To analyze the global hydroclimate response during the Younger Dryas cold event, we evaluate climate model results that have been constrained with proxy-based temperatures from the North Atlantic region. We find that both the temperature and the hydroclimate response have a clear global signature. A marked cooling is simulated over the North Atlantic Ocean (more than 5 °C) and the downwind continents (2–4 °C). This response is related to the weakening of the Atlantic meridional overturning circulation under influence of meltwater discharges. The hydroclimate response is most expressed over Eurasia in a belt between 40 and 60°N, and over Northern Africa in the Sahel region. In both areas, a strong decrease in soil moisture is simulated (up to 20% reduction). In contrast, a striking increase in moisture is found over southeastern North America (15% increase), where southerly atmospheric flow brings moist air to the continent. Outside these areas that are clearly affected by the cold North Atlantic Ocean, the responses of temperature and moisture are decoupled, with different causes for these temperature and hydroclimate responses. In the tropics, the hydroclimate response is governed by the southward shift of the intertropical convergence zone (ITCZ) due to the cooling of the North Atlantic Ocean. This causes drier conditions north of the equator and wetter conditions in the Southern Hemisphere tropics. The associated changes in soil moisture are relatively gradual here, taking up to two centuries to complete, suggesting that the impact of the ITCZ shift on the tropical hydroclimate is building up. Our experiment indicates that Southern Hemisphere continents experienced a small cooling (less than 0.5 °C) during the Younger Dryas, caused by the negative radiative forcing associated with reduced atmospheric methane concentrations and enhanced dust levels. In our simulation, the bi-polar seesaw mechanism is relatively weak, so that the associated warming of the South Atlantic Ocean is not overwhelming the reduction in radiative forcing. Our results thus indicate that in the tropics and/or Southern Hemisphere, the cooling is a response to the negative radiative forcing, while the hydroclimatic changes are predominantly resulting from ITCZ variations. Consequently, when interpreting hydroclimatic proxy records from these regions, data should not be compared directly to key records from high latitudes, such as Greenland ice core stable isotope records. |
