摘要:本文通过与观测和再分析资料的对比，评估了LASG/IAP发展的气候系统模式FGOALS的两个版本FGOALS-g2和FGOALS-s2对南亚夏季风的气候态和年际变率的模拟能力，并使用水汽收支方程诊断，研究了造成降水模拟偏差的原因。结果表明，两个模式夏季气候态降水均在陆地季风槽内偏少，印度半岛附近海域偏多，在降水年循环中表现为夏季北侧辐合带北推范围不足。FGOALS-g2中赤道印度洋"东西型"海温偏差导致模拟的东赤道印度洋海上辐合带偏弱，而FGOALS-s2中印度洋"南北型"海温偏差导致模拟的海上辐合带偏向西南。水汽收支分析表明，两个模式中气候态夏季风降水的模拟偏差主要来自于整层积分的水汽通量，尤其是垂直动力平流项的模拟偏差。一方面，夏季阿拉伯海和孟加拉湾的海温偏冷而赤道西印度洋海温偏暖，造成向印度半岛的水汽输送偏少；另一方面，对流层温度偏冷，冷中心位于印度半岛北部对流层上层，同时季风槽内总云量偏少，云长波辐射效应偏弱，对流层经向温度梯度偏弱以及大气湿静力稳定度偏强引起的下沉异常造成陆地季风槽内降水偏少。在年际变率上，观测中南亚夏季风环流和降水指数与Ni?o3.4指数存在负相关关系，但FGOALS两个版本模式均存在较大偏差。两个模式中与ENSO暖事件相关的沃克环流异常下沉支和对应的负降水异常西移至赤道以南的热带中西印度洋，沿赤道非对称的加热异常令两个模式中越赤道环流季风增强，导致印度半岛南部产生正降水异常。ENSO相关的沃克环流异常下沉支及其对应的负降水异常偏西与两个模式对热带南印度洋气候态降水的模拟偏差有关。研究结果表明，若要提高FGOALS两个版本模式对南亚夏季风气候态模拟技巧，需减小耦合模式对印度洋海温、对流层温度及云的模拟偏差；若要提高南亚夏季风和ENSO相关性模拟技巧需要提高模式对热带印度洋气候态降水以及与ENSO相关的环流异常的模拟能力。
关键词:南亚夏季风/
模式偏差/
气候态与年际变率/
FGOALS模式
Abstract:Based on comparison with observational and reanalysis data, we assess the performances of two versions of the IAP/LASG Flexible Global Ocean-Atmosphere-Land System (FGOALS), FGOALS-g2 and FGOALS-s2, in simulating the climatology and interannual variability of South Asian summer monsoon (SASM). Moisture budget analysis is applied to explain the precipitation biases. FGOALS-g2 and FGOALS-s2 both underestimate precipitation over the continental monsoon trough but overestimate precipitation over the adjacent ocean. The northward seasonal migration of continental convergence zone is weaker than observation. The east-west sea surface temperature (SST) biases in the equatorial Indian Ocean (IO) simulated by FGOALS-g2 lead to weak southern intertropical convergence zone (ITCZ) over the eastern equatorial IO, while the south-north SST biases over the IO simulated by FGOALS-s2 result in southwestward shift of the ITCZ. Moisture budget analysis shows that precipitation biases in the FGOALS models are mainly attributed to the convergence of vertically integrated moisture flux biases, especially biases in the vertical dynamic moisture transport term. On the one hand, cold SST biases in the Arabian Sea and the Bay of Bengal along with warm SST biases in the tropical western IO reduce moisture flux over the Indian subcontinent in both models. On the other hand, cold biases of tropospheric temperature in the FGOALS models are most prominent in the upper troposphere over northern India. The FGOALS models also simulate weak longwave cloud radiative effects over the monsoon trough region due to their negative biases of cloud fraction over South Asia. The subsiding branches linked with the reduced meridional tropospheric temperature gradient and strengthened gross moist stability decrease climatological precipitation in the continental monsoon trough region. The FGOALS models cannot reasonably simulate the ENSO-SASM relationship at interannual time scale. The descending branch of the anomalous Walker circulation and corresponding negative precipitation anomalies are shifted to the tropical central-western IO to the south of the equator. The heating anomalies asymmetric about the equator enhance the northward cross-equatorial monsoon circulation and further cause erroneous positive precipitation anomalies over southern India. The shifts of the anomalous Walker circulation and negative precipitation anomalies are associated with the model biases in simulating climatological precipitation over the southern tropical IO. Our results show that reducing IO SST biases, tropospheric temperature biases and cloud biases is necessary for better simulation of mean state SASM by climate system models. On interannual time scale, the reasonable simulation of ENSO-monsoon relationship relies on successful simulation of climatological precipitation over the tropical IO and ENSO related circulation anomalies.
Key words:South Asian summer monsoon/
Model bias/
Climatology and interannnual variability/
FGOALS model
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