Meeting with an old friend

Where I was? What was I doing? Perhaps, a cruise in the Arctic? Perhaps, conferences, meetings? No, that was not the case. It was something much more exciting: I was on maternity leave until mid-June, and that is a time to enjoy with your baby, and simply do nothing else. I could tell you thousands of things about it, but for that there are marvelous maternity blogs. After that, I was quite busy starting to work again, but on the top of that, honestly, I didn’t feel inspired to write in the blog. You can be thinking: ‘Really? Do you need any kind of inspiration to write a science blog?’ And, you’re right in some sense. Science is not art, but still. I’m convinced that I do better Science when I feel inspired, or let’s say really motivated.

¿Dónde he estado y qué he estado haciendo? ¿Quizá una campana en el Ártico? ¿Quizá, conferencias, reuniones? Pues no. Ha sido algo mucho mejor que todo eso: He estado de baja maternal hasta mediados de Junio, y no me he planteado hacer nada más que disfrutar del tiempo con mi hijo. Os podría contar miles de cosas sobre ello, pero para eso hay cientos de blogs maravillosos sobre maternidad. Después, he estado muy ocupada incorporándome al trabajo, pero sobretodo, la verdad, es que no me he encontrado con ganas de escribir en el blog. Podéis pensar: ‘¿De verdad? ¿Hace falta inspiración para escribir un blog sobre ciencia?’ Y en parte no os falta razón. La Ciencia no es arte, pero aun así. Estoy convencida de que hago mejor mi trabajo, que al final es sobre lo que escribo, cuando estoy inspirada, o dejémoslo en muy motivada.

What ever happened to Deep Convection in the Greenland Sea?

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The deep Greenland Sea is warming faster than the World Ocean

Ayer, en la página de facebook os dejaba el enlace a la nota de prensa que ha publicado el AWI (por si alguien anda perdido, AWI son las siglas del Alfred Wegener Instititute donde trabajo) en alemán a ráiz del interés despertado en distintos medios tras la publicación del artículo 'Increasing amounts of Arctic Ocean deep waters in the Greenland Sea' en la revista Geophysical Research Letters, del que os he hablado (aquí o aquí) . Se me hace raro publicar una especie de entrevista a mí misma en mi blog, pero ahí la tenéis. Creo que os será bastante fácil de entender, y si no ya sabéis click arriba en dejar un comentario y preguntad lo que queráis.

Yesterday on the facebook page, I left you the link to the press release that AWI (in case someone is lost, AWI is the acronym of the Alfred Wegener Institute where I work) has published in German due to the interest shown by different media after the publication of the article 'Increasing Amounts of Arctic Ocean deep waters in the Greenland Sea' in the scientific journal Geophysical Research Letters. I have previously told about it (here or here). It's kind of strange to post a sort of interview to myself on my blog, but there you have it. I think it will be quite easy to understand, but you know to do in case you have any question: click above 'comentarios' and ask whatever you want.

Las aguas profundas del Mar de Groenlandia se calientan más rápido que el resto del Océano.

Bremerhaven 23 de septiembre 2013. El calentamiento actual de las aguas profundas del Mar de Groenlandia es alrededor de diez veces mayor que el calentamiento medio estimado para el resto del Océano. Los científicos del Instituto Alfred Wegener  -Centro Helmholtz para la Investigación Polar y Marina- han publicado recientemente estos resultados en la revista Geophysical Research Letters. Para su estudio, analizaron los datos de temperatura disponibles desde 1950 hasta 2010 en el Mar de Groenlandia, la cual es una zona oceánica situada justo al sur del Océano Ártico.

The deep Greenland Sea is warming faster than the World Ocean

Bremerhaven 25th September 2013. Recent warming of the Greenland Sea Deep Water is about ten times higher than warming rates estimated for the global ocean. Scientists from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research recently published these findings in the journal Geophysical Research Letters. For their study, they analysed  temperature data from 1950 to 2010 in the abyssal Greenland Sea, which is an ocean area located just to the south of the Arctic Ocean.

Someday...after tomorrow? (in English :-))

Theatrical release poster.Source

I guess that for most of you the title of this post resembles that famous film titled ‘The day after tomorrow’. It’s not a coincidence that I have chosen this titled to talk you about some of the Myths and Legends related with one of the most well-known process affecting our climate. That is the concept of Thermohaline Circulation, maybe also known by some of you as the Meridional Overturning Circulation. In the failure, collapse, halt of that circulation, the argument of this film is based on. I should watch the film again, because that is basically all I remember about it. I don’t write this post to criticize the film; I want to leave that clear.

And without more delay, I start because otherwise writing the post in Spanish and English finally takes me three days (It took me more at the end. Sorry for the delay in the English version ). As a consequence of antropogenic greenhouse emissions, oceanographic models predicted a decrease in the Thermohaline Circulation and Meridional Overturning Circulation circulation (Velinga and Mood, 2002; IPCC, 2007) associated to a reduction of the deep water formation, especially in the North Atlantic. The Achilles' heel of this sequence of events was the stop of the process known as ‘deep convection’, responsible for the deep water formation in the Labrador and Greenland Seas in the North Atlantic.

Although perhaps only known because of the film, I guess these ideas are not completely strange for you. However, in the last decade, some of these ideas have begun to be questioned. Why? One of the most notorious reasons is that in the Greenland Sea, one of the places that are normally associated with this process  (later I will explain why I underline this sentence), there is no deep water formation since three decades ago. Fortunately, the collapse of Thermohaline Circulation predicted by ocean models has not taken place. Whew! 

Then, however, you will probably wonder how that can be. Is there something wrong with the concept of the Thermohaline Circulation and Meridional Overturning Circulation, and that I represent schematically in the figure below? Or in other words, is there something wrong in the sequence: (1) at high latitudes due to very low air temperatures surface waters cool and become denser sinking in the water column and resulting in the formation of deep water; (2) those waters flow to lower latitudes (equatorial) through the ocean bottom; and (4) the deep waters formed at high latitudes are replaced by warmer water flowing at the surface from lower latitudes while (3) the bottom water returns to the ocean surface? Well, yes and no.

 

Figure 2. Summary of processes involved in the Thermohaline Circulation and Meridional Overturning Circulation. (1) Deep water formation at high latitudes (subpolar and polar areas, and for this reason the reference Santa Claus now that we are close to the Christmas time), (2) transport of these colder and denser  waters towards lower latitudes, (3) deep water return to the surface (a process known as upwelling in Oceanography), and (4) transport of warm water at the surface from low latitudes to polar and subpolar regions

We can interpret the previous scheme in two different ways:

Version (1):                            Key Process = deep water formation (1)

- The water flowing at depth (2) (derived from deep water formation (1)) must be in balance with the water flowing at the surface from low-latitudes (4).

Version (2):                Key Process: Deep-water return to the surface (3)

- Since the deep water must return to the surface (3), the balance of water circulating at depth (2) and at surface (4) depends on the deep water return to the surface (3) and not in deep water formation (1). 

In general, transports (how big are the arrows in our scheme) of deep water towards low-latitudes (2) or of warm surface water towards high-latitudes (4) determine the intensity of the Thermohaline Circulation or Meridional Overturning Circulation, respectively. At the end, such intensity controls how much heat is exchanged from equatorial and subtropical to subpolar zones, and vice versa, to regulate our climate.

An important difference between the two interpretations of ours scheme is that the latter depends on a process that occurs throughout the ocean, deep water return to the surface (3), while the former depends on the cooling conditions in a specific place, which result in the formation of deep water (1).

Presented in a simplified way, this is the conclusion that Walter H. Munk and Carl Wunsch got in their work 'Abyssal recipes II: Energetics of tidal and wind mixing' (1998) after a review of the previous work 'Abyssal recipes' of Walter H. Munk (1966): the intensity of the Meridional Overturning Circulation is primarily determined by the power available to return deep water back to the surface layers, and that comes from the wind and tides, and not by cooling conditions responsible for deep water formation at high latitudes. For those of you not scared by an oceanographic research article including formulas, I recommend their reading. I really enjoy them.

Thus, our Thermohaline Circulation and Meridional Overturning Circulation don’t depend on "local processes" that are subject to larger variability, but on larger-scale processes and therefore more stable.

Another ‘little big detail’ among the reasons why the halt of deep water formation in the Greenland Sea has not led to a collapse of the Thermohaline Circulation is that although there is (was) deep water formation in the Greenland Sea, the resulting deep waters are too dense to cross the different ridges that would enable it flowing towards low latitudes (see Figure 3). Returning to our previous scheme (Fig. 2), it means that we can have (1) in the Greenland Sea, but it doesn’t contribute to (2), and so either to (3) or (4), not affecting the Thermohaline Circulation and Meridional Overturning Circulation. For this reason, at the beginning, I emphasized the fact that Greenland Sea is one of the places that are usually associated to this circulation, but that is not an accurate representation of reality. It doesn’t mean that what happens in the Greenland Sea is not important; in fact, this basin plays a crucial role in our climate but not for its contribution to the Thermohaline Circulation.

Map of the North Atlantic where you find the Labrador Sea and the Greenland Sea at its northern limits. As you can see by the color bar at the bottom, the depth of the Labrador Sea is about 4000 m. communicating directly with the remaining deep North Atlantic. However, although the Greenland Sea is about 3600 m. depth, it is surrounded by ridges (orange colors in the surroundings of the Greenland Sea) rising to 1500 m. below the sea surface. This means that if we imagine that these two seas are two glasses of water, the deep water formed in the Labrador Sea can leave the glass through a tap at the bottom, and so deep waters can flow to the rest of the North Atlantic. However, the deep water formed in the Greenland Sea can only leave the basin (the glass) through a tap located just above the ridges surrounding it (there is no other possible exit), and hence the densest waters generated by deep water formation in Greenland Sea can’t flow to the rest of the North Atlantic.

Altogether explains why even when there is no deep water formation in some areas of the North Atlantic as the Greenland Sea since the mid-80s, the Thermohaline Circulation hasn’t stopped and we haven’t suffered a cataclysm.

I hope you understood something. If not, don’t hesitate to ask me please

References:

Vellinga, M., Wood, R.A., 2002. Global climatic impacts of a collapse of the Atlantic thermohaline circulation. Climatic Change 54, 251–267.

IPCC,2007. Summary for policy makers. In:Solomon,S., Qin,D., Manning,M., Chen, Z., Marquis,M., Averyt,K.B., Tignor,M., Miller,H.L.(Eds.), Climate Change 2007: The Physical Science Basis, Contribution of Working Group to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, NewYork.

W. H. Munk and C. Wunsch, 1998. Abyssal recipes II: Energetics of tidal and wind mixing. Deep Sea Research Part I, Vol. 45,12,Pages 1977–2010

W. H. Munk, 1966. Abyssal recipes. Deep Sea Research Part I, Vol. 13, 4, Pages 707–730

Part of what I have told here is included in the article that I saw you the other day on Facebook but not published yet. Thus, if anyone wants to use this information I would be grateful if you get in contact with me to cite it properly.