Friday, 26 February 2016

The weakenng el-Nino

As a Titanic El Nino Begins to Fade, What Fresh Trouble Will a Record Warm World Bring?

26 February, 2016

Today the globe is feeling quite a bit of backlash from a human-warmed sea surface and atmosphere. As it ends up, Dr. Kevin Trenberth was right. Deep ocean warming set off by heat-trapping fossil fuel emissions and building up through the first two decades of the 21st Century did re-surge from the depths to haunt us in 2014, 2015 and 2016. In that wrenching global climate system shift to the hot side of natural variability, a titanic El Nino emerged. It was one of the top three strongest such events in the modern record. One that by NOAA’s measure appears to have tied the extreme event of 1998 at its peak intensity.
(Sea surface temperature departure from average in the benchmark Nino 3.4 zone shows surface ocean heat anomalies for the 2015-2016 El Nino equaled peak 1997-1998 values. Image source:NOAA/CPC.)

Expected Heat, Drought, and Storms Together With a Few Ominous Surprises

Amidst this great upheaval of global heat, the world also experienced yet one more wave of freak droughts (this time over Northern South America, the Caribbean, large swaths of Africa and Southeast Asia), heat-related mass casualty eventsfloodsand strongest hurricanes on recordArctic and global sea ice measures are once again plunging to new record lows. A global coral bleaching event, perhaps the worst such instance ever experienced, was also set in motion.
The predicted patterns and potential worse-case events (such as heatwave mass casualties, coral bleaching, and sea ice loss) were also contrasted by a number of surprises. The first and perhaps most ominous was the failure of El Nino to bust the California drought. Though the West Coast of the US did experience a number of storms, the pattern was more typical of normal Winter moisture for the Northwestern US even as drought continued throughout the Southwest.  Moisture instead tended to split fire-hose fashion — with storms either cycling northward into Alaska, the Aleutians, or the Bering Sea, or south over Southern Mexico or Central America, up across the Gulf and on out into a particularly severe storm zone forming in the North Atlantic.
(Over the last 30 days the southwest drought re-emerged as a blocking pattern again began to take hold over Western North America and the Eastern Pacific. Image source: NOAA/CPC.)

This continued loss of moisture for the US Southwest despite a record El Nino is particularly apparentin the Climate Prediction Center’s most recent precipitation anomaly measure for the last 30 days. Here we find that large parts of Central and Southern California have received just 10 to 50 percent of typical rainfall for this period. Coupled with 1-3 C above average temperatures for the month, this loss of rainfall during what would typically be California’s wettest period has come as a disappointment to many who were hoping a strong El Nino would help break the state out of a crippling drought. Now, the window for late Winter and early Spring rains is starting to close even as the blocking pattern appears to be strongly re-established in both the present weather pattern and in the forecast model runs.

But perhaps the biggest surprise coming from this El Nino year was a set of weather events in the North Atlantic that were likely more related to climate change. 
There, severe storms hammered a flood-beleaguered UK as a greatly distorted Jet Stream heaved Equatorial heat and moisture northward — rushing it up over a ridiculously warm and apparently backed-up Gulf Stream before slamming it on into a likely Greenland ice melt-outflow related cool pool. There the heat and moisture collided with cold to produce the epic storms that then vented their fury upon the UK.
(December 29th saw temperatures rise above freezing at the North Pole — the first time temperatures have warmed so much for this high Arctic region so late in the year. Image source: Earth Nullschool.)

During one such event, a daisy chain of heavy-hitting North Atlantic lows hurled high winds, heavy rains and epic surf at the UK even as the meridional flow set up by these powerful beasts shoved above-freezing temperatures all the way to the North Pole during late December. Yet one more unprecedented and unexpected event during a record warm year. One that looks more like a human forced warming which has overcome the traditional influences of El Nino, rather than an El Nino related impact in itself.

As El Nino Fades, Equatorial Heat Tends to Move Pole-ward

Though we may see these two events — the failure of El Nino to provide heavy rains to the US West Coast, and the massive northward pulses of storms, heat and moisture hitting the North Atlantic — as unrelated, the twain patterns appear to be linked to an ongoing polar amplification. Overall, heat within the Arctic has tended to weaken the Northern Hemisphere Jet Stream over these two zones. And even during El Nino, when the Jet would have typically strengthened, we have continued to see high amplitude wave patterns forming over these regions.
But as El Nino weakens and the Equator cools, the Jet Stream would tend to slow even more. Such an atmospheric state would tend to further exaggerate already significant Jet Stream wave patterns — transferring still more low-Latitude heat poleward. In addition, the ocean gyres tend to speed up as El Nino fades or transitions to La Nina. The result is an amplified pulse of warmer waters emerging from southern Latitudes and entering the Arctic.
It’s for these combined reasons — tendency to amplify south to north atmospheric heat transfer into the Arctic post El Nino and tendency to flush warmer waters toward Arctic Ocean zones during the same period that it appears we are entering a high risk time for potential new sea ice melts and possible related Greenland land ice melts during 2016 and 2017.
(Northeastern Pacific Hot Blob remains at high intensity even as its size is predicted to expand through July. Meanwhile, very warm sea surface temperatures are predicted to remain in place off the Eastern Seaboard. The net effect of these two hot blobs may be to shove the Jet Stream far northward over North America during the summer of 2016 — potentially increasing the risk of widespread and potentially record heat and drought. Predicted very warm sea surfaces in the region of the Barents and Greenland seas — in excess of 3 C above average for a large region — is also cause for concern. This is not only due to risk for sea ice loss through this zone, but also due to its potential to set off blocking pattern and heat dome formation over Eastern Europe and Western Russia. Image source:NOAA/CFS.)

In addition, we are at serious risk of seeing the high amplitude blocks and wave patterns re-establish and persist, especially in the zone over Western North America were a related Northeastern Pacific Hot Blob is expected to restrengthen as El Nino fades. In fact, large regions of the US may fall under record to near record heat and drought this summer due to the combined influences of two very warm ocean zones surrounding her shores. Models now indicate a particular late spring drought risk for the Great Lakes region as well as an extended period of far above average temperatures for pretty much all of the Continental US during summer. Meanwhile, predicted above average spring-time precipitation for the Southwest appears less and less likely to emerge.

Finally, extreme above average sea surface temperatures are predicted to intensify over the Barents and Greenland seas through to end of Summer 2016. This is an area to watch. The added ocean heat would tend to pull the Jet Stream northward over Eastern Europe and Western Russia — generating risk of heatwaves and drought for this region even as Central Asia fell under risk of floods. Long range CFS precipitation and temperature model runs for Europe have not yet picked up this risk. However, given the intensity of heat predicted for Barents sea surfaces and the related tendency of warmth over oceans and in the far north to influence the formation of blocking patterns, heat domes, and high amplitude troughs, it’s worth keeping a weather eye on the situation.

El Nino to Weaken and Then Return; or is a Shift to La Nina Now Under Way?

Related to a polar and ocean warming-enhanced tendency to generate high amplitude Jet Stream waves — as well as associated persistent heatwaves, droughts, and floods — is the heat balance of the Equatorial Pacific. Strong El Ninos, or even a tendency to remain in or near an El Nino state, has historically aided in the breaking of new record global high temperatures when linking up to the greenhouse gas warming trend. Meanwhile, the shift toward La Nina has tended to enhance a range of global heating related issues including record rainfall events and large injections of heat toward the poles in the drop off from El Nino to La Nina.
The cause for increased risk of major precipitation events is due to the fact that El Nino is providing a massive moisture bleed into the atmosphere at times of peak intensity. With the current El Nino topping out near record levels and with global temperatures at above 1 C higher than 1880s averages, global atmospheric moisture levels are hitting new record highs at this time. If global temperatures subsequently drop by around 0.1 to 0.2 C during a transition into La Nina (into a range about 0.9 to 0.8 C hotter than 1880s values) then the atmosphere will be unable to keep a larger portion of that extra moisture in suspension and it will fall out as precipitation — primarily wringing out where the major trough zones tend to set up. We should be very clear here in saying that the drought risk related to a global warming intensification of ridge and heat dome formation is not reduced during such instances — just that the risk of extreme precipitation events is enhanced.
(During 2011, as the 2010 El Nino faded into La Nina conditions, a high amplitude wave in the Jet Stream set off record heat, drought and wildfires over Russia even as Pakistan was hit by a month-long deluge that was the worst rainfall event for the region in the last 1,000 years. La Nina’s tendency to wring excess water out of the atmosphere can enhance the risk for such events to occur in a warming climate state. Image source: NASA.)

As for risks to sea ice, we’ve provided some of the explanation above. However, it’s also worth noting that the mobility of heat poleward tends to be enhanced during the periods when El Nino drops off toward La Nina. During these times, Equatorial heat tends to propagate in wave fashion toward the Poles — especially toward the Northern Hemisphere Pole which has already lost its strong Jet Stream protection warding away warm air invasions.
These two factors are major issues when considering whether La Nina or an ENSO Nuetral state will appear post El Nino during 2016. But there is a third — rate of global temperature rise. Though the primary driver of global warming is a massive human fossil fuel emission, the response of the world ocean system can significantly wag the rate of atmospheric temperature increases on a decadal time scale. If the ocean tendency is for La Nina, this would tend to somewhat suppress the overall decadal rate of temperature increase — and we saw this during the 2000s. But if the ocean tendency is to produce El Ninos (in a switch to a positive Pacific Decadal Oscillation, as appears to be happening now), then the overall pace of global atmospheric temperature increase would tend to be enhanced.
( IRI/CPC consensus model runs show a drop off to a weak La Nina by late in the year. However, CFS model runs [image below] have shown a tendency to predict a resurgence of El Nino conditions by Fall. Image source: NOAA/CPC.)

To this point we find that the official model forecast consensus published by NOAA (IRI/CPC figure above) shows a transition to ENSO neutral states by May, June, and July which then proceeds on to a very weak La Nina by Fall. In such a drop off, we would likely still see record global high temperatures during the period of 2016 (in the range of 1.03 to 1.15 C above 1880s values).
However, the late 2016 and 2017 tendency for temperatures to recede from new record highs would be somewhat enhanced (likely dropping below the 1 C above 1880s mark in 2017 or 2018 before again making a challenge to the 2015-2016 record with the potential formation of a new El Nino in the 3-5 year time-frame of 2019 through 2021). It’s worth noting that this scenario shows an increased risk of a stronger warm air pulse heading toward the Northern Polar zone together with added fuel for extreme precipitation events as global temperatures would tend to drop off more swiftly from late 2015 and early 2016 peaks.
(CFSv2 model run — shows El Nino continuing on through the end of 2016. Over recent months, the CFSv2 series has shown a high accuracy. However, NOAA’s current forecast preference is for the IRI model set predictions [previous image above]. Image source: NOAA/CPC.)

In contrast, the CFSv2 model forecast from NOAA (above image) shows El Nino only weakening through to July and then re-strengthening in the October-November time-frame. This CFS model scenario would result in higher atmospheric temperatures in 2016 — practically guaranteeing a lock on an unprecedented three back-to-back-to-back record warm years for 2014, 2015, and 2016. But such a scenario — implying that the Pacific Ocean had entered a new period of El Nino tendency — would also tend to keep atmospheric temperatures nearer to the newly established record highs.
Under the CFSv2 scenario, we may expect annual average global temperatures to rise as high as 1.08 to 1.2 C above 1880s values during 2016 (a very extreme departure and one uncomfortably close to the 1.5 C warming mark). These extreme values would, perhaps, recede to around between 0.9 and 1.1 C during 2017 so long as the second El Nino pulse did not remain in place for too long. However, if the bounce back toward El Nino conditions was strong enough in late 2016, there would be an outside chance that the globe may experience not 3, but an absolutely obnoxious 4 back-to-back record warm years.
(During 2015 global annual temperature rocketed to above 1 C hotter than 1880s values. There’s at least an even chance that 2016 will be hotter still. Considering the considerable heating tendency imposed by a fossil fuel-forced warming of the world, how much worse can it get during the 21st Century’s second decade? Image source: NASA GISS.)

Meanwhile, the warm air pulse heading toward the poles may be somewhat muted under this scenario. A statement that should be qualified by the fact that we’ve already seen a substantial amount of El Nino heat heading poleward during the present event. In addition, potentially heavy rainfall events may not receive the added oomph of a decent global temperature drop to wring out more moisture. A statement that requires the further qualification that overall atmospheric moisture loading is enhanced by rising global temperatures — so comparatively less heavy rainfall is a relative term here.
A transition to ENSO-neutral is likely during late Northern Hemisphere spring or early summer 2016, with a possible transition to La Nina conditions by fall.”
However, it’s worth re-iterating that the CFSv2 model forecasts have been quite accurate in predicting the path of the current record El Nino to date.
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Hat tip to Caroline

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