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El Niño and NW Europe

EL NIÑO: Summary of some published articles relating to links between variability of the ENSO patterns, and weather in NW Europe.

0. Outline:

During the early part of 1998, when the warm-phase ENSO event, commonly referred to as 'El Niño' was at its peak, there was much discussion in the uk.sci.weather newsgroup as to just what, if any, the effect on the weather would be/was over the UK and elsewhere in Europe. This note does not break new ground, but attempts to pull together references to articles, publications etc., that deal with the subject, so that readers can form their own opinions. However, at some points in this note, I have interjected my own (strictly personal) comments, and to clarify these passages, I have italicised them in square brackets so they are clearly not part of the original referenced work. Bear in mind that I do not claim to be a specialist in this subject, or indeed in climatology as such, but like others, I have a keen interest in the wider workings of the atmosphere. This note was updated in autumn 2006.

The note concentrates upon effects found (or suspected) during the European winter (December - February, or January - March in some studies). This is not to say that there are no effects at other times: indeed some researchers have found tentative signals during the summer, but the over-whelming body of evidence at present (2006) is that where there is a detectable effect, it is most pronounced in our winter. Closer to the seat of the 'disturbance' of course, then the effects will be more pronounced throughout a greater part of the year.

1. What is 'El Niño'?

El Niño is the name now usually given to the phenomena of large-scale and long-lasting weakening of the northeast / southeast convergent trade winds and consequent warming of the surface layers (due to reduced evaporative cooling/reduced cold-water upwelling) in the eastern and central equatorial Pacific Ocean. These phenomena occur rather irregularly: roughly every 2 to 7 years. The event typically lasts 12-18 months, and can be detected by 'swings' in the Southern Oscillation, an inter-annual see-saw in tropical sea level pressure between the eastern and western hemispheres, thought to be produced in large part by the aforesaid changes in focus of warm / cold anomalous water. During a marked (warm-phase ENSO) El Niño, notably higher than average mean sea level pressures occur in the western tropical Pacific and Indian Ocean regions, and significantly lower mean sea level pressures develop in the south-eastern tropical Pacific. The event is credited to the year of first observance of the warm (or cold) water anomaly: thus the 1997/98 event is usually noted as the El Niño of 1997. [ based on the notes obtained from the U.S. Dept of Commerce, NOAA, 'TOGA-TAO' web site.]

For more on definitions, terminology..... http://www.pmel.noaa.gov/toga-tao/ensodefs.html
and for some notes on the Southern Oscillation etc...... http://www.cru.uea.ac.uk/tiempo/floor2/data/soi.htm

[ The name El Niño is long established in the local dialect of the peoples of the west coast of Peru and Ecuador, and simply refers to the annually occurring warm/southward flowing current of water that (a) led to a much reduced fish catch in the region, sometimes with no fish catch at all, and (b) seemed to coincide with the Christmas-tide period, hence THE Child/El Niño. Only within the latter part of the 20th century has the name been attached to the unusually strong warmings and total devastation of fish stocks etc., and moreover, the name is now synonymous with the entire Pacific-wide (and adjacent areas) event during which the waters of the equatorial east Pacific warm significantly above normal values. El Niño, and the Southern Oscillation are coupled via the acronym: ENSO, and strictly an El Niño event should be called a warm ENSO event (or warm phase of ENSO). We now have La Niña, and it is not clear whether this has just been thought up (i.e. the female equivalent of El Niño), or was long established. I suspect the former. It must be clearly understood that the La Niña event refers only to markedly colder waters, not just a return to a 'normal' pattern.]

2. What are the known direct effects?

From: Reference (2)

J. Bjerknes (Reference (12), is noted as suggesting a teleconnection between the shifting of the equatorial Pacific zone of convection and a change in the upper level circulation at mid-latitudes in the North Pacific/North America and more tentatively, over the North Atlantic. He analysed the 1957-1958 winter [NB: only one event] and is quoted thus in that paper: .. "a positive temperature anomaly in the tropical Pacific (extending over about 90 deg from South America to the mid-Pacific) strengthens the mid-latitude zonal wind system within this sector; the associated negative pressure anomaly ( a deeper Aleutian Low ) in the extratropical cyclone belt anchors the phases of the prevailing stationary waves in the upper westerlies so that a succession of positive and negative stationary wave anomalies appears downwind over North America/Southwestern Greenland and the north-eastern Atlantic/north-western Europe."

[Although only based initially on the one case study, these direct effects as regards the Pacific/North America region have now been accepted by-and-large, to the extent that the altered flow patterns/strength aloft over the North American region can be related to the incidence of 'anomalous' or extreme events in these areas; the suppression of the Atlantic hurricane activity (*), and altered precipitation patterns across the USA are two examples often quoted. See also Reference (5) for another re-statement of the direct and accepted link between ENSO swings and extra-tropical latitude circulations.]

[(*)=for more on how the various ENSO patterns affect Atlantic (and other basin) hurricane activity, see the FAQ: HURRICANES, TYPHOONS, AND TROPICAL CYCLONES at: http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html]

[ However, even before this work, Sir Gilbert Walker* early in the 20th century had demonstrated connections of at least a general nature....]
[* head of the Indian Meteorological Service in the 1920's, and responsible for the theory behind the 'Walker circulation']

From: Reference (6) it is well known that Walker, in his work on predictability of the Indian Monsoon, attempted to find links within datasets he had access to coupling events right across the equatorial regions, from the east Pacific to central Africa. His work resulted in a rather complex series of 'indicators' although the one that has come to us as the most useful is based on a noting of the difference from the average of mean sea level pressures at two stations: Darwin and Tahiti. Walker's work was simplified by an Australian meteorologist, Troup, and what is now known as the Southern Oscillation Index (SOI), can also be found listed as the Troup Index.

When the msl pressure is anomalously high in the monsoon regions of SE Asia / north Australasia and it is lower than average in the equatorial east Pacific, then this defines a warm-event ENSO (-ve value), and vice-versa for a cold-event ENSO.

Walker in fact used data-sets covering the entire globe (such as they were in the 1920's), and went so far as to postulate connections globally and of course this is the subject with which we are most interested.

[It is important to note here that Walker, and Troup, were looking for broadscale, mass-related connections, not local/regional scale 'weather' effects. Indeed, given the data sparsisty/integrity and more importantly, the short records that Walker in particular had to work with, it would be very surprising if they had found direct connections in terms of local weather. That was Walker's original aim: the seeking after a method of long-term forecasting of the onset/intensity of the Indian Monsoon. However, modern reappraisal of the record seems to suggest that whatever connection there may be between the Monsoon over India and phase-changes of ENSO do not appear very reliable, since researchers have found that warm-phase ENSO years are associated with both particularly wet and notably dry years. It seems to be accepted by modern researchers in the field that the warm/cold switch in the ENSO signal is coupled in some way with the timing/intensity of the SW (or Asian) monsoon, but that there are other factors at work which offset the signal. There are no doubt complicated feed-back mechanisms at work here which may still not be understood fully. Certainly Walker did not achieve his primary aim of finding the answer to predictability of the Monsoon: something that is still sought after today.]

From: Reference (4) [ Most researchers now accept some strong, directly coupled mechanisms between ENSO events (warm-neutral-cold), and weather patterns in the immediate equatorial Pacific region: indeed, so confident are climatologists of the direct links that routine forecasts, based on fluctuations in the equatorial Pacific, are produced by NOAA for North America & other 'Pacific Rim' areas.]

From: Reference (5) As noted earlier, as the trade wind flow weakens, upwelling in the eastern Pacific decreases: this is because the wind blowing across the surface of a body of water exerts a drag on the top-most layers, deflected by the Coriolis effect in such a way that divergence of the surface layers occurs, requiring the aforementioned upwelling of sub-surface waters to preserve a balance - however this water is cold, relative to the top-most layers. Thus, if the upwelling is significantly reduced, sea surface temperatures tend to rise, and not just due to the absence, or reduction of cold water injection, but because the amount of evaporation also decreases (due to reduced wind velocity). Further, in a mature El Niño, the warm water of the western and central Pacific spreads eastwards (as there is now a reduced fetch across the surface so the Pacific waters attempt to achieve a level) and the combined effect of movement of the warm water eastwards, and local warming in the east mean that the atmospheric convergence zone (which will be most vigorous where the waters are warmest) migrates towards the central Pacific ... the subsidence over South America decreases to such an extent that a new convective region is allowed to appear here, associated with intense/heavy rain and floods in the normally arid countryside. It is now accepted that at least on this regional scale, the eastward displacement of the Indo-Australian enhanced convergence zone results in large-scale drought conditions in Australia and Indonesia [hence all the fuss about the forest fires/smoke etc.]: and, conversely, locally intense rainfall and hurricane formation in the central Pacific, where such phenomena are normally rare. [ These effects are now accepted such that no further qualification is needed. However, it is worth pointing out that each warm ENSO event, and indeed its opposite, will vary both in timing and intensity, and so the consequent effects will not be the same from event to event. There seems to be some, almost hysterical, assumption that every incidence of anomalous warming in the central/east equatorial Pacific will lead to the same level of 'mayhem' across the region...this is not so.]

From: Reference (5) As well as these latitudinally-bounded (to the equatorial region of the Pacific-rim) effects, the movement of the Pacific convergence zone (with its associated moisture/momentum/heat mechanisms) alter the upper tropospheric atmospheric circulation over the north Pacific, and it is suggested, a large part of the globe [ see, for example, Reference (7)]. This is because of the effect that these alterations have on stationary waves that tend to form in the upper troposphere (Rossby waves), which in turn leads to the displacement of the alternating zones of semi-persistent low and high pressure systems coupled to the 'loops' in the wave-train leading away from the central/north Pacific towards high latitudes and then eastwards across North America. A consequence of this altered pressure pattern is the southward diversion of the upper jet, and therefore of the associated frontal systems in the North Pacific and high rainfall in the western and southern USA.

[This effect too seems to meet with general acceptance. Indeed, mean maps of zonal velocity/u-component wind at jet-stream altitudes in the NE Pacific do show higher values when the equatorial east Pacific is warmer than average, and this should be expected from Polar Front jet-stream theory: a greater degree of warmth through an atmospheric column will lead to higher pressures aloft, relative to poleward locations, and lead, if only temporarily, to an enhancement of upper level jet strength, and downwind propagation of the jet, with a consequent displacement of its associated entrances/exits. Further, altered jet-strength across the Rockies, will lead to altered CAV trajectories which in turn will lead to a displacement and altered configuration of the downstream long-wave trough. All these changes too seem to be accepted as 'direct' effects of the warm-phase ENSO. However, with time during any event, there will tend to be a 'levelling-out' , so later in a winter season, the effects should be less detectable, but this may not be the case for a significantly warm/cold event. Further, higher latitude effects will also impinge on this relatively simple story, which may or may not be directly coupled to ENSO, but perhaps be tied more to the weather of the previous season. It is therefore dangerous to couple every anomalous event even in the immediate Pacific area, to ENSO variation. ]

3. What are the suspected effects, direct or indirect?

From: Reference (6)

Walker in his paper, suggested that relationships exist between other geographically remote regions: indeed his concern, as already stated, was to find some large-scale methodology of relating what we now call the Southern Oscillation to weather/climate changes elsewhere. He specifically noted a suspected link between msl patterns [ he would not have been able to comment upon the upper-air aspects ], in the Pacific and the North Atlantic. It is worth quoting Walker directly thus ..... ' there are swayings on a much smaller scale between the Azores and Iceland and between the areas of high and low pressure in the North Pacific Ocean .... and that all appeared to be related: .... there is a marked tendency for the highs of the last two swayings to be accentuated when the pressure in the Pacific is raised and that in the Indian Ocean is lowered '... [i.e. a cold-ENSO event - however, it is not clear to me what Walker was talking about here: does he mean 'high' in terms of pressure, or 'high' in terms of extreme? Nevertheless, the fact that this 'swaying' (or I suppose we would now say 'oscillation') in pressure pattern distribution was noted early in the 20th century, and the fact that we have (see later) good statistical evidence for such alterations, not only at the surface but aloft, means we have to accept the broad-scale links between ENSO variability and patterns over the NE Atlantic, particularly during strong ENSO episodes.]

From: Reference (5)

This reference accepts that the climate of other ocean basins (outwith the Pacific) can be altered by El Niño events, and notes the eastward shifting of the convergence zone over Indonesia (during a warm ENSO event) which is suspected to have an impact on the Indian Summer Monsoon through a weakening of its land-sea circulation. This is thought to lead to a failure of the monsoon rains across India. [ However, as pointed out above, this link is disputed by some.] This reference also states that Africa is affected to a significant degree. The author notes that an indirect link has been found between severe drought years in the Sahel and El Niño events ( from Reference (13)), and also with drought frequency in southern Africa, and states that this is probably the result of circulation modifications above the Atlantic and Indian Oceans respectively."

[ It is difficult to refute these statements, however it is pertinent to ask about the integrity of the data-sets used to establish the suspected links, particularly the length of the series, and note the difficulty of obtaining long-period accurate data-sets of sea surface temperature in data-sparse areas. It is also worth noting the primary effect of the warm ENSO pattern is achieved because of direct effects due to changes in evaporation/upwelling etc., and these effects cannot of themselves impact upon areas further west. The effects must be indirect, involving perhaps changes in the upper easterly flow, changes in TUTT(*) orientation/location, and perhaps delayed relative to the primary region affected.]

[ (*)=Tropical Upper Tropospheric Trough: An upper level weakness at low latitudes - to see their importance to tropical forecasting, particularly for tropical cyclone formation, see the FAQ: HURRICANES, TYPHOONS, AND TROPICAL CYCLONES at: http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html ]

[ For this Section, see also References (8) & (9) ]

4. Some notes summarising articles relating to possible effects over Europe

From: Reference (3) & Reference (14)

These authors have no doubt that ENSO variation has a direct effect upon the synoptic climatological patterns of the NE Atlantic/European region. It is further stated that the effect is largest/best detected in the winter months of January and February (& for the second paper also March) following the year of a warm or cold event. [ However, careful reading of Ref. (3) and we have....] " There is more variability between individual warm event winter months, whereas the response to cold episodes is relatively uniform."

[This article goes on to say that model results confirm the teleconnections between varying ENSO events and the northern hemispheric extra-tropical circulation. However, it is well known that such models are crude at best, particularly where ocean/atmosphere interactions are required ... however, such as they are, they do confirm ideas expressed, and accepted earlier.]

"Not unexpectedly, the largest response is observed in the NH winters following the year of the event, in particular in January and February.".... "Warm events are associated with highly variable winters; that is, there are large differences observed in the high winter season (January and February) of one episode to another one. Cold events, however, appear to produce a more uniform response...."

Many sources quote from earlier work (e.g. Reference (10) and Reference (11)) which found significant pressure and temperature anomalies due to the mid-latitude response noted earlier downstream of an intensified (for a warm event) Aleutian Low [this is noted elsewhere, and generally accepted] and a coupled low-pressure anomaly over the North Atlantic which reveals a pattern similar to the North Atlantic oscillation. [And which latter has been strongly correlated to weather patterns across the British Isles/NW Europe sector. It is also worth a re-emphasis that Walker picked this up over 60 years earlier, and reproduced at Reference (6). The pedigree for this idea, from Walker, through Bjerknes to latter-day researchers is therefore sound, and as the data have been tested over varying time-sets etc., it would be churlish not to acknowledge the effect on the North Atlantic/NE Europe upper air region from upstream anomalies in the North Pacific. ]

[ Interestingly,Reference (2), contains the following statement....] " Finally, it should be mentioned that this ENSO response analysis .... serves as an interpretation after the effect. The prognostic value and dynamic interpretation needs further investigation; Europe is farthest away from the key regions of ENSO so that it is not surprising if at times the influence of the oscillation is superseded by other effects" .... "For example, the extratropical wintertime atmospheric circulation also is sensitive to certain extratropical sea-surface temperature anomalies and not only to anomalies associated with El Niño". However, Reference (14), notes that later work has 'firmed-up' the link, and finds that ENSO events are well correlated with NAOI values provided the north Pacific sea-surface temperature anomalies reinforce the broadscale Northern Hemispheric pattern: this occurs roughly in 4 cases out of 5.

From: Reference (1)

This author draws on other published work referenced here-under, but also interprets the response in terms of circulation types across the British Isles, using Lamb's Weather Types (LWT). He notes that the A-type appears to be more sensitive to the prevailing ENSO than the C-type, with the former showing positive anomalies during cold events and negative anomalies during warm events. This accords well with statements made in the papers which concentrated more upon effects across 'central' Europe. However, the article goes on to state that although C-type patterns occur on average more frequently during warm-ENSO events, the percentage of correct signals was notably lower than for the A-type: only 50% (C) compared with 68% (A).

[ This would appear to confirm the doubts expressed about the predictive ability of the warm-ENSO event as noted elsewhere. Further doubts are expressed later .... there is a figure of C-type frequencies during a warm ENSO extreme and this does identify the enhanced prevalence in mid-February which corresponds to the singularities previously detected in the European Grosswetter (Reference (3)). The pattern for the A-type during a cold-ENSO extreme is less clear with below average frequencies in early-mid January, and above average prevalence during most of February ].

The author performed a statistical analysis on these data and this gives rise to the following statement... " In other words, the A-type winter frequencies display significant anomalies during ENSO extremes (particularly in February) which are sensitive to the ENSO type. The signal for the C-type, however, is less conclusive.".....(and further....) " These findings support those of (Reference (3) who suggested that warm extremes are associated with highly variable winters, whilst cold extremes tend to produce a more uniform response, with less variation between episodes, and therefore a higher predictability in long-range forecasting.

Reference (14) is a useful updating of the above ideas (to 2006): Using reconstructions back to the 1500's, they appear to show that provided the events are 'strong' (well-defined & long-lasting), then the tendency is as follows:-

Strong "El Niño" (-ve SOI) event: -----> -ve NAOI (i.e. tending to blocked pattern)
Strong "La Niña (+ve SOI) event: -----> +ve NAOI (i.e. tending to mobility)

However, this study confirms other findings that even for 'strong' events, roughly a fifth show no correspondence to the above: it appears that conditions (of SSTA) in the northern Pacific must act to enhance the forcing of the warm/cold ENSO patterns for downstream effect in Europe to be marked. It is most important to understand that whilst there is a potential link, it is not an unvarying one.

5. Summing up, and the Q/A issued with the uk.sci.weather FAQ.

So, what do we know, or more importantly, what do we think we know?

Strong signals are there, and, as for the 1997 and 1982 events, global effects have been found in datasets from widely separated regions of the globe, and, according to researchers working in Europe, can be found in the relative frequency of broad weather types (e.g. anticyclonic versus cyclonic) between cold and warm forcing. However researchers point out the variability for each individual event, and make the point that the coupling is picked up as a bias towards one type or another in a long-term record.

For a region as far away from the seat of the disturbance as ours is, most researchers make the point that events closer to home will either significantly modify the effect, or perhaps completely swamp any effect such that only a residual signal is to be found by careful statistical analysis. To predict any one winter/spring season over the British Isles/NW Europe on the basis of any one event in the equatorial east Pacific is problematical; however, it is true to say that solid links are being found in the historical record between strong events in the equatorial Pacific & the circulation in the North Atlantic / European region.

This is the Q/A that is published in the long and html versions of the uk.sci.weather FAQ:

Q. What impact does 'El Nino' have on the weather over Europe?

A. The 'El Nino' phenomenon, or more strictly the warm El Nino -Southern Oscillation (ENSO) event is coupled closely to remarkable shifts in weather patterns in the immediate Pacific basin, and adjacent areas: e.g. parts of North America. For example, it is clear that the altered distribution of warm/cold water across the equatorial Pacific is the primary reason why excessive rain can fall in places like Peru, and a general deficit of rainfall is experienced in Indonesia, parts of Australia and the Philippines. There is also an accepted link between a less-than-'normally' active Atlantic hurricane season and the notably warm event that characterises what has come to be called, THE El Nino.

It is becoming clear from recent studies that we can now rule out the 'No Effect' case: this leaves us with two options -

(a) There IS an effect, but it is on a scale that is dwarfed by regional variations closer to home, e.g. long-term thermal inertia in SST distribution in the N. Atlantic, or continental/oceanic temperature differences across the North America - North Atlantic - Eurasian 'super-region'.

(b) There is a direct, and marked effect that leads to verifiable modification of the weather types across the NE Atlantic/European - Mediterranean region.

(a) appears to be the most likely if we take the year overall; indeed, even in studies published which set out to prove the link between warm/cold ENSO regimes, and impacts over Europe, caution is always advised relating to local/regional scale modification.

(b) is climbing higher in the 'probability' stakes, at least if the 'winter' season only is considered. There are an increasing number of studies published that show a direct link between a warm ENSO season, and, for example, altered rainfall/temperature anomalies across west/central Europe. No lesser person than J.Bjerknes postulated in 1966 that altered activity in the equatorial Pacific appeared to significantly alter the strength/orientation of the PFJ over and downwind of the NE Pacific, which in turn must have at least some effect on the long-wave structure downstream. This appears to have been accepted in later studies & developed further using datasets going back over two centuries or more.

also, have a look at the following sites:

[WMO home page] http://www.wmo.ch/

[NOAA/TOGA-TAO site for real-time data, advisories, further definitions etc.] http://www.pmel.noaa.gov/toga-tao/el-nino/home.html

... and of course, a search of the WWW will throw up many active sites dealing with El Nino.


Reference (1)

PUBLICATION: Weather
DATE: August, 1993
VOLUME, ISSUE NUMBER ETC: Volume 48, pp234-239
TITLE: 'Evidence of ENSO in the synoptic climate of the British Isles since 1880'
AUTHOR(S)/EDITOR(S): Robert Wilby
AFFILIATION(S): Geography Department, University of Loughborough.

Reference (2)

PUBLICATION: International Journal of Climatology
DATE: January/February, 1992
VOLUME, ISSUE NUMBER ETC: Volume 12, pp25-31
TITLE: 'Climate anomalies in Europe associated with ENSO extremes'
AUTHOR(S)/EDITOR(S): Klaus Fraedrich and Klaus Müller
AFFILIATION(S): Institut für Meteorologie, Freie Universität, Berlin.

Reference (3)

PUBLICATION: International Journal of Climatology
DATE: January/February, 1990
VOLUME, ISSUE NUMBER ETC: Volume 10, pp 21-31
TITLE: 'European Grosswetter during the warm and cold extremes of the El Niño/Southern Oscillation.'
AUTHOR(S)/EDITOR(S): Klaus Fraedrich
AFFILIATION(S): Institut für Meteorologie, Freie Universität, Berlin.

Reference (4)

PUBLICATION: WMO WWW HOME PAGE
DATE: February, 1998
VOLUME, ISSUE NUMBER ETC: El Niño update, No.4
TITLE: 'El Nino update'
AUTHOR(S)/EDITOR(S): None directly credited.
AFFILIATION(S): WMO

Reference (5)

PUBLICATION: Weather
DATE: January, 1990
VOLUME, ISSUE NUMBER ETC: Volume 45, pp 2-8
TITLE: El Niño and the Southern Oscillation
AUTHOR(S)/EDITOR(S): G.R. Bigg
AFFILIATION(S): School of Environmental Sciences, University of East Anglia.

Reference (6)

BOOK: Monsoons
DATE: 1995
CHAPTER: The variable and interactive monsoon
AUTHOR(S)/EDITOR(S): Peter J. Webster (ed: JS Fein and PL Stephens)
PUBLISHERS: John Wiley and Sons.

 

Other relevant articles not surveyed in detail:

Reference (7)

PUBLICATION: Monthly Weather Review
YEAR: 1981
VOLUME, ISSUE NUMBER ETC: Volume 109, pp785-812
TITLE: 'Teleconnections in the geopotential height field during the Northern Hemisphere winter'
AUTHOR(S)/EDITOR(S): Wallace, J.M., and Guetzler, D.S.

 

Reference (8)

PUBLICATION: Science
YEAR: 1983
VOLUME, ISSUE NUMBER ETC: Volume 222, pp 1195-1201
TITLE: 'Meteorological aspects of the El Niño/Southern Oscillation.'
AUTHOR(S)/EDITOR(S): Rasmusson, E.M., and Wallace, J.M.

Reference (9)

PUBLICATION: Journal of Climatology
YEAR: 1988
VOLUME, ISSUE NUMBER ETC: Volume 8, pp 67-86
TITLE: 'A detailed examination of the extratropical response to tropical El Niño/Southern Oscillation events'.
AUTHOR(S)/EDITOR(S): Hamilton, K.

Reference (10)

PUBLICATION: Monthly Weather Review
YEAR: 1981
VOLUME, ISSUE NUMBER ETC: Volume 109, pp 1150-1162
TITLE: 'The Southern Oscillation. Part I. Global associations with pressure and temperature in northern winter'.
AUTHOR(S)/EDITOR(S): Van Loon, H., and Madden, R.A.

Reference (11)

PUBLICATION: Monthly Weather Review
YEAR: 1981
VOLUME, ISSUE NUMBER ETC: Volume 109, pp 1163-1168
TITLE: 'The Southern Oscillation. Part II. Associations with changes in the middle troposphere in the northern winter'.
AUTHOR(S)/EDITOR(S): Van Loon, H., and Rogers, J.C.

Reference (12)

PUBLICATION: Tellus
YEAR: 1966
VOLUME, ISSUE NUMBER ETC: Volume 18, pp 820-829
TITLE: 'A possible response of the atmospheric Hadley circulation to equatorial anomalies of ocean temperature'.
AUTHOR(S)/EDITOR(S): Bjerknes, J.

Reference (13)

PUBLICATION: Nature
YEAR: 1986
VOLUME, ISSUE NUMBER ETC: Volume 320, pp602-607
TITLE: ' Sahel rainfall and worldwide sea temperatures.'
AUTHOR(S)/EDITOR(S): Folland, C.K., Palmer, T.N. and Parker, D.E.

Reference (14)

PUBLICATION: Dynamical Climatology
YEAR: 2006
TITLE: ' ENSO influence on Europe during the last centuries. '
AUTHOR(S): Brönnimann, S., Xoplaki, E., Casty, C., Pauling, A., Luterbacher, J.

Martin Rowley: 24 OCT 2006