Glacial changes in tropical climate amplified by the Indian Ocean

From DiNezio et al. (2018): "We used CESM1, a model that simulates realistic IPWP climate and is sensitive to changes in the configuration of land masses over the Maritime Continent—a response that is important for simulating glacial climates. CESM1 consists of coupled general circulation models of the atmosphere and ocean, as well as sea ice and land models. Other components of the Earth system, such as the carbon cycle and marine ecosystems, can also be simulated using CESM1; however, we kept them inactive because our focus here is on the climate response to glacial boundary conditions. The climate at the LGM was simulated by prescribing the following boundary conditions: (i) reduced GHG concentrations, (ii) insolation changes due to the orbital configuration at 21 ka, (iii) orography changes due to ice sheets and corresponding roughness length, (iv) surface albedo changes due to ice sheets, and (v) changes in the land-sea distribution and altitude due to lower sea level. A simulation of preindustrial climate was used as control. A series of simulations forced with individual glacial boundary conditions, also known as “single forcing” runs, were used to isolate the climate responses to different glacial drivers. The climate responses and single forcing simulations used to compute them are listed in Table 1. Full details on the implementation of the LGM boundary conditions and the single forcing simulations are available in the Supplementary Materials. The ensemble was augmented by simulations in which the CESM1 atmosphere and land models (CAM5 and CLM4) were coupled to ocean models of varying complexity. These simulations allowed us to explore the importance of ocean-atmosphere coupling in response to ice sheets. First, we replaced the fully dynamical ocean (POP2) with a model of the ocean mixed layer. In this model, the effect of vertical mixing, entrainment, and horizontal currents is prescribed as a seasonally varying heat source or Q-flux. Changes in SSTs computed by this “slab” ocean model can only be influenced by energy exchanges with the atmosphere, such as changes in evaporation or clouds. In the second case, the ocean model consists of climatological SSTs and sea ice extent from the preindustrial control. In this configuration, the air-sea heat fluxes are computed but cannot change the prescribed climatological SST and sea ice extent. Therefore, climate changes simulated in this configuration are due to “uncoupled” atmosphere or land changes. “Thermally coupled” and uncoupled responses were computed from each configuration as in the full-coupled cases by differencing the simulations as specified in Table 1."本内容引自DiNezio等（2018）的研究：我们采用了CESM1（Community Earth System Model 1）模式，该模式可逼真模拟印度洋-太平洋暖池（Indo-Pacific Warm Pool, IPWP）气候，且对海洋大陆（Maritime Continent）区域陆块配置的变化具有敏感性——这一响应特征对冰期气候的模拟至关重要。CESM1由耦合的大气与海洋环流模式、海冰模式及陆面模式组成。地球系统的其他组分（如碳循环与海洋生态系统）也可通过CESM1进行模拟，但本研究未启用这些模块，因我们的研究重点是冰期边界条件下的气候响应。 末次冰盛期（Last Glacial Maximum, LGM）的气候通过预设以下边界条件实现模拟：（i）降低的温室气体（Greenhouse Gas, GHG）浓度；（ii）距今21 ka（约2.1万年前）的轨道配置引发的日射变化；（iii）冰盖导致的地形变化及对应粗糙度长度；（iv）冰盖引发的地表反照率变化；（v）海平面下降引发的海陆分布与海拔变化。本研究以工业革命前气候模拟作为对照试验。 我们还开展了一系列仅由单个冰期边界条件强迫的模拟，即‘单强迫’试验，以分离不同冰期驱动因子对应的气候响应。用于计算气候响应的试验及单强迫模拟详见表1。关于LGM边界条件的实现方案与单强迫模拟的完整细节，可参见补充材料。 本研究的集合模拟还扩充了一类试验：将CESM1的大气与陆面模式（CAM5（Community Atmosphere Model 5）与CLM4（Community Land Model 4））与不同复杂度的海洋模式耦合。这类试验用于探究海气耦合过程对冰盖响应的重要性。首先，我们将完全动力海洋模式POP2（Parallel Ocean Program 2）替换为板状海洋模式（slab ocean model）：该模式中，垂直混合、卷夹与水平流的效应被设置为季节变化的热源或热通量（Q-flux）。该板状海洋模式计算得到的海表温度（Sea Surface Temperature, SST）变化仅受大气能量交换影响，如蒸发或云量变化。第二种配置下，海洋模式采用工业革命前对照试验的气候态海表温度与海冰范围，此时海气热通量虽可被计算，但无法改变预设的气候态海表温度与海冰范围。因此，该配置下模拟的气候变化仅由‘非耦合’的大气或陆面变化导致。 参照全耦合试验的方法，我们通过表1中指定的试验差值，分别计算了‘热耦合’与非耦合响应。