Climate Dynamics (2003) 21: 539–557 DOI 10.1007/s00382-003-0351-3 C. S. Jackson Æ A. J. Broccoli Orbital forcing of Arctic climate: mechanisms of climate response and implications for continental glaciation Received: 9 July 2002 / Accepted: 10 July 2003 / Published online: 4 November 2003 Ó Springer-Verlag 2003 Abstract Progress in understanding how terrestrial ice of terrestrial ice growth seen within the marine d18O volume is linked to EarthÕs orbital configuration has record. Both dynamical and thermal effects contribute to been impeded by the cost of simulating climate system the increases in snowfall during these periods, through processes relevant to glaciation over orbital time scales increases in storm activity and the fraction of precipi- (103–105 years). A compromise is usually made to rep- tation falling as snow. The majority of the mid- to high resent the climate system by models that are averaged latitude response to orbital forcing is organized by the over one or more spatial dimensions or by three- properties of sea ice, through its influence on radiative dimensional models that are limited to simulating par- feedbacks that nearly double the size of the orbital ticular ‘‘snapshots’’ in time. We take advantage of the forcing as well as its influence on the seasonal evolution short equilibration time (310 years) of a climate model of the latitudinal temperature gradient. consisting of a three-dimensional atmosphere coupled to a simple slab ocean to derive the equilibrium climate response to accelerated variations in EarthÕs orbital configuration over the past 165,000 years. Prominent 1 Introduction decreases in ice melt and increases in snowfall are sim- ulated during three time intervals near 26, 73, and 117 The growth and decay of terrestrial ice sheets during the thousand years ago (ka) when aphelion was in late Quaternary ultimately result from the effects of changes spring and obliquity was low. There were also significant in EarthÕs orbital geometry on climate system processes. decreases in ice melt and increases in snowfall near 97 This link is convincingly established by Hays et al. and 142 ka when eccentricity was relatively large, aph- (1976) who find a correlation between variations of elion was in late spring, and obliquity was high or near terrestrial ice volume and variations in EarthÕs orbital its long term mean. These ‘‘glaciation-friendly’’ time eccentricity, obliquity, and longitude of the perihelion. intervals correspond to prominent and secondary phases One of the earliest and most widely known hypotheses concerning the effects of orbital configuration on glacial cycles is described by Milankovitch (1941), who presents C. S. Jackson (&) Program in Atmospheric and Oceanic Sciences, an argument that a decrease in summer insolation is Princeton University, critical for glacial initiation. The usual interpretation of Princeton, NJ 08542, USA the Milankovitch hypothesis is that a reduction in E-mail: [email protected] summer insolation directly affects the amount of ice melt A. J. Broccoli that occurs during that season to a point when snow NOAA/Geophysical Fluid Dynamics Laboratory, fields can build upon previous yearÕs accumulation. In Princeton, NJ 08542, USA this usual interpretation, the Milankovitch hypothesis is Present address: C. S. Jackson centered on the effects of orbital configuration on snow Institute for Geophysics, melt (or ablation) rates. The John A. and Katherine G. Jackson School of Geosciences, Other hypotheses have been presented as well. Young The University of Texas at Austin, 4412 Spicewood Springs Rd., Bldg 600, Austin, TX 78759, USA and Bradley (1984) argue that the meridional insolation gradient is a critical factor for the growth and decay of Present address: A. J. Broccoli Department of Environmental Sciences, terrestrial ice through its control over poleward moisture Rutgers University, New Brunswick, transports and snowfall. Ruddiman and McIntyre NJ 08903 USA (1981), Miller and deVernal (1992), and Imbrie et al. 540 Jackson and Broccoli: Orbital forcing of Arctic climate (1992) suggest the North Atlantic Ocean circulation is cover from orbital forcing alone. The prescription of at important for modulating ice volume. Ruddiman and least an initial meter of snow over land points north of McIntyre (1981) and Miller and deVernal (1992) found 45°N is also insufficient to create conditions favorable evidence within the geologic record that the North for glacial initiation in several experiments (Oglesby Atlantic remains warm during periods of ice growth and 1990; Rind et al. 1989; Phillipps and Held 1994). In conjecture that enhanced meridional temperature gra- contrast to other experiments, the model employed by dients and evaporation rates during winter enhance the Dong and Valdes (1995) is sufficiently sensitive to radi- delivery of snow to the nascent ice sheets of northeastern ative changes at 115 ka to reach the critical threshold of Canada. Imbrie et al. (1992) propose that orbital forcing summer cooling noted by Rind et al. (1989) that allowed affects North Atlantic deep-water formation and, the annual accumulation of snow over many grid cells of through a chain of causality involving the Southern the high Arctic, although not necessarily where the Ocean, atmospheric CO2 levels. geologic record suggests glacial initiation took place. Three-dimensional climate models provide opportu- One reason some climate models may lack the necessary nities to explore some of these hypotheses within a sensitivity is due to a warm bias in summer surface air physically consistent framework. Unfortunately, the temperatures within the northern high latitudes, a large computational cost of simulating more than a common feature in many model predictions of modern century or two with the most comprehensive climate climate (Vettoretti and Peltier 2003a, b). The reduction models imposes an important constraint on modeling of this bias leads to an enhanced sensitivity and per- glacial-interglacial cycles. This constraint has focused manent snow cover over part of the northern high lati- research efforts towards more feasible targets, such as tudes in response to insolation anomalies at 115 ka. identifying the necessary factors that allow glaciation to The failure of many climate models to simulate per- occur at the inception of the most recent glacial cycle at manent snow cover over Eurasia and North America at approximately 115 thousand years before present (ka). 115 ka could indicate that those models have neglected a The 115 ka time period is of interest not only because critical feedback. Vegetation feedback is one possible it corresponds to a time of early growth of land ice, but candidate. Cooler summers were found to be a factor in also because the distribution of land ice and atmospheric forcing a conversion of high-latitude taiga and decidu- concentration of CO2 were similar to present. The main ous forests to higher albedo tundra, a process that boundary condition that may account for differences contributed to glacial initiation in the modeling studies between present and 115 ka is the difference in orbital of de Noblet et al. (1996) and Gallimore and Kutzbach configuration. A number of modeling studies have at- (1996). Recently, the coupled, three-dimensional, atmo- tempted to simulate glacial inception by prescribing the sphere-ocean modeling study of Khodri et al. (2001) 115 ka orbital geometry in climate models consisting of found the amplifying effects of orbital forcing on the an atmospheric general circulation model (AGCM) North Atlantic sea surface temperatures to be critical to coupled to a simple slab ocean (Rind et al. 1989; Oglesby glacial initiation, through reductions in the strength of 1990; Mitchell 1993; Phillipps and Held 1994; Dong and the thermohaline circulation in addition to increased Valdes 1995; Gallimore and Kutzbach 1995). These poleward moisture transports within the atmosphere. modeling studies discuss the importance of feedbacks For a variety of reasons, which we will discuss in more involving sea ice and soil moisture. Sea-ice feedbacks detail toward the end of this study, the importance of play a role in amplifying the annual mean cooling as well these feedbacks in the real climate system remains as shaping seasonal response to changes in insolation. uncertain. Moreover, sea-ice dynamics serve as a negative feedback While identifying the necessary factors that led to on sea ice thickness, positive feedback on sea ice con- glacial initiation at 115 ka is important to quantifying centration, and a positive feedback on surface air tem- some aspects of orbital forcing of glacial cycles, we also perature for an orbital configuration at 115 ka (Vavrus need to identify processes that are important to the 1999). Increases in soil moisture in the mid- to high evolution of terrestrial ice volume that may or may not latitudes are also noted in several studies and could be be the same as those processes emphasized from studies playing a significant role in maintaining cooler surface of climate at 115 ka. Time series analysis of oceanic air temperatures as well as increasing precipitation over sediments reveals that ice volume has varied linearly land at 115 ka (Mitchell 1993; Phillipps and Held 1994). with obliquity and precessional forcing since at least An increase in storm activity, which leads to increased 800 ka (Imbrie et al. 1984). This suggests that orbital precipitation rates in simulations of orbital forcing at forcing has affected climate in a systematic fashion, and 115 ka, is found by Kageyama et al. (1999) in an AGCM that knowledge gained from examining time slices other with specified sea surface temperatures (SSTs). The than 115 ka would be relevant to identifying processes change in storm activity is directly related to increases in that link orbital configuration with glacial-interglacial the meridional temperature gradient forced by a win- cycles. In fact, the most direct way to identify such tertime warming of the continents consistent with the processes is to directly model the temporal evolution of change in radiative forcing.