value. The ambiguity in the current state of the PDO makes me
question if it can be reliably used in seasonal forecasting for the fall
months, and possibly for the winter months if these conflicting signals
continue into late summer and early fall. Seasonal forecasters should take heed
of the ambiguity and weight longer-term teleconnections accordingly.
Andrew
