Potential predictability and skill assessment of boreal summer surface air temperature of South Asia in the North American multimodel ensemble

摘要

The potential and actual forecast skill of surface air temperature (SAT) during boreal summer season (June to August: JJA) over South Asia (SA: 65 degrees-100 degrees E and 5 degrees-30 degrees 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 degrees-240 degrees E and 5 degrees S-5 degrees 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 degrees-110 degrees E and 20 degrees S-20 degrees 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.