Saturday, June 22, 2019

June 30 - July 4 Potential Storm System

A potential storm system looks to impact the United States in the June 30th through July 4th timeframe.

MSLP and precipitation forecast for 1 AM Central Time, June 24.
Source: Tropical Tidbits
Model guidance sees a storm system moving northeastward parallel to the coast of Japan on June 23rd and 24th, as depicted in the image above. Upper-level flow indicates this system will be the finale for broadly stormy weather seen over the East Asian region as of late, and it is plausible that this will be the case in the United States as well.

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 30 - July 4 period.

Forecasted 500-millibar geopotential height anomalies for 7PM Central Time, July 1st.
Source: Tropical Tidbits
Ensemble guidance suggests ridging will become a predominant theme over the western two-thirds of the country as an upper-level low nudges its way on over Greenland to promote +NAO conditions (which bring about more zonal upper-level flow over North America), and troughing in the Gulf of Alaska supports a modest ridge in western Canada. The consequence looks to be lower heights evolving over the waters just offshore of the Eastern Seaboard, and it seems plausible that any storm system that comes about during this timeframe will be encouraged to traverse the South (or the nation's midsection) on a rather-zonal path before being drawn northeast as it approaches the Mid-Atlantic, with the storm seeking the path of least resistance towards those lower geopotential heights. Coming days will provide for more accurate forecasts as the potential storm approaches, of course, but at this time it appears as though any potential storm system in the June 30 - July 4 period will most likely impact the South and East U.S., as opposed to a storm track that takes in northeast across the Plains like we saw last month into early June.

To Summarize:

- There is potential for a storm system to impact the United States in the June 30 - July 4 period.
- Based on the projected atmospheric flow, any storm that does develop would be most inclined to move west-to-east across the South before perhaps trying to shift a bit northeast near the Eastern Seaboard.
- As always, considerable uncertainty exists, especially with regards to the projected track of any storm system.


Friday, June 21, 2019

June ENSO Update: El Nino Increasingly In Danger for Fall and Winter

It seems to me that the El Nino is in increasing danger for its survivability during the fall and winter months, with a variety of recent observations suggesting the El Nino has weakened and that near- to medium-term prospects are similarly downbeat. Click on any image to enlarge it.

We first begin with a look at the latest sea surface temperature (SST) raw values and anomalies.

Observed sea surface temperatures (top panel) and SST anomalies (bottom panel) for the seven-day period centered on June 12th.
Source: CPC
The actual observed temperatures, as shown in the top panel, are arguably more pessimistic with regard to the current state of the El Nino than temperature anomalies reflect. Indeed, where the top panel seems to suggest a total erosion of warmer waters and a filling-in of cooler waters (which appears symptomatic of a La Nina), the anomalies in the bottom panel are more sanguine, reflecting aggregate-neutral anomalies from the western coast of Ecuador to about 130 degrees West longitude. To the west of that, a swath of above-normal SSTs reflect the presence of El Nino conditions in at least some part of the Equatorial Pacific. 

In other words, while sea surface temperature anomalies (SSTAs) are rather mixed in the eastern Equatorial Pacific, it certainly looks more like an El Nino than a La Nina, a relief for fans of winter weather in the eastern United States. Of course, it’s never as simple as merely viewing one or two pieces of data. This situation gets far more intricate. In fact, there’s four different regions of the Equatorial Pacific that each make the situation rather intricate.

The four regions used for monitoring the ENSO phenomenon.
Source: CPC
There are four different parts of the Equatorial Pacific that meteorologists monitor to determine the state of the El Nino-Southern Oscillation (ENSO), the phenomenon that defines an El Nino or La Nina. When these sea surface temperatures are above normal, we call it an 'El Nino' event. When these anomalies are below-normal, we call it a 'La Nina' event. While we monitor the entire Equatorial Pacific to analyze the ENSO phenomenon, these four primary "zones" are:

Nino 1+2. This is a small slice of the Pacific located between the Equator and the 10º South latitude line, extending from the far western tip of Peru to the 90º West longitude line.

Nino 3. This is a larger slice of the Equatorial Pacific which spans from 5º North to 5º South latitude lines, and from 90º West to 150º West longitude lines.

Nino 4. This is also a larger slice, and also extends between 5ºN and 5ºS on the latitude markers. For Nino 4, however, the space is spread by longitude from 150º West to about 160º East, crossing the dateline in the process.

Nino 3.4. This is the critical area to watch, and is typically viewed as the primary space with which to assess the state of the ENSO phenomenon. Spatially, it extends from 5ºN-5ºS latitudinally, and 120º West to 165º West longitudinally.

Why do we break this space up into four different pieces rather than just average out the sea surface temperature anomalies and call it a day? A number of scientists with far more knowledge and research than I have come to determine that there can be more than one type of El Nino - where typically El Nino's bring warmer than normal waters to the eastern Pacific, an "El Nino Modoki" event brings warm waters to the western Pacific, and cooler waters to the eastern Pacific. This is not a trivial difference, but for our purposes here, we won't dive into that topic. For now, the key is understanding there are four different regions in which we monitor the ENSO phenomenon, with the Nino 3.4 region broadly being of most importance.

Now that we have an understanding of the parts of the Equatorial Pacific that are most important when analyzing the state of the ENSO phenomenon, we can proceed on to other observational data as of late.

SST Anomalies over the last year for the four ENSO monitoring regions.
Source: CPC
When breaking down observed SST anomalies over the four ENSO regions for the past year, as in the image above, it’s clear that this El Nino event is not clean-cut across the board. In Nino region 4, anomalies were recently seen about 1 degree C, a level that has been breached a number of times over the last year and a pretty strong signal of an El Nino event being in place – at least for that region. In Nino region 3.4, while positive SSTAs have been consistently seen over the past year, they aren’t at the same magnitude as in region 4, with a recent peak at the end of May only barely reaching 1.0 degree C above normal. Still, with this region consistently exhibiting anomalies above +0.5 degrees C, this region is also indicative of an El Nino, even if the signal is not as strong as in region 4.

Continuing eastward, region 3 is where this analysis begins to run into some troubles. While the recent history of region 3 anomalies has been positive and mostly above +0.5 degrees C, this same region saw anomalies turn negative as recently as early September 2018, with most-recent readings looking to dip below +0.5 degrees C to possibly a four-month low. Again, anomalies in this region have been pretty consistent in remaining above the +0.5 degree C level, which tells us an El Nino seems to be present. But the last year has shown anomalies that aren’t as stable as those seen in the last few months, which (at the very least) informs us that the current El Nino seems less established in eastern Eqautorial Pacific waters as compared to waters further west. Indeed, moving to Nino 1+2, this concept is confirmed. Anomalies are only now beginning to exit marginally-negative levels in this region, anomalies on a magnitude that set a ~10 month low earlier this month. Nino 1+2 seems to be more reflective of an ENSO-neutral set-up as opposed to the El Nino environment portrayed by the other three ENSO monitoring regions.

So far, we’ve established that there is indeed an El Nino present as per sea surface temperature anomalies. This is nothing new; the Climate Prediction Center has maintained an El Nino Advisory for some time now to reflect this. Let’s now turn to convective anomalies along the Equatorial Pacific and discuss something called the Walker Circulation.

If you’ve learned about the weather to some degree, you’ll know that a critical component to meteorology in the aggregate is the concept of heat transport. Whether it’s certain large-scale circulations like the Walker Circulation or Hadley Cell, or smaller-scale but still significant features like Rossby Waves or Kelvin Waves, the atmosphere has a plethora of ways by which it moves heat around the planet. Here, we’ll discuss the Walker Circulation.

Typical Walker Circulation pattern in a La Nina.
Source: Wikipedia
The Walker Circulation, in a nutshell, is an atmospheric circulation along the Equatorial Pacific that is closely intertwined with the ENSO phenomenon. Indeed, the ENSO phenomenon itself is a consequence (and, to a degree, an instigator) of heat transport in the atmosphere and in the ocean. The Walker Circulation changes with regard to longitudinal position of rising and sinking air depending on the state of the ENSO phenomenon, but the constant is that it involves an area of rising air, movement aloft of this air either to the west or to the east, and subsequent sinking air back to the surface before it moves either west or east (opposite the direction the air moved when it was aloft) back to the starting point to create that ‘circulation’ feature. 

The image above shows the Walker Circulation in a La Nina event as an example of how the circulation works. How can we tell this is a La Nina orientation of the Walker Circulation (aside from it being given by accessing the image)? Note the above-normal water temperatures in the far western Equatorial Pacific, just offshore Australia. This prompts convective activity, which sends warm air aloft. The air then travels to the east before cooling and then sinking over the waters just offshore of Peru. This sinking motion discourages thunderstorm development by suppressing air from rising. This air hits the surface of the water offshore Peru, and easterly surface winds transport that air back to where it started, just offshore of Australia. As a consequence of those easterly surface winds, water “piles up” offshore of Australia, a consequence of those surface winds pushing the water up against land. My favorite comparison to this phenomenon comes from the University of British Columbia where the author compares this “piling up” of water to someone in a bathtub blowing air towards the other end of the bathtub. In this scenario, the water is pushed to the other side of the bathtub and “piles up” there as well – albeit on a far smaller scale. To be sure, the actual piling up of water in the western Pacific from a La Nina Walker Circulation is also very small relative to the depth of the ocean, but it is still noticeable enough to address it. 

Let’s use observational data now to see if we can identify how the Walker Circulation is acting, to try and ascertain if the atmosphere is joining sea surface temperatures in signaling the presence of an El Nino.

OLR Anomalies over the Tropical Pacific from May 23rd through June 17th.
Source: CPC
The image above is titled with a strange acronym – “OLR”. OLR stands for Outgoing Longwave Radiation, and while much smarter people with much more research can provide a much better interpretation of it, for the purposes of this post we only need to understand how it relates to convection. Negative values of OLR correspond to increased convective activity, while positive values of OLR correspond to suppressed convective activity. Therefore, rising air / thunderstorms in the above image are shown by cooler colors, while sinking air / suppressed convective activity is shown by warmer colors.

Taking a gander at the above image, enhanced convection is seen across a good chunk of the Equatorial Pacific, namely between the longitude lines of about 150 degrees East to 150 degrees West. In contrast, broadly neutral to slightly positive OLR anomalies are seen east of the aforementioned area, extending all the way to the western coasts of Peru and Ecuador. With regard to the Walker Circulation, it seems as though the area of rising air is located east of Papua New Guinea, with that air then transported to the east and allowed to sink over the waters west of Ecuador. The surface winds would then appear to transport that wind to the west, completing the circulation. 

The only problem here is, this sort of circulation resembles the Walker Circulation in a La Nina, not an El Nino. In other words, if the Walker Circulation really is oriented like the paragraph above describes, water temperatures may support an El Nino but the atmosphere is more supportive of a La Nina.

Observed 200-millibar vector winds from May 23rd through June 17th.
Source: ESRL

Observed surface wind vectors from May 23rd through June 17th.
Source: ESRL
We can identify what the Walker Circulation is doing by checking out some composites of atmospheric variables as of late. For the benefit of comparison, I will view these variables using the same timeframe presented in the OLR graphic of May 23rd to June 17th. 

The top image above shows mean 200-millibar winds over the Tropical Pacific basin for this timeframe. In this graphic, we see the wind vector arrows looking like they’re ‘blossoming out’ away from a center point roughly located along the 180 degree line of longitude. This is called divergence, and represents air rising and then spreading out aloft. In severe weather, divergence aloft is crucial to sustaining thunderstorms, and the process is no different here. It is also no coincidence that the divergence aloft is juxtaposed with observed enhanced convection over roughly the same area – if there was convergence aloft and those arrows pointed in towards each other, convection would be suppressed. After the air rises east of Papua New Guinea, it has been transported to the east as shown in the graphic, before those vector arrows seem to converge at around the 120 or 130 degree West line of longitude along the Equator. This is that convergence phenomenon I just discussed, and represents air aloft that is coming together and now sinking.

After the air sinks back towards the surface, the second panel allows us to see how surface winds have been behaving over this ~month timeframe beginning in late May. After reaching the surface at around the 120 degree West line of longitude, surface winds have sent this air to the west and back to the starting point at about the 180 degree line of longitude, where the circulation starts over again.

In summary, over the last month the Walker Circulation has seen convection / rising air over about the 180 degree line of longitude, with the air then transported to the east aloft before converging and sinking at roughly the 120-130 West line of longitude. Once back at the surface, this air has been transported to the west back to its starting point, completing the circulation. Is this representative of an El Nino or La Nina? Let’s view the two Walker Circulation composites for each situation below.

Typical Walker Circulation cells in an El Nino state, with shading indicating SST anomalies.
Source: UBC

Typical Walker Circulation cells in a La Nina state, with shading indicating SST anomalies.
Source: UBC
When comparing the above two Walker Circulation composites for the two ENSO states to what was described / observed above, the circulation observed over the last month doesn’t line up exactly with either image. Looking solely at the recent flow aloft and comparing it to the above two images, the recent Walker Circulation has certainly been more akin to that seen in a La Nina. As both the La Nina image above and the recently observed flow show, upper-level winds are shown to be moving eastward after the 180 degree longitude line, and move westward over the waters offshore of Peru and Ecuador. This same pattern is shown in the La Nina image above, while the opposite is shown in the El Nino composite. At the surface, winds also reflect a La Nina pattern to some degree, with westward winds both observed and shown in the La Nina composite west of the 130 degree West longitude line, but observed winds are still easterly from the 130 degree West line of longitude to the coast of Ecuador, opposite of what a La Nina would see for that area. Also posing trouble in our interpretation of the state of the Walker Circulation is that observed areas of enhanced and suppressed convection don’t really line up with either composite graphic precisely, and again only seem to “lean” more towards a La Nina than an El Nino Walker Circulation. 

What do we then gather from all of this? Based on the behavior of the Walker Circulation, it seems that the atmosphere is more reflective of a La Nina than it is of an El Nino, in contrast to observed SST anomalies which promote a solid El Nino. This can be confirmed by viewing the Southern Oscillation Index (SOI), which has been seen at weakly-negative values (indicative of an El Nino) but also seen as “consistently near zero”, perhaps quantifying the somewhat-murky atmospheric features described above.

Recent history of the Southern Oscillation Index (SOI).
Source: BOM
So, we’ve got oceanic temperatures which are in favor of an El Nino, and atmospheric patterns which are broadly more akin to a La Nina than an El Nino. To try and remedy this discrepancy, we now look to the behavior of water temperatures in the Pacific basin below the surface.

Upper-ocean heat anomalies along the Equator.
Source: CPC
El Nino and La Nina events can be driven by Equatorial Kelvin Waves, and whether the wave moving eastward along the Equatorial Pacific is upwelling or downwelling. If that sentence made you raise an eyebrow, you're most likely not alone. I can assure you, though, it's actually pretty simple to understand. Let's break it down.

The phrase 'Equatorial Kelvin Wave' seems intimidating, so for our purposes here all we need to know is that, from time to time, these Equatorial Kelvin waves develop in the western part of the Equatorial Pacific and gradually move eastward along the Equator. When they move eastward along the Equator, they can be either 'downwelling' or 'upwelling' waves.
Consider the explanation of a 'downwelling' Equatorial Kelvin wave as described by the NOAA:

"Normally, winds blow from east to west across the tropical Pacific, which piles up warm water in the western Pacific. A weakening of these winds starts the surface layer of water cascading eastward..."

In other words, if this wind pattern that blows winds from east to west breaks down, that warmer than normal water begins pushing eastward along the Equatorial Pacific. This anomalously warm water works its way eastward gradually and tends to sustain itself in the process. As a consequence, downwelling Equatorial Kelvin waves tend to be associated with El Nino events. You can see my annotations of downwelling Kelvin waves as solid lines on the above image.

On the flip side, an 'upwelling' Equatorial Kelvin wave can be thought of as the ocean waters trying to get itself a little more in balance in the wake of this very warm downwelling wave. Thus, an upwelling wave again features a Kelvin wave slowly progressing eastward, but this time it cools down the upper-ocean waters to a degree that upwelling Kelvin waves are generally associated more with La Nina events. I’ve made an attempt to outline downwelling Kelvin Waves with gray lines, and upwelling Kelvin Waves with dark blue lines in the image above.


Over the last year, we have seen several instances where warmer than normal waters traverse the Equatorial Pacific, as shown by the streaks of warmer colors. I’ve annotated these as downwelling Kelvin Waves. We have also seen a couple instances where either the warmer than normal anomalies subside in an eastward-moving fashion, or the anomalies outright flip to below-normal levels. I have highlighted these as upwelling Kelvin Waves in the above graphic. There are a couple items in particular I want to discuss with regards to the graphic.

First is how the recent downwelling waves seem to have been weaker with each iteration since a strong one traversed the basin beginning in late September 2018. With the exception of the wave which eventually strengthened significantly in March, recent downwelling waves have proved underwhelming. This poses a risk to the sustainability of the El Nino, as weaker downwelling waves make the El Nino more vulnerable to a deterioration to neutral-ENSO conditions, or even marginal La Nina conditions, especially if a strong upwelling wave propagates through with the atmosphere already unconvinced over the presence of an El Nino.

Second is how the far western Pacific has become increasingly cooler over the last several months. Indeed, anomalies on the order of between -1 and -1.5 degrees C have most recently been spotted right at the 150 degree East line of longitude, which marks the coldest anomalies over that part of the basin in at least a full calendar year. If this is the start of another upwelling Kelvin Wave and these deeper negative anomalies sustain themselves, there is a risk that the El Nino takes a severe hit in SSTAs, perhaps threatening its sustainability into the fall and winter months.

Model guidance for the Nino 3.4 region sea surface temperature anomalies.
Source: CPC
Seasonal model guidance is picking up on the increasingly-fragile El Nino. Where there is a good deal of spread between the individual model solutions in the above graphic, the general framework sees the Nino 3.4 region warming slightly this month into next before weakening through the fall, with a modest recovery back into weak-El Nino territory for the winter months. These models have a number of ensemble members of their own, which can all be tracked on a single image. As you might imagine, that image is very messy and does more harm than good in my opinion with regard to trying to explain what’s going on, hence why I’m not posting it here. However, I can tell you that the suite of ensembles and models show quite a spread in the forecast, implying substantial uncertainty over how the El Nino will evolve in the coming months. There is, of course, always uncertainty in these kinds of forecasts, but even for only a two-month forecast, the variation in ensemble forecasts for Nino 3.4 anomalies is from about -0.7 degrees C to about +1.75 degrees C. By December’s forecast, that variation blows out to a range of about -1.0 degrees C to more than +2.5 degrees C. The variation speaks to model uncertainty over how the El Nino will transpire over the next several months, including questions as to if it will be able to survive in what could be a hostile environment.

To Summarize:
- Current oceanic conditions indicate the presence of an El Nino, with warmer than normal SSTs present across most of the ENSO monitoring regions.
- Atmospheric conditions are more ambiguous, appearing more in line with vaguely-La Nina conditions as opposed to El Nino conditions.
- Further weakening of the El Nino appears likely moving into late summer and fall, which could bring the survival of the El Nino into question. 
- For the time being, it seems prudent to continue with the assumption of a weak El Nino moving into fall and early winter, but close monitoring is needed over the coming months as the El Nino is increasingly fragile.


Wednesday, June 19, 2019

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 1st through June 16th.
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 1st 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.