Summer Rainfall-SST Relationships in the Western North Pacific Simulated by the FGOALS Model with Ocean Assimilation
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摘要: 评估了耦合气候系统模式FGOALS海洋同化试验对西北太平洋夏季降水和SST相关关系的模拟技巧,并对比了相应的观测海温强迫试验(AMIP)和历史气候模拟试验结果。结果显示,FGOALS海洋同化试验对亚洲季风区大部分海域夏季SST年际变化有较高的模拟技巧,但其对菲律宾以东海域模拟技巧较低。在西北太平洋夏季降水-SST相关关系方面,同化试验部分地再现了南海和菲律宾以东海域降水超前SST变化1个月和同时二者的负相关关系,优于AMIP试验但逊于自由耦合模拟试验。同化试验对SST倾向-降水相关关系的模拟技巧亦介于AMIP试验和自由耦合试验之间。观测中,西北太平洋夏季降水与环流异常受日界线附近和赤道东印度洋海洋大陆地区海温异常的遥强迫,并通过改变到达海表的净短波辐射通量影响局地SST异常,导致局地海温-降水和局地海温倾向-降水的负相关关系。在AMIP试验中,遥强迫导致的西北太平洋地区环流异常较之观测偏弱,由于缺少局地海气耦合过程,在西北太平洋多数地区表现为海温对大气的强迫作用,即SST-降水正相关关系。FGOALS同化试验和自由耦合试验考虑了局地海气耦合过程,虽然低估了遥强迫对西北太平洋地区夏季环流异常的影响,依然部分模拟出局地降水-SST负相关关系但较之观测偏弱。同时,自由耦合试验高估了西北太平洋20°N以南地区海温异常对大气环流异常的强迫,使得其对中国南海和日本岛以南海域SST-降水负相关关系的模拟稍优于同化试验。Abstract: This study evaluates the performance of FGOALS (Flexible Global Ocean-Atmosphere-Land surface-Sea ice coupled model) with ocean assimilation in the simulation of summer rainfall-SST relationship during 1979-2005 in the western North Pacific (WNP), and compares the results with corresponding simulations forced by observed sea surface temperature and FGOALS historical simulation. Results show that the FGOALS with ocean assimilation well captures the interannual variability of summer SST over the WNP except that over east of the Philippines. For the interannual variability of precipitation, it barely demonstrates any skill over Asian summer monsoon region, which is comparable to the AMIP (Atmospheric Model Intercomparison Project) simulation. However, for the summer rainfall-SST relationship, the observed negative correlations over South China Sea and east of Philippines are partly reproduced in the FGOALS with ocean assimilation, in particular when the precipitation leads SST by one month and is concurrent with SST. The simulated skill is better than the AMIP simulation, but is inferior to the historical simulation. Based on observations, anomalous convection and circulation in the summer over the WNP are primarily driven by SST anomalies over the area near the dateline and the eastern Indian Ocean-Maritime Continent. The induced anomalous convections affect solar radiation reaching the sea surface, which contributes significantly to local SST anomalies and leads to negative SST-rainfall correlation and SST tendency-rainfall correlation. In the AMIP simulation, the anomalous circulation over the WNP driven by the remote forcing is underestimated. Since the AMIP simulation is forced by observed SST, the anomalous convection and circulation are forced by underlying SST over some places of the WNP, leading to positive rainfall-SST correlation. Although the anomalous circulations over the WNP driven by the remote forcing are also underestimated in both FGOALS with ocean assimilation and historical simulation, weaker than observed negative SST-rainfall correlations are produced since local air-sea coupling is included. In addition, the historical simulation tends to overestimate the forcing from SST anomalies over the WNP south of 20°N, which leads to better simulated SST-rainfall correlation than the FGOALS with ocean assimilation over South China Sea and south of Japan islands.
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图 1 FGOALS海洋同化试验模拟的1979~2005年夏季(a)SST异常和(b)降水异常与观测的相关系数分布,(c)为FGOALS AMIP试验模拟的1979~2005年夏季降水异常与观测的相关系数分布(等值线代表相关系数通过0.05和0.1显著性水平检验)
Figure 1. Simulated skills of June–August (JJA) (a) SST anomalies and (b) rainfall anomalies for the period of 1979–2005 by the FGOALS (Flexible Global Ocean–Atmosphere–Land surface–Sea ice coupled model) with ocean assimilation measured by correlation coefficients with observations. (c) is the corresponding skills of JJA rainfall anomalies simulated by the FGOALS AMIP (Atmospheric Model Intercomparison Project) simulations. Counter lines represent the 0.05 and 0.1 siginificance levels
图 2 (a) 观测、(b)同化试验、(c)AMIP试验和(d)自由耦合模拟试验模拟的1979~2005年夏季降水异常与局地SST的相关系数分布(线条代表相关系数通过0.05和0.1显著性水平检验)
Figure 2. Spatial patterns of correlation coefficients between the JJA SST and precipitation anomalies of 1979-2005 for (a) observations, (b) FGOALS with ocean assimilation, (c) AMIP simulation, and (d) historical simulation. Counter lines represent the 0.05 and 0.1 siginificance levels
图 3 基于1979~2005年每年5~10月资料,计算的观测(第一行)、同化试验(第二行)、AMIP试验(第三行)和自由耦合试验(第四行)模拟的降水超前SST 1个月(左列)、同时(中)和滞后1个月(右列)SST与降水异常的相关系数空间分布(每个时间序列的样本数为108,线条代表相关系数通过0.05和0.1显著性水平检验)
Figure 3. (First row) Observed lead-lag correlation between monthly mean SST and precipitation anomalies for May through October of 1979-2005: (a) The correlation coefficients when precipition leads SST by one month, (b) is concurrent with SST, and (c) lags SST by one month. The sample size is 108 for the time series at each grid. The second row is the same as the first row, except that the correlations are calculated from the FGOALS with ocean assimilation. The third (fourth) row is the same as the first row except for AMIP (historical) simulation. Counter lines represent the 0.05 and 0.1 siginificance levels
图 5 观测的1979~2005年(5°N~35°N,110°E~140°E)区域平均夏季SST倾向标准化时间序列与(a)SST (单位:℃)、(b)SST倾向(单位:×5 ℃/month)、(c)降水(单位:mm/d,填色)和1000 hPa风场(单位:m/s,矢量)、(d)潜热通量(单位:×5 ℃/month,向上为正)、(e)10 m风速(单位:m/s)、(f)海表净短波辐射(单位:×5 ℃/month,向下为正)的回归系数。黑点区域代表回归系数通过0.1显著性水平检验
Figure 5. Simultaneous regression with respect to normalized JJA SST tendency in the region of (5°N-35°N, 110°E-140°E) for the period of 1979-2005 in the observations: (a) SST (℃); (b) SST tendency (×5 ℃/month); (c) rainfall (units: mm/d, shading) and wind at 1000 hPa (units: m/s, vector); (d) latent heat flux (units:×5 ℃/month, upward is positive); (e) wind speed at 10 m (m/s); (f) surface shortwave radiation (units:×5 ℃/month, downward is positive). The black dotted areas indicate that the regression coefficients are statistically significant at the 0.1 level by a two-tailed student's t test
图 6 AMIP模拟的1979~2005年(5°N~35°N,110°E~140°E)区域平均夏季SST标准化时间序列与(a)SST (单位:℃)、(b)降水(单位:mm/d,填色)和1000 hPa风场(单位:m/s,矢量)、(c)潜热通量(单位:×5 ℃/month,向上为正)、(d)10 m风速(单位:m/s)、(e)海表净短波辐射(单位:×5 ℃/month,向下为正)的回归系数。黑点区域代表回归系数通过0.1显著性水平检验
Figure 6. Simultaneous regression with respect to normalized JJA SST in the region of (5°N-35°N, 110°E-140°E) for the period of 1979-2005 derived from AMIP simulation: (a) SST (℃); (b) rainfall (units: mm/day, shading) and wind at 1000 hPa (units: m/s, vectors): (c) latent heat flux (units:×5℃/month, upward is positive); (d) wind speed at 10m (m/s); (e) surface shortwave radiation (units:×5℃/month, downward is positive). The black dotted areas indicate that the regression coefficients are statistically significant at the 0.1 level by a two-tailed student's t test
表 1 所用到的试验及其特点说明
Table 1. Description of numerical simulations and their characteristics
试验名称 SST 特点 FGOALS AMIP试验 观测月平均海温 完全没有考虑海气耦合过程 FGOALS同化试验 耦合模式模拟,但同化了观测海温 部分地考虑了海气耦合过程 FGOALS historical试验 耦合模式模拟 完全考虑了海气耦合过程,但模拟SST较之观测有漂移 -
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