Wednesday, September 18, 2013

New paper finds current climate models are 'unable to reproduce present or future climate accurately'

More problems for the models: An article published today in Nature says "a new player on the scene" called "internal lee waves" are "a player in ocean dynamics that may make an important contribution to deep-ocean mixing...Climate models that do not represent this mixing appropriately will be unable to reproduce present or future climate accurately." According to the article, current IPCC climate models do not include this deep-ocean mixing from lee waves, which "warrant serious consideration for inclusion in the next generation of climate models."


Internal lee waves are a player in ocean dynamics that may make an important contribution to deep-ocean mixing. They warrant serious consideration for inclusion in the next generation of climate models. 

Turbulent mixing in the ocean interior plays a crucial part in driving downward transport of heat and dissolved  greenhouse gases and upward transport of  biologically essential nutrients. It also ultimately supplies the deep ocean with the  energy that drives a global network of currents known as the meridional overturning  circulation. Climate models that do not represent this mixing appropriately will be unable  to reproduce present or future climate accurately. 

Many previous studies have focused  on the breaking internal waves that are driven  by tides and winds as the dominant source  of that turbulence. In a paper published in  Geophysical Research Letters, Nikurashin  and Ferrari2 describe a previously under-  appreciated dynamic mechanism — internal  lee waves — that may significantly contribute  to mixing in the deep ocean. Away from the direct influence of surface  forcing, most turbulent mixing in the ocean  interior is driven by breaking internal gravity  waves3. These propagate along and across den- sity interfaces within the ocean, similar to the  interfacial waves you might see between oil and  vinegar in a glass or even between coffee and  milk in a well-made cappuccino. Compared  with the more familiar surface waves, internal  waves are much slower, with periods of hours  instead of seconds. 

Their breaking is in many  ways analogous to that of surface waves on the  beach, albeit in a slow-motion, larger-than-life  way — the wave height may reach tens or even  hundreds of metres3. But unlike waves at the  beach (which lose all of their energy to heat  or sound with each thunderous crash), some  of the energy lost by breaking internal waves  increases the potential energy of the ocean by  mixing stratified water and raising its centre  of mass. It is this potential energy that is even- tually converted into the kinetic energy of the  meridional overturning circulation. Over the past decade, most of the focus of  oceanographers has been on the geography  and life cycle of internal waves created by  winds and tides4. 

In the deep ocean, inter- nal waves with tidal frequencies have been  assumed to be a dominant mechanism for  turbulent mixing. These internal tides are  produced when the surface tide, generated by  the Sun and Moon, forces dense water up and  over sea-floor topography. This happens in the  same way that tides pull and push water up and  down the beach once or twice a day. As water  goes back and forth, up and over, it perturbs  the normally flat interfaces between density  layers known as isopycnals, and creates inter- nal waves along those surfaces. Some of that  energy dissipates locally, producing a pattern  of enhanced turbulence over rough topogra- phy (Fig. 1). Recent analysis5 of climate models  has shown that modelled ocean circulation and  heat content are sensitive not only to the aver- age level of turbulent mixing in the deep ocean,  but also to its detailed geography. 

In their study, Nikurashin and Ferrari dis- cuss a related but relatively new player on the  scene — internal lee waves. These are internal waves that are produced by comparatively  steady flow over sea-floor topography. As  with internal tides, dense water is forced up  and over topographic obstacles and thus per- turbs density surfaces. The idea is similar to  the standing waves that delight river kayakers,  or to atmospheric mountain waves over abrupt  geographical features — such as Mount Rainier in Washington — that are often visible as  lenticular clouds. Although these waves seem  stationary, energy propagates up into the  overlying moving fluid. When the flow speed  changes or waves become large, these internal  lee waves can also break and produce turbulent  mixing (Fig. 1). In the atmosphere, inclusion  of the dynamic consequences of mountain  waves is known to be important for accurate  weather forecasts. Nikurashin and Ferrari argue that internal  lee waves may be prominent in places where  strong deep flows encounter rough topogra- phy. In particular, their theoretical calcula- tions highlight the Southern Ocean, where the  strong Antarctic Circumpolar Current flows  over the rough topography of Drake Passage,  which lies between South America and the  Antarctic Peninsula. They propose that turbulent mixing associated with breaking internal  lee waves, when globally integrated, contributes one-third of the mixing required to trans- form the deepest dense water into less-dense  water. 


Settled science update: Oceanographers find enormous deep-sea waves that 'play a crucial role in long-term climate cycles'

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