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The Bjerknes Centre is a collaboration on climate research, between the University of Bergen, Uni Research, the Institute of Marine Research, Nansen Environmental and Remote Sensing Centre.

Natural and anthropogenic climate changes

IMMUNITY was a strategic project at the Bjerknes Centre (SKD) in the years 2011–2014.
 

The primary object of IMMUNITY is to integrate novel palaeoclimatic time series at unprecedented temporal resolution and instrumental data together with coupled atmosphere–ocean–sea-ice general circulation model simulations, in order to explore the long-term climate variations during the last 1500 years.

Analyses so far indicate that paleo reconstructions of ocean state variables generated by IMMUNITY deliver a temporal resolution that reflects the main decadal trends of instrumental data and thus can be used with confidence to develop a longer-term climate perspective (see figure). IMMUNITY has generated promising multiproxies for the reconstruction of key climatic features, such as deep overflows through the Iceland–Scotland ridge (Mjell et al., in prep) as well as frequency of storms and floods on land in Norway during the past millennia.

Major efforts have been devoted into a computationally efficient lower-resolution version of NorESM to make it suitable for millennial simulations and decadal prediction ensemble integrations. In this regard, the comparison of simulated and observed circulation of the North Atlantic gyre has given promising results.

Progress also has been made in elucidating the governing mechanisms behind multi-decadal climatevariability. The use of virtual drifters shows that, even in a coarse climate model, it is unusual for large volumes of warm surface water to be transported by the Gulf Stream into the Nordic Seas and Arctic (Medhaug and Czaja, in prep). Instead, observations and modelling agree that warm surface waters mostly recirculate in a loop and rarely reach high latitudes. In another investigation, the probability of having negative regional and global temperature trends for a control climate, historical observations and simulations, and for future scenarios using 17 models from CMIP5, has been analysed (Medhaug and Drange, in prep.). This analysis shows that the hiatus we have experienced the last decade or two is not uncommon even though we are experiencing a general warming of the global temperatures.

Potential mechanisms for decadal variability in the Pacific and East Asian summer monsoon have also been addressed during the past year. The role of anthropogenic forcing in shaping the weakened East Asian summer monsoon and associated anomalous precipitation in eastern China over the late 20th century (Wang et al., 2013) is highlighted. Finally, model output from the Bergen Climate Model and observations suggest that intensified solar radiation in combination with a lull in volcanic activity during the 1920s–1950s can explain much of the early 20th century Arctic warming (Suo et al., 2013).

 

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Figure caption: Example of high-resolution (~ 5 years) paleo reconstructions generated by IMMUNITY. The plot shows δ13C composition of G.bulloides. The amplitude of the signal is large, with a maximum in δ13C associated with the Little Ice Age. Strong negative values predominate during the Medieval Warm Period.

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Medhaug,  I.  and Czaja,  A.  Surface  exchange  between  the  subpolar  and  subtropical  North  Atlantic using  virtual drifters, in prep.
Medhaug, I. and Drange, H. Negative global and regional temperature trends in a warming climate, in prep.
Mjell,  T.  L.,  Langehaug,  H.  R.,  Otterå,  O.  H.,  Eldevik,  T.,  Ninnemann,  U.  and  Kleiven,  H.  F.  Reconstructing
ocean circulation and climate based on the Gardar drift, in prep.
Suo,  L.,  Otterå,  O. H.,  Bentsen,  M.,  Gao,  Y.  and  Johannessen,  O. M.  (2013):  External  forcing  of  the  early  20th century Arctic warming. Tellus A, 65, 20578.
Wang,  T., Wang,  H.,  Otterå,  O. H.,  Gao,  Y.,  Suo,  L.,  Furevik,  T.  and Yu,  L. (2013):  Anthropogenic  forcing  of shift  in  precipitation  in  Eastern  China  in  late  1970s. Atmos.  Chem.  Phys.,  13,  12433-12450, doi:10.5194/acp-13-12433-2013.