The Formation Mechanism Of The Winter Mixed Layer In Subtropical Northeast Pacific And The Response To Increased Radiation Forcing

The mixed layer is deep in winter over the Subtropical Northeast Pacific (160°W-110°W,10°N-40°N), which plays an important role in transporting the subtropical ocean-atmosphere interaction signal to the tropical ocean through the subduction and advection processes, so the depth of the mixed layer in winter over this region is important for the climate change. The mixed layer depth (MLD) spatial pattern in present climate simulation and future projection are investigated based on the coupled general circulation models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Compared with the Levitus and ARGO observation data, it is found that the spatial pattern of the MLD is non-uniform in present climate, and the contribution of the ocean heat advection is shown quantitatively. The responding characteristics and mechanism of the MLD with increased radiation forcing is analyzed. Ekman pumping, sea surface net heat flux and ocean heat advection change are all effective with increased radiation forcing. Finally, the characteristics of subduction rate in present climate simulation and future projection are also estimated quantitatively and the MLD non-uniform spatial pattern is important for the subduction process. The main innovations of this research are as follows:1. According to the ARGO observation data, the spatial pattern of February-March mean MLD over the subtropical Northeast Pacific is non-uniform:the MLD maximum (130m) appears at the (28°N,140°N), and most of the MLD in (25°N-32°N, 150°N-135°N) is more than 90m. The MLD shoals from the maximum center, faster to the south than to the north, and it forms a banded MLD front at the south MLD maximum region, with the intensity over 0.24m/km. The Levitus data shows a similar pattern. All the nine CMIP5 models display the similar MLD non-uniform spatial pattern in the historical simulation, especially the MLD front at the south MLD maximum region.2. The contributions of the main factors on the MLD non-uniform spatial pattern in the historical simulation of the nine CMIP5 models are calculated quantitatively. Existed in the eastern part of the subtropical gyre, the thermocline in the subtropical Northeast Pacific is shallower than that in the Western Pacific. By changing the thermocline, the seasonal variability of the Ekman pumping could affect the location of the MLD maximum in winter, especially on the northeast boundary of the maximum region; sea surface net heat flux can contribute to deepening the MLD in winter and determine the northwest boundary of the maximum region; while the main reasons for the banded MLD front are the different temperature advections on both sides of the MLD front:to the north of the MLD front, the upper ocean cold advection helps deepen the MLD by about 21.4m/mon, which is almost the same as the contribution of the sea surface net flux (about 30.4m/mon); to the north of the MLD front, the upper warm advection shoals the MLD by about 29.0m/mon, which is opposite to the contribution of the net flux (about 33.4m/mon), leading to the MLD south of the MLD front shallower than that north of the MDL front.3. Compared with the present climate in the nine models, the response of the MLD in subtropical Northeast Pacific to increased radiation forcing is also non-uniform in spatial pattern. The MLD maximum shoals about 50m with increased radiation forcing in the ensemble mean result of the nine CMIP5 models. The MLD to the north of the MLD front shoals more than that to the south of the MLD front. As a result, the MLD front intensity weakens obviously from 0.24m/km to 0.15m/km. In warmer climate, the upper ocean is more stratified with the risen temperature and the MLD shoals in the whole region, but the weakened warm advection, caused by the reduced northeast trade wind, leads to the MLD shoals not that much to the south of the MLD front.4. It is shown in the ensemble mean result of the nine CMIP5 models that the Ekman pumping rate and the lateral induction rate have the similar contribution to the subduction rate in the major subduction region (20°N-28°N,145°W-120°W), but the non-uniform spatial pattern of the subduction rate is primarily dominated by the lateral induction rate, as the spatial differences of the Ekman pumping rate is very small. So the MLD front is the main subduction region and the main source region of the North Pacific Eastern Subtropical Mode Water. The subduction rate in ensemble mean is weakened by 0.47×10-6 m·s-1 with increased radiation forcing. As the Ekman pumping rate changes small, the reduction of the lateral induction rate (by 0.40－10-6 m·s-1) make a contribution to the reduced subduction rate more than 85%. The non-uniform shoaling of the MLD causes the weakening of the MLD front and then leads to the reduction of the lateral induction rate. As a result, the mode water volume is decreased.