Saturday, October 5, 2019

Hurricanes, Ireland and Lorenzo

There has been a lot of discussion around hurricanes and Ireland this past week and a bit, associated with the remnants of Hurricane Lorenzo, coming as it has climatologically speaking hot on the heels of ex-hurricane Ophelia. Lorenzo ended up having relatively minor impacts upon the island of Ireland and there have been accusations it was over-hyped. There have also been, frankly wild, messages around hurricanes, climate change and the future impacts on Ireland bandied around. I thought it may be useful to put some thoughts down on the matter.

Was Lorenzo unusual?

While there are records for parts of the North Atlantic basin that go back for in excess of a Century, we only started observing the whole basin with the advent of satellite records in the 1970s. The eastern Atlantic basin before this time was fitfully observed, which means we can't be certain about earlier behaviour. Hurricanes are also infrequent events and it is thus hard to know whether single events are unusual in the context of a short record - did we just not observe such events by chance in the short record rather than them actually being truly unusual?

Caveats out of the way, Lorenzo was by far and away an outlier geographically in where Category 5 hurricanes (the strongest classification) have been known to have occurred. You'll note that for the <24 hours it was likely at category 5 it was very far away from Ireland.
Unusualness of the position of Lorenzo when category 5 in a historical record context. Source: Robert Rohde (@RArohde)

Other aspects of Hurricane Lorenzo were far more typical. The storm started out as a disturbance coming off Africa, moved westwards, strengthened, recurved to the north then north-east before becoming entrained in the mid-latitude circulation and transitioning to a post-tropical storm.
Storm histories for all recorded historical systems including their genesis, intensification and eventual decay either in-situ in the tropics or as extra-tropical systems. Colours denote intensity (see colour bar). Source: NOAA


Were the potential impacts on Ireland overhyped?

Let's start with the official forecasts from Met Eireann, which as the national meteorological service should be seen as the sole source for authoritative information (similarly the UK Met Office for Northern Ireland). From about a week out it was consistently suggested by the premier global forecast model from ECMWF that the post-tropical remnants of Lorenzo could impact Ireland.
 ECMWF deterministic model for 00Z on 4/10 initialised a week in advance. Note the tightly packed isobars off Cork which were the remnants of Lorenzo as forecast at that time. Source: www.wetterzentrale.de

By the start of the week, based upon continued run to run consistency of the forecast for the post-tropical remnants of Lorenzo to be over / near Ireland it was absolutely right of Met Éireann to start providing advice to the public warning of potential disruption. However, there was still considerable uncertainty in the track and intensity which persisted until much nearer the time. This is not unusual - predicting how a tropical storm system will interact with the mid-latitude system is not a simple problem. By 48 hours in advance and with Lorenzo undergoing extra-tropical transition just north of the Azores the predicted state had changed slightly:
Forecast of ex-hurricane Lorenzo initialised 48 hours in advance from ECMWF. Source: www.wetterzentrale.de

However, even this close in there were substantial disagreements between predictions arising from different global modelling groups. ECMWF and the Met Office bought the system in over Donegal / Mayo rapidly weakening as it moved over to Wales and SW England. Several other modelling groups took it up to the west of Scotland or even recorded it toward Greenland.

It was only this close in that Met Eireann issued amber and yellow specific warnings. These were appropriate to what the models were suggesting would occur. These warnings were then updated regularly and downgraded appropriately as it became clear that the system was weakening. Impacts and measured windspeed were clearly entirely consistent with the amber warnings issued. Thus Met Eireann did not overhype the storm or its likely impacts.

Members of the media and certain social media may be less innocent in the matter. Lorenzo was a category 5 hurricane several thousand kilometres and about 7 Days prior to eventual arrival in Ireland. The storm was never going to be that strong anywhere near Ireland. Yet, much reporting implied that it was still a category 5 hurricane. Arguably responsible reporting should have concentrated upon the actual state of the system upon arrival on Irish shores (a strong post-tropical system) rather than what it had been in, meteorologically speaking, a former lifetime.

What Lorenzo actually looked like at landfall in Donegal according to the Met Office analysis: i) confirms the typical post-tropical system structure and ii) the ECMWF model forecast from two days or so prior.
Met Office analysis chart for 00Z on Friday 4th Oct


Could Ireland be hit by an actual hurricane?

There have been some suggestions reported that Ireland could be hit by a hurricane in the near future. Not putting too fine a point on this: those suggestions are utterly false. Hurricanes are warm cored systems that require the sustained availability of ocean waters over 26 degrees to maintain their structure. Present day climatological sea surface temperatures of the surface Ocean (not coastal waters) in the proximity of Ireland are about 16 degrees:
Climatological SSTs for September from NOAA. Hurricanes can only be maintained over regions above 26 degrees (the oranges) which are very far from Ireland.

The 26 degree isotherm is several thousand kilometres to the SW of Ireland. Tropical systems can only transect for a very limited time over ocean surface temperatures below 26 degrees without losing their tropical characteristics:

Model output from the GFS model showing the transition from a warm cored hurricane to an extra-tropical system for Lorenzo. Colours denote temperatures, white lines denote isobars. Note how the system takes on very different structure and how pressure field broadens (weaker peak winds). Source: Simon Lee (@simonleewx)

That is before getting into finer points such as the effects of vertical shear that can rip apart a tropical system. The hurricane season in the North Atlantic is driven by the availability of a low shear environment which is a seasonally occurring phenomenon. Shear in the midlatitudes would tend to tear apart storms even if, somehow, the ocean surface temperature warmed sufficiently.

For a hurricane to hit Ireland would require late summer season sea surface temperatures to be sustained all the way from the tropics to Ireland at or above 26C, along with a sufficiently low shear environment along the path. That would require warming of the North Atlantic basin of the order 10 degrees. Because oceans warm slower than land this would equate to a global mean surface temperature warming of even more - likely somewhere north of 15 degrees. That is a warming not seen in even the most sensitive climate models under the most pessimistic emissions scenarios by 2100.

So, no, Ireland is not about to get hit by a hurricane. It is extremely unlikely to occur within the lifetime of anyone reading this blog irrespective of our collective choices as global society.

There are more than sufficient reasons for concern and subsequent undertaking of meaningful actions to address the significant challenges of climate change. This is not one of them.

But what about Ophelia?

Ophelia was also extra-tropical (an ex-hurricane) when it hit. Despite having hurricane force winds it was not a hurricane when it hit County Cork. It was, however, a very special case, transitioning much closer to Ireland than any known system. This was a very particular dynamical set-up whereby the hurricane interacted with a strong polar jet streak which maintained an outflow aloft and allowed it to retain warm-core tropical characteristics over colder than usual ocean surface temperatures. Even that  somewhat unique circumstance was insufficient to maintain tropical storm structure all the way to Ireland.

Saturday, December 22, 2018

Addressing stated concerns from Ray Bates around the SR1.5

Ray Bates has provided a critique of the IPCC Special Report on 1.5C promoted as a ‘paper’  (it is no such thing in that it is not a rigorously peer-reviewed piece) by the GWPF – a highly questionable think tank.

This piece has garnered some interest in the Irish press and I am quoted as critical of it and the associated views in a piece in the Irish Times. To be crystal clear: a free press is an essential component of a healthy, vibrant democracy and it would be strange for the media to completely censure views. Furthermore, journalists only have a finite amount of real estate to work with. Also, journalism is a real-time business and a tough gig. Hence here I am taking the opportunity to more clearly lay out from a scientific viewpoint why the piece being reported upon is fundamentally flawed and the IPCC SR1.5 report stands on its merits.

The societal response and policy aspects I shall leave for another day but suffice to say I am more optimistic and view national leadership on acting on climate change, if done right, as a real national opportunity for all Irish citizens.

Wilful misinterpretation of AR5 attribution findings

Ray Bates begins by claiming that the central attribution statement of AR5 is:
It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together.
And that the statement did not necessarily assert that more than half of the warming is due to human influences. He goes on to contend that SR1.5 departs from AR5 in contending that all the warming is due to human influence. 

Why is this a wilful misinterpretation of AR5? Well, the complete statement rather than a partial lift is:
It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together. The best estimate of the human-induced contribution to warming is similar to the observed warming over this period. 
Spot the difference? It’s this:
The best estimate of the human-induced contribution to warming is similar to the observed warming over this period. 
I see no other way to describe this presentation other than wilful misinterpretation. Don’t believe me?  p17. Section D.3 first bullet. Want more evidence? Try to reconcile only half the change being explained by anthropogenic influences in Figure SPM 6:
Figure SPM6 from IPCC AR5 WG1 report. Changes for simulations with human influences (pink) and without (blue) for a range of global and continental and ocean / cryosphere indicators. Human influences explain effectively all the change seen since the early 20th Century.

Or to reconcile the statement with Figure 10.5 from the attribution chapter in the report:
Figure 10.5 showing attributable contributions to the observed change (black bar) from Grrenhouse gases, all anthropogenic forcings, non-greenhouse gas anthropogenic forcings, natural forcings and internal variability. The best estimate is that anthropogenic contribution to warming slightly exceeds that observed (balanced by variability / natural forcings). It is in no way only half the warming.

So, does SR1.5 start from a distinct premise from the AR5 conclusions around attributable warming contribution from us humans? That’ll be a resounding no.

Appropriate use of temperature period and series

Ray Bates goes on in his piece to imply nefarious intent behind the IPCC considering only the post-1960 series of GMST in their SPM figure. The choice is reasonable because the attribution statement in AR5 (and the prior figure) pertained to post-1950 changes. Furthermore, in the underlying chapter 1 Figure 1.2  the full series from 1850 is shown. The SR1.5 is hardly ignoring the early period as implied is it? 

Ray Bates then goes on to show a satellite dataset that shows less warming than the surface. It is true that the UAH dataset does, indeed, show less warming than the surface. What’s the issue? Well, satellite data are no gold standard. You put a delicate instrument on top of a controlled explosion to send it into space then expose it to vagaries in space weather. Unlike the surface there are not hundreds of instruments there is one. It can drift in the sampling and it can degrade in performance over time. What happens if another group also go to the trouble of creating a product from the same instruments in space? Well, they find greater warming than at the surface. I’m not going to argue one of these is right and the other wrong. I’m just pointing out that satellites aint no gold standard. Indeed, AR5 concluded that the surface was robust whereas the tropospheric changes while certain of the sign (warming) were uncertain in magnitude. As I lead that portion of the AR5 assessment I am uniquely qualified to make this statement.

Ocean red herrings

Ray Bates goes on to throw in a couple of red herrings on the Oceans for good measure. He states that an emerging signal of warming of land faster than oceans in the observed record is problematic. Well, no, its actually what we expect as an emerging signal of climate change and it will only get worse. As SR1.5 points out much of the global land domain already experiences seasonally or annually changes exceeding 1.5C. 

Don’t believe me? Here is Figure 12.11 of the SPM of AR5. Note how the land warms more than the oceans in all cases. Note also how our choices matter in determining future generations climate.
Figure 12.11 from AR5. Land robustly warms more than oceans. I sure don’t want my descendants inheriting that bottom right world. Can we please make efforts to make the top right reality?

Ray Bates then goes on to contend that CERA-20C ocean heat content in the 0-300m horizon casts doubt on the significance of ocean heat content changes. The changes in the early period of that record are clearly spurious. Variations of the degree shown would lead to changes in sea level that simply are not present in the tide gauge records around the world. The abstract of the paper makes clear that the result is experimental: 
These preliminary results are considered of interest for the ongoing community efforts focusing on coupled data assimilations.
Why would IPCC be expected to rely upon an experimental data assimilation system analysis? Well, exactly.

Climate model tuning

The section on climate model tuning is very confused indeed. I was involved, when at the Met Office, in the model tuning and evaluation exercises. I saw lots of effort concerned around getting critical processes such as El Nino, mid-latitude storm tracks and monsoon variations adequately as well as the mean state. I never saw an effort to tune the model to reproduce the observed trends or to determine some state in 2100 in the projections. I also saw significant efforts to run perturbed ensembles and these have been done even more so in the citizen science www.climateprediction.net experiments. To claim that the model tuning is an issue that is unexplored or a major issue is incorrect.

Equilibrium Climate Sensitivity

The section on ECS is, to put it politely, selective. Fortunately I don’t have to waste many words here as there has been a recent major review piece written by Reto Knutti and colleagues - the figures are not behind the paywall and any number of these give a much more holistic assessment as to the state of our knowledge on the topic. The bottom line is that contrary to the assertion of Ray Bates if anything the lower bound on ECS has increased since AR5. The good news here is there is a whole chapter devoted to it and related matters in AR6.

To conclude

Ray has had a long and distinguished career. But that career has been in atmospheric dynamics and not climate. Yes, both are to do with the atmosphere, but when your toilet is backing up you call the plumber and not the electrician. In the same way when looking for guidance on climate change it is advisable to listen to the climate scientists of which there are many thousands the vast majority of whom (and I mean vast) concur with the broad findings of the IPCC and various national assessments and national academies that climate change is real, its due to us, and that our choices now are of critical import. Equally, if you want to discuss the intricacies of atmospheric dynamics please don’t come knocking at my door!

The analysis of Ray Bates is not a peer reviewed paper and finding substantive flaws in it is really not that hard. This has taken me all of an hour of an evening. Compare and contrast to the rigor of the IPCC assessment process with multi-author teams, multiple drafting meetings, substantive rounds of in-depth review, and final sign off on the summary word by word with the governments of the world. Only the most robust findings survive such a process. It is to borrow a colloquialism like contrasting night and day. I know who I’m going to pin my choices as to the future for my kids on. It’s the guys saying this:







Thursday, August 2, 2018

The ongoing flash drought is (probably) the most intense experienced at Phoenix Park since records began

Simon Noone, Conor Murphy and Peter Thorne

Drought has begun to fade from the national conversation following a week of rain, but observations show that Dublin remains in the grip of the one of the most intense flash droughts, probably the most, since at least the middle of the nineteenth Century.

Defining drought

There are numerous definitions of drought. None are perfect. In Ireland we have tended to use an index called the Standardised Precipitation Index (SPI) (McKee et al. 1993). This makes sense in that most of the time it is a lack of rainfall that leads to drought in our relatively moderate climate. But there are other indexes such as the Standardised Precipitation and Evapotranspiration Index (SPEI) (Vicente-Serrano et al.2010) which, in addition, account for removal of water via evaporation and plant transpiration. This index is arguably better for characterising flash droughts that tend to occur in the warm season. It uses a combination of monthly rainfall and temperature records.

The long-term context

The Noone et al. (2015) assessment of variability and change in precipitation over the period 1850–2010 indicates positive trends in winter and negative trends in summer precipitation. Noone et al. (2017) produced a 250 year drought catalogue for the Island of Ireland by applying SPI-12 ( a 12 month drought index) to identify long hydrological drought rich periods. The results show that Ireland is drought prone but recent decades are unrepresentative of the longer-term drought climatology. Ireland has experienced seven long drought rich periods over the period 1850-2015 impacting the whole of the island of Ireland; (1854–1860, 1884–1896,1904–1912, 1921–1923, 1932–1935, 1952–1954 and 1969–1977). But it is also possible to have ‘flash’ droughts, such as that in 2018.

Phoenix Park

Phoenix Park is one of the longest continuous meteorological stations in Ireland. Records started in the early 1800s and continue through present. This long record is thanks to the significant efforts of Met Eireann, Ordnance Survey Ireland and others. Although instrumentation has changed over time, the site has always stood in the grounds of Ordnance Survey Ireland and the regional environment has remained largely unchanged. The precipitation series was homogenised by ICARUS as part of Noone et al. (2015). The temperature record has not been homogenised to date.* Met Eireann make monthly and daily summaries available. To perform the present assessment we have appended the July daily values aggregated to monthly averages** to the long monthly series and calculated SPEI-3 (a 3 month drought index) for the entire series.

Phoenix Park site photo sourced from Met Eireann53°21‘50” N, 06°20’00’’ W, 48 m above mean sea level

SPEI-3 at or very close to record levels

The provisional value of SPEI-3 for May to July is -2.70 which is the most extreme value on record, beating October 1995 by a whisker***. The next most extreme was August 1995. Before that the index values are sufficiently different to conclude that either 2018 or 1995 represent the worst flash drought since modern meteorological records began in Ireland, with 2018 marginally more likely. 

SPEI-3 values calculated at Phoenix Park January 1850 to July 2018. Red lines show 3 month accumulative deficits and blue lines show 3 month accumulative surplus.

One important thing to note is that the two most extreme prior months occurred in the same year. This shows that drought can appear to diminish only to quickly return. We may well have had a week of rain (although even that in Phoenix Park amounted to only 47% of the long term monthly average rainfall for July), but that does not even begin to undo months of deficit. 

Rank
year
Month
PP_SPEI3
1st
2018
7
-2.70
2nd
1995
10
-2.69
3rd
1995
8
-2.60
4th
1938
4
-2.57
5th
1964
2
-2.47
6th
1959
9
-2.46
7th
1943
4
-2.45
8th
1854
4
-2.40
9th
1896
6
-2.34
10th
1933
9
-2.33
Table of the ten most extreme values of SPEI-3 in the Phoenix Park record. Negative values denote deficits. The more negative the index the more severe the drought conditions.

Perhaps more worrying still is the seasonal forecasts for the coming two months which show a real possibility of the meteorological set-up that led to the current drought returning and persisting. Don’t be fooled by a week of recent rains. We are potentially far from done yet.

Current seasonal forecast output from the US National Oceanic and Atmospheric Administration CFSv2 system. Daily deficits of >-0.2mm/day are predicted. Note large uncertainties exist in seasonal prediction systems which should only be used as indicative guidance.


*A reason to say probably rather than definitively
** Another reason to say probably rather than definitively in that monthly summaries have quality control applied by Met Eireann which may lead to a mismatch between the aggregated to monthly daily reports and eventual monthly summary of record.
*** Yet another reason to say probably rather than definitively.

References

McKee TB, Doesken NJ, Kleist J. (1993). The relationship of drought frequency and duration of time scales. In Eighth Conference on Applied Climatology. Am. Meteorol. Soc. January 17–23, 1993, Anaheim CA, 179–186

Noone S, Murphy C, Coll J, Matthews T, Mullan D, Wilby RL, Walsh S. (2015). Homogenization and analysis of an expanded long-term monthly rainfall network for the Island of Ireland (1850–2010). Int. J. Climatol. 36: 2837–2853, doi: 10.1002/joc.4522.

Noone S, Broderick C, Duffy C, Matthews T, Wilby R. and Murphy C. (2017), A 250‐year drought catalogue for the island of Ireland (1765–2015). Int. J. Climatol, 37: 239-254. doi:10.1002/joc.4999


Wednesday, April 18, 2018

Reflections on the Citizens Assembly consideration of climate change

The final report from the deliberations of the Citizens' Assembly on How the State can make Ireland a leader in tackling climate change is now available and delivered to the Oireachtas (and to our non-Irish readers, yes, whenever the political institutions of Ireland are discussed there is indeed a hint of Lord of the Rings). That means I am now allowed, apparently, to discuss this as a member of the Expert Advisory Group (other members were: Anna Davies, Margaret Desmond, John Garry, Aine Ryall, and Diarmund Torney).

The Citizens' Assembly is an exercise in deliberative democracy. There are people far more qualified than I to provide a definitive outline of what that is. But, at its most basic, it is picking randomly from those eligible to vote and bringing them together to hear evidence, discuss and then vote on issues within their charge. The Citizens' Assembly has considered 5 topics of which climate change was the third.

The assembly members were given two weekends to discuss the issues and then vote on a series of resolutions. The role of the Expert Advisory Group was to aid the Chair and secretariat to shape these deliberations. I got to know how to get to Government buildings rather well.

What were the key issues?

Finding the angle: The scope given was very broad and could have been taken in any number of directions. At the same time there were existing national activities around the national mitigation and adaptation planning and several existant bodies.  Ultimately, the citizens wanted to know what would change in their lives were Ireland to become a climate leader. So, the final structure was to spend the first day as a scientific primer and scene setting and then dive into the three major sectors of energy, transport and agriculture. Each sector should show what the status quo is, what 30 years hence may look like with leadership and take examples of how we are currently doing (ideally drawn from Irish exemplars).

Finding the right mix of speakers: Citizens needed to hear from speakers who could speak to the range of issues objectively. This was a challenging and rewarding puzzle to help solve. Particularly important was to hear inspirational examples of actual leadership in the here and now.

Helping find questions that answer the charge: The charge was how the State can make Ireland a leader so questions needed to be cast in the context of legal frameworks, incentives, policy and taxation. Questions needed to be realistic and recognise the need for trade-offs (no questions enabling having one's cake and eating it), and most importantly of all actionable.

The weekends themselves were fascinating. I'd urge people to look at the video presentations to see what was presented to the citizens. The citizens really engaged thoughtfully with the issue and asked challenging questions.

The final voting was very strongly (>80% in all cases) for a range of policy and fiscal measures which, if enacted in full would, indeed, make Ireland a leader in tackling climate change.

What happens next? Well, that, ultimately is up to members of the Oireachtas to decide. The report contains a wealth of information for their consideration.

It was a tremendous honour to serve the Assembly on this topic. It took me somewhat outside my comfort zone quite often but it was a real privelige to see the process in action and work with experts I wouldn't ordinarily interact with. The Chair and secretariat did sterling effort, but most of all the citizens impressed me with how much they engaged.

It is just a pity that this level of civic engagement is so much the exception rather than the norm in our modern democracies. When our governments do not care because we do not care or don't wish to become informed we quite frankly get the governments we deserve. Exercises such as the Citizens' Assembly offer a tantalising glimpse of an alternative.

Wednesday, January 10, 2018

The Atlantic Overturning Circulation and Ireland

 



Palm trees in Sligo


Figure 1:  Palm trees grow in Mullaghmore in Sligo (left), which is as far north as glaciated Grytviken, South Georgia (right) is south, where glaciers grow.[image modified from:Wikipedia]

Sometimes it’s easy to forget how far north Ireland is. Palm trees grow in Sligo on the northwest Atlantic coast whereas glaciers---though in retreat ---overshadow Grytviken, South Georgia (Fig. 1). Grytviken is as far south as Sligo is north, so why the big difference? 


Frequently people will cite the Gulf Stream as the reason for Ireland’s mild climate. This is an idea dating back to Matthew Fontaine Maury who coined the idea that the Gulf Stream was responsible for Europe’s mild climate in 1855. But the Gulf Stream itself is not inherently unique. It is an ocean current, called a ‘Western Boundary Current’ due to its location in the western North Atlantic, analogues of which exist in every major ocean basin. In the North Pacific, the Kuroshio is a western boundary current just as vigorous as the Gulf Stream. But when we look at surface temperatures relative to relative to the average for that latitude (Fig. 2), the Atlantic does stand out as anomalously warm.



Figure 2: Deviations from zonally averaged temperatures based on NCEP/NCAR reanalysis (annual mean) in degrees Celsius.[image: Joel Hirschi, NOC]

Atlantic Exceptionalism


The ocean process that drives the anomalous warmth in the Atlantic is the Atlantic Overturning Circulation. This is a system of currents that carries warm water northwards, through the Gulf Stream and its extended current, the North Atlantic Current. This water is warm and salty. As it loses heat, through a variety of processes, it forms deep, cold water that returns southwards. This exchange of warm and cold water leads to the largest movement of heat by any ocean. This heat is then released from the Atlantic and is carried by the prevailing winds over northwest Europe. This combination of ocean and wind is the reason for the exceptionally mild climate of northwest Europe.

Figure 3: Dublin is around 9C warmer than St Johns in Newfoundland and 4C warmer than the similar maritime climate of Seattle on the Pacific Northwest.[image from McCarthy et al., 2015, Weather]


The combination of winds and ocean is important and leads to a subtlety in discussing this Atlantic Exceptionalism. It might be tempting to look across from the cliffs of Moher in winter to the frigid Labrador coast of Canada and ascribe the almost 10 degrees C difference in temperature to the Atlantic Overturning. However, much of this temperature difference is due to this coast being downwind of the frigid Canadian landmass whereas Ireland is downwind of the ocean. A better comparison to reveal the impact of the Atlantic overturning on climate is to compare Ireland to similar maritime climates of the Pacific northwest. Dublin is 4C warmer than Seattle in winter and this difference can be mainly ascribed to the influence of the Atlantic Overturning Circulation. While not as dramatic a comparison as with the coast of Labrador, a mean difference of 4C is approximately the difference in temperature between the climate of Ireland and the climate of Northern Portugal.



Threats to the Overturning Circulation

The Atlantic Overturning has changed in the past, with large fluctuations in climate in the last glacial period associated with collapsing and recovering of the Overturning circulation, and it is predicted to decline in the future---the Intergovernmental Panel for Climate Change ranking as 'very likely' that the Overturning Circulation will weaken by 30% in the coming century.  If the overturning declines, this will have a serious impact on Irish climate. Evidence for a declining overturning is already mounting. Temperatures globally continue to rise due to anthropogenic global warming but, in the North Atlantic, a cold spot appears where a small temperature fall is occurring. 
 

Here at ICARUS, work is ongoing to understand the ongoing changes in the Atlantic, including understanding the drivers and the impacts that has on Ireland, northwestern Europe and beyond.




Figure 4: Global surface temperatures show a general increase due to anthropogenic climate change. A blue area in the North Atlantic shows where temperatures have not risen and could be indicative of a slowdown in the Atlantic Overturning Circulation.

Leagan Gaeilge

Crainn phailme i Sligeach



Is fusa dearmad a dhéanamh ar cé chomh fada thuaidh is atá Éire. Fásann crainn phailme i Sligeach ar an gcósta thiar ó thuaidh. Ar an leithead céanna, i nGrytviken, sa tSeoirsia Theas (Fig. 1), feictear oighearshruthanna, cé go bhfuil siad ag cúlú de bharr athrú aeráide. Cad í an chúis don difríocht seo?

Go minic, deirtear gurb é an teas a thugann Sruth naMurascaille leis i dtreo na hEorpa cúis na h-aeráide chineálta in Éirinn. Matthew Fontaine Maury a mhol an smaoineamh sin i 1855. Ach ní hé Sruth na Murascaille amháin gur cúis le seo. Is sruth aigéin é Sruth na Murascaile ar a glaotar ‘Sruth Teorann Iartharach’ de bharr a shuíomh in iarthar an Atlantaigh Thuaidh. Agus tá a leithéid de na sruthanna seo i ngach aigéan. San Aigéan Ciúin, is sruth teorann iartharach é an Kuroshio atá chomh láidir le Sruth na Murascaille. Ach, nuair a fhéachaimid ar na teochtaí dromchla i gcomparáid le meán-teocht an leithid (Fig. 2), seasann teochtaí arda aimhrialta an Atlantaigh amach. 

Eisceachtúlachas Atlantaigh


Glaotar Iompu Imsruthu an Aigéin ar an gcóras aigéin a bhogann an teas seo i dtreo iarthuaisceart na hEorpa. Is córas sruthanna aigéin é Iompú Imshruthú na Mara a bhogann uisce te ó thuaidh, i Sruth na Murascaille mar shampla. Tá an t-uisce seo te agus goirt. Agus an t-uisce seo ag cailliúint teasa de bharr próiseas éagsúla, cruthaíonn sé uisce fuar, domhain a fhilleann ó theas. Cruthaíonn an malartú den uisce fuar agus uisce te seo an iompar teasa is mó san aigéan. Scaoiltear an teas seo ón Atlantach agus iompaítear é i dtreo na hEorpa ag na príomhghaotha. ‘Sé meascán an aigéan agus an ghaoth cúis don aeráid chineálta eisceachtúil atá in iarthuaisceart na hEorpa.




Is tábhachtach é an meascán de ghaotha agus aigéan sa chomhrá faoi Eisceachtúlacht Atlantaigh. Ba éasca é féachaint ó Aillte an Mhóthair i rith an gheimhridh go dtí cósta fuaránta Labrador igCeanada  agus an difríocht 10 gcéim Celsius a chur i leith iompú an Atlantaigh. Ach is í cúis an difríocht teochta seo ná go bhfuil Labrador le cóir na gaoithe ó mhórchríoch fuaránta Cheanada, agus tá Éire le cóir na gaoithe ón aigéan. Is fearr an comparáid chun tionchar iompú an Atlantaigh ar an aeráid a léiriú, ná an comparáid idir Éire agus aeráid mhara den chineál céanna in iarthuaisceart an Aigéin Chiúin. Tá Baile Átha Cliatha 4C níos teo ná Seattle sa gheimhreadh agus is de bhuíoch iompú an Atlantaigh an difríocht seo. Cé nach bhfuil an comparáid seo chomh géar leis an gcósta Labrador, is ionann é difríocht 4C idir aeráid na hÉireann agus aeráid Portaingéil thuaidh.

Bagairtí i leith an Sruthú Iompaithe


Tá Iompú an Atlantaigh athruithe, le luaineacht mór san aeráid san oighearthréimhse deireanach ceangailte le titim agus athbheochan an sruthú iompaithe. Táthar ag tuar go dtiocfaidh meath ar an sruthú sa todhchaí – deir an Painéal Idir Rialtais don Athrú Aeráide go bhfuil an-seans go laghdóidh an Sruthú Iompaithe de 30% sa chéad atá amach romhainn. Má thagann an meath seo ar an sruthú, beidh tionchar ollmhór ar aeráid na hÉireann. Tá fianaise de seo ag méadú cheana féin. Tá teochtaí domhanda ag ardú ar bhonn leanúnach de bharr téamh domhanda antrapaigineach ach, san Atlantach Thuaidh, feictear spota fuar áit a bhfuil teocht íseal ag tarlú.



Ag ICARUS, táimid i mbun oibre chun tuiscint a fháil ar athruithe leanúnacha an Atlantaigh, chomh maith le tuiscint a fháil ar an tionchar atá ag sin ar Éirinn, ar iarthuaisceart na hEorpa agus ar áiteanna níos faide ó bhaile.