Ocean Circulation and Climate Dynamics

North Atlantic Air-Sea Interactions and Remote Impacts on the North Atlantic

What is the role of the global ocean circulation in low-frequency climate variability in the North Atlantic region? More than 50 years ago, Bjerknes suggested that the character of large-scale air-sea interaction over the mid-latitude North Atlantic Ocean differs with timescales: the atmosphere was thought to drive directly most short-term (interannual) sea surface temperature (SST) variability, and the ocean to contribute significantly to long-term (multidecadal) SST and potentially atmospheric variability. Such a relationship is also suggested by a number of global climate models in which the Atlantic Meridional Overturning Circulation (AMOC) plays a major role in driving multidecadal SST variability.

The first part of the Bjerknes hypothesis has been well studied by now, but the second part still required some verification by data; understanding Atlantic Multidecadal Variability (AMV) of SST, due to limited ocean observations, remained a challenge. AMV is nonetheless of major socio-economic importance because it is linked to important climate phenomena such as Atlantic hurricane activity and Sahel rainfall, and it hinders the detection of anthropogenic signals in the Tropical and North Atlantic sector. Direct evidence of the oceanic influence on AMV can only be provided by surface heat fluxes, the 'language' of ocean-atmosphere communication. Gulev et al. (2013) have provided for the first time observational evidence that in the mid-latitude North Atlantic and on timescales longer than 10 years, surface turbulent heat fluxes are indeed driven by the ocean and may force the atmosphere, whereas on shorter timescales the converse is true, thereby confirming the Bjerknes hypothesis. Consistent with this picture, Klöwer et al. (2014) described the important role of the AMOC in enhancing decadal predictability of North Atlantic SST. The studies of Gulev et al. (2013) and Klöwer et al. (2013) suggest that the predictability of mid-latitude North Atlantic air-sea interaction could extend beyond the ocean to the climate of surrounding continents. Martin et al. (2015) find evidence in the Kiel Climate Model for Southern Ocean centennial variability influencing the North Atlantic sector climate through a delayed AMOC response. The impact of the Southern Ocean centennial variability on regional as well as North Atlantic sea level in the model is of the same order of magnitude as the rise of global average sea level during the 20th century, which amounts to about 15-20cm. This suggests that internal variability on a centennial time scale cannot be neglected a priori in assessments of 20th and 21st century AMOC and regional sea level change.

 

References:

  • Gulev, S. K., M. Latif, N. S. Keenlyside, and K. P. Koltermann (2013): North Atlantic Ocean Control on Surface Heat Flux at Multidecadal Timescales. Nature, 499, 464-467.
  • Klöwer, M., M. Latif, H. Ding, R. Greatbatch, and W. Park (2014): Atlantic Meridional Overturning Circulation and Prediction of North Atlantic Sea Surface Temperature. Earth Pl. Sci. Lett., 406, 1-6.
  • Martin, T., W. Park and M. Latif (2015): Southern Ocean Forcing of the North Atlantic at Multi-centennial Timescales in the Kiel Climate Model. Deep-Sea Research II, DOI: 10.1016/j.dsr2.2014.01.018.