Ambiguous SSTAs, Wind Patterns Could Render PDO Ineffective for Fall

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.

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

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.

SST anomalies over the globe, as of June 17, 2019.

Source: NOAA

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.

Surface winds over the Pacific basin, from April 1^st through June 16^th.

Source: ESRL

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.

Seasonal correlation of 500-millibar geopotential heights with the PDO in the months of April through June.

Source: ESRL

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.

500-millibar geopotential height anomalies from April 1^st through mid-June. 

Source: ESRL 

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

June 21-25 Potentially Strong Storm System

It appears a potentially strong storm system will move through the country in the June 21-25th time period.

Forecasted 500-millibar geopotential height anomalies valid for 7pm June 15th.
Source: Tropical Tidbits

By the evening of June 15th, model guidance sees a formidable trough sweeping across Japan, negatively-tilted (oriented from a northwest-to-southeast direction) with substantial negative height anomalies. In general, when an upper-level trough is oriented in that negative-tilt direction, it implies that the storm system it is associated with has reached its mature phase, and is broadly at or near its strongest point in the storm's life cycle.

Regular readers of this blog in the past will know that I frequently use a teleconnection whereby weather phenomena occurring over and near Japan correlate to similar phenomena in North America roughly 6-10 days later. I am employing this again here, and extrapolating the above model graphic gives us a potential storm system occurring in the United States in the June 21-25 period.

Forecasted MSLP and 6-hour average precipitation rates valid for 7pm June 15th.
Source: Tropical Tidbits

At the surface, we see why the 500-millibar geopotential height anomalies are so negative. Indeed, at the same time as the first image, a surface low with a minimum sea level pressure of 986 millibars is forecasted to be placed right over Japan. While it is not unusual for surface lows to plumb minimum pressure values firmly below 1000 millibars in the Pacific Ocean, particularly in the open waters near the Aleutian Islands, the substantial negative 500-millibar height anomalies at the top of this page and the substantial negative MSLP normalized anomalies below illustrate that even around Japan this storm system is somewhat strong.

Forecasted MSLP normalized anomalies valid for 7pm June 15th.
Source: Tropical Tidbits

So, what can we expect for this June 21-25 timeframe?

Forecasted MSLP  anomalies via the GFS Ensembles valid for 1pm June 24th.
Source: Tropical Tidbits

We can get a sense as to the general pattern shaping up, particularly since it's too far out to try and pick out individual model runs. Above, the forecasted MSLP anomalies from the GFS ensembles on the afternoon of June 24th are shown, and it provides a glimpse at what could be on the way.

Naturally, given these images are the average of the model's ensembles, we aren't able to identify the exact strength or placement of the storm - as stated, we're aiming to identify the broad pattern. The ensembles see below-normal sea level pressure anomalies developing over the central U.S., maximized over the central and northern Plains. This would likely be the storm that would be correlated with the storm expected to impact Japan over the next few days, but it will still take some time for model guidance to reliably latch on to (or even lose) the storm system.

Forecasted 850-millibar temperature anomalies valid for 1pm June 17th.
Source: Tropical Tidbits

In the wake of the storm system, an airmass featuring substantially below-normal temperature anomalies looks to move in over Japan, which should then correlate with below-normal temperatures again invading the United States to round out the month of June.

To Summarize:
- A potentially strong storm system looks to impact the United States in the June 21-25 period.
- Notably cooler than normal weather is expected to impact the country to end June, after the storm system moves through.

Andrew

Does Siberian Snow Cover in October Really Predict North American Winters?

Over the last several winters, an interesting phenomenon that has been discussed is that of Siberian snow cover trends over the month of October. The idea is that if snow cover is anomalously widespread over Siberia in October, the following winter in North America will lean towards being colder than normal. Similarly, if snow cover over Siberia is lackluster in October, temperatures across North America will tend to be warmer than normal during the following winter.

This theory has been advanced by Dr. Judah Cohen of MIT, and I have been particularly fascinated by the concept. In an effort to see how reliable this method could be, in this post I present an admittedly-crude analysis of Eurasian snow cover in the month of October from 1967-2018 and view the subsequent winter seasons.

I would like to make it explicitly clear that this is only meant to be a crude analysis of this phenomenon - I have a great deal of respect for Dr. Cohen, and there are doubtless other far more intelligent and far more able-minded scientists who can analyze this topic more closely and more accurately. This post is more of a preliminary and basic analysis on the topic. Feel free to read more about the concept as well: link

---

Let's begin with an analysis of the data I will use in this analysis. I use data from Rutgers University's Global Snow Lab, which provides content on the extent of snow cover across different swaths of the Northern Hemisphere, as well as the Northern Hemisphere in the aggregate. This data is almost completely continuous, save for October data in the year 1969. As such, we will work with 51 samples in this analysis. For this post, I am using snow cover data for the Eurasia region specifically, over the month of October only. I am using data on an anomaly basis: per the Rutgers GSL, the average snow cover in the month of October in Eurasia is approximately 9,497,000 square kilometers.


I. Winters Following Top Ten October Eurasia Snow Cover Anomalies

First, I'll create a composite of winter-season temperature anomalies over North America for the winters following the ten years of highest positive snow cover anomalies in October over Eurasia. The composite is presented below. Click on any image to enlarge it.

Surface temperature anomalies for the ten winters following the ten Octobers featuring the highest Eurasia snow cover anomalies.
Source: ESRL

The ten years featuring the highest positive anomalies of snow cover in October over Eurasia, in order, are 1976, 2014, 2016, 2002, 1970, 2013, 2015, 1971, 1968 and 1972. I have graphed the composite surface temperature anomaly (in degrees Celsius) for the December-January-February period that followed each year in the above graphic. When putting these top ten years / top ten winters all together, the result is actually pretty in line with what this concept states, that stronger snow cover over Eurasia in the October preceding these winters should result in colder than normal winters for North America in the winters themselves.


II. Winters Following Bottom Ten October Eurasia Snow Cover Anomalies

Next, let's look at the temperature composite for the bottom ten years - that is, the winters that followed the ten Octobers where the snow cover anomaly over Eurasia was the lowest. The image is shown below.

Surface temperature anomalies for the bottom winters following the ten Octobers featuring the lowest Eurasia snow cover anomalies.
Source: ESRL

The picture becomes less clear-cut when looking at the ten winters which followed the ten Octobers with the lowest Eurasian snow cover anomalies. While the below-normal temperature anomalies in North America have become less centralized from what we saw in the first temperature composite, we do not see predominantly above-normal temperatures over these ten winters. Indeed, the majority of the country is actually at neutral or slightly below-normal temperatures, with much of Canada entrenched in firmly below-normal temperature anomalies.

However, I do see an area where this concept becomes useful.


III. 500-Millibar Geopotential Height Anomalies

I want to take a look at those same top-ten and bottom-ten winters, except this time instead of viewing surface temperature anomalies over North America we'll go over 500-millibar geopotential height anomalies over the Northern Hemisphere.

500-millibar geopotential height anomalies for the ten winters following the ten Octobers featuring the highest Eurasia snow cover anomalies.
Source: ESRL

500-millibar geopotential height anomalies for the ten winters following the ten Octobers featuring the bottom Eurasia snow cover anomalies.
Source: ESRL

Where the surface temperature anomalies were rather murky when trying to differentiate the winters following top-ten and bottom-ten Octobers in terms of snow cover anomalies over Eurasia, the picture becomes far more focused when expanding to 500-millibar geopotential height anomalies.

The first image of the above two shows 500-millibar height anomalies for those ten winters following the ten Octobers with the highest positive snow cover anomalies over Eurasia. That composite shows that the winters featured a firmly-disrupted tropospheric polar vortex, with strong ridging evident from the north Pacific through the Bering Sea, across the Arctic Circle and into western Europe. This led to the tropospheric polar vortex being forced to lower latitudes, as shown by the deep negative anomalies in North America, far western Europe, and northern Eurasia. In contrast, the second image - showing 500-millibar height anomalies for those ten winters following the ten Octobers with the deepest below-normal snow cover anomalies over Eurasia - shows essentially the opposite. A stronger than normal tropospheric polar vortex is observed across the upper latitudes, with generalized and widespread ridging prevailing around the mid-latitude regions.

This does seem to validate Dr. Cohen's theory, with more expansive snow cover over Siberia in October leading to a weaker stratospheric *and* tropospheric polar vortex in the Northern Hemisphere for the following winter.


To Summarize:
- A crude analysis of Eurasian snow cover data seems to confirm Dr. Judah Cohen's theory that anomalously high (low) snow cover over Eurasia in October corresponds to an anomalously weak (strong) stratospheric and tropospheric polar vortex in the following winter, with broadly-attendant chances for colder (warmer) than normal temperatures for the winter.
- While I was admittedly skeptical of this tool, the data in this preliminary analysis don't lie: this does seem to be a worthwhile tool for seasonal forecasting.

Andrew