A systematic study of the impact of freshwater pulses with respect to different geographical locations

Didier M. Roche 0 1 Ane P. Wiersma 0 1 Hans Renssen 0 1 0 A. P. Wiersma Deltares, Subsurface and Groundwater systems, Utrecht, The Netherlands 1 D. M. Roche (&) H. Renssen Section Climate Change and Landscape Dynamics, Department of Earth Sciences, Vrije Universiteit Amsterdam , De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands The first comparative and systematic climate model study of the sensitivity of the climate response under Last Glacial Maximum (LGM) conditions to freshwater perturbations at various locations that are known to have received significant amounts of freshwater during the LGM (21 kyr BP) climate conditions is presented. A series of ten regions representative of those receiving most of the meltwater from decaying ice-sheets during the deglaciation is defined, comprising the border of LGM ice-sheets, outlets of rivers draining part of the melting ice-sheets and iceberg melt zones. The effect of several given freshwater fluxes applied separately in each of these regions on regional and global climate is subsequently tested. The climate response is then analysed both for the atmosphere and oceans. Amongst the regions defined, it is found that the area close by and dynamically upstream to the main deep water formation zone in the North Atlantic are most sensitive to freshwater pulses, as is expected. However, some important differences between Arctic freshwater forcing and Nordic Seas forcing are found, the former having a longer term response linked to sea-ice formation and advection whereas the latter exhibits more direct influence of direct freshening of the deep water formation sites. Combining the common surface temperature response for each respective zone, we fingerprint the particular surface temperature response obtained by adding freshwater in a particular location. This is done to examine if a surface climate response can be used to determine the origin of a meltwater flux, which is relevant for the interpretation of proxy data. We show that it is indeed possible to generally classify the fingerprints by their origin in terms of sea-ice modification and modification of deep-water formation. Whilst the latter is not an unambiguous characterization of each zone, it nonetheless provides important clues on the physical mechanisms at work. In particular, it is shown that in order to obtain a consistent see-saw temperature pattern, addition of freshwater in the Northern Hemisphere at sites dynamically close to the deep water formation zones is needed. Finally a preliminary datamodel comparison for the time of the Heinrich event 1 suggests that those sites are indeed the most favourable to explain the pattern of climate variability recorded in proxy data for this period. More importantly, this modeldata comparison enables us to clearly reject a substantial fraction of the zones tested as potential source for large freshwater entering the ocean at that time. 1 Introduction The ocean circulation in the Atlantic Ocean is widely accepted to be of crucial importance for the Northern Hemisphere climate. Indeed, the upper branch of the socalled thermohaline circulation (THC) is transporting warm and saline water to the Northern Hemisphere where it releases its heat to the atmosphere, subsequently warming the climate (Trenberth and Caron 2001). Perturbation of this system by modifying the surface buoyancy fluxes has first been recognised by Stommel (1961) as a potential cause for changing the ocean state. Subsequently, Broecker et al. (1988) put forward the importance of the modification of the THC in changing the surface climate, at first in the context of the Younger-Dryas cold period during the deglaciation. Since then, numerous studies have aimed at assessing the impact of imposed freshwater perturbations to the surface ocean (known as hosing experiments) in different climate models. Most studies (Rahmstorf 1995; Manabe and Stouffer 1995; Ganopolski and Rahmstorf 2001; Vellinga and Wood 2002; Stouffer et al 2006) have directed the perturbation to the North Atlantic Ocean, where the addition of freshwater modifies the deep water formation and slow down the THC. In turn, such modifications of the ocean dynamics promote drastic climate changes over (and around) the North Atlantic region. Recently, some additional studies have investigated the impact of a freshwater perturbation in the Southern Ocean (Knorr and Lohmann 2003; Stouffer et al. 2007) in order to investigate the symmetry of climate changes caused by perturbing the North Atlantic Ocean and Southern Ocean with freshwater. In addition, Peltier et al. (2006) have demonstrated that forcing the Arctic Ocean with freshwater leads to comparable results as perturbing the North Atlantic, albeit with different processes at work, in particular involving a strong sea-ice feedback. Finally, the importance of delivering freshwater to coastal regions instead of spreading it over large regions of the oceans has been investigated, showing the importance of the interplay between atmosphere and oceans (Saenko et al. 2007). Until now however, no study has provided a comprehensive and systematic picture of the sensitivity of ocean circulation to freshwater perturbations at the various geographical regions that have been proposed as a source of freshwater eventually leading to abrupt climate events. Such a study is of crucial importance if one aims at correctly simulating the climate evolution at times when there is strong meltwater from decaying ice-sheets directed to the ocean, as was the case during the last deglaciation (218 kyr BP) when decaying ice-sheets were delivering huge amounts of freshwater to the ocean. Potentially, this might also be relevant for the near future, since the Greenland ice sheet is experiencing accelerated melting in response to anthropogenic warming, which is also likely to affect the balance of the West-Antarctic ice sheet. Here we aim at providing a first step towards the comprehensive simulation of the last deglaciation by systematically studying the effect of freshwater forcing in the regions that are known to receive important freshwater contributions during the deglaciation. Therefore, we use the Last Glacial Maximum (LGM) climate as a background climatic state. 2 Definition of the freshwater perturbation zones Hosing experiments focusing on the North Atlantic generally use a crude geographical spreading of the freshwater anomaly: in most case, the pulse is spread evenly over a wide longitudinal band (e.g. (Ganopolski and Rahmstorf 2001; Stouffer et al. 2006)). Although this scheme does present the advantage of being simple and can potentially be used in any climate model (including zonally averaged ocean models), it is an highly unrealistic forcing considering how most freshwater forcing enters the ocean (either today or in past climates). Therefore, if one is looking for a fruitful model data comparison, there is a need of testing the sensitivity of coupled climate mo (...truncated)