Dynamic boreal summer atmospheric circulation response as negative feedback to Greenland melt during the MIS-11 interglacial

. The unique alignment of orbital precession and obliquity during the Marine Isotope Stage 11 (MIS-11) interglacial produced perhaps the longest period of planetary warmth above preindustrial conditions in the past 800 kyr. Reconstructions point to a signiﬁcantly reduced Greenland ice sheet volume during this period as a result, although the remaining extent and volume of the ice sheet are poorly con-strained. A series of time slice simulations across MIS-11 using a coupled climate model indicates that boreal summer was particularly warm around Greenland and the high latitudes of the Atlantic sector for a period of at least 20 kyr. This state of reduced atmospheric baroclinicity, coupled with an enhanced and poleward-shifted intertropical convergence zone and North African monsoon, favored weakened high-latitude winds and the emergence of a single, uniﬁed midlatitude jet stream across the North Atlantic sector during boreal summer. Consequent reductions in the lower-tropospheric meridional eddy heat ﬂux over the North Atlantic therefore emerge as negative feedback to additional warming over Greenland. The relationship between Greenland precipitation and the state of the North Atlantic jet is less apparent, but slight changes in summer precipitation appear to be dominated by increases during the remainder of the year. Such a


Introduction
The Marine Isotope Stage 11c interglacial (approximately 424 ka to 395 ka; hereafter MIS-11) is the longest and one of the warmest interglacials of the past million years (e.g., Lisiecki and Raymo, 2005;Raymo and Mitrovica, 2012). Its climatological significance lies in the extent to which sea levels rose during this period, estimated at around 6-13 meters above that of the present day (Dutton et al., 2015). A substantial percentage of this rise is attributed to the melt of the 25 Greenland ice sheet (GrIS), which may have contributed as much as 4 m to 7 m of its estimated 7.4 m of present-day sealevel equivalent water content (Morlighem et al., 2017;Robinson et al., 2017). Antarctica has recently been estimated to have contributed another 6.7-8.2 m (Mas e Braga et al., 2021). The prolonged warmth of this period therefore is of direct relevance to understanding the processes that cause GrIS melt, a highly pertinent question as planetary warming is likely to continue in the near future. 30 As suggested by the wide range in sea-level rise estimates, considerable uncertainty exists with regards to both the degree of melt of the GrIS and the global and regional temperature anomalies during MIS-11. Pollen records indicate the development of some boreal coniferous forest around the margins of southern Greenland at some point during MIS-11 (de Vernal and Hillaire-Marcel, 2008;Willerslev et al., 2007), indicating both the prevalence of ice-free ground and sufficient summer warmth to support tree growth. Peak temperatures during this time remain poorly constrained, however. While some ice-core 35 data (e.g., Masson-Delmotte et al., 2010) and SST reconstructions (Dickson et al., 2009) suggest global temperatures 1-2°C warmer than pre-industrial, with Arctic anomalies potentially several degrees higher (Melles et al., 2012), orbital parameters and greenhouse-gas (GHG) measurements are broadly similar to those of the Holocene (Berger and Loutre, 2003). As such, https://doi.org/10.5194/cp-2021-118 Preprint. Discussion started: 13 September 2021 c Author(s) 2021. CC BY 4.0 License. climate models have historically struggled to replicate the temperature anomalies implied by both the limited paleotemperature records and the implied degree of GrIS melt (e.g., Robinson et al., 2017;Reyes et al., 2014). 40 A mechanism invoked as potential means for achieving and sustaining higher temperatures during MIS-11, particularly in the Arctic during boreal summer, is a strengthened Atlantic meridional overturning circulation (AMOC; Rachmayani et al., 2017). The authors of that study ascribe this strengthening primarily to salinity increases in the North Atlantic, in turn a product of favorable alterations of the surface wind and pressure fields inducing stronger ocean surface currents. The extent to which factors related to atmospheric transports of heat and moisture are involved is left as a question for future research, 45 and one that we attempt to address further in our analysis.
Multiple modelling approaches have been utilized to attempt to refine understanding of the MIS-11 interglacial climate.
Computing resources are a major limitation to simulating multi-millennial timescales with modern complex climate models, thereby inducing a choice to use simplified or low-resolution models like Earth system models of intermediate complexity (EMICs; e.g., Yin and Berger, 2012) or simulating shorter "snapshots" of the climate state at representative key periods (e.g., 50 DeConto and Pollard, 2003;Herold et al., 2012;Stone et al., 2013;Milker et al., 2013;Rachmayani et al., 2016Rachmayani et al., , 2017. The latter approach is often referred to as the time-slice method, and is frequently adopted when utilizing complex coupled atmosphere-ocean global climate models (AOGCMs). A complex AOGCM such as the Community Earth System Model (CESM; Hurrell et al., 2013;Gent et al., 2011) would require many months of computation time to complete a transient simulation of the MIS-11 interglacial even on a high-performance computer; thus the time-slice approach is more practical 55 for capturing the evolution of the climate over such a long period.
Still others have utilized a combined approach, running both an EMIC and an AOGCM over several time slices to compare with each other and reconstructions. Kleinen et al. (2014) produced broadly similar climate states with both CLIMBER2 (an EMIC) and CCSM3 (an AOGCM), though the increased resolution of CCSM3 enabled much better identification of regional climate features, such as the enhanced African summer monsoon during the 410 ka and 416 ka periods of MIS-11. The 60 limited spatial and temporal resolution of records and reconstructions during MIS-11 makes verification difficult (e.g. Milker et al., 2013), but identifying regional climatic changes with teleconnection potential is important to understanding the climate mechanisms influencing Greenland.
A key aspect of replicating the regional distribution of temperature changes under different climate forcing regimes is adequately capturing feedback mechanisms internal to the climate system. Orbital forcing in particular has widespread 65 consequences, as different distributions and intensities of surface heating cause the atmospheric and oceanic circulations to respond in different ways (e.g., Merz et al., 2015;Fischer and Jungclaus, 2010). Despite relatively modest changes in the magnitude of seasonal insolation values throughout most of MIS-11, the latitudinal distribution of insolation is still notably different relative to the pre-industrial period. The high Northern Hemisphere summer insolation during a long interval of MIS-11 was responsible for both enhancing the African monsoon (Rachmayani et al., 2016;Mohtadi et al., 2016;Wu and 70 Tsai, 2021) and weakening the mean hemispheric baroclinicity. Both weakened midlatitude baroclinicity in the atmosphere and enhanced tropical forcing have been identified as mechanisms for shifting the preferred state of the North Atlantic upper https://doi.org/10.5194/cp-2021-118 Preprint. Discussion started: 13 September 2021 c Author(s) 2021. CC BY 4.0 License. tropospheric jet stream from a split regime (separate subtropical and polar front jets) to a unified hybrid jet regime (Lee and Kim, 2003;Son and Lee, 2005;Andres and Tarasov, 2019). Altered jet regimes in turn have consequences for the development and propagation of atmospheric eddies, thus affecting a major source of atmospheric heat at high latitudes (e.g., 75 Nakamura and Oort, 1988;Overland and Turet, 1994;Serreze et al., 2007).
Internal climate system mechanics contributing to the pronounced melt of the GrIS during MIS-11 remain largely unidentified. In the present study, we utilize some of the highest-resolution climate simulations performed to date under MIS-11 conditions in order to parse these mechanisms further. In particular, what atmospheric changes across the North Atlantic sector were most consequential for mass balance changes in the Greenland ice sheet? We therefore explore to what 80 extent insolation-induced changes in the jet stream may have led to feedbacks affecting the poleward transport of atmospheric heat and moisture.

Model configuration
The climate model chosen for this study is the Community Earth System Model (CESM) v1.2.2, a fully-coupled atmosphere-85 ocean general circulation model with coupled sea ice, land, and runoff components. The CESM and Community Climate System Model (CCSM) family has been widely utilized in paleoclimate studies. Our particular configuration utilizes the Community Atmosphere Model version 5 (CAM5). The land and atmosphere models have an approximate resolution of 1.9° latitude by 2.5° longitude with 30 sigma-hybrid coordinate vertical layers in the atmosphere. The ocean and sea-ice grids are comprised of an orthogonal curvilinear grid at nominally 1° resolution with the north pole displaced over Greenland to avoid 90 singularities in flux calculations in the Arctic Ocean.
Fixed present-day ice sheet topography was assumed for all MIS-11 time-slice experiments, and therefore the land ice model component was disabled. This is perhaps the largest assumption of these simulations, but enables isolation of the effects of changing orbital and greenhouse gas (GHG) forcings on the simulated climate. Options for applying different ice sheet configurations were also severely limited by the sparse Greenland ice-core data extending back through MIS-11, and only a 95 small number of modelling studies have produced transient reconstructions (e.g., Robinson et al., 2017).
A control run was conducted which adheres to the standards set forth by the Paleoclimate Modelling Intercomparison Project 4 (PMIP4) for a pre-industrial baseline (Otto-Bliesner et al., 2017). This simulation was integrated for 2500 years to enable full equilibration of the surface climate and quasi-equilibration of deep-ocean temperatures. The experimental MIS-11 runs were branched from year 1500 of the control integration, with only orbital and GHG parameters altered as described below. 100

Experimental design and parameters
The "time slice" technique (see e.g., Stone et al., 2013;Rachmayani et al., 2017) Table 1 For each experimental simulation, orbital parameters were calculated following Laskar et al. (2004) at the representative time period. Greenhouse-gas concentrations were obtained from the European Project for Ice Coring in Antarctica (EPICA) record, primarily consisting of data from Dome C (Siegenthaler et al., 2005;Luethi et al., 2008). Following the experimental 115 setup detailed in Otto-Bliesner et al. (2017), a nominal +23 ppb adjustment was applied to Antarctic CH4 values, accounting for the fact that methane persistently exists in higher concentrations in the Northern Hemisphere during interglacials. GHG values represent means of a 5 kyr window around the representative time (i.e., 423 ka GHG values are given by a mean of all values between 425.5 ka and 420.5 ka), which accounts for the inherent uncertainty in GHG values due to short-term variability (centennial and millennial scale) and analysis techniques. 120 A sensitivity experiment was also conducted to ensure that the results of the time slice experiments were not dependent upon initialization. To this end, additional simulations for 418 ka were conducted, branching from year 500 of the pre-industrial control run and from year 500 of the 423 ka run. A comparison of the equilibrated periods (final 100 years of each integration) showed no statistically significant differences existed between them. four. In reality, 600 total seasonal values likely have a much greater number of degrees of freedom, although each time slice's 100 years of data is best classified as a red-noise time series and therefore does not have 100 degrees of freedom (Thomson, 1982). Significance of correlations presented are therefore conservative.

Surface temperature response 135
The surface temperature response is the most obvious effect of the varying orbital and GHG conditions throughout MIS-11 ( Fig. 1). Global-mean 2 m air temperatures were at or modestly above pre-industrial levels for each of the 423-408 ka simulations (dark blue). As expected, the temperature pattern is considerably amplified over Greenland, with positive anomalies reaching 2-3°C above pre-industrial during the warmest period (413 ka). The spatial distribution and magnitude of boreal summer temperature anomalies across the North Atlantic sector also varied notably throughout the analysis period (Fig. 2). Anomalies as high as 4-5°C above pre-industrial are evident around northern Greenland and the Canadian Arctic during the peak warmth of 413-408 ka due to the combined effects of high insolation and regional feedbacks, including reduced sea-ice cover. More modest and relatively uniform warming is present across much of 150 the rest of the Atlantic basin, with the exception being areas immediately surrounding the Mediterranean Sea. A narrow but stark band of 1-2°C cool anomalies stands out in sub-Saharan Africa during the 423 ka and 413-408 ka simulations, the result of increased cloud cover and surface cooling induced by evaporation associated with enhanced monsoonal and intertropical convergence zone (ITCZ) convection. Also evident is the abrupt return of surface air temperatures to near-preindustrial conditions by 403 ka and substantial cool anomalies by 398 ka, consistent with conditions potentially favorable for 155 renewed glaciation.  One clear consequence of stronger warming at high latitudes, especially when paired with tropical surface cooling over Africa, is the reduction of the mean equator-to-pole surface temperature gradient, and thus the contribution of the thermal wind balance to the geostrophic flow. A dynamic adjustment of baroclinic processes is therefore to be expected, including attendant changes in jet stream and baroclinic eddy behavior. 165

Atmospheric circulation response
Particularly robust changes are present in the lower-tropospheric eddy heat flux (EHF) anomalies across the North Atlantic sector. Figure   Two features in the total EHF anomaly fields stand out. First, the couplet of positive and negative anomalies over southwestern Europe and the central Mediterranean during 423-408 ka is indicative of a robust shift in the favored storm track. Positive EHF anomalies are indicative of either an increased and anomalous meridional temperature gradient or frequent large southerly wind anomalies (transient or stationary), or more likely, some combination thereof. Given that the mean temperature gradient appears unchanged or even slightly weaker (Fig. 2), the likely source of this anomaly is increased 185 frequency and/or intensity of wave activity and meridional transport in this region. Conversely, reductions exist over the central Mediterranean, consistent with a shift in baroclinic wave activity away from this region. A second region of widespread reduction in total eddy heat fluxes is present over much of Greenland and the open North Atlantic. The magnitude of this shift is less significant than the anomalies over Europe and the Mediterranean, but still implies a reduction in wave activity and/or intensity, a tendency towards weaker warm advection, or both. 190 As implied by the eddy flux maps, a strong negative correlation exists between the temperature averaged over Greenland and its immediate surroundings (the region enclosed by the green box in the top-left panel of Fig. 2) and the mean eddy heat flux over the North Atlantic region (the larger region enclosed by the green box in Fig. 3). The strength of this relationship is also strongly dependent on the season (Fig. 4). While both boreal winter-mean and summer-mean EHF-temperature relationships 200 pass significance at 95% confidence, the correlation is much stronger in boreal summer and with a much-reduced 95% rvalue confidence interval. This can be partly explained by the much larger magnitude and variability of seasonal-mean North Atlantic eddy heat fluxes in winter, but is also indicative of a more robust dynamic response in the summer season.

Jet stream response
While reduced meridional temperature gradients play a role in reducing eddy heat fluxes, changes in eddy activity are the 210 dominant cause. Eddy activity in the midlatitudes has a symbiotic relationship with the jet stream, which both drives and is driven by wave activity. An attendant change is therefore expected in jet behavior, which is apparent in the daily jetfrequency matrices (Fig. 5). A clear shift in the favored latitude and strength of the boreal summer North Atlantic jet is evident, with weaker and lower-latitude daily maximum 300 hPa winds clearly favored in the 418 ka, 413 ka, and 408 ka experiments. These three warm periods also show a slight reduction in the frequency of low-latitude jet maxima, denoted by 215 the slight negative frequency anomalies on the lower flank of the robust couplet. This is consistent with a decreasing tendency to have wind maxima in the typical (modern) latitude of the subtropical jet, an observation which is further supported by the presence of negative mean 300 hPa wind anomalies (Fig. 6).  Thus, both purely eddy-driven high latitude jet maxima and purely subtropical jet maxima are reduced (the southern flank 225 subtropical wind weakening is even more clearly visible in the u-component of the wind field; not shown), with the dominant state instead emerging as a broad hybrid eddy-thermal jet in the middle latitudes. This result is a confirmation of the observation made by Son and Lee (2005), who demonstrated that either weakened midlatitude baroclinicity or stronger tropical ascent could cause the jet to transition to a unified state. Both mechanisms are present during MIS-11 boreal summers. Baroclinicity changes are clear in the temperature anomaly patterns (Fig. 2) and implied by the robust negative 230 correlation between jet strength and mean 2 meter air temperature over the North Atlantic (Fig. 7). Meanwhile, negative temperature anomalies and greatly increased precipitation across the ITCZ corridor of the North Atlantic and sub-Saharan African (see Fig. 11) are indicators of significantly enhanced convective activity (and thus tropical ascent) in this sector.

Eddy-jet relationship
Notably, the region with the large couplet in EHF anomalies over western Europe and the Mediterranean (Fig. 3)  240 corresponds with the largest changes in jet stream wind strength (Fig. 6). Since the changes in EHF are dominated by the transient component, it is clear that the emergence of these anomalies is associated with a significantly altered North Atlantic storm track. A much more baroclinically active JJA storm track across this region appears to be the consequence of the merged jet state, which is also consistent with the trapping of atmospheric waves equatorward of a merged jet (e.g., Nakamura and Sampe, 2002).  Interestingly, correlations between EHF anomalies and jet characteristics fail to paint quite so clear of a picture. While hints of a relationship between jet latitude and North Atlantic-mean EHF (mean over a box spanning 40-80°N and 70°W-0°E) 255 exist, the correlation does not reach significance at the 95% confidence level (Fig. 9). However, EHF exhibits a strong positive correlation with boreal summer jet strength in the North Atlantic. Given the association our simulations demonstrate between larger jet wind maxima and the split-jet state, this would imply greater EHF during split-jet regimes, and reduced EHF during the prevailing merged-jet regime of MIS-11 boreal summers.

Precipitation and the storm track
With such a pronounced change in eddy behavior as a result of the shifted jet, a logical consequence would seem to be a 265 positive correlation between jet strength and precipitation over Greenland. However, exactly the opposite appears to be the case (Fig. 10), with a significant but small negative trend in precipitation associated with higher North Atlantic wind https://doi.org/10.5194/cp-2021-118 Preprint. Discussion started: 13 September 2021 c Author(s) 2021. CC BY 4.0 License. maxima. As is the case with the EHF correlations (Fig. 9), only the relationship between maximum jet velocity and precipitation obtains 95% significance. Competing effects appear to be influencing precipitation over Greenland in these simulations: intuitively, increases in eddy heat flux should correspond to increases in eddy moisture flux (EMF). However, comparing the jet state to total EMF over 275 the North Atlantic directly results only in a statistically insignificant correlation (not shown). On the other hand, we have already demonstrated that warmer North Atlantic temperatures are associated with weaker jets, and lower-tropospheric air temperature places a strong cap on atmospheric moisture. The apparent negative correlation between maximum jet winds and precipitation over Greenland (Fig. 10) therefore appears to be overwhelmingly driven by the inverse relationship between temperatures over Greenland and jet strength. Behavioral changes in the jet and eddies exhibit only a secondary 280 influence.
Spatially, the largest boreal summer precipitation changes across the North Atlantic sector appear related to the strength of the African monsoon (Fig. 11). Specifically, a drying is noted across much of the midlatitude Atlantic and much of central and southern Europe, consistent with the general shift of the preferred storm track and suggestive of increased subsidence from the poleward flank of Hadley cell. The magnitude of subtropical drying appears to be roughly proportional to the 285 increase in precipitation over sub-Saharan Africa, strongly suggesting an influence for the tropical convection upon the altered jet state. Precipitation rate increases across west/central Africa associated with the increased monsoon are evident through the 423-403 ka periods, peaking at up to 1.5 to 2 mm day -1 under 408 ka conditions. Attendant decreases of up to 0.5 mm day -1 over Europe and 1.0 to 1.5 mm day -1 across the Caribbean and western subtropical Atlantic are also evident during 413 and 408 ka. 290 Over Greenland, boreal summer precipitation changes during the warm 423-403 ka period resemble that of orographic precipitation during dominant westerly/northwesterly wind regimes: positive precipitation anomalies along the western slopes and negative anomalies along and just off the southeastern coast. The anomalies are relatively small, on the order of 0.1-0.3 mm day -1 . However, the mean modelled precipitation over Greenland is just under 2 mm day -1 , so these changes are potentially significant (though values in coastal regions and along terrain gradients are considerably higher). Coupled with 295 the negative eddy heat flux anomalies, this precipitation pattern would appear to support the notion that baroclinic wave activity during boreal summer is reduced in the vicinity of Greenland, with background westerlies becoming the prevailing pattern.
In the annual mean, however, a near-opposite pattern emerges in precipitation anomalies, particularly for southeastern Greenland (Fig. 12). Simulated annual average precipitation rates increase there by 0.1-1.0 mm day -1 for the 418-408 ka 300 period. Since this precipitation increase occurs in the cool season, it is overwhelmingly snow (not shown but confirmed by model precipitation-type output), and therefore contributes to an increase in mass balance of the ice sheet across southeastern regions.

Discussion
Our results compare favorably with the limited previous modelling studies conducted to assess MIS-11 climate. Kleinen et 310 al. (2014) examined key periods of MIS-11 climate using both an EMIC (CLIMBER2) and an AOGCM (CCSM3), finding both the pronounced high-latitude warming and narrow region of tropical cooling that we have identified here with CESM1.2 in both of their models. They similarly identified the enhancement of the African monsoon and the migration and strengthening of tropical convection over the eastern Atlantic, albeit with a more limited spatial extent of increased precipitation than in our simulations. Further analysis of the same two CCSM3 time slices in Rachmayani et al. (2016) 315 confirmed these temperature and precipitation relationships.
Consensus among recent climate modelling studies therefore indicates a robustly strengthened African monsoon during MIS-11 in boreal summer. Despite severe limitations in the availability and spatial coverage of reliable proxy records from this time period, the data that exists appears to support this. Decreased deposition of terrigenous iron in the deep ocean off the coast of northwestern Africa during the period 420-396 ka indicates a substantially less dusty (i.e., wetter) northwest African 320 climate during this time (Helmke et al., 2008). Also coincident with this development was the occurrence of increased deposition of organic matter in the eastern Mediterranean (sapropel 11), approximately dated to 407 ka and likely the result of increased freshwater input from the Nile River as eastern African rainfall increased (Kroon et al., 1998;Lourens, 2004).
Timing the African wet period to a resolution greater than a few thousand years or determining precisely the magnitude of the rainfall changes is still beyond the precision afforded by these records; however, they do offer some general verification 325 of the signal produced by the model consensus.
Primary responsibility for the changes in both the African monsoon and the midlatitude baroclinicity naturally lies with the insolation changes. Both inter-hemispheric and intra-hemispheric changes to the insolation gradient have roles to play, with the relatively high obliquity conditions of the early-mid MIS-11 interglacial (ca. 423-408 ka) a chief contributor to the lowbaroclinicity conditions in boreal summer. High obliquity conditions are responsible for increased high-latitude insolation in 330 the summer hemisphere, thus driving the reduction in meridional temperature gradient seen in our simulations (Mantsis et al., 2014). Furthermore, high obliquity enhances the inter-hemispheric temperature gradient, sustaining a stronger Hadley circulation and causing a poleward shift in the Hadley cell in the summer hemisphere (Mantsis et al., 2014). Climatic precession is known to have the dominant effect on the state of the north African monsoon (e.g., Bosmans et al., 2015), which is illustrated in our simulations by the emergence of the strongest monsoon conditions during 408 ka (and, conversely, 335 the weakest monsoon precipitation anomalies nearest precession maxima at 418 and 398 ka). The favorable alignment of moderate-low precession and moderate-high obliquity throughout much of MIS-11 thus produces sustained Northern Hemisphere boreal summer warmth as well as monsoon and jet responses over an anomalously long period.
As others have already noted, the changes in baroclinicity seen during MIS-11 will likely be mirrored to some extent by present and future GHG-induced global warming, suggesting the possibility of a strengthened African monsoon and 340 northward-shifted Atlantic ITCZ in the not-so-distant future (Mohtadi et al., 2016, and references therein). In the present https: //doi.org/10.5194/cp-2021-118 Preprint. Discussion started: 13 September 2021 c Author(s) 2021. CC BY 4.0 License.
climate, it has been demonstrated that an improved understanding of western African convection can significantly improve weather forecast skill across Europe and the North Atlantic due to teleconnection patterns (e.g., Pante and Knippernitz, 2019). Long-term and large-scale mean circulation anomalies in the west African monsoon region thus have far-reaching implications for regional or global weather and climate patterns, including over Greenland. Improved understanding of the 345 two-way dynamic linkages between patterns of tropical convection during boreal summer and changes in the North Atlantic circulation are necessary in order to optimize predictions of future Greenland ice sheet behavior.
It also remains to be determined to what extent the jet-eddy response identified here is robust to all models, given the variable degree to which the North Atlantic storm track tends to be southward-biased in Climate Model Intercomparison Project (CMIP) models (Harvey et al., 2020). Numerical models can also struggle to replicate the magnitude and latitudinal 350 extent of the west African monsoon circulation, which, given the sensitivity of the midlatitude jet to tropical forcing in our simulations, suggests another potential bias of concern (e.g., Brierley et al., 2020). Our study also does not test the sensitivity of the jet-eddy response to varying levels of GHGs under identical orbital forcings, which are subject to some uncertainty for paleo-periods like MIS-11 given millennial-scale fluctuations and the relatively low temporal resolution of GHG records (variable between a few hundred and approximately 1000 years). Natural limitations on the availability of quality 355 reconstructions will almost certainly continue to prevent true "validation" of modelling results as well.
The use of a fixed modern-day Greenland ice sheet in our climate simulations represents probably our most significant assumption, although the effects of this are minimized during the summer season. Elevation reductions on the order of several hundred meters across the GrIS would result in 2 m air temperature increases of several degrees Celsius, further enhanced by albedo reductions due to the melting of the ice sheet and emergence of bare rock and soil across marginal 360 regions. In fact, modelling sensitivity studies examining future scenarios have identified these factors as contributing to notably larger GrIS ablation areas and sea-level rise from Greenland melt when accounted for in a two-way coupled simulation (Le clec'h et al., 2019). In contrast, it is possible that these feedbacks may result in further reduced baroclinicity across the North Atlantic sector, possibly further reinforcing the unified-jet state and leading to additional reductions in eddy heat fluxes. Competing effects on precipitation are also possible: increased atmospheric moisture content is expected over a 365 warmer, lower-elevation Greenland surface; however, the diminished topography in large sections of Greenland would be expected to reduce the production of orographically-induced precipitation. We plan to investigate the dynamic atmospheric response to a greatly-reduced GrIS under MIS-11 climate conditions in a future sensitivity study, and thus hope to better assess the appropriateness of the fixed ice sheet assumption.

Conclusions 370
Using CESM time-slice simulations of only insolation and GHG forcing conditions corresponding to the MIS-11 interglacial, we have uncovered a robust dynamic atmospheric response that includes negative feedbacks to further melting of the Greenland ice sheet. Increased high-latitude insolation due to a favorable alignment of high obliquity and relatively https://doi.org/10.5194/cp-2021-118 Preprint. Discussion started: 13 September 2021 c Author(s) 2021. CC BY 4.0 License. low-amplitude climatic precession variations across a time interval of around 20 kyr drives pronounced, sustained boreal summer surface warming across Greenland and the surrounding high-latitude oceanic regions. It remains a future task to 375 determine if this insolation-driven surface warming alone is sufficient to produce GrIS melt of the magnitude proposed by previous studies (e.g., Alley et al., 2010;Reyes et al., 2014;Robinson et al., 2017). Resolving this question is complicated by the two negative feedbacks we have identified in the atmospheric response to MIS-11 interglacial forcing: (i) a transition in preferred jet stream behavior leading to reductions in poleward eddy heat flux over the Greenland sector, and (ii) increased annual-mean precipitation over the ice sheet. The mass balance implications of these feedbacks will be explored in a planned 380 future ice sheet modelling study.
The preferred boreal summer jet response to MIS-11 forcing identified here, driven by the combined effects of weakened midlatitude baroclinicity and enhanced tropical ascent over the Atlantic sector, is more consistent with the merged-jet state considered to be characteristic of glacials rather than interglacials (e.g., Andres and Tarasov, 2019;Merz et al., 2015).
Further study, and comparison with other interglacial climate simulations, may clarify whether this is a unique feature of 385 MIS-11 or typical of strong interglacials. Earlier studies utilizing medium-resolution CCSM3 simulations (Herold et al., 2012;Rachmayani et al., 2016) suggest that an enhanced African monsoon and warmer high-latitude temperatures are typical features of late-Quaternary interglacial states, indicating that at least the primary mechanisms behind the jet changes tend to be present. However, spatial patterns of warming and the degree and duration of monsoon enhancement appear to vary considerably between interglacials, and it therefore remains to be seen whether the dynamic conditions present in other 390 interglacials favor the same northern summer jet configuration.
Author contributions. This research was performed as part of the PhD studies of lead author Brian Crow, who advised by authors Matthias Prange and Michael Schulz. Dr. Prange and Prof. Dr. Schulz provided computing resources, guidance for experimental design, and extensively discussed results. Both also provided considerable constructive feedback and edits to 395 the manuscript.
Competing interests. The authors declare no competing interests that impacted the research process or the development of this manuscript.