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Tuesday, March 4, 2008

Stu Ostro: Update on Global Warming, Weather, and the Arctic

Note to readers: One of my particular concerns has been the "odd," "strange," and "unusual" (to quote from articles that I have read) changes in the North Atlantic Oscillation, the polar vortices, and the like.

The post below was taken from another blog. I will be contacting the author to seek his permission and further comments.

It was taken from this blog:

(Please take careful note of the second figure, which shows a very "unusual" high pressure zone over the Arctic.)

by Stu Ostro, Senior Meteorologist

In my "A Connection Between Global Warming and Weather" blog in September I linked to a PDF of the latest installment of a PowerPoint presentation I had been giving at and outside of TWC, and I recently presented an updated version at the 2008 Steamboat Weather Summit. I wanted to again make the content available, so here is a new PDF (large >20MB file). For those who have seen the previous version, the new material is mainly the first few slides and then many of them from #205 on. Here are some highlights and things to expound upon.

My ongoing goal is to analyze and understand what's happening to our weather and why. If 10 years from now I'm still alive and blogging, and we look back and see that the recent goings-on were just a blip, then that's what I'll write. But for now at least, the types of patterns I noted in that September blog have continued to occur. First there were a couple of events about which I posted follow-up entries: the extreme warm wave which affected the Chicago Marathon and the Yankees/Indians game, and in the same month the record largest October tornado outbreak (and one which was unusually far north for that time of year).

Then in late October across the conterminous U.S., anomalously high 500 millibar heights (please refer to the September entry for a thorough explanation of that stuff) interacted with a large, retrograding, and bizarre-for-October cutoff low. That took place at the time of the exceptionally strong Santa Ana wind episode; and, although there were non-meteorological factors involved in the catastrophe in the Mexican state of Tabasco, the southern extension of the trough associated with the cutoff triggered thunderstorms with heavy rain that helped initiate the flooding.

In mid-December there was a very strong, toasty subtropical ridge, on the periphery of which the historic ice storm in the central U.S. occurred and Olga developed in the Caribbean region, one of the latest tropical storms on record there (and maybe the latest to make landfall in the U.S., depending on what NHC officially does with Olga's regeneration).

Recently there's been a topsy-turvey upper-level pattern in Asia and Europe, which led to stormy and unusually cold and snowy weather in some places. To reiterate: despite its paradoxical nature, not only is that sort of outcome not mutually exclusive with global warming, it is consistent with the types of patterns in recent years that have been driven by strong ridges of high pressure aloft at mid and high latitudes directly related to the overall warming.

Fig. 1.

And since the new year commenced, in between a couple of cold but unremarkable-for-January bouts of Arctic air in the U.S. (including the current one) there was the exceedingly warm, April-like pattern which begat that amazingly-far-north tornado outbreak.

And the above can't all be attributed to La Nina.

From a "synoptic meteorology" standpoint, though, perhaps the most extreme anomaly was in early December.

There was an incredible ridge of high pressure aloft for that far north at that time of year (500 mb heights > 570 dm) in and around the Alaskan Arctic. To the south of that was a gigantic and fierce trough, which resulted in the "Great Coastal Gale" in Oregon and Washington that was unusual in its combination of intensity (gusts as high as 129 mph), duration, and how far away from the coast the surface low was at the peak of the storm.

Fig. 2.

This NCEP reanalysis data analyzed 500 milibar height anomalies over the Arctic Ocean in excess of 600 meters above the climatological mean. That's outrageous. None of the many previous cases I had investigated since 2005 had an anomaly higher than ~425 meters, and indeed, Barrow and Fairbanks set their December records for 500 millibar heights (period of record going back to 1948).

There were also surface temperature anomalies underneath that ridge upwards of 25C ... and this happened in the vicinity of where the stunning sea ice loss was in late summer and near where in early December there was still a chunk missing ... pure coincidence?

Of course, since the September minimum, the waters of the Arctic Ocean have refrozen, but the ice area is still nearly a million square kilometers below the 1979-2000 mean. Much of the new ice is thinner than average, and it'll be very interesting -- and important -- to see what happens next summer and the few summers thereafter. Sheldon Drobot recently posted a good analysis of the situation in these TWC Forecast Earth blog pages, and Bob Henson did the same in the UCAR Quarterly. Bob also addresses the much-ballyhooed ice increase in the Southern Hemisphere. An above-average sea ice area in the Southern Hemisphere and global warming are not mutually exclusive!

But ... be that as it may ... can't all of this simply be explained by naturally-occurring phenomena, such as the Pacific Decadal Oscillation (PDO), and the associated Great Pacific Climate Shift which commenced in the mid-1970s? That's what the viewpoint was in this article, which cited me by name and alluded to my September GW/weather blog.

Actually, I'm in agreement that the PDO and other types of natural variability, such as the Atlantic Multidecadal Oscillation (AMO), which was also cited in that article, play an important role in weather and climate, and clearly what took place a few decades ago was no exception.

However, I have a few questions.

Here is the surface temperature correlation to the PDO which was cited:

Fig. 3.

And the anomalies from the late 1970s through 1997, which were also cited, because of the pattern's similarity to the above map:

Fig. 4.

But here are the surface temperature anomalies for the past 10 years using the same scale (followed by a graph of the PDO index showing that it peaked from the late 1970s into 1998 and has on average been lower than that since then). Notice anything which stands out? Why was this anomaly map for the past decade left out of that article? And if the PDO does not explain this, what does?

Fig. 5.

There's a new study by Meehl, Hu, and Santer on this mid-1970s climate shift being presented in New Orleans this week at the Annual Meeting of the American Meteorological Society, which I am attending. The abstract reads:

"A significant shift of Pacific climate occurred in the mid-1970s with effects that extended globally. One view is that this change was entirely natural and a product of internally-generated decadal variability of the Pacific climate system. However, during the mid-1970s there was also a significant increase of global temperature and changes to a number of other quantities that have been associated with changes of external forcings, particularly increases of greenhouse gases from the burning of fossil fuels. We analyze an un-forced control run from a global coupled climate model as well as 20th century simulations with changes in external forcings to show that the 1970s climate shift had a contribution from changes in external forcing superimposed on what was likely an inherent decadal fluctuation of the Pacific climate system. Thus this inherent decadal variability delayed to the 1970s what would have been a forced climate shift in the 1960s."

Maybe, though, you're skeptical of climate models' capabilities. Referring back to that article which touted the role of the PDO, could the combination of the PDO and the AMO explain everything?

Well, Figure 8 therein shows the value of the PDO+AMO dropping sharply during the past couple of years.

Nevertheless, what has happened recently?

The idea that planetary warming has ceased in the past decade has recently been in the news and analyzed. Has global warming stopped? And if so, is that further evidence that it's all a hoax?

Well, let's look a little closer ...

Positive temperature anomalies overwhelmed negative ones worldwide in 2007, with the warmth being particularly dramatic in Asia:

Fig. 6.

Yes, the land+ocean temps peaked a decade ago and although that level has been rivaled it has not been appreciably exceeded. But the 1998 spike was associated with the exceptionally strong El Nino at the time. In 2007, even with notable Arctic blasts in the U.S. in February and April and much-ballyhooed bouts of cold weather in South America, and despite ocean cooling (in large part due to the massive negative anomalies in the Pacific which extended well away from the equator as La Nina developed and flourished), the year was the warmest on land in the period of record going back to 1880.

Fig. 7.

There's always a certain amount of natural year-to-year variability, but clearly the overall land trend continues to be steadily upward. A recent study modeled in these natural variations in recent decades and projected the entire global average (land+ocean) out through the next one, with at least half the years after 2009 predicted to exceed the existing warmest year on record. By 2020 we'll have the luxury of those additional years of perspective and be able to see how that forecast verified. In the meantime, remember that 2007 was in the context of what happened in the Arctic during the summer. Those who are skeptical of global warming, its causes, and future prospects (as I once strongly was) like to point out that there's inherent forecast uncertainty, and that climate models can be wrong. I agree. But that can cut both ways: overestimating or underestimating the rapidity of the changes ... and so far when it comes to the Arctic, a crucial component of the Earth's climate system, it's stunningly been the latter in regard to sea ice. This chart from Bob Henson's article linked to above, and in turn courtesy of Marika Holland and the Community Climate System Model, shows how the ice decline (blue line) has not only radically deviated from the ensemble mean forecast (red line), it has exceeded that of any ensemble member (orange shading).

Fig. 8.

So what does it all mean? Well, if the Earth's weather patterns have already gone out of kilter and the nature of extremes is changing, and the Arctic has already passed a tipping point, then that has implications for adaptation to the effects of climate change. Now, I know this is a sensitive topic; there was even an entry on these pages a month ago titled, "Adapt! The Cry of the Coward"! So before my words are misconstrued, let me be clear that I'm not suggesting that this is totally in place of addressing the challenging issues associated with the mitigation of the human role in global warming. We won't be around a couple hundred years from now but to those who will be, what we do this century matters. And in general, as I've written in the past, we are tenants on this planet, not its owners, and we have a moral responsibility to coexist with it in a reasonable balance.

That having been said, the political and economic realities involved with deciding how far to go in forcing the issue of mitigation are difficult ones, while there is no question that the need to adapt is a given. Regardless of what happens in future centuries, we have the next 50 years and 20 years and decade and year and tomorrow to deal with. And it's not a question of which climate, a relatively cold or warm one, is "better," it's the rapidity of change which is problematic for human civilization (and the animal and plant worlds), as well as vulnerability to weather and climate phenomena such as floods, droughts, temperature extremes, and intense cyclones (tropical or otherwise). That's the case regardless of global warming, and a shift to a more volatile weather/climate regime would exacerbate the situation.

Although Roger Pielke Jr.'s points of view and mine may not always be identical, when I go back and read his paper on adaptation from 10 years ago, I'm hard-pressed to find anything in there I disagree with.

The bottom line is this: in addition to long-term trends, we need to focus on being able to deal with shorter-term anomalous and/or severe manifestations of global warming including day-to-day weather, as that's where the climate rubber hits the road.

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