Abstract The potential and actual forecast skill of surface air temperature (SAT) during boreal summer season (June to August: JJA) over South Asia (SA: 65°-100°E and 5°-30°N) is assessed in six ensemble prediction systems from the North American Multimodel Ensemble (NMME) project over the common period 1982–2016, and two different lead times (April and May). All models generally show the observed climatological pattern and variation of the JJA SAT over the region. However, there are sizeable errors in the simulation of the climatological mean SAT mainly over the topographically complex quarters in the area. A statistically significant positive correlation (0.39) is obtained between SA-SAT index and sea surface temperature (SST) anomalies in the El Nino Southern Oscillation (ENSO) region (190°-240°E and 5°S-5°N), which shows that ENSO modulates the summer SA-SAT, inducing warm (cold) conditions during El Nino (La Nina). Also, the SA-SAT index is highly positively correlated (0.75) with the SST anomalies in the tropical Indian Ocean (TIO: 40°-110°E and 20°S-20°N). Each prediction system captures the observed relationship between spatially averaged SA-SAT with SST and 200 hPa geopotential height during JJA, with varying details. Over South Asia, the resulting forecasts are characterized by high potential and moderate actual skill at both leads. Skill drops with the increase of lead time, though some unique models show even higher anomaly correlation coefficient for the predictions started at April observed initial condition. A multimodel ensemble (MME) average of six models shows lower potential predictability as compared to individual models at both lead times. However, the actual skill is higher as compared to different models at both lead times, showing an advantage of MME in predicting boreal summer SA-SAT. Together, these results indicate that current prediction models have quite high potential predictability and moderate forecast skill of boreal summer SAT over the South Asian region.

中文翻译：

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北美多模式集合中南亚北方夏季地表气温的潜在可预测性和技能评估

摘要 对南亚（SA：65°-100°E 和 5°-30°N）夏季（6 月至 8 月：JJA）地表气温（SAT）的潜在和实际预报技巧进行了六个集合评估。来自北美多模型集合 (NMME) 项目的预测系统在 1982 年至 2016 年的共同时期以及两个不同的提前期（4 月和 5 月）。所有模型一般都显示了该地区观测到的 JJA SAT 气候模式和变化。然而，主要在该地区地形复杂的地区，气候平均 SAT 的模拟存在相当大的误差。SA-SAT指数与厄尔尼诺南方涛动（ENSO）区域（190°-240°E和5°S-5°N）的海面温度（SST）异常之间获得统计上显着的正相关（0.39），这表明 ENSO 调节夏季 SA-SAT，在厄尔尼诺（拉尼娜）现象期间诱发温暖（寒冷）条件。此外，SA-SAT 指数与热带印度洋海温异常（TIO：40°-110°E 和 20°S-20°N）呈高度正相关（0.75）。每个预测系统都捕获了 JJA 期间空间平均 SA-SAT 与 SST 和 200 hPa 位势高度之间观察到的关系，具有不同的细节。在南亚，由此产生的预测的特点是潜力高，实际技能中等。技能随着提前期的增加而下降，尽管一些独特的模型显示出更高的异常相关系数，用于从 4 月观察到的初始条件开始的预测。与两个提前期的单个模型相比，六个模型的多模型集成 (MME) 平均值显示出较低的潜在可预测性。然而，与两个提前期的不同模型相比，实际技能更高，显示了 MME 在预测北方夏季 SA-SAT 方面的优势。总之，这些结果表明，当前的预测模型对南亚地区的北方夏季SAT具有相当高的潜在可预测性和中等预测能力。