Tuesday, September 30, 2014

Minimum Temperature Heat Wave

September has really been remarkable over western Washington in one way:  the consistently high minimum temperatures.   I have mentioned this before, but it just doesn't end.  Here are the September temperatures at Sea-Tac Airport (yellow line) and the average max (red) and min (blue) based on climatology.  For every day but one, the minimum temperatures were above normal.  Many days in the last two weeks had minimum temps of 2-5F above normal.
Now this pattern is not limited to western Washington...take a look at a map of the average difference between climatology and the observed minimum temperature over the west during the past month.   Nearly the entire region had very high minima, with extensive regions being 2-6 F above normal.

The effects of these high minima have been clear in my vegetable garden.   Of great importance to me, my tomato plants are healthy and still producing.

So why have the minimum temps been so high?

First, we have had a very persistent area of low pressure offshore, with enhanced southerly flow on the western side of the low (see the map of the difference of sea level pressure from normal).   Remember that air goes counterclockwise around lows in the northern hemisphere.

In addition, the water over the eastern Pacific has been much warmer than normal-- the warm blob we have discussed a lot (see graphic).  In this map (SST differences from climatology over the past week), we see warm water (1.5-4C above normal) off the West Coast.   So southerly flow moving over warm water keeps us toasty at night.

We had a few rainy days, but that is over now and the daytime temperatures will surge again, but with a different pattern:  a major ridge of high pressure will form over the eastern Pacific.  Here is the forecast for Sunday at 5 AM PDT from the European Center.  Big ridge.  But it will get wet along the BC coast and there is a chance of some light shower on Saturday before the ridge amplifies on Sunday.   Want to be sure of the weekend weather...go south of Olympia.

And the 6-10 day forecast from Climate Prediction Center? Above normal over the western U.S. and colder than normal over the east.  This is looking a lot like the persistent pattern last winter.  Getting scary.

Sunday, September 28, 2014

Why is the Northwest U.S. warming? Natural variations or mankind's greenhouse gases?

During the past week, a major debate within the scientific community has reached the popular media: 

To what degree is the modest warming over the northwest U.S. during the last century caused by (1) natural variations or (2) by increases in greenhouse gases emitted by mankind?

This is a critical question with huge implications for public policy, climate adaptation, and scientific understanding of local climate change.

The two viewpoints

On one hand, there are some scientists saying that nearly all of the warming is due to mankind's emission of greenhouse gases.   For example, here is a quote (made to KUOW) by Dr. Phil Mote, a scientist at Oregon State and Oregon State climatologist:

“As far as the 1.3 degree warming over the last hundred years or so," Mote said, "that’s all because of human activity.”

He is claiming that all of the warming of the last 100 years is due to anthropogenic (human-related) forcing.  And  Dr. Mote  goes even farther than that, suggesting that the local warming has accelerated

"what is particularly significant is that the rate of warming is increasing"

Support for his claims is found in a paper he published in the Journal of Climate last April with John Abatzoglou (the lead author, University of Idaho) and David Rupp (Oregon State):

On the other hand, a recent paper by Drs. Jim Johnstone and Nate Mantua in the prestigious journal Preceedings of the National Academy of Sciences (PNAS) came to the OPPOSITE conclusion:

"these changes are not likely related to historical anthropogenic and natural radiative forcing"

As Dr. Mantua said to the Seattle Times
"We do not see a human hand in the warming of the West Coast,”

In their paper, Dr. Johnstone and Mantua demonstrate that natural variability explains virtually all of the temperature evolution of the past century over Northwest U.S.

So we have two groups of scientists, both with Ph.Ds and having published results in well-known academic journals, coming to the opposite conclusions about the origin of the roughly 1.3 degree F warming that has occurred over the Northwest in the last century.


How can two groups of scientists looking at the same basic data come to radically different conclusions?

Both can not be correct.

As explained below, I believe the Johnstone/Mantua paper makes a far better case:  human-caused warming over the Northwest has been minimal, with natural variability dominating.

Both teams of atmospheric scientists agree that the earth and the Pacific Northwest will warm due to increases in greenhouse gases, what they disagree upon is the impact of greenhouse gases in the past compared to natural variability.

The warming of the Pacific Northwest

One things nearly everyone agrees upon is that the Pacific Northwest has warmed over the past century, but NOT in a continuous way.  Here are two plots of temperature change (from a base period) over the Northwest, one from the Abatzzoglou, Rupp, and Mote paper (upper figure) and the other from the summary report of the UW Climate Impacts Group (bottom).  The top figure has the temp variations from three different sources as well as lines that attempt to smooth out the yearly variations.  The second figure has a single trend line (which really doesn't make sense since the variation is so complex)

These plots can be summarized as follows:
  • Since 1900, the temperatures have warmed by about about 1.3 F
  • The temperature has NOT warmed in a steady way.
  • The largest warming was concentrated in two periods: 1915-1940 and 1975 to 1985
  • Temperature has hardly changed from roughly 1985, the last 30 years.
So it is immediately obvious that the claims of some local scientists that the temperature change in the Northwest is accelerating is simply not true.  I wish they would stop saying that; it completely undercuts their credibility and projects poorly on the rest of the scientific community.

Natural versus human-forced variability

So what is forcing the temperature changes shown above?

Let's begin by human-connected temperature changes forced by increasingly greenhouse gases (such as CO2).  The Intergovernmental Program for Climate Change (IPCC) has published a series of authoritative reports, including an estimate of the global impacts on radiation in the atmosphere due to mankind's influence on greenhouse gases.  Here is a plot of their estimates over time;  pay particularly attention to their estimates of the influence of Long Lived Greenhouse Gases (LLGHG, red line))
Such gases slowly increased from 1850 onward, but REALLY accelerated around 1960 as population and use of fossil fuels exploded.  So if the radiative effects from greenhouse gases was were only thing going on, we would have seen slow, steady warming until the mid-20th century, followed by sharp warming after 1960.  Some other factors have influenced the radiative balance of the earth as well, like more reflective particles from combustion, volcanic eruptions, and changes to the land surface.   Considering all those effects produces the black line, which suggests that the big warming due to greenhouse gases should have begun around 1965.

Clearly, the temperature change in the Northwest looks very different than this, so more must be going on!

For example, because of the non-uniform nature of the earth's surface and weather systems, some places would warm up more or less than others.  We can get a handle on that by looking at the average of many climate simulations for the upcoming century (as shown in the figure below, which shows the impact of  greenhouse gas increases on surface temperatures by the year 2100). The Arctic warms up the most, the continents warm more than the oceans, and the eastern oceans warm up less than the western oceans.  The Northwest is downstream of the eastern Pacific and thus would warm less than most places (the simulations below did not have the resolution to describe our cooling by the Pacific, higher-resolution simulations show this).

So what should you conclude?    The radiative effects of greenhouse gases were relatively small before roughly 1965 and that our area should experience less warming than most places.  

There is nothing controversial in this statement.  Thus, one would certainly not expect the warming in the early part of the century over our regon to be forced by mankind's greenhouse gas emissions.

But although the Northwest should not have been highly impacted by global warming from greenhouse gases, we ARE highly impacted by natural variability, changes in atmospheric circulations that occur due to complex interactions within the atmosphere and with the oceans and land surface.

One example many of you know:  El Nino and La Nina, in which sloshing water in the tropical Pacific causes an oscillation of warming and cooling of equatorial waters over a period of 4-7 years.  El Nino/La Nina, often called ENSO (El Nino Southern Oscillation), has weather impacts all over the world.

But there is another type of natural variability that has a huge impact on the weather/climate of the Pacific basin:  The Pacific Decadal Oscillation.   This climate feature, discussed in depth for the first time by Dr. Nate Mantua (yes the same Mantua of the paper noted above), has a period of around 50 years, oscillating between warm and cold cycles (see figure).  It was in a cool cycle between roughly 1950 and 1977, then a warm cycle until around 2005, and more recently looks to be in a cooler cycle.

If you compare the variations of the PDO with the Northwest temperature traces shown above, it is very obvious that the variations of NW temps seems to closely follow the PDO changes, suggesting our temperatures are highly controlled by this mode of natural variability.

Thus, when some local scientists say that the temperature changes experienced here in the Northwest are mainly due to greenhouse gas emissions they are certainly incorrect.  We live in an area where the greenhouse gas signal is small and where natural variability (as forced by the PDO and ENSO) are quite large (and there are other modes of natural variability I have not even discussed).

The Controversy

As noted earlier, the media has been full of reports and stories about this controversy of natural versus greenhouse gas forcing here in the Northwest.  There have been literally hundreds of stories in national/international media outlets and the Seattle Times even wrote an editorial about the subject.

Many of the media reports were stimulated by the Johnstone/Mantua paper in PNAS.    This paper presented stunning results.   They showed that one could explain the sea surface temperature pattern off the West Coast if one knows the pressure/wind field.  Amazingly, they could produce a nearly perfect evolution of the temperatures over the past 100 years using pressures/winds (a.k.a., the circulation).

 Here is an example of their results from this paper.  SLP1 is their pressure/circulation index, SSTarc is the sea surface temperatures off the West Coast, and SATarc are air temperatures over land along the West Coast.  Folks, this is amazing correspondence.

Then they plotted the observed temperature change over the last century (left panel below) and took out the contribution of changes in circulation (wind/pressure)--right panel (residual trends).  The temperature change over the past hundred years goes away for most locations (right side) when you take our trends in atmospheric circulation.

You might ask, could the changes in the winds/pressure that explain the temperature changes be explained by greenhouse gas increases?   At this point in time, there is no theoretical or observational evidence to suggest so, and tree ring data suggests long term circulation changes have occurred for thousands of years in the past.  Furthermore, when Johnstone/Mantua examined the pressure/circulations changes produced by global climate models over the next century, they did NOT find any suggestions of changes in natural variability resulting from greenhouse gas increases.

So the bottom line of the Johnstone/Mantua paper is that natural variability has dominated temperature changes during the past century over the Northwest.   I believe their  arguments are very strong.

In contrast, there is the paper of Abatzoglou, Rupp, and Mote (2014) that suggests greenhouse gases are the dominant source of warming.  Specifically, they stated that anthropogenic forcing was "the leading contributor to long term warming."  In their paper, they used a technique called multiple linear regression to determine the forcing of temperature by several forcing mechanism (natural variability, volcanoes, solar variability, and human greenhouse gas forcing).  Their results suggested that anthropogenic contributions (greenhouse gases) were dominant.  But unfortunately, their approach has some critical problems that make their conclusions insupportable.  Problems so severe I am surprised they got through the review process.

The biggest deficiency--and this is going to get technical-- is that they detrended (over time) their index of natural variability (they detrended the 500-hPa height anomalies they used).  In doing so, they removed the ability of natural variability to explain the long-term temperature trends.  In other words, they threw away EXACTLY what Johnstone/Manuta showed to be important.

And the Abatzoglou paper had other problems, like using a poor index of natural variability (they did not use the PDO or anything like it).

I know this has been a long blog, but the issues are very important.  What should you come away with?

  • Global warming is not globally uniform, some places will warm more or less quickly.  The Northwest is a regional where warming will be slow due to our proximity of the eastern Pacific.
  • Natural variability in climate can be quite large and in some locations it is currently far larger than the contribution of mankind from greenhouse gas emissions  and other impacts.
  • For the West Coast, natural variability has been far larger than greenhouse gas warming.

As the century progresses, the human-caused global warming signal will increase while natural variability should remain about the same. Thus,the lack of human-caused warming on the West Coast does not imply that anthropogenically forced global warming is unimportant.  It is a serious issue mankind must deal with.

This situation also shows you some of the strengths and weaknesses of the scientific process.   Some papers get published that have problematic results.   Some scientists say things that are not supported by rigorous research.   But the system has a way of righting itself and self correcting over a period of time.  The Johnstone/Mantua paper has made a substantial contribution to recentering the discussions regarding Northwest climate change.

Saturday, September 27, 2014

Summer is NOT OVER

A number of the local media outlets were musing that summer is over now.

That clouds and rain are our destiny.

Don't believe it.

We will start with a perfect weekend.  Yes perfect, with sunny skies and temperatures getting into the lower 70s.  What could be better than that?

Weather will decline on Monday through Wednesday as a trough pushes through the region.  Good for work or school.  But a substantial ridge will develop mid-week and fine...if not warm...weather will end the week and extended into the weekend.  

Let's take a look at the upper level (500 hPa, about 18,000 ft) maps from the European Center for Medium Range Weather Forecasting (ECMWF).  For each panel the left side gives the average of an ensemble of many forecasts.   Averages of many forecasts tend to be more skillful than a single forecast.  The right panel is the forecast from their very high resolution single forecast.  The shading gives you a measure of uncertainty in each (green indicates more certain on the left panel).

For Wednesday at 4 PM the ridge is starting to build in the eastern Pacific.

Further development on Thursday at 4 PM.

By Friday at 4 PM, we are talking about a major ridge over the West Coast and the ensembles have some confidence in this solution!

Saturday at 4 PM:  BIG RIDGE!
Sunday at 4 PM.  More ridging.

The Climate Prediction Center extended (8-14 day) forecasts for October 4-10th is for warmer than normal and drier than normal over us.

We will see sun and temperatures into the 70s later in the week and next weekend.

 And what about the warm "blob" over the eastern Pacific that has contributed to above-normal temperatures over us?

As shown in the figure below, the blob is still there!  I like the blob.

One final thing, although our max temps have been knocked down a bit lately, we have been much warmer at night (perhaps with the help of the blob).  The figure below provides the proof.  Every day of the last two weeks has had a minimum above normal at Seattle-Tacoma Airport.
Our Friend

Thursday, September 25, 2014

Long-lived atmospheric river hits the Northwest

During the last few days we have experienced an early and unusually sustained atmospheric river event that has dropped 1-4 inches over the Northwest.

As many of you know, atmospheric rivers are associated with a narrow tongue of large amounts of water vapor that move northward or northeastward out of the tropics/subtropics into the midlatitudes.  When they hit our regional mountains, they can dump large amounts of precipitation.

Atmospheric rivers can be observed from weather satellites that sense atmospheric moisture.  Here is an example from the current event (this image shows the water vapor content of the atmosphere from 5 AM to 5 PM on Tuesday).  Lots of moisture in the tropics and you can see the plume heading
straight into us from the SW.  An alternative view is from a simulation of the atmospheric river for 5 PM on Tuesday (made by the UW WRF model).  The blues are area of very high water vapor content.

Since the origin of this and many other atmospheric rivers that affect us is from near Hawaii, we affectionately often call them pineapple expresses.

Here is a close up view of our recent pineapple express using visible satellite imagery....nice river of moisture, as evident in the clouds on Tuesday (top picture) and Wednesday (bottom picture)

Now this river has dumped serious moisture around here.  Take a look at the 24h precipitation totals ending 11:29 AM on Wednesday.  2-3 inches, with heaviest amounts on the southeast side of the Olympics and in the north Cascades.   1 inch plus was observed around Puget Sound and over SW Washington.

And the 48h rainfall total from Seattle RainWatch shows a similar pattern.  Nice rainshadow around Sequim!  Those folks were playing golf while the rest of us were getting poured on.

This river has been very long lived because the weather pattern has not been progressive.  The river slowly slid eastward yesterday (Wed)--see graphic.

And even this morning was along the West Coast.

A fortunate byproduct of the current atmospheric river is that heavy precipitation is hitting northern California--several locations have gotten 2-3 inches.  This is a little drop in the bucket for them, but hopefully some water will get into their two main reservoirs:  Trinity and Shasta.

The weekend looks mainly dry in our area, but precipitation return on Monday.

One final thing.  The strong southerly flow over the West Coast had a major impact on aviation, resulting in very quick flights from San Francisco to Seattle.   Some flights were making the trip in just a bit over an hour.   The upper level map (300 hPa, around 35,000 ft) this morning illustrates the amazing winds.  Yellow colors indicate a wind speed of  more than 60 meter per second (134 miles per hour or more).

Tuesday, September 23, 2014

The weather turns....

Mother nature must be watching her seasonal clock very carefully, because as we officially transition to fall today, a major storm has moved into our offshore waters and rain has returned to the Northwest.    

Let's show you the forecast precipitation.  Here is the 24-h total ending 5 PM Tuesday.  Wet offshore and light to moderate rain over much of Washington.

 But the next 24h, in which a front crosses our region, brings heavier precip, with 1-2 inches over the coastal mountains, north Cascades, and the southern Willamette Valley.

And it is still wet for the 24 h ending 5 PM Thursday.

Add it all up and what do you get?   Here is the answer...soggy...with isolated locations enjoying 2-5 inches.

But even more impressive is the strong low pressure offshore (972 hPa), low pressure that will produce strong winds over the Pacific.

Here is the forecast sustained winds at 2 AM Tuesday.  An area around that low has SUSTAINED WINDS over 50 knots!

This low will move slowly to the NE.  Big low system, strong winds, and slow movement implies the generation of big waves.  An NOAA's Wavewatch III ocean prediction system forecasts exactly that (see image), with 8-9 meter waves (25-30 ft) forecast offshore.  Such low pressure, strong winds,and waves is bad for the warm "blob" off our coast.

Saturday, September 20, 2014

Time Travel for Northwest Weather

How would you like to enter a time machine and experience the weather of 50 years from now?

Interestingly, many of you have already done this and haven't realized it.

I am talking about this summer's warm weather in the Pacific Northwest.

Many locations on both sides of the Cascades experienced their all-time warmest temperatures for July and August and although it is not official yet, several locations, on both sides of the Cascades will have their all-time record summer (June 21-Sept 21) temperatures.  As I noted in my earlier blogs, this warmth appears to be the result of natural variability, which created persistent high pressure over the eastern Pacific last fall.  Not anthropogenic global (or local) warming.

How warm has the last three months been?  Here are the temperature anomalies (difference from normal or climatology, generally the 1970-2000 is used for climatology today) over the past 90 days for Washington and Oregon (note the scales are a bit different in the figures).  Nearly all of these states have been well above normal;  averaging roughly 3F above normal, with eastern Washington more like 3.5F above normal

Now how far in the future would you have to "time travel" for such temperatures to be normal?

To be put it differently, how far in the future will the average summer be like the summer we just went through?

The UW Climate Impacts Group has published a report outlying their prediction of the future Northwest climate.  Here is a figure from that report showing predicted temperature change (relative to 1960-1989) with time.  They show the warming from various scenarios or RCPs (Representative Concentration Pathways).  RCP 8.5 (solid red line) is the most aggressive.   For reasons that I won't go into, many of my climate colleagues believe it it too aggressive and the RCP 4.5  (blue line) will prove closer to what will happen.  Actually, a number of model simulations were made, with the solid lines being the average of many runs (the range of these simulation is shown by the lighter lines).

Reading off the graph, it looks to me that a 3F warming from 1970-1999 would be about a rise from roughly .5 F during 1970-1999 to 3.5 F around 2050.  Quite frankly, even if we choose the more aggressive RCP 8.5 scenario the answer would not be that much different (maybe 2045).

Bottom line:  the warm summer we have experienced is very much like the average summer around 2050.

Thus, we all have been time travelers, experiencing the average weather roughly 35 years into the future!  Congratulations, time traveler.

Is the warming we experienced a good or bad thing?  

I will let each of you decide for yourself.  For my tomatoes, it has been a good thing.  For my desiccated lawn, a bad thing.    For those who like sustained warmth and sun, a good thing.  Those who long for Northwest clouds and cool weather, a bad thing.

                                          When I was a kid, one of my favorite TV shows was the Time Tunnel.   

I do have an editorial comment that will undoubtedly get me into trouble.   When folks talk about climate change and more specifically global warming, it is always described in negative terms.  I was at the Northwest Climate Conference at the UW two weeks ago.  NO ONE mentioned a single positive attribute of a warming climate here in the Northwest.

Yes, rapid climate change is often (and perhaps generally) negative for fauna, flora, and people accustomed to the current climate, but is there nothing positive about a cool, cloudy part of the U.S. getting a bit warmer?   God knows, I am going to get a lot of flack saying this, but there it is....