{"id":225,"date":"2005-11-30T17:43:08","date_gmt":"2005-11-30T21:43:08","guid":{"rendered":"\/?p=225"},"modified":"2007-08-16T10:17:46","modified_gmt":"2007-08-16T15:17:46","slug":"decrease-in-atlantic-circulation","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2005\/11\/decrease-in-atlantic-circulation\/","title":{"rendered":"Decrease in Atlantic circulation? Ralentissement de la circulation Atlantique?<\/lang_fr>"},"content":{"rendered":"
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by Gavin Schmidt and Michael Mann<\/small><\/p>\n

In a sure-to-be widely publicized paper in the Dec. 1 Nature<\/i>, Bryden et al.<\/a> present results from oceanographic cruises at 25\u00b0N across the Atlantic showing a ~30% decline in the ocean overturning circulation. These cruises have been repeated every few years since 1957, and the last two cruises (in 1998 and 2004) show notable changes in the structure of the deep return circulation. In particular, the very deepest part of the return flow (at around 3000 to 5000 m) has reduced and moved up in the water column compared to previous decades. How solid is this result and what might it imply for climate?par Gavin Schmidt et Michael Mann (traduit de l\u2019anglais par T. de Garidel)<\/small><\/p>\n

Dans un article largement comment\u00e9 dans la presse (voir par exemple ici<\/a> et la<\/a>) dans le num\u00e9ro du 1er d\u00e9c. de Nature<\/i>, Bryden et al. <\/a> pr\u00e9sentent des r\u00e9sultats de croisi\u00e8res oc\u00e9anographiques \u00e0 25\u00b0N \u00e0 travers l’Oc\u00e9an Atlantique qui montrent un d\u00e9clin d\u2019environ 30% de la circulation oc\u00e9anique \u201cg\u00e9n\u00e9rale\u201d\u2013dite circulation thermohaline-. Ces croisi\u00e8res ont \u00e9t\u00e9 r\u00e9p\u00e9t\u00e9es r\u00e9guli\u00e8rement depuis 1957, et les deux derni\u00e8res croisi\u00e8res (en 1998 et 2004) montrent des changements notables de la structure de la circulation de retour profonde. En particulier, le flux dans la partie la plus profonde du courant de retour (entre environ 3000 et 5000 m) a diminu\u00e9 et est remont\u00e9 dans la colonne de l’eau par rapport aux d\u00e9cennies pr\u00e9c\u00e9dentes. Quelle est la robustesse de ces r\u00e9sultats et quelles en sont les implications potentielles pour le climat ?
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The first question that is asked is usually how these calculations are done. Due to the predominantly “geostrophic” nature of the ocean circulation (i.e. velocity is generally horizontally perpendicular to pressure gradients because of the Coriolis effect), you can calculate changes in North-South velocities by only considering the East-West changes in temperature and salinity. So given a section across the ocean (say 25\u00b0N), oceanographers can estimate the transport across that section. The error in these numbers is a little hard to know, but Bryden et al estimate around +\/- 6 Sv (1 Sv is 106<\/sup> m3<\/sup>\/s, the Amazon output is around 0.1 Sv for perspective). <\/p>\n

What did Bryden et al find? Their calculations indicate that the Gulf Stream itself has been remarkably stable over the almost 40 years, and this accords with other measurements of the Gulf Stream flow itself. Since what goes north must eventually go south (after taking into account the very small amounts of atmospheric transport and the amount of flow through the Bering Strait), all of the other changes will balance. They show that the amount of deep return flow seems to have gone down about 8 Sv (out of 25 Sv), and the amount of mid-ocean to surface transport has gone up by about the same amount. This corresponds to a roughly 30% apparent weakening in the so-called “Thermohaline Circulation” (see our previous discussion here<\/a>). Since the surface flow is warmer than the deep flow, there is a consequent decrease in the northward heat flux of about 0.2 PW (or about 15%). <\/p>\n

It should be stressed that should this be a sustained feature (and not affected by the +\/- 6 Sv uncertainty estimated in the paper), this would be extremely significant. Modelling experiments suggest that this kind of decrease should be associated with a decrease in ocean temperatures in the North Atlantic of up to 2\u00b0C or so, and maybe 0.5\u00b0 over Europe. Since these changes have not been observed (both the North Atlantic and Europe have warmed significantly over this time period), it might be premature to assert that the circulation definitely has changed. Alternatively, the models may not entirely be capturing the fairly complex oceanic processes involved. Continuous monitoring of this section has already been funded through the UK RAPID program and should provide much better data in the future, and a potential solution to this and other remaining puzzles.<\/p>\n

It will take some time to integrate the findings of this study with other evidence of changes in North Atlantic ocean circulation, including the changes seen in salinity<\/a>, changes in the so-called Atlantic Multidecadal Oscillation (AMO)<\/a> (see e.g. Knight et al, 2005<\/a> and references therein) and other indicators of Atlantic climate change (e.g. Dickson et al, 2002<\/a>). Right now, there isn’t an obvious synthesis of what these disparate studies are telling us.<\/p>\n

While this is quite a serious issue, there are a few amusing points. Firstly, this study does present some awkward reading for some who hold that natural cyclical changes in the thermohaline circulation (rather than, say, anthropogenic influences), are responsible for the anomalous increase in Atlantic Hurricane activity<\/a> in recent decades. Hurricane prognosticator William Gray (whose public statements we have commented on previously<\/a>), has, in his recent senate testimony<\/a>, confidently asserted that a putative increase<\/em> in the intensity of the Atlantic Thermohaline circulation over recent decades was entirely responsible for this increase:<\/p>\n

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\nThe Atlantic has large multi-decadal variations in major (category 3-4-5) hurricane activity. These variations are observed to result from multi-decadal variations in the North Atlantic Thermohaline Circulation (THC) – Fig. 4. When the THC is strong, it causes the North Atlantic to have warm or positive Sea Surface Temperature Anomalies (SSTA) and when the THC is weak, cold SSTAs prevail. Figure 5 shows these North Atlantic SSTAs over the last century with a projection for the next 15 years.\n<\/p>\n<\/blockquote>\n

By Gray’s very clearly articulated reasoning, there should have been a downturn<\/em>, not the observed upturn<\/em> in major Atlantic hurricane activity over the past several decades (in the absence of other—including anthropogenic–influences on tropical Atlantic climate) if Bryden et al.’s results are correct. It will be interesting to see if Gray, and others, will change their line of argument in the face of this new study. Today, the last day of the official 2005 Atlantic Hurricane season, might be a fitting opportunity for them to do so.<\/p>\n

Secondly, since the Gulf Stream itself is remarkably stable in these analyses, headline writers may have to adjust the standard ‘Gulf Stream may reverse’ titles that they normally come up with when dealing with this topic (though we note that The Independent<\/a> succumbed anyway) . See here<\/a> for a previous discussion. <\/p>\n

And finally, for those of you who frequent some of the more contrarian websites, JunkScience.com<\/a> tried to pull a bit of a fast one in predicting ‘imminent’ headlines on this issue on Tuesday (Nov 29th). This was before the official release of the paper, but after<\/i> the embargoed copies of this paper had been sent out, which as journalists they would have seen, but their readers had not. You didn’t need to be Nostradamus to predict the headlines here! They were trying to suggest that ‘alarmists’ will automatically blame the ocean circulation for the current wintry weather in Europe. Since the Bryden paper is talking about a multi-decadal trend, this week’s weather is obviously not relevant… <\/p>\n

References:<\/b>
\nHarry L. Bryden, Hannah R. Longworth and Stuart A. Cunningham, Slowing of the Atlantic meridional overturning circulation at 25\u00b0 N, Nature<\/i>, 438<\/b>, 655-657. 2005
\nDickson et al. Rapid freshening of the deep North Atlantic Ocean over the past four decades, Nature<\/i>, 416<\/b>, 832-836. 2002
\nKnight, J.R., Allan, R.J., Folland, C.K., Vellinga, M., Mann, M.E., A signature of persistent natural thermohaline circulation cycles in observed climate, Geophysical Research Letters, 32, L20708, doi:10.1029\/2005GL024233, 2005.
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\nLa premi\u00e8re question qui est habituellement pos\u00e9e est comment ces calculs sont-ils faits. En raison de la nature principalement “g\u00e9ostrophique” de la circulation oc\u00e9anique (c.-\u00e0-d. la vitesse est g\u00e9n\u00e9ralement perpendiculaire \u2013horizontalement- aux gradients de pression en raison de l’effet de Coriolis), il est possible de calculer les changements de vitesses Nord-Sud en se basant uniquement sur les variations Est-Ouest de temp\u00e9rature et de salinit\u00e9. Ainsi en consid\u00e9rant une section longitudinale \u00e0 travers l’oc\u00e9an (25\u00b0N par exemple), les oc\u00e9anographes peuvent estimer le transport \u00e0 travers cette section. L’erreur d\u2019estimation est un chiffre assez difficile a estimer, mais Bryden et al. l\u2019estiment a environ +\/- 6 Sv (1 Sv correspon a 106<\/sup> m3<\/sup>\/s, le d\u00e9bit de l\u2019’Amazone \u00e9tant de 0.1 Sv pour avoir un ordre de grandeur).<\/p>\n

Qu\u2019est-ce que Bryden et al. ont trouv\u00e9 dans ce travail ? Leurs calculs indiquent que le Gulf Stream lui-m\u00eame a \u00e9t\u00e9 remarquablement stable au cours des 40 dernieres ann\u00e9es, et ceci est en accord avec d’autres mesures du flux du Gulf Stream. Comme ce qui va au nord doit a la fin repartir au sud (apr\u00e8s avoir tenu compte d’une petite quantit\u00e9 d\u2019\u00e9changes avec l\u2019atmosphere et du courant oc\u00e9anique passant le d\u00e9troit de Bering), tous les autres changements s\u2019\u00e9quilibrent. Ces auteurs prouvent que le flux de retour en profondeur semble avoir diminu\u00e9 d\u2019environ 8 Sv (sur 25 Sv), alors que le flux a mi-profondeur a augment\u00e9 d\u2019environ de la meme quantit\u00e9. Ceci correspond \u00e0 une baisse apparente d\u2019approximativement 30% de la “circulation thermohaline” (voir notre pr\u00e9c\u00e9dente discussion ici<\/a>). Puisque le flux de surface est plus chaud que celui profond, il y a une diminution cons\u00e9quente du flux de chaleur vers les plus hautes latitudes d’environ 0.2 PW (ou environ 15%).<\/p>\n

On devrait noter que si ce bilan \u00e9tait maintenu durablement (et non affect\u00e9e par +\/-6 Sv, l’incertitude estim\u00e9e dans cet article), ce serait extr\u00eamement significatif. Des exp\u00e9riences de mod\u00e9lisation sugg\u00e9rent que ce genre de diminution devrait \u00eatre associ\u00e9 \u00e0 une diminution des temp\u00e9ratures oc\u00e9aniques dans l\u2019 Oc\u00e9an Atlantique Nord atteignant jusqu’\u00e0 2\u00b0C environ, et peut-\u00eatre 0.5\u00b0 en Europe. Puisque ces changements n\u2019ont pas \u00e9t\u00e9 observ\u00e9 (l’Oc\u00e9an Nord -Atlantique et l’Europe se sont r\u00e9chauff\u00e9s significativement au cours de cette meme p\u00e9riode de temps), il est peut-\u00eatre pr\u00e9matur\u00e9 d\u2019affirmer que la circulation a certainement chang\u00e9. Alternativement, les mod\u00e8les peuvent ne pas capturer enti\u00e8rement les processus oc\u00e9aniques assez complexes impliqu\u00e9s. L\u2019\u00e9tude continue de cette section oc\u00e9anique a \u00e9t\u00e9 d\u00e9j\u00e0 financ\u00e9e par le programme UK RAPID et devrait fournir des donn\u00e9es bien meilleures \u00e0 l’avenir, et une solution potentielle \u00e0 cette question et \u00e0 d’autres questions restants en suspens.<\/p>\n

Cela prendra du temps d’int\u00e9grer ces r\u00e9sultats avec d’autres r\u00e9sultats de changements de la circulation de l’Oc\u00e9an Nord-Atlantique, y compris les changements d\u00e9crits de la salinit\u00e9<\/a>, des changements de l\u2019Oscillation Atlantique Multid\u00e9cennalle (AMO) <\/a> (voir par exemple. Knight et al., 2005<\/a> et r\u00e9f\u00e9rences incluses) et d’autres indicateurs du changement du climat Atlantique (par exemple Dickson et al., 2002<\/a>). A ce jour, il n’existe pas de synth\u00e8se \u00e9vidente et coh\u00e9rente des indications apport\u00e9es par ces diff\u00e9rentes \u00e9tudes.<\/p>\n

Alors que c’est un probleme s\u00e9rieux, il y a quelques points amusants. Premi\u00e8rement, cette \u00e9tude pr\u00e9sente des points malcommodes pour ceux qui soutiennent que les changements cycliques normaux de la circulation thermohaline (plut\u00f4t que par l\u2019 influence anthropique) sont responsables de l’augmentation anormale de l’activit\u00e9 des ouragans dans l\u2019Atlantique<\/a> au cours des dernieres d\u00e9cennies. William Gray, le pronostiqueur d’ouragan (dont nous avons comment\u00e9s ses rapports publics on pr\u00e9c\u00e9demment<\/a>), a, dans son audition r\u00e9cente au s\u00e9nat Am\u00e9ricain<\/a>, affirm\u00e9 avec confiance qu’une augmentation putative de la circulation Thermohaline Atlantique au cours des d\u00e9cennies r\u00e9centes \u00e9tait enti\u00e8rement responsable de cette augmentation :<\/p>\n

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\nThe Atlantic has large multi-decadal variations in major (category 3-4-5) hurricane activity. These variations are observed to result from multi-decadal variations in the North Atlantic Thermohaline Circulation (THC) – Fig. 4. When the THC is strong, it causes the North Atlantic to have warm or positive Sea Surface Temperature Anomalies (SSTA) and when the THC is weak, cold SSTAs prevail. Figure 5 shows these North Atlantic SSTAs over the last century with a projection for the next 15 years.\n<\/p>\n<\/blockquote>\n

En suivant le raisonnement tr\u00e8s clair de Gray, il devrait y avoir eu une diminution, et non pas la reprise observ\u00e9e dans l’activit\u00e9 des ouragans majeurs Atlantique au cours des dernieres d\u00e9cennies (en l’absence de tout autre -y compris anthropog\u00e8ne — influence sur le climat atlantique tropical) si les r\u00e9sultats de Bryden et al. sont corrects. Il sera int\u00e9ressant de voir si Gray, et d’autres, changeront leur argumentation face \u00e0 cette nouvelle \u00e9tude. Aujourd’hui, au dernier jour de la saison officielle des ouragans de l’Oc\u00e9an atlantique 2005, pourrait \u00eatre l\u2019occasion qu’ils en conviennent.<\/p>\n

Deuxi\u00e8mement, puisque le Gulf Stream lui-m\u00eame est remarquablement stable dans ces analyses, les auteurs d\u2019articles de journaux vont sans doute devoir ajuster leur titre standard \u2018un renversement du Gulf Stream’ au regard de cette nouvelle \u00e9tude. Voir ici pour une pr\u00e9c\u00e9dente discussion<\/a> de ce sujet.<\/p>\n

References:<\/b>
\nHarry L. Bryden, Hannah R. Longworth and Stuart A. Cunningham, Slowing of the Atlantic meridional overturning circulation at 25\u00b0 N, Nature<\/i>, 438<\/b>, 655-657. 2005
\nDickson et al. Rapid freshening of the deep North Atlantic Ocean over the past four decades, Nature<\/i>, 416<\/b>, 832-836. 2002
\nKnight, J.R., Allan, R.J., Folland, C.K., Vellinga, M., Mann, M.E., A signature of persistent natural thermohaline circulation cycles in observed climate, Geophysical Research Letters, 32, L20708, doi:10.1029\/2005GL024233, 2005.
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\n<\/lang_fr><\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

by Gavin Schmidt and Michael Mann In a sure-to-be widely publicized paper in the Dec. 1 Nature, Bryden et al. present results from oceanographic cruises at 25\u00b0N across the Atlantic showing a ~30% decline in the ocean overturning circulation. These cruises have been repeated every few years since 1957, and the last two cruises (in […]<\/p>\n","protected":false},"author":12,"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":[1,13,21,19],"tags":[],"class_list":{"0":"post-225","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-climate-science","7":"category-faq","8":"category-hurricanes","9":"category-oceans","10":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/225","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\/12"}],"replies":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/comments?post=225"}],"version-history":[{"count":0,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/225\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=225"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=225"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=225"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}