{"id":159,"date":"2005-05-26T03:21:54","date_gmt":"2005-05-26T07:21:54","guid":{"rendered":"\/?p=159"},"modified":"2007-08-23T12:05:56","modified_gmt":"2007-08-23T17:05:56","slug":"gulf-stream-slowdown","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2005\/05\/gulf-stream-slowdown\/","title":{"rendered":"Gulf Stream slowdown? Est-ce que le Gulf Stream ralentit ?<\/lang_fr>"},"content":{"rendered":"
\n

There has been an overwhelming popular demand for us to weigh in on recent reports in the Times Britain faces big chill as ocean current slows<\/a> and CNN Changes in Gulf Stream could chill Europe<\/a> (note the interesting shift in geographical perspective!).
\npar Gavin Schmidt (traduit par Pierre Allemand)<\/small><\/p>\n

Nous avons \u00e9t\u00e9 submerg\u00e9s de demandes d\u2019\u00e9valuation des rapports r\u00e9cents du Times
\n“Britain faces big chill as ocean current slows<\/a>” (la Grande Bretagne soumise \u00e0 un refroidissement important en raison du ralentissement de courants oc\u00e9aniques<\/em>) et de CNN “Changes in Gulf Stream could chill Europe<\/a>” Les changement du Gulf Stream pourrait refroidir l\u2019Europe<\/em>), (notez l\u2019int\u00e9ressant glissement de perspective g\u00e9ographique !).<\/p>\n

(suite…)<\/a>
\n<\/lang_fr>
\nAt the heart of the story was a statement at the recent EGU meeting by Peter Wadhams<\/a> from Cambridge University, that convection in a normally active area of the Greenland Sea was much reduced last winter. Specifically, in an area where a dozen or so convective ‘chimneys’ form, only two small chimneys were seen. (Unfortunately, I can’t seem to be able to find a relevant abstract of Dr. Wadhams talk, and so I have to rely on the Times’ news reports for the specifics).<\/p>\n

<\/a>
\nConvective chimneys in the seas bounded by Greenland, Iceland and Norway occur when intense cooling of the ocean, usually associated with a low-pressure system passing through, breaks down the normally stable ocean layers and causes the now colder, denser water to convect and mix down to a relatively deep layer. This area of the world is one of only a few places where the underlying ocean column is marginally stable enough that this process can occur in the open ocean and lead to convective chimneys going down 2000 to 3000 meters. The deep water masses formed in this way are then exported out of the area in deep currents that eventually make up “North Atlantic Deep Water” (which also contains contributions from the Labrador Sea and entrainment of other water masses). This process is part of what is called the ‘thermohaline’ or ‘overturning’ circulation and is associated with a significant amount of heat transport into the North Atlantic, which indeed keeps Britain and the rest of the North Atlantic region 3 to 6 degrees C warmer than they otherwise would be. The figure gives two model estimates for the impact of this circulation (Stocker, 2002<\/a>).<\/p>\n

This heat transport is often associated with the Gulf Stream in the media and among the public. However, my pedantic side obliges me to point out that the Gulf Stream is a predominantly wind-driven western boundary current that moves up from the Gulf of Mexico along the US coast to Cape Hatteras, at which point it heads off into the central Atlantic (see also this letter<\/a> by Carl Wunsch). It then turns into the North Atlantic Drift which is really the flow of water responsible for the anomalous northward heat transport in the Atlantic. There is good evidence from past climates, theoretical studies and climate models that large changes, a slowing down or even a complete collapse, in the North Atlantic Drift and the thermohaline circulation can happen. Indeed climate models generally (though not exclusively) forecast a slowdown in this circulation by 2100. This occurs mainly as a function of increased rainfall in the region which strengthens the ocean layering and reduces the amount of convection in the region. It is probably futile to insist on it at this point, but a collapse of the overturning circulation is not the same as a collapse or reversal of the Gulf Stream (which as I mentioned above is predominantly wind-driven).<\/p>\n

Getting back to the statement by Peter Wadhams though, how does this relatively small-scale observation get translated into headlines forecasting changes in the Gulf Stream and chilly times ahead for Europe? The major problem is that the background story and the climate model results are now very well known, and any scientific result that appears to project onto this storyline therefore gets a lot of attention.<\/p>\n

However, it is a long way from the Greenland Sea to the Gulf Stream and some important points did not get a mention in the news stories. Firstly, we know that there is a great deal of decadal variability in how much and where deep convection takes place. Indeed, it was reported by Schlosser et al (1991), that based on CFC measurements, very little convection had occured in the Greenland Sea over the previous 7 years. Subsequently, convection was renewed. Similarly, convection in the Labrador Sea (the other main component) has also oscillated, possibly out of phase with the convection in Greenland. Studies by Dickson et al (1999, 2002<\/a>) showed that properties of the deep water overflowing the Denmark Strait (between the Greenland Sea and rest of the Atlantic) appear to be related to patterns of variability like the North Atlantic Osillation<\/a>, and this may help explain some of the variabilty.<\/p>\n

To be sure, there are some long term trends that are becoming discernable. There is a freshening of the North Atlantic visible since the 1950s. Long continuous records of temperature and salinity at Ocean Weather Station M<\/a> in the Norwegian Sea indicate that the deep water has also warmed noticeably. However, monitoring networks are now starting to be put in place (Osterhus et al, 2005<\/a>) and better integrated data will be available in the future. It is important to bear in mind that while the changes being seen are indeed significant given the accuracy of modern oceanography, the magnitude of the changes (a few hundredths of a salinity unit) are very much smaller (maybe two orders of magnitude) than the kinds of changes inferred from the paleo data or seen in climate models. Thus while continued monitoring of this key climatic area is clearly warranted, the imminent chilling of the Europe is a ways off yet. <\/p>\n


\nL\u2019origine de l\u2019histoire, est une affirmation, au cours d\u2019une r\u00e9union de l\u2019EGU<\/a> de Peter Wadhams<\/a> de l\u2019Universit\u00e9 de Cambridge, selon laquelle la convection, dans une zone normalement active de la Mer du Groenland avait \u00e9t\u00e9 tr\u00e8s r\u00e9duite pendant l\u2019hiver dernier. Plus pr\u00e9cis\u00e9ment, dans une des zones o\u00f9 une douzaine de \u2018chemin\u00e9es\u2019 convectives se forment, on avait observ\u00e9 seulement deux petites chemin\u00e9es. (Malheureusement, je n\u2019ai pas \u00e9t\u00e9 capable de trouver un r\u00e9sum\u00e9 des propos du Docteur Wadhams digne de confiance, et j\u2019ai d\u00fb m\u2019en remettre \u00e0 l\u2019article du Times pour les d\u00e9tails). <\/p>\n

<\/a>
\nLes chemin\u00e9es convectives des mers bord\u00e9es par le Groenland, l\u2019Islande et la Norv\u00e8ge se mettent en route lorsque le refroidissement intense de l\u2019oc\u00e9an, normalement associ\u00e9 \u00e0 un passage de basses pressions, d\u00e9truit la stabilit\u00e9 des couches normalement stables de l\u2019oc\u00e9an, et provoque la convection et le m\u00e9lange de l\u2019eau refroidie et rendue plus dense pour former une couche relativement plus profonde. Cette r\u00e9gion du monde est un des seuls rares endroits o\u00f9 la colonne inf\u00e9rieure des couches oc\u00e9aniques est assez stable pour que ce processus puisse se produire au large, et conduise \u00e0 des chemin\u00e9es convectives descendant jusqu\u2019\u00e0 2 \u00e0 3000 m\u00e8tres.
\nLes masses d\u2019eau profonde issues de ce ph\u00e9nom\u00e8ne sont ensuite \u00e9vacu\u00e9es de cette r\u00e9gion, pour former \u00e9ventuellement les courants profonds des “North Atlantic Deep Water” (eaux profondes nord atlantique<\/em>) (auxquels participent aussi des eaux provenant de la Mer du Labrador, et d\u2019autres masses d\u2019eaux). Ce processus constitue une partie de ce qu\u2019on appelle la ‘circulation thermohaline\u2019<\/a>. Il est associ\u00e9 aux quantit\u00e9s de chaleur relativement importantes transport\u00e9es dans l\u2019Atlantique nord, qui maintiennent la Grande Bretagne et le reste de l\u2019Atlantique nord 3 \u00e0 6 \u00b0C plus chauds que ce qu\u2019ils devraient normalement \u00eatre. La figure illustre les chiffres estim\u00e9s \u00e0 partir de deux mod\u00e8les pour l\u2019impact de cette circulation. (Stocker, 2002<\/a>). <\/p>\n

Ce transport de chaleur est souvent associ\u00e9 au Gulf Stream dans les m\u00e9dias et le grand public. Cependant, mon cot\u00e9 p\u00e9dant m\u2019oblige \u00e0 pr\u00e9ciser que le Gulf Stream est essentiellement un courant p\u00e9riph\u00e9rique provoqu\u00e9 par les vents d\u2019ouest qui remontent depuis le Golfe du Mexique le long des c\u00f4tes am\u00e9ricaines vers le Cap Hatteras d\u2019o\u00f9 il bifurque vers l\u2019Atlantique central (voir aussi cette lettre de Carl Wunsch). Il devient ensuite le Courant Nord Atlantique qui est, de fait le courant responsable du d\u00e9placement de chaleur anormal vers le nord de l\u2019Atlantique. Il y a de fortes chances, en se fondant sur des \u00e9tudes th\u00e9oriques et des mod\u00e8les climatiques des climats pass\u00e9s, pour que d\u2019importants changements, un ralentissement ou m\u00eame un arr\u00eat complet puissent se produire dans le Courant Nord Atlantique et la circulation thermosaline. Les mod\u00e8les climatiques pr\u00e9voient en g\u00e9n\u00e9ral ind\u00e9niablement (mais pas exclusivement) un ralentissement de cette circulation vers 2100. Ce ph\u00e9nom\u00e8ne est d\u00fb \u00e0 l\u2019accroissement des pr\u00e9cipitations dans la zone, ce qui renforce la formation de couches oc\u00e9aniques et r\u00e9duit le mouvement de convection. Il est probablement futile d\u2019insister sur ce point, mais un arr\u00eat de la circulation du grand vecteur oc\u00e9anique n\u2019est pas la m\u00eame chose qu\u2019un arr\u00eat ou un renversement du sens de circulation du Gulf Stream (qui, comme je l\u2019ai mentionn\u00e9 plus haut, est essentiellement gouvern\u00e9 par les vents). <\/p>\n

Pour en revenir aux affirmations de Peter Wadhams, comment ces observations \u00e0 petite \u00e9chelle peuvent \u00eatre traduites en titres de presse concernant des pr\u00e9visions de changement du Gulf Stream et un rafra\u00eechissement \u00e0 venir pour l\u2019Europe ? Le probl\u00e8me majeur est que le sujet et les r\u00e9sultats du mod\u00e8le climatique sont maintenant bien connus, si bien que n\u2019importe quel r\u00e9sultat scientifique concernant ces pr\u00e9visions retient \u00e9norm\u00e9ment d\u2019attention.<\/p>\n

N\u00e9anmoins, il y a beaucoup de chemin entre la Mer du Groenland et le Gulf Stream, et un certain nombre de points importants n\u2019apparaissent pas dans ces nouveaux sc\u00e9narios. Tout d\u2019abord, nous savons que la variabilit\u00e9 d\u00e9cennale concernant l\u2019amplitude, la profondeur de la convection et sa profondeur est importante. En fait, il a \u00e9t\u00e9 rapport\u00e9 par Schlosser et al. (1991) que des mesures concernant les CFC avaient montr\u00e9 une convection tr\u00e8s faible dans la Mer du Groenland au cours des 7 ann\u00e9es \u00e9coul\u00e9es. Par la suite, la convection a repris. De fa\u00e7on similaire, la convection dans la Mer du Labrador (l\u2019autre \u00e9l\u00e9ment majeur) a aussi oscill\u00e9, peut-\u00eatre sans relation de phase avec la convection du Groenland. Les \u00e9tudes de Dickson et al. (1999, 2002<\/a>) ont montr\u00e9 que les propri\u00e9t\u00e9s des eaux profondes circulant dans le D\u00e9troit du Danemark (entre la Mer du Groenland et le reste de l\u2019Atlantique) \u00e9taient reli\u00e9es \u00e0 des ensembles de variabilit\u00e9 comme l\u2019Oscillation Nord Atlantique<\/a> et que cela pouvait expliquer une partie de la variabilit\u00e9.<\/p>\n

Il est ind\u00e9niable que des tendances \u00e0 long terme deviennent discernables. Un refroidissement de l\u2019Atlantique nord est visible depuis les ann\u00e9es 1950. De longues mesures continues de la temp\u00e9rature et de la salinit\u00e9 \u00e0 ‘Ocean Weather Station M’ (NdT : Station m\u00e9t\u00e9o oc\u00e9anique M) en Mer de Norv\u00e8ge indiquent que les eaux profondes se sont notablement r\u00e9chauff\u00e9es. De plus, des r\u00e9seaux de mesures commencent \u00e0 se mettre en place (Osterhus et al, 2005) et des mesures mieux int\u00e9gr\u00e9es entre elles seront disponibles dans le futur. Il est important de garder \u00e0 l\u2019esprit que si les changements observ\u00e9s sont, \u00e9videmment, significatifs compte tenu de la pr\u00e9cision de l\u2019oc\u00e9anographie moderne, l\u2019amplitude des changements (quelques centi\u00e8mes d\u2019unit\u00e9 de salinit\u00e9) est beaucoup plus petite (peut-\u00eatre deux ordres de grandeur) que celle rapport\u00e9e par les donn\u00e9es pal\u00e9oc\u00e9anographiques ou observ\u00e9e dans les mod\u00e8les climatiques. Ainsi, bien que la surveillance continue de cette zone climatique soit clairement justifi\u00e9e, l\u2019imminence d\u2019un refroidissement en Europe est encore bien lointaine. <\/p>\n

<\/lang_fr><\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

There has been an overwhelming popular demand for us to weigh in on recent reports in the Times Britain faces big chill as ocean current slows and CNN Changes in Gulf Stream could chill Europe (note the interesting shift in geographical perspective!). par Gavin Schmidt (traduit par Pierre Allemand) Nous avons \u00e9t\u00e9 submerg\u00e9s de demandes […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_exactmetrics_skip_tracking":false,"_exactmetrics_sitenote_active":false,"_exactmetrics_sitenote_note":"","_exactmetrics_sitenote_category":0,"_genesis_hide_title":false,"_genesis_hide_breadcrumbs":false,"_genesis_hide_singular_image":false,"_genesis_hide_footer_widgets":false,"_genesis_custom_body_class":"","_genesis_custom_post_class":"","_genesis_layout":"","footnotes":""},"categories":[1,13,19,2],"tags":[],"class_list":{"0":"post-159","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-climate-science","7":"category-faq","8":"category-oceans","9":"category-paleoclimate","10":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/159","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/comments?post=159"}],"version-history":[{"count":0,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/159\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=159"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=159"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=159"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}