Overshoot Phenomenon Of Thermohaline Circulation During B(?)lling-Aller(?)d Warming Event

As the oceanic deep circulation system with global scale, and one important factor of meridional heat transport in earth climate system, Thermohaline Circulation (THC) is critical to the formation and variation of the earth climate. Overshoot phenomenon is a transient behavior of THC, defined as the temporal enhanced THC intensity with hundreds years scale during the THC recovery period after perturbed. The existence of overshoot enlarges the magnitude of THC change, and then induces more intense climate change. In the first transient simulation of last deglaciation with fully coupled model (TraCE-21000), THC overshoot is proposed to be one key factor to the onset of Bφlling-Allerφd Event (BA, the most pronounced abrupt climate change event within past 20,000 years). After the concept of THC overshoot is formally brought forward for the first time in this simulation, there is still no works to study this phenomenon systematically, so far. Through the comprehensive analysis of TraCE-21000 simulation results, additionally with the implementation of box-model, multiple water-hosing experiments, "Partial blocking" sensitivity experiments and paleoclimate records, we systematically investigate its dynamical basis, historic occurrence, formation processes, formation mechanism and climatic impact to China region in this PhD thesis. The main results are listed below:1 The theorical analysis and simulation results of box-model show that, the dynamical basis of overshoot relies on the "Oscillatory Stable Mode" of THC, the magnitude of overshoot increase exponentially during the THC mean state approaching Stommel Bifurcation Point, the anomaly southward deep salinity gradient induces the occurrence of THC overshoot during recovery period.2 Investigation of TraCE-21000 simulation shows that, the robust response of deep-circulation to THC overshoot at BA are regionally, only specific regions can capture the THC overshoot signal in deep-circulation. The different appearances of paleoclimate records to THC overshoot at BA are consistent with TraCE-21000 simulation, according to the location of records. Our results heighten the reliability of the occurrence of THC overshoot at BA.3 Investigation of TraCE-21000 simulation shows that, the recovery process of THC during BA onset has a "two-stage" feature, the reinitiation of North Atlantic Deep Water (NADW) first occur at Labrador Sea in first stage, and then occur at GIN Sea in second stage. THC overshoot is generated in the second stage. The consequent result of overshoot occurrence at BA is the mean state transition of THC from glacial to interglacial.4 "Partial Blocking" sensitivity experiments show that, the reinitiation process of NADW in GIN Sea dominates the occurrence of THC overshoot at BA. Comparing to the mechanism proposed in box-model, the mechanism shown in this part is more realistically and reliable.5 "Partial Blocking" sensitivity experiments also show that, the occurrence of THC overshoot at BA significantly impact the climate of China region, enlarging the magnitude of warming (the maximum value is over 2℃), and intensifying the northward shift of rain belt during BA onset. In this PhD thesis, the research about THC overshoot at BA derive from the coupled model results of the transient simulation for last deglaciation, idealized water-hosing experiments, diagnostic sensitivity experiment, and the theorical analysis and simulation results of box-model, also the paleoclimate records. This work systematically investigates series of questions in THC overshoot, and the results contribute to the understanding of this new concept of THC variation, and the mechanism of abrupt climate change.Meanwhile, a new measure of NADW formation amount in two origins is proposed and successfully implemented in series of works of this PhD thesis, which based on the Atlantic Meridional Overturning Circulation stream function. Comparing to the traditional measures, the new measure can provide more quantitatively information, and the calculation is more convenient, objective and high compatible to different THC variation situations. The implementation of the new measure could significantly increase the analysis skill of THC.