Decadal variability of the thermohaline circulation





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Decadal variability of the thermohaline circulation

   
 
 

is one main theme of the German Ocean/CLIVAR-project:
"The changing North Atlantic Ocean: mechanisms of circulation changes".

During the intensive field phase of the World Ocean Circulation Experiment (WOCE) from 1990 until 1998 a comprehensive high-quality hydrographic data base was collected, especially for the northern North Atlantic. Meanwhile there exist nine realisations of the so-called "48°N"-section (WOCE/A2) along the line English Channel-Grand Banks of Newfoundland, for example. Seven of the high-density, full-depth section were carried out during WOCE (during GOOS since 1998, respectively) and two in previous decades, namely 1957 and 1982.

First analyses of the coherent hydrographic data set along the WOCE-section A2 during the Nineties show significant interannual changes in the climate relevant key parameters of the large-scale circulation in the North Atlantic such as heat and freshwater transports. With a phase lag of one year the transport values show an almost linear correlation with changes in the dominant mode of low-frequent atmospheric variability in the North Atlantic, the North Atlantic Oscillation (NAO). The values for two historic cruises confirm this behaviour (Lorbacher, 2000).

The line of zero wind stress curl shifts its position in concert with the NAO exciting long baroclinic Rossby waves. These build a sink of potential energy of the large-scale ocean circulation; the superposition of these Rossby waves leads to intense mesoscale variability. Along A2 this variability manifests itself in the meandering of the North Atlantic Current in the central Newfoundland Basin. One year after a low NAO-index (weak westerlies and a southward shift of the line of zero wind stress curl) the heat transport drops by 60%. It seems also that the "dynamic oceanic response" is more sensitive to a weakening of the zonality of westerlies than strengthening.

The propagation of long baroclinic Rossby waves forced in the eastern North Atlantic through the meridional movement of the line of zero wind stress curl could explain the spatial structure of the top-kilometer variability along A2 and the phase lag of one year between changes in the NAO and the oceanic dynamical response such as the variability of heat and freshwater transports. At intermediate depth (the depth of Labrador Sea Water (LSW)) the transport variability can be linked with a time lag of 3-5 years to the atmospheric variability in its source region. With the hydrography along 24.5°N (WOCE/A5) and 36°N (WOCE/A3) the decadal variability in the North Atlantic is described in form of a bimodal structure of the vertical profile of the Meridional Overturning Circulation (MOC): a single meridional cell in 1982/83 with higher volume transports of the upper and deeper layers than the intermediate layer of the LSW and two meridional cells in 1957/58 and 1992/1993 with a more pronounced LSW transport, whilst the upper and deeper transports are drastically reduced (Koltermann et al., 1999). To relate the flip between the two modes to just one controlling mechanism is difficult, because the variability in deeper levels is coupled to the distance from the source region, and so the two modes as a function of depth reflect variability with different phase lags to that of the atmospheric forcing.

These results lead to new questions: Is the linear correlation between the atmospheric forcing and the observed variability of heat and freshwater transports statistically significant and could it therefore be used as a parameter for forecasting? Are they therefore representative of the climate variability of the whole North Atlantic? This observed temperature variability seems more of adiabatic than of true climate origin. If we can understand the mechanisms controlling the oceanic variability on different time scales, it would be possible to decide if the observed changes are a superposition of mesoscale variability to a decadal trend which is reflected in the changed vertical structure of the MOC. To go one step further in answering these questions, we intend in CLIVAR to synthesize the observed results achieved with classical oceanographic methods with results of temporally and spatially high-resolving assimilation- and numerical models.

High-resolution Atlantic circulation modeling C. Böning (IfM Kiel) Reconstruction of the Atlantic circulation 1990-1998 J. Schröter, D. Olbers (AWI Bremerhaven)

NAO_corr.gif


upper panel: time series of the winter (DJF) NAO-index after Löwe and Koslowski [1998] and (black line) the five year running mean of the NAO-index. ”time series” of transport estimates across the "48°N"-section in the North Atlantic (red: heat, green: freshwater and blue: meridional overturning rate). (yellow line) latitude of the zonally integrated (between 30°-45°W) annual mean of zero wind stress curl.

lower panel: correlation between the heat transport and the meridional overturning rate and between the overturning rate and the NAO-index for a phase lag of one year; explained variance of the correlation between the transport estimates and the NAO-index for different phase lags.

 

 
 © 2016 Bundesamt für Seeschifffahrt und Hydrographie Last Update: Apr 19, 2013 6:03:47 PM  
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