RETRO-AGENDA


RETRO BACKGROUND
It is increasingly clear that the ocean’s current mode of ventilation is not unique but can, and has, switched rapidly between dramatically different states with severe and far-reaching climate repercussions. Yet, the various processes hypothesized to have driven these past ocean reorganizations remain largely unconstrained, preventing reliable assessments of the ocean’s vulnerability to future changes. Most models attempting to simulate these ocean mode switches activate them by disturbing the low to high latitude density gradients supporting meridional overturning circulation (MOC).
Pasted Graphic 1
Perhaps the most commonly invoked theories to explain MOC changes focus on the importance of Northern Hemisphere ice sheet-ocean interactions due to the ability of ice sheets to act as bouyancy capacitors — slowly building large stores of freshwater which are then discharged catastrophically. Model simulations introducing freshwater (low density) anomalies near sites of deep-water formation in the North Atlantic are able to throttle deepwater production and incur climate changes across much of the globe.


PALAEO-OCEANOGRAPHY
Paleoceanographic evidence indicates that the tropical and extratropical Atlantic are tightly linked on centennial to millennial timescales. Sediment records from the Cariaco Basin show that tropical Atlantic Ocean and atmosphere changes correlate with variations in the North Atlantic region since the last glacial period Specifically, southward shifts of the ITCZ and changes in the north-easterly trades regularly accompany cooling episodes recorded in Greenland ice cores. Model results suggest that these regions may be connected through the atmosphere, whereby changes in the extent of high latitude sea ice can affect meridional displacements of the ITCZ. In contrast to SST variability in the Cariaco basin that shows an in-phase relationship with North Atlantic high latitudes, tropical SST variations within the Guyana/Caribbean Current show an anti-phase relationship with North Atlantic high latitudes (The contrasting climate histories of these two proximal locations suggest that the atmosphere is not the only pathway affecting tropical ocean changes in the past. Indeed, the anti-phased response exhibited by the Guyana/Caribbean Current site is more consistent with that expected due to changes in the MOC. Ocean circulation model experiments, and reconstructions from proxy data indicate that changes in the MOC are associated with ocean-wide reorganisation in heat transport and temperature distribution, notably in conjunction with rapid climate events, such as Dansgaard/Oeschger (D/O) events and Heinrich events. Specifically, reduced deep-water formation in the North Atlantic results in cooling in the North Atlantic region, but warming in the tropical thermocline and much of the southern hemisphere. While current data indicate these possibilities, the hypotheses concerning the oceanic role are poorly constrained by available observations and core material with enough temporal and vertical resolution.

retrologo2_small
The Benguela/Cape of Good Hope region is an important site of warm water and salt import into the permanent thermocline of the Atlantic. There is a supply of Indian Ocean water which originates in the Agulhas Current and slips into the South Atlantic Ocean around Cape of Good Hope. The remaining water is subantarctic surface water added at the Subtropical Front or intermediate water that enters the South Atlantic Ocean through Drake Passage, upwells into the thermocline, warms up and flows eastward with the South Atlantic Current. The salt and heat import from the Indian Ocean, the so-called Agulhas leakage, is considered to be an important process to maintain the global thermohaline circulation. Foraminiferal records suggest that the Agulhas spillage was indeed enhanced during early deglaciations suggesting a crucial role for glacial terminations. However, whether the heat and salt transport was also affected at the thermocline level is largely unknown, and cannot be properly constrained by currently available data. Contemporary descriptive physical oceanography relies on three main pillars: three-dimensional observational networks; measurements of conservative and density-related parameters (temperature and salinity in particular); and dynamical measurements (either of flow rate or of water ‘age’). Ideally, palaeo-oceanography would rely on the very same observational pillars, although in practice they are far more difficult to achieve. This is primarily due to the necessity to work with ‘proxy’ measurements for palaeoenvironmental parameters (few of which are easily linked to density and dynamics in particular); however it is also due to the necessity to make careful choices with regard to sediment core locations. Previous coring campaigns have provided a wealth of site survey data and short ‘pilot’ gravity cores from the equatorial and South Atlantic. In the Caribbean, however, such site survey data remain sparse. To achieve the high temporal resolution and the appropriate stratigraphic coverage (last 60,000 years) that are required in order to study the behaviour of the climate system in the past (and under a wider range of possible states than is possible using historical data alone), long piston cores from high accumulation sites are needed.

RETRO MISSION STATEMENT
If the most fundamental questions concerning the behaviour of the ocean circulation, and its role in rapid climate change, are to be answered it is clear that these observational needs must be addressed. RETRO -research therefore focussus on investigating the full range of time-scales that are recorded in the marine geological archive, from seasonal or interannual to multi-millennial, and aims emulation of contemporary physical oceanographic observations more closely. Based on geostrophic theory it will only be possible to interpret past changes in the dynamics of the Atlantic MOC, if ‘palaeo-property’ gradients can be estimated over depth transects perpendicular to main ocean currents feeding the MOC. This forms the basis for the design of RETRO, in which we join forces to construct centennially resolved time series of the tropical Atlantic surface, thermocline, intermediate and deep waters through key transitions of the Meridional Overturning Circulation (MOC) and climate.