By Rasmus Benestad & Raymond Pierrehumbert
This is the first part of a planned mini-series of 3 posts on tropical climate, circulation, and oceanic response in conjunction with a global warming. Climate change related to a global warming is more than just temperature and precipitation -massive atmospheric circulations change too, and these changes can have consequences.
The name ‘El Niño’ originally was given to a change in the coastal current (usually flowing from south to north) near the Peruvian coast during anos de abundancia. 
Paita sailors who used to sail north-south direction along the coast called the counter-current ‘El Niño’, after the Child Jesus because it had a tendency to appear soon after Christmas (the reason for this seasonality is not yet fully understood, and some of the strongest events peaked earlier in the year). The counter-current (reversal to north-to-south flow) usually would appear in concert with rains in otherwise dry regions. El Niño is also associated with warm surface water in the eastern tropical Pacific. 
However, the name ‘El Niño’, which originally has its origin from changes in the ocean, is linked to changes in the atmospheric circulation. The understanding of the atmospheric circulation changes, later to be discovered to be connected to the appearance of El Niño events, was originally motivated by reasons other than ocean currents. Sir Gilbert Walker (photo to the left) was motivated by the question why the South Asian Monsoon sometimes failed from one year to another. There was a catastrophic drought and a subsequent famine in 1877, and the question was why do such events take place? Walker discovered that the sea level pressure fluctuations over the Indian Ocean and tropical Pacific tend to vary with opposite phase. He named this dipole of opposite variations ‘the Southern Oscillation’. The atmospheric circulation associated with this dipole pattern is known as the Walker circulation. The Walker circulation refers to the mean (steady) ciculation where air over the warm pool in the western part of the tropical Pacific rises, being fed by the easterly surface trade winds across the Pacific, and subsidence over eastern Pacific. The Southern Oscillation refers to the inter-annual variations in this circulation.
Figure showing the Walker circulation (left) and the Hadley circulation (right).
It is important not to confuse the Walker Circulation with the Hadley Circulation (also known as the ‘Hadley Cell’), which also involves deep convection in the tropics. Whereas the Walker Circulation (or ‘Walker Cell’) refers to an air flow parallel with the equator – all in the tropics – the Hadley Cell involves air rising in the tropics (follows the solar equator and gives rise to the Inter-Tropical Convergence Zone (ITCZ) which then flows polewards before sinking in the subtropics. The Walker Circulation involves an east-west asymmetry, whereas the Hadley Cell in principle does not.
It was not until 1969 that Jacob Bjerknes (photo to the left) proposed that there was a physical connection between the oceanographic and atmospheric variations on the year-to-year (inter-annual) time scales, and now the oceanic and atmospheric aspects are combined in the term ‘El Niño Southern Oscillation’ (ENSO) that encompasses both the ocean and the atmosphere (also see the IRI link, the Australian Bureau of Meteorology, National Geographic, and a discussion on Wikipedia). El Niño events tend to recur every 3-8 years. The last El Niño as of today was in 1997-98, and was the strongest or second strongest (after 1982-83, depending on what you look at) event observed in modern times. The Bureau of Meteorology (BOM) in Australia provides an Internet page on ENSO with a nice ENSO wrap-up for up-dated information. Another resource for keeping up-to-date with ENSO is the TAO-array. The seasonal migration of both the ITCZ and the South Pacific Convergence Zone (SPCZ) are affected by the presence of El Niño.
What is La Niña?
La Niña usually refers to the opposite state to an El Niño: low sea surface temperatures in the eastern part of the eastern tropical Pacific. Intense trade winds and strong uppwelling along a region near the equator, known as the cold tongue and caused by Ekman pumping, bringing up cold and nutrient water from the deep sea. Note that Ekman pumping does not penetrate deep into the oceanic interior, but since the trades advect the surface waters westward, the upper layer of warm sea water is deeper in the west than in the east. Underneath this layer lies cold ocean water, and the Ekman pumping reaches sufficients depths in the east to bring some of this up to the surface.
As an aside, it’s amusing to note that in some early papers, the opposite of El Niño was described as the ‘anti-El Niño’ but given the religious connotations described above, this usage did not get a lot of support…
Why does ENSO arise?
The background conditions are essential for the existence of ENSO. The prevailing surface winds over the tropical Pacific blow from east-to-west (easterlies), and tend drive a surface current, pushing (advecting) the warm surface water westward. The winds are known as ‘trade winds’ and have played an important role in the world history in terms of the ship routes for sailing vessels. The western tropical Pacific is known as the ‘warm pool’ with the highest sea surface temperature (SST) in the world (on average). The trade winds ‘pile’ up water masses in the west, resulting in a slightly higher sea level in the west. The higher sea level near the western ocean boundary creates a west-east pressure difference in the ocean, that results in the equatorial undercurrent flowing from west to east below the surface.
Mechanisms:
The usual pattern is that an El Niño event gradually builds up between June and December, peaking around Christmas time (the reason for this seasonality is not yet fully understood). There are several explanations as to why there are fluctuations in the Walker circulations/trade winds, and the ocean currents. The different explanations do not exclude the others, and it is possible that more than one of these may take place. Here is a simple desciption of the main mechanisms:
–Delayed Oscillator: Adjustments in the oceans as a whole, such as response to changes in the surface wind stress, happens through wave propagation. Ocean serves as the memory whereby slow oceanic Rossby waves and Kelvin waves propagate through the basin and affect the depth of the oceanic surface layer of warm water. Through waves, wind forcing in one location may have an effect on a remote location. Kelvin waves can travel eastward along the equator and poleward on the coasts along the eastern boundary of the ocean basins, but not in the ocean interior. Rossby waves, however, can travel westward and away from the equator and the coasts. Model results have shown that these waves can account for some of the irregular appearances of ENSO.
–Coupled Mode: The notion involves unstable air-sea coupling, where a local region deep convection enhances the trades to the east and reduces the surface winds to the west. These local wind alterations produce changes in the Ekman pumping.
–Noise forced and volcanoes: The idea is that weather events kick off El Niños due to unstable conditions. The system is ‘charged up’, and El Niño events are then initiated by a ‘kick’ (eg the Madden-Julian Oscillation or westerly wind bursts).
What are the effects of ENSO?
Although ENSO represent ‘modest’ shifts in the weather patterns world-wide, El Niño events may, according to the ‘Report to the Nation’, have dire social and economical consequences. Increased rain in the Andean states due to El Niño can have devastating effects, as can the increase in corresponding increase in drought in Indoneasia – and this can lead to big increases in forest fires and air pollution. There is also a clear statistical relationship between El Niños and hurricanes/typhoons. Whereas the number of hurricanes in the Atlantic basin tend to drop (~50%) during El Niño years, the number of typhoons tend to increase in the Pacific. Drought in the American West is associated with La Niña conditions. The effects can also be felt as far away as Antarctica where a ‘dipole’ of sea level pressure between the Bellinghausen and Weddell Seas is highly correlated to the ENSO phase and can have important effects on sea ice and Antarctic ecology.
When will next El Niño take place?
This is always the exciting question around June. After the notorious spring barrier the predictive skill of ENSO models increase, and our confidence in the ENSO prognoses are higher. The BOM provides a nice summary of a number of different model prognoses. At the moment, there are no indication of an El Nino for the end of this year.
How will the El Niño phenomenon be affected by a global warming?
This is what the Dutch Meteorological Institute (KNMI) and the Max-Planck Institute (Germany), Matt Collins of Univ. Reading (U.K.) think. There is even a short entry about global warming and ENSO in Wikipedia. The brevity of this entry may reflect the fact that the question about how ENSO will respond to a global warming is still not settled. However, it seems that one common trait among some climate models is the indication that a global warming may result in a more a general El Niño-type average state (eg. Collins et al. 2005, Climate Dynamics, 24, 89-104. 19 and here).
How could a change in the background state affect ENSO? There are a number of different theories which go in different directions, but part of the difficulty is that we cannot put our finger on one mechanism and say ‘this is the one!’. If we look into the ocean, then changes in the vertical temperature profiles may plausibly affect oceanic wave propagation, thus perturbing the conditions to which the delayed mechanism is sensitive. Furthermore, a deeper upper layer of warm surface water may weaken the cold tongue if the Ekman pumping doesn’t reach down below the thermocline to bring up colder water, and weakened trade winds would have a similar effect through reduced Ekman pumping near the equator. Some ENSO experts, such as Mark Cane, points out that the upwelling impact on SST in the East implies that in the transient warming, the warming might happen faster in the West than the East thus strengthening the Walker circulation – some of the supposed volcanic-El Niño connections would support that. Changes in convection and cloud formations through altered air moisture (CAPE) could have implications for the coupled mode mechanisms, as would a deeper thermocline (usually situated near the bottom of the warm surface layer). Another question is for the noise-forcing mechanism, whether the high frequency forcing will change or whether the state at which the ocean becomes sensitive to such forcing will be altered (eg ‘charging’ at a different rate or reaching ‘charged level’ at a different threshold). The picture may be even more complex, and a response in ENSO may even involve perturbations of the carbon cycle. The models are all over the place, and most climate models yield patterns with some bias either in geographical character, amplitude or time scales. Analysis by Collins of climate model simulations indicated that increased CO2 may result in ENSO events becoming larger in amplitude and more frequent than under present conditions. This conclusion was based on version 2 of the Hadley Centre Coupled Model (HadCM2). However, in a subsequent analysis based on version 3 of the Hadley Model (HadCM3), Collins found that he could not detect a change in magnitude or frequency of ENSO as greenhouse gases increased, thus contradicting the results of his earlier study. These differences highlight the level of uncertainty associated with ENSO and global warming.
A Science paper by Huber and Caballero (2003) on Eocene El Nino reports on similar ENSO in the Eocene warm climate as the model has in the modern climate. They suggested that the transient changes in El Nino (before the deeper water tapped by upwelling has warmed) may be different from the state of El Nino after the ocean has come into equilibrium.
Rasmus, Ray- Here is some additional information on societal impacts:
1. The ENSO shifts the burden of disasters around, but does not appear to be more or less extreme globally under any phase of the cycle. See:
L. Goddard and M. Dilley, 2005. El Nino: Catastrophe or Opportunity, Journal of Climate, 18:651-665.
An excerpt from the abstract:
“Global economic losses of tens of billions of dollars are attributed to extremes of ENSO (i.e., El Nino and La Nina), suggesting that these events disproportionately trigger socioeconomic disasters on the global scale. Since global El Nino/La Nina – associated climate impacts were first documented in the 1980s, the prevailing assumption has been that more severe and widespread climate anomalies, and, therefore, greater climate-related socioeconomic losses, should be expected during ENSO extremes. Contrary to expectations, climate anomalies associated with such losses are not greater overall during ENSO extremes than during neutral periods”
2. There is a clear shift in the economic impacts of tropical cyclones in the Atlantic:
Pielke, Jr., R.A., and C.W. Landsea, 1999: La Nina, El Nino, and Atlantic Hurricane Damages in the United States. Bulletin of the American Meteorological Society, 80, 10, 2027-2033.
http://sciencepolicy.colorado.edu/admin/publication_files/resource-85-1999.14.pdf
Thanks!
A nice coincidence as I was just looking El Nino from a different POV. before I get to my question, I’ll go through the steps I reached to get there so that if there’s a flaw along the way it’ll be obvious.
I assume that the years for El Nino are given as (say) 1997-1998 because the effect peaks just after Christmas so affects the following year?
Now I’ve seen mentions that (strong) El Nino years will make the global annual average higher – e.g. 1998 was so warm partly because of El Nino, and that this is due to the fact that sub-surface warmer water is brought up and allowed to affect the air temperature.
I used a listing of El Nino years from here:
http://www.upei.ca/~physics/p261/projects/elnino1/elnino1.htm
and plotted the global temp anomalies from the CRU dataset against them. One of the plots was earlier year as one line and latter year as another. Before he 1950′ the earlier year was consistently warmer than the latter year and after a period of roughly equal temperatures to about 1970, the latter years have (so far) been consistently warmer then the earlier years.
Is this a known effect and is there a reason for it? Or is the whole line of reasoning too simplistic or flawed?
Rereading my comment (2) I think it may be a little unclear what I mean. Basically taking three El Nino years and how each year pair compared we get
1902-1903: 1902 was warmer than 1903
1951-1952: both years were about the same
1997-1998: 1998 was warmer than 1902
Re 3: Erm, 1998 was warmer than 1997 not 1902 (it was warmer than 1902, but that wasn’t what I meant).
I’ve not put this across very well have I?
Thanks for the lesson. I remember the 90s El Nino. I read about it being very strong, and nearly continuous (or frequent) over several years. And I thought, ah-ha, more evidence of GW.
I also heard weathermen attribute certain harmful weather events to El Nino (end of discussion). It seemed they (esp the one connected to oil) were almost gleefully pinning it on EN, so that people wouldn’t look further into the possibility that GW might be increasing the ENs, and implying we could all sleep easy blaming it on “we can’t do anything about it” Mother Nature. Off the personal responsibility hook to reduce our GHGs, and good news for the oil companies to keep promoting Hummers.
So it seems the actual situation may be between my vivid imagination (stronger EN as more evidence of GW) & those placid weathermen (it’s only nature in the form of EN, nothing at all to do with AGW). And of course, this in no way detracts from the more well accepted harms from GW, or the increase in hurricane intensity.
I also remember reading around that time about a big decline in fish due to a huge plankton kill-off due to high SST and/or lack of upchurning nutrients (I think somewhere in the eastern Pacific).
So when in doubt leave it out (emitting GHGs).
You really are terribly good at making science accessible to the non-professional and I found the posting very informative – thanks.
Not off topic because nothing ever is and I didnt know where to post this question, but following a question from me several months ago on the distribution of “carbon” gases in the atmosphere to which the response, I believe from William, was that gases are pretty well mixed and evenly distributed apart from ozone, I read this in The Guardian today:
“As if to underline the environmental message of David Cameron’s visit to an Arctic glacier last month, Norwegian scientists have now discovered European smog just a few miles from where he staged his photo op, writes David Fickling.
Scientists at the Alfred Wegener Institute on Svalbard recorded pollution levels that would put a British city centre to shame at their weather centre in the tiny settlement of Ny Alesund.
Their instruments recorded levels of PM10 aerosols that are equivalent to standing next to the Marylebone Road in central London, and high enough to exceed European Union air quality directives.
Ozone was recorded at 160 micrograms per cubic metre, a level considered moderate-to-high on the UK’s pollution index and almost twice as high as Aberdeen, which had the UK’s worst ozone levels at time of writing.
A vivid illustration of the pollution is provided by two photographs published by the Institute, showing Svalbard before and after the smog blew in.
Svalbard is traditionally seen as one of the most unpolluted inhabited places on earth. The nearest major city, St Petersburg, is more than 1,300 miles to the south across the Arctic Ocean.
The Institute’s Dr Andreas Herber blamed an unusual weather system dumping eastern European pollution within the Arctic circle, and said that a temperature spike could be expected.
“The present air pollution is more than 2.5-fold higher than values measured in spring 2000. As a result, we expect significantly increased warming,” he said.
But he added that further data would be needed to know whether this incident is an anomaly or the start of a long-term trend.”
So the question is this, with me bashing on on the same issue : are there hotspots and if there are how do they arise and to what extent are they important in affecting the overall climate? I take the point about aerosols being cold spots too but lets not get too picky about the question I pose.
If anyone has any relevant reading lists then I am confident enough to have a go at understanding the issues myself thanks. I take a very simple and basic view on scientific issues, such as : if there are “local” phenomena in the ocean such as El Nino why shouldnt there be local phenomena in the atmosphere even though the time scale for “locality” may be at a different pace. After all they are “fluids/gases” albeit at different densities. Is it significant? I would guess (not a scientific word I know…. but) that it is.
Why are El Nino years warmer than other years? Because sub-surface water is warmer than surface water?
Also re comment 1, thanks, I didn’t realize that El Nino’s and La Nina’s aren’t worse overall than other years — however, some of us live where extremes are usually worse. It is true that less of the carbon we emit is absorbed during El Nino years, because of extra forest fires presumably. When calculating reductions in carbon emissions needed by 2050, is the shift to more El Nino like conditions included, or is not likely to be important by 2050?
[Response: Regarding the effect of El Nino on air temperature, llewelly put it well in Comment #6 to the post “Gray and Muddy thinking…” In a La Nina year, the warm water is all bunched up in a thick layer with little area exposed to the atmosphere. In an El Nino, it spreads out, allowing more area to be exposed to the atmosphere. There are other things going on, but this simple effect probably is a good part of the explanation. This argument shows that a permanent El Nino would have a different effect on global temperature than a transient one, since all that newly exposed warm water would eventually cool off. The subsurface water, by the way, is colder than the surface water.
On the subject of damages caused by El Nino years, please keep in mind that the references Roger Jr. provided in #1 are only for a limited set of economic damage metrics. It’s useful information, but one shouldn’t oversimplify the conclusion to the statement that El Nino years are no better or worse overall. Among other things, there are distributional issues and natural ecosystem impacts to consider, For that matter, as Amartya Sen points out, simple aggregation of costs and benefits does not always make sense as a measure of social welfare. Social impact of climate change is a really tough problem, and I’m glad I’m not in charge of sorting that out. –raypierre]
Remember, pollution they’re talking about in the Arctic is fallout of dust and particulates carried by there by weather systems/winds and dropped there.
For gases, the mixing takes time (in fact takes a couple of years across the equator, if I recall correctly). So you can have a plume of dust, diesel exhaust and carbon monoxide and ozone carried by a weather system. The gases eventually mix with the rest of the atmosphere. The particles fall out, wherever the air drops them.
I guess we don’t talk about the ocean having “weather” but that may be what you’re thinking of — patterns that last relatively briefly: http://www.nasa.gov/worldbook/weather_worldbook.html
Pinatubo ash:
http://www.geo.arizona.edu/palynology/geos462/dustvolc.gif
Thanks Hank Roberts, but the issue is the extent and rapidity with which the pollution (if it is pollution) is dispersed or “averaged”. I understand the difference between weather and climate. Back to my hot spots again. I just find that the measurements from Svalbard are strange.
Raypierre, Dont you believe it! in your response to the impacts of global warming. We are all in charge.
Good luck.
6, 8, 9 — Eachran, I think you understood this but I’m not sure: “PM10 aerosols” are suspended for a while (“haze” described at Svalbard). They won’t become evenly mixed in the atmosphere. Low level sources get carried with local air systems; once packets of air get to the Arctic, I think there’s less chance they’ll move out again before dropping what they carry. Expertise welcome, anyone?
“PM10: Aerosols with a diameter smaller than 10 micrometers, on which the EPA has based current NAAQS. Larger aerosols in this size range (larger than 2.5 micrometers) are less effective in creating haze than the smaller ones….”
Except for the early 1930s, the periods with strong El Nino were warmest of record at climate stations in the Midwest (periods based on 5 year annual moving averages).
http://www.mnforsustain.org/mn_dewpoints_neuman_p_special_report.htm
http://www.pmel.noaa.gov/~kessler/ENSO/soi-shade-ncep-b.gif
The 1930-1934 exception (above) leads me to conclude that the intensity of solar radiation at the surface created the intense heat of the dust bowl years, and that the 1939-1942 strong EL Nino period was triggered by the intense solar radiation from 1930-1934 penetrating a large surface area in the Pacific.
To what extent does the rain shadow of the tropical Andes effect the Walker circulation?
Thanks Hank Roberts. I think I shall explore further but it still begs the question of what is “local” and how these local features interact.
For example it as an extraordinary (perhaps you will tell me it is not?) result that someone elses “weather” (in its totality I assume) gets dumped on another part of the globe 2000 km away and stays there at a slow rate of mixing (?) for how long I wonder? The report doesnt say what else the station measured but Dr Herber seems to think that temperatures will go up and if they do then it is not a specially good place to have increased temperatures I would have thought.
I shall do some research but if anyone can help me shorten the research time please let me know.
Don’t forget (I’m sure you won’t):
Vecchi, G.A. et al. (2006) Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing. Nature, 441, 73-76.
[Response: That’s what’s coming in Part II. –raypierre]
Hank Roberts and others. I have done the usual suspects like wiki and links but my problem is duration of mixing (particularly at the poles) and the idea that “pollution” of the sort briefly described increases temperature.
The link is:
http://www.awi-bremerhaven.de/AWI/Presse/PM/pm06-1.hj/060511ArcticSmog-e.html
Eachran, why don’t you contact the webmaster at that link (bottom of the page you link to above) and ask them to help you understand it?
They explain that “Aerosols from eastern Europe have been transported into the Arctic atmosphere due to a particular large-scale weather situation.”
You could ask them to help you find a satellite picture from the beginning of May that shows how this happened. Maybe they could add it to their web page, so you could see the weather system they describe.
You could ask them to explain how, when a mass of warm and dirty air from Eastern Europe moves north into their location, they get warmer temperatures.
They’re trying to explain this, but obviously they haven’t yet — your questions might help them improve their explanation.
Re 15 & 16 you might find this site of use when looking at recent weather systems over Europe: Wetterzentrale
“Ocean serves as the memory whereby slow oceanic…”
The scientists are looking for an energy storage system that has a time constant related to the frequency of oscillation. The first place I would look would be energy storage in the deeper ocean basins. Has there been any studies of temperature changes in deeper waters in phase with El Nino?
Thanks all.
Minor La Nina note. I had made some commentary on earlier threads regarding the cold and wet winter (especially late winter) and early spring we had on the W. Coast. So, here it is, one week before Memorial Day Weekend, and the current NWS forecast is calling for what is said, at this point, to be a “brief” return to the Siberia Express we had going until about 4 weeks ago (seemingly ad infinitum). Brief – assuming of course no new block sets in, etc. Forecast calls for the snow level to be down at Lake Tahoe level by early next week. I’ll be all over it, getting in some more skiing prior to the Memorial Day weekend crowds. Sodden thought: What would a year without a summer ala 1815 do to us in this day and age? I reckon it would be a real disaster.
The post states: “El Nino events tend to recur every 3-8 years. The last El Nino as of today was in 1997-98”
I imagine this is just a matter of how you define events that are near the threshold… but a look at the NOAA website http://www.ncdc.noaa.gov/img/climate/research/teleconnect/eln-5-pg.gif suggests that 2002-3, and possibly also 2004-5, were also El-Nino events.
RE: #21. Tha actual ENSO page would say you are wrong.
re 1, response to 7:
when judging the impact of extreme climactic changes, instead of just measuring damage in dollars (as link in 1 does), it would make sense to perform the same calculation using fatalities. This would answer the question of whether or not the damages are similar, or are simply being shifted to less economically developed areas. If El Nino effects the hurricane season by simply sparing Florida and nailing Japan, then el and la years should be equitable on both dollar and fatality scales. If El Nino spares Florida and causes a similar dollar amount of damage in poor countries, then the damage measured in fatalities will probably be much higher, since poor countries generally suffer more fatalities per dollar of weather damage incurred. And dollars and fatalities are both relatively easy to measure.
At some point, we may have to conclude that the North Atlantic Oscillation has and is being affected as well. That the Walker circulation is changing because of warming will not be the end of the story.
When that lynchpin is conclusively removed, the contrarians may find themselves dangling from the yard arm.
>21, 22
Steve Sadow, you refer to “the actual El Nino page” — link please?
This is the closest I found, last updated in 2004:
http://www.cdc.noaa.gov/people/klaus.wolter/MEI/#ElNino
“… How does the 2002-04 El Nino event compare against the seven previous biggest El Nino events since 1949? …”
Correcting my typo, Steve, you refer above to the “Actual ENSO” page — what page?
Siberia Express in place, major cold front came through yesterday, from the Mexican border well up into the Pac NW. Snow level down below 8 thou, yesterday had a wintry mix much lower. What is the latest date that a Bering Sea storm encroached upon latitudes lower then 37N, along the W. Coast? Will we break that record this year?
global warming is resolved. you still search for the donkey?
Amazing …. on the cusp of June …
“A CHANGE WILL TAKE PLACE ON FRIDAY AS THE UPPER LOW DIGS INTO THE NORTHERN SIERRA. AS THE COLD POOL ALOFT MOVES OVER THE REGION HEIGHTS AND ASSOCIATED THICKNESS VALUES WILL DROP. THE GFS PROGS 700 MB TEMPERATURES OF -9C OVER THE NRN SIERRA BY 06Z SATURDAY. WITH THE INCREASED LAPSE RATES -SHRA CAN BE EXPECTED OVER THE NRN ZONES FROM TAHOE TO LOVELOCK NORTH. SNOW LEVELS WILL DROP TO 7000 FT OR LOWER OVER THIS REGION SO LIGHT SNOW IS POSSIBLE IN THE HIGHER ELEVATIONS OF THE NRN SIERRA FRIDAY NIGHT. O`HARA”
Someone had asked me to post “the” ENSO web site. In my book, that would be:
http://www.elnino.noaa.gov/
I have a confession to make … I am ….. an ENSO geek.
Do I understand this right? Paleo-proxy reconstructions indicate that a lower insolation due to volcanic aerosols (or else) leads to a lower east-west SST gradient/a higher eastern equatorial SST/ a higher El Nino variability? A higher insulation would lead to a more La Nina-like state with less ENSO variability? If this is true, should’nt global warming lead to a more La Nina like state?
[Response: Ah yes, you’ve hit on the fundamental question. This was indeed the argument originally put forward by Cane et al in their ’97 Science article ‘Twentieth-Century Sea Surface Temperature Trends’. However, one potential criticism is that the model (the Cane-Zebiak model) does not allow for important extratropical feedbacks such as subducting water masses from the extratropics that eventually can influence the equatorial Pacific thermocline. These feedbacks act on multidecadal timescales, and could potentially dampen the reponse to any long timescale changes in forcing. This would likely apply to long-term solar forcing as well. On the other hand, these feedbacks are too slow to have any significant impact on the the impulsive response to high-frequency (e.g. volcanic) radiative forcing, and indeed the evidence seems to be most compelling that this is where the predicted signal is clearly detectable. As regards the true nature of the response of the tropical Pacific ocean-atmosphere system to anthropogenic forcing, the real-world evidence seems to be a bit mixed. There have certainly been some large El Nino events over the past couple decades, and this leverages any linear trend estimates of the long-term behavior (such as those shown in the recent Vecchi et al paper, which we’ll be talking about more in a follow-up post to this). However, if one downweights these two events (either by eliminating or, as in Cane et al ’97, using a ‘robust’ trend), then an argument can be made for a long-term pattern which is in some respects more ‘La Nina’-like, i.e. little warming in the eastern and central equatorial Pacific, and far more warming in the western equatorial Pacific and Indian oceans, associated with a strengthening, not weakening, of the negative equatorial Pacific zonal SST gradient. Indeed, Hoerling and Kumar, in their ’03 Science article ‘The Perfect Ocean for Drought’, make a fairly compelling argument that this trend is associated with a trend towards enhanced drought in regions such as the desert southwest of the U.S., a pattern we typically associate with ‘La Nina’. Needless to say, this is not, by any stretch, a settled matter, and it remains one of the key mysteries in our understanding of anthropogenic climate change, with potentially huge societal ramifications that lie in the balance. –mike]
No matter what one might conclude about causitive factors, on the West Coast, where I have spent my 40 plus year life, indeed, La Nina like conditions have seemed to increase. Winters have brought increases in low elevation snow events and increases in the wide and wild swings typical of “less droughty” La Ninas (well, less droughty for the coast north of about San Luis Obispo, that is …). Springs have become limited to non existent. Summers tend to have more onshore push, deeper marine layers and more “unseasonal” cold fronts and insider slider lows. We see more cold core systems earlier in fall. So, depending on which GCM you subscribe to, and depending on the extent to which you elevate arthropogenic modification of the atmospheric gas mixture as a causitive factor of specific long term dynamics, yes, La Nina may be a proxy, of sorts, of the AGW theory assuming it is true. Frightening thought – if and only if the AGW centric prediction of future climate is either not completely correct, or out right wrong, consider extreme scenarios which would result in a drastically (and painfully) different outcome than the prophecied sea level rise / climatic tropical expansion / northerly movement of species model. Flip that on its head and imagine it. Pretty rough stuff, and in my mind, even worse than the outcome of the global warming tipping point scenario. Colder may not be better!
Steve, I’m in Berkeley, I watch the weather here too. Remember we had a ‘rare’ cold storm last year, the last week of June. If you know a forum for our local weather, please point me there for appropriate followups.
RE 29 (Sadlov):
Steve, in the early ’90s almost the same pattern occurred over Mem day weekend (forgotten what year), but our long weekend in Calaveras County was spent in the rain. Look at your climo.
West coasts of continents are occasionally subject to persistent longwave troughing in spring.
Best,
D
>31 Thanks to Heiko for asking and Mike for answering in depth. Looking forward to your further postings about this.
[Response: Thanks Hank, the next installment of the series should be up sometime soon. –mike]
RE: #34. What I’ve described covers all the seasons, not only Spring.
And the meta message – are you sure, are you really, really sure? We are winding up the masses to believe that the future will be quite a warm one. Maybe even Jurassic warm. Warm I can deal with. However, if either A we are simply dead wrong about the impact of GHGs and / or B we are missing the forest (solar/astronimical and tectonic things) for the trees (gas mixture things) and the actual future, among the several possible futures, turns out to be one of cooling – possibly the outright end of the current interglacial, then all those people wound up to believe in a warm future are going to be cold, hungry and out for blood. I, for one, would not want to be anywhere near that little shindig. I see Skilling and Lay are in the news today ….
25, 30
Here is another gov’t ENSO page. More charts and a nice list of references.
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml
Changes in the state of the Pacific Decadal Oscillation need to be better understood as well.
I took a look at the wikipedia entry under Atlantic Multidecadal Oscillation, which seems to try and draw strong correlations between phases of the AMO and hurricane intensity. In contrast, the above article points out that “…Whereas the number of hurricanes in the Atlantic basin tend to drop (~50%) during El Nino years, the number of typhoons tend to increase in the Pacific.”
I’m not quite sure who wrote that wikipedia article, but if you follow the links and references you might get a hint or two. Ignoring the El Nino effect doesn’t seem to be valid if one is looking at hurricane frequency and intensity.
Thanks for the very clear article, too.
1998 absolutely was the warmest year! In my country, the impact not only in environment that cause forest fires and drought but also in social situation, many demonstration and riots happened. That’s horrible!
Just another La Nina field observation here. ~ 40 N, near the W. Coast of the USA. With the exception of a couple of almost-warm days in April, wedged in between the endless early spring train of Siberia Express cold fronts we had, and two notably (and seasonably) hot days in May (along the lines of what we’d normally expect in late April), this Spring has been a real dud. Not to complain too much about it, had wonderful late skiing in the Sierra both the weekend before Memorial Day weekend and even better during Memorial Day weekend (freshie). Yesterday, we had a cold front through here that was more along the lines of what you’d expect in late March or early April. Finally, at long last, the forecast is calling for a halfway decent ridge and some warmth, late this week. Normally, due to sea breezes and persistent coastal anticyclonal advection induced stratus during Summer, Fall is our warmest season and Spring the second warmest. But this Spring, nada in terms of normal warmth. We’ll see how Summer turns out. And how long the climatic Summer actually lasts (and how early climatic Fall conditions set in).
Dang … even the Weather Service got faced by this one … the front was through last night and we were supposed have good ridging by now. Alas, an inside slider came in a couple hours ago, more rain … La Nina served a funky pitch…