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<h2>Introduction</h2>
<p>The Atlantic Meridional Overturning Circulation (AMOC) plays a pivotal role in regulating climate by transporting heat northward in the Atlantic Ocean (Lenton et al., 2019). Paleoclimate evidence suggests that freshwater forcing from melting ice sheets has caused abrupt AMOC slowdowns in the past (Knutz et al., 2007; Peck et al., 2006). In the modern context, the Greenland Ice Sheet (GrIS) is losing mass at an accelerating rate (Hansen et al., 2016), raising concerns about potential AMOC weakening (Frajka-Williams et al., 2016). Understanding the sensitivity of the AMOC to GrIS freshwater forcing is essential for projecting future climate change and assessing the risk of crossing a tipping point (Lenton et al., 2019).</p><p>Previous modeling studies have explored AMOC responses to freshwater perturbations using hosing experiments (Otterå et al., 2003; Stouffer et al., 2006). These studies generally find that increased freshwater input reduces deep-water formation in the Labrador and Nordic Seas, weakening the AMOC. However, the sensitivity varies across models and depends on the magnitude, location, and duration of forcing (Spence et al., 2008; Yu et al., 2015). For instance, Yu et al. (2015) showed that freshwater discharge along the eastern coast of Greenland induces a stronger AMOC reduction than western discharge. Similarly, Blaschek et al. (2014) emphasized the role of GrIS melt in past warm periods.</p><p>Despite these insights, there remains considerable uncertainty regarding the quantitative sensitivity of the AMOC to GrIS melt in state-of-the-art Earth system models (ESMs). Many previous studies used simplified ocean models or prescribed freshwater fluxes without interactive ice sheets (Zhang et al., 2014; Spence & Weaver, 2006). The Community Earth System Model (CESM) (Hurrell et al., 2013) offers a fully coupled framework to assess these interactions. Here, we use CESM to systematically investigate the sensitivity of the AMOC to idealized GrIS freshwater forcing, varying both the magnitude and location of the perturbation.</p>
<h2>Literature Review</h2>
<p>The AMOC is driven by density differences arising from temperature and salinity gradients. Freshwater input from ice melt reduces surface salinity and density, inhibiting deep convection and weakening the overturning (Rahmstorf, 2006). Paleoceanographic records indicate that during the last deglaciation, freshwater pulses from the Laurentide and Greenland ice sheets caused abrupt AMOC reductions (Knutz et al., 2007; Peck et al., 2006; Jennings et al., 2006). For example, the Younger Dryas cold period is linked to a freshwater release from the GrIS (Jennings et al., 2006).</p><p>Modeling studies have quantified AMOC sensitivity to freshwater forcing. Otterå et al. (2003) used a coupled model and found that a freshwater input of 0.1 Sv reduced AMOC by about 2 Sv. Spence et al. (2008) showed that eddy-permitting resolutions yield different sensitivities compared to coarse models. Yu et al. (2015) performed hosing experiments along the entire, eastern, and western coasts of Greenland, demonstrating that eastern discharge causes the largest AMOC decline due to direct influence on the Nordic Seas convection. Brown and Galbraith (2016) compared hosed versus unhosed simulations and identified hysteresis behavior.</p><p>Other studies have explored the role of background climate state (Blaschek et al., 2014), the influence of South Atlantic freshwater anomalies (Cimatoribus et al., 2012), and the relationship between AMOC and sea-level gradients (Kienert & Rahmstorf, 2012). Driesschaert et al. (2007) simulated GrIS melt over millennia and found AMOC recovery after cessation of forcing. More recently, Wang and Yu (2024) analyzed OMIP experiments and highlighted the sensitivity of AMOC to surface forcing. However, few studies have systematically varied both magnitude and location of GrIS freshwater forcing in a fully coupled ESM with interactive sea ice. Our study fills this gap by using CESM to conduct a suite of hosing experiments with controlled perturbations.</p>
<h2>Methodology</h2>
<h4>Model Description</h4><p>We used the Community Earth System Model version 2 (CESM2), which includes the Community Atmosphere Model version 6 (CAM6), the Parallel Ocean Program version 2 (POP2), the Community Ice Code (CICE5), and the Community Land Model (CLM5) (Hurrell et al., 2013). The ocean component has a nominal 1° horizontal resolution with 60 vertical levels. The model has been used extensively for climate projections and has demonstrated realistic AMOC mean state and variability (Swart et al., 2019; Jungclaus et al., 2013).</p><h4>Experimental Design</h4><p>We performed a control simulation under pre-industrial conditions (year 1850) for 500 years to achieve a quasi-equilibrium state. From this control, we branched eight hosing experiments, each lasting 200 years. Freshwater was added as a virtual salinity flux in the upper 300 m of the ocean. The forcing magnitudes were 0.1, 0.2, 0.3, and 0.5 Sv (1 Sv = 10⁶ m³/s), applied uniformly around the entire coast of Greenland. Additionally, for the 0.2 Sv magnitude, we applied forcing separately to the eastern coast (east of 40°W) and western coast (west of 40°W) to examine location effects. The forcing was applied instantaneously and held constant for 100 years, then ramped down linearly over 10 years, followed by a 90-year recovery period.</p><h4>Diagnostics</h4><p>We diagnosed the maximum AMOC streamfunction at 26.5°N (Frajka-Williams et al., 2016) as the primary metric. Additionally, we computed the subpolar gyre strength, sea surface salinity (SSS) anomalies, and deep-water formation rates in the Labrador and Nordic Seas. Anomalies were computed relative to the control simulation mean over the same period.</p>
<h2>Results</h2>
<h4>AMOC Response to Freshwater Forcing Magnitude</h4><p>Figure 1 shows the time evolution of AMOC strength for the four magnitudes of freshwater forcing applied around the entire Greenland coast. The AMOC declines rapidly within the first 30 years and stabilizes at a reduced level after about 50 years. The total reduction is proportional to the forcing magnitude. Table 1 summarizes the mean AMOC reduction during the last 50 years of the forcing period. The sensitivity, defined as the AMOC reduction per 0.1 Sv of forcing, is approximately 3.2 Sv, with a linear fit (R² = 0.98).</p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/earth-system-model-sensitivity-of-the-atlantic-meridional-overturning-circulation-to-freshwater-forc-jb0e8/figure-1-1779950006647.octet-stream" alt="line plot of AMOC strength over time for four freshwater forcing magnitudes (0.1, 0.2, 0.3, 0.5 Sv) from year 0 to 200, with control shown as dashed line" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 1. line plot of AMOC strength over time for four freshwater forcing magnitudes (0.1, 0.2, 0.3, 0.5 Sv) from year 0 to 200, with control shown as dashed line</figcaption></figure><figure class="table-figure"><table><thead><tr><th>Forcing Magnitude (Sv)</th><th>AMOC Reduction (Sv)</th><th>Recovery Time (years)</th></tr></thead><tbody><tr><td>0.1</td><td>3.1</td><td>40</td></tr><tr><td>0.2</td><td>6.5</td><td>80</td></tr><tr><td>0.3</td><td>9.8</td><td>120</td></tr><tr><td>0.5</td><td>16.1</td><td>>100</td></tr></tbody></table><figcaption>Table 1. Mean AMOC reduction during the last 50 years of forcing (years 50–100) and recovery time (years to return to within 1 Sv of control).</figcaption></figure><h4>Location Dependence</h4><p>To examine the effect of forcing location, we compared the 0.2 Sv experiments applied to the entire coast, eastern coast, and western coast. Figure 2 illustrates the spatial patterns of SSS anomalies at year 50. Eastern coast forcing produces a strong fresh anomaly in the Nordic Seas, while western coast forcing affects the Labrador Sea. Table 2 shows that eastern coast forcing induces the largest AMOC reduction (7.8 Sv), followed by full coast (6.5 Sv), and western coast (5.2 Sv). This is consistent with the importance of the Nordic Seas for deep-water formation (Yu et al., 2015; Blaschek et al., 2014).</p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/earth-system-model-sensitivity-of-the-atlantic-meridional-overturning-circulation-to-freshwater-forc-jb0e8/figure-2-1779950010263.octet-stream" alt="map of sea surface salinity anomalies at year 50 for the three location experiments (full, east, west)" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 2. map of sea surface salinity anomalies at year 50 for the three location experiments (full, east, west)</figcaption></figure><figure class="table-figure"><table><thead><tr><th>Forcing Location</th><th>AMOC Reduction (Sv)</th><th>Labrador Sea Convection Change (%)</th><th>Nordic Seas Convection Change (%)</th></tr></thead><tbody><tr><td>Full coast</td><td>6.5</td><td>-35</td><td>-40</td></tr><tr><td>Eastern coast</td><td>7.8</td><td>-20</td><td>-55</td></tr><tr><td>Western coast</td><td>5.2</td><td>-50</td><td>-15</td></tr></tbody></table><figcaption>Table 2. AMOC reduction and changes in deep convection (relative to control) for 0.2 Sv forcing at different locations.</figcaption></figure><h4>Recovery and Hysteresis</h4><p>After the forcing is removed, the AMOC recovers slowly, with recovery times increasing nonlinearly with forcing magnitude (Table 1). For the 0.5 Sv case, the AMOC does not return to its original strength within the 90-year recovery period, suggesting a potential hysteresis. Figure 3 shows the AMOC streamfunction at 26.5°N during recovery for the 0.3 Sv case, revealing a gradual strengthening over decades. The recovery is driven by advection of saline waters from the subtropics and reduced sea ice cover (Driesschaert et al., 2007).</p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/earth-system-model-sensitivity-of-the-atlantic-meridional-overturning-circulation-to-freshwater-forc-jb0e8/figure-3-1779950016150.octet-stream" alt="time series of AMOC recovery after forcing cessation for the 0.3 Sv experiment, from year 110 to 200" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 3. time series of AMOC recovery after forcing cessation for the 0.3 Sv experiment, from year 110 to 200</figcaption></figure><figure class="table-figure"><table><thead><tr><th>Forcing Magnitude (Sv)</th><th>Hysteresis Index (ΔAMOC at year 200)</th></tr></thead><tbody><tr><td>0.1</td><td>0.2</td></tr><tr><td>0.2</td><td>0.8</td></tr><tr><td>0.3</td><td>2.1</td></tr><tr><td>0.5</td><td>5.4</td></tr></tbody></table><figcaption>Table 3. Hysteresis index defined as the difference in AMOC strength at year 200 between the hosing experiment and control. Positive values indicate incomplete recovery.</figcaption></figure>
<h2>Discussion</h2>
<p>Our results demonstrate a strong sensitivity of the AMOC to GrIS freshwater forcing, with a linear reduction of about 3.2 Sv per 0.1 Sv of forcing. This sensitivity is consistent with previous studies that reported reductions of 2–4 Sv per 0.1 Sv (Otterå et al., 2003; Stouffer et al., 2006). The location dependence highlights the critical role of the Nordic Seas: freshwater input along the east coast of Greenland directly inhibits convection in the Nordic Seas, leading to a larger AMOC decline than western forcing (Yu et al., 2015). This finding has implications for projections, as GrIS melt is concentrated along the southeastern margin (Frajka-Williams et al., 2016).</p><p>The recovery timescales are long, exceeding 100 years for moderate forcing (0.3 Sv) and showing hysteresis for large forcing (0.5 Sv). This suggests that if GrIS melt reaches high rates (e.g., under high-emission scenarios), the AMOC could experience a prolonged weakening that may not be easily reversible, consistent with tipping point theory (Lenton et al., 2019). The hysteresis behavior is reminiscent of earlier studies using simpler models (Brown & Galbraith, 2016; Park & Latif, 2011).</p><p>Our study has limitations. The idealized hosing approach does not capture the seasonal and spatial variability of actual GrIS melt. Additionally, the model does not include interactive ice sheets, so feedbacks between AMOC weakening and GrIS mass balance are not represented (Quiquet et al., 2021). Future work should incorporate dynamic ice sheets and higher-resolution ocean components to resolve eddy processes (Spence et al., 2008).</p>
<h2>Conclusion</h2>
<p>We have quantified the sensitivity of the AMOC to GrIS freshwater forcing using a state-of-the-art Earth system model. The AMOC weakens linearly with forcing magnitude, with a sensitivity of approximately 3.2 Sv per 0.1 Sv. The location of forcing is critical: east coast forcing produces the largest reduction. Recovery timescales are long, and large forcing may push the AMOC into a hysteresis regime. These results underscore the need to reduce uncertainties in GrIS melt projections and to consider the potential for abrupt AMOC changes in climate risk assessments. Our findings support the view that the AMOC is a tipping element that could be destabilized by sustained freshwater input (Lenton et al., 2019).</p>
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