Physical paleoceanography and modelling

Agatha de Boer (group leader) , Martin Jakobsson, Rezwan Mohammed, Christian Stranne, Robert Graham.

Our research is focussed around the dynamics of the large scale ocean circulation and its interaction with climate on all time scales. Insight is gained from theory, present day observations, paleo proxies, and models of various complexities. Modern observations of the ocean and climate are very short compared to the timescale on which the deep ocean circulation changes and the carbon cycle equilibrates so it is essential to look at paleo proxies to study climate variability on decadal and longer timescales. Our knowledge of the modern ocean-atmosphere system helps us to determine the control of past climate events. In turn, the paleo proxies of the past inform us about the dynamics of the modern ocean and how it may change in the future. 

Regions of interest: Southern Ocean, Agulhas Region, Atlantic Ocean, Arctic Ocean, and Global Ocean.
Time periods: Cretaceous, Quaternary, Holocene, present day, next century.
Tools and models: Satellite data, Argo data, proxies, Nemo ocean model, Mom4 ocean model, HiGEM coupled climate model, CM2Mc earth system model, box models.

Specific research areas:

» Southern Ocean: Fronts and winds

Our Southern Ocean work focusses around the identification of dynamic fronts (i.e., jets) and the study of the control on their position. We illustrated that the position of dynamics fronts is best observed using sea surface height gradients on not sea surface height contours. Using a combination of satellite observations and high resolution coupled climate model output we showed that the Subtropical Front is a different feature in the western and eastern side of basins. Only in the west of basins is it associated with a strong current which we call the Dynamic Subtropical Front. Our work shows an overarching control of the bottom topography on the position of the Southern Ocean fronts, even over flat topography. Seasonal changes are negligible south of the Subtropics. The Subtropical front is not aligned with the zero windstress curl. We also used Chlorophyll satellite data to deduce that the primary source of iron to the Southern Ocean is coastlines. Finally, our group is involved in the modelling and synthesis of observations of Southern Hemisphere Westerly Winds during the Last Glacial Maximum.


Fronts identified as local maxima in satellite sea surface temperature (green) and height (black) gradients. Orange lines are climatological positions of the Sub-Antarctic and Subtropical Front from Orsi et al. [1995]. Pink lines are the Dynamical Subtropical Front from Graham and De Boer [JGR, 2013].


» Meridional Overturning Circulation: Controls on structure and variability

Our group studies what controls the structure and strength of the MOC. This work includes questions about what the circulation was like in past climatic states and also how it may change in the next century. We found that on long time scales the wind and mean ocean temperature is more important to the global overturning circulation than the hydrological cycle. That means that strong winds and warm temperatures drive a distributed sinking regime where deep water forms equally in the Pacific, Atlantic and Southern Oceans. In colder climates and weak winds the deep water formation is more focussed in the Atlantic. We further found that the strength of the Atlantic MOC (AMOC) is not controlled by meridional density gradients but rather by meridional pressure gradients and that these are not always the same as is commonly assumed. Changes in the zonal structure of the AMOC is determined by how much the upper ocean transport is in Sverdrup balance, i.e., controlled by winds. 


Vorticity terms in the 15 year time averaged Sverdrup balance in (a,c,e) ECCOGODAE and (b,d,f) HiGEM. (a,b) V (using integration depths of 1400 m and 1000 m respectively),(c,d) Sverdrup transport calculated from the windstress curl, (e,f) The difference between the ocean and Sverdrup transports. Units are m2s−1.


» Quaternary glacial cycles and carbon: Role of ocean circulation

An ongoing interest is the climatic forcing that drove the Quaternary ice age cycles and the role that the ocean played therein. Using a circulation-nutrient statistical box model, we found that atmospheric CO2 is more sensitive to ocean circulation changes during glacial states and more sensitive to productivity changes during interglacial states, thus highlighting the importance of starting with the correct control run when doing sensitivity studies. We further showed in a theoretical box model that the reduction of vertical mixing from the bottom to the surface in the ocean could explain how the glacial ocean came to be filled with a large volume of southern sourced deep water that overturned very slowly. Finally, we suggested that the hypothesis originally put forward by Adkins [2005], that the glacial ocean could act as a thermobaric capacitor, may explain a huge fluctuation of CO2 observed in the stomatal record at the start of the Younger Dryas. 

Glacial and interglacial states from the statistical nutrient-circulation box model of De Boer et al. [2010]. The circulation (top) and nutrient utilization rate (bottom) parameters of the interglacial states (right) and the glacial states (left) are shown here against the preformed nutrient concentration in the deep ocean. The top axis relates the difference in preformed nutrient between the glacial and interglacial states to changes in atmospheric pCO2.


» Arctic Ocean Circulation: Role of and control of strait flows and river runoff

Our research in the high latitudes focusses on both straits flows and the general Arctic circulation. We suggested before that the Bering Strait through flow is connected to the Atlantic Meridional Overturning Circulation and the global wind field. We now aim to understand the longer term variability of the Bering Strait transport in more detail and also determine its importance for the Arctic and large scale circulation. For the Arctic we also look at the role of runoff for the circulation, stratification, and sea-ice distribution. 


Development of the Arctic barotropic streamfunction in a 0.25 degree NEMO model setup in a 250 year control simulation (left) and no-runoff simulation (right). Contour intervals are 4 Sv and the white background is centred around 0 Sv.

 Glacial and interglacial states from the statistical nutrient-circulation box model of De Boer et al. [2010]. The circulation (top) and nutrient utilization rate (bottom) parameters of the interglacial states (right) and the glacial states (left) are shown here against the preformed nutrient concentration in the deep ocean. The top axis relates the difference in preformed nutrient between the glacial and interglacial states to changes in atmospheric pCO2.

Department of Geological Sciences
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