Persistence of systematic errors in the Asian-Australian monsoon precipitation basic states in climate models: a way forward
- Univ. of Hawaii, Honolulu, HI (United States)
- Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Kanagawa (Japan)
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Indian National Centre for Ocean Information Services (INCOIS), Hyderabad (India)
- Centro Euro-Mediterraneo per i Cambiamenti Climatici (CMCC)/Istituto Nazionale di Geofisica e Vulcanologia, Bologna (Italy)
- UK Meteorological Office, Exeter (United Kingdom)
- Collaboration for Australian Weather and Climate Research (CAWCR), Melbourne (Australia)
The annual cycle of the Asian-Australian monsoon (AAM) system can be regarded as the seasonal displacement of the large-scale Intertropical Convergence Zone (ITCZ), which is anchored by the north-south migration of the Indo-Pacific warm pool. In the respective hemispheres, intense solar heating over land during spring and early summer provide the necessary thermodynamic conditions for the occurrence of deep convection off the equator. The rainfall and diabatic heating associated with the AAM is perhaps the most vigorous of all the regional monsoon components in the globe. Yet, skill in monsoon prediction (days to seasons) by dynamical models remains low, partly due to our lack of understanding of the entirety of the monsoon system and our inability to model the interactive processes that govern it. Simulating the monsoon precipitation climatology remains a grand challenge. The multi-model mean (MMM) errors for summertime precipitation relative to GPCP observations have shown little improvement in CMIP5 as compared to CMIP3. For the Asian Summer Monsoon, the MMM monsoon rainfall is weaker over South Asia, the western equatorial Indian Ocean and tropical West Pacific. For the Australian summer monsoon, it is excessive over most of the Maritime Continent, and deficient over northern Australia. Throughout the year, excessive rainfall is simulated over South China Sea – Maritime Continent regions, and CMIP5 models do not capture the annual cycle of the AAM. One implication is that uncertainties in future projections (e.g., IPCC 2013) of AAM mean rainfall may not have reduced from CMIP3 to CMIP5. Solutions from an intermediate model show that diabatic heating (Q) associated with the AAM influences the global circulation. Compared to the reanalysis, many models tend to have maxima at the mid-troposphere but their simulated amplitude is overestimated in the lower troposphere (900-700 hPa) and underestimated from 600-300 hPa, a feature readily apparent in the MMM composite (dashed black line). The lower troposphere peak may be attributed to misrepresentations in shallow convection. Some outliers, such as CSIROMk3- 6-0 and ACCESS1-3, do not show any appreciable vertical structure, and the simulated monsoon over South Asia is virtually absent in these models. TRMM observations indicate that over the monsoon region the contribution from stratiform rainfall is about 40% to the Q intensity. In contrast, most of the CMIP3 models produce too much convective rainfall (95% of the total) and too little stratiform precipitation. Given the persistence of errors, we speculate that errors in the partitioning of total rainfall into convective-stratiform still persist in CMIP5, and may be one of the reasons for underestimation (overestimation) of Q in the layer 600-300 (900-700) hPa. In summary, the rather slow progress in modeling in the last decade or so led us to wonder: Has the scientific community reached a “plateau” in modeling mean monsoon precipitation?
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC52-07NA27344
- OSTI ID:
- 1178403
- Report Number(s):
- LLNL-TR-663804
- Country of Publication:
- United States
- Language:
- English
Similar Records
Intraseasonal, Seasonal, and Interannual Characteristics of Regional Monsoon Simulations in CESM2
Sources of errors in the simulation of south Asian summer monsoon in the CMIP5 GCMs