A mechanism for land–ocean contrasts in global monsoon trends in a warming climate
A central paradox of the global monsoon record involves reported decreases in rainfall over land during an era in which the global hydrologic cycle is both expected and observed to intensify. It is within this context that this work develops a physical basis for both interpreting the observed record and anticipating changes in the monsoons in a warming climate while bolstering the concept of the global monsoon in the context of shared feedbacks. The global-land monsoon record across multiple reanalyses is first assessed. Trends that in other studies have been taken as real are shown to likely be spurious as a result of changes in the assimilated data streams both prior to and during the satellite era. Nonetheless, based on satellite estimates, robust increases in monsoon rainfall over ocean do exist and a physical basis for this land–ocean contrast remains lacking. To address the contrast’s causes, simulated trends are therefore assessed. While projections of total rainfall are inconsistent across models, the robust land–ocean contrast identified in observations is confirmed. A feedback mechanism is proposed rooted in the facts that land areas warm disproportionately relative to ocean, and onshore flow is the chief source of monsoonal moisture. Reductions in lower tropospheric relative humidity over land domains are therefore inevitable and these have direct consequences for the monsoonal convective environment including an increase in the lifting condensation level and a shift in the distribution of convection generally towards less frequent and potentially more intense events. The mechanism is interpreted as an important modulating influence on the “rich-get-richer” mechanism. Caveats for regional monsoons exist and are discussed.
KeywordsMonsoon Climate change Tropical convection
This research was sponsored by NASA Awards NNX09AH89G-S01 and NNG06GB91G. I would like to acknowledge the modeling groups, the Program for Climate Model Diagnosis and Intercomparison (PCMDI) and the WCRP’s Working Group on Coupled Modeling (WGCM) for their roles in making available the WCRP CMIP3 multi-model dataset. Support of this dataset is provided by the Office of Science, US Department of Energy. The author would like to thank two anonymous reviewers for their constructive comments to improve this manuscript.
- Arkin PA, Smith T, Sapiano MRP, Janowiak J (2010) The observed sensitivity of the global hydrological cycle to changes in surface temperature. Environ Res Lett 5. doi: 10.1088/1748-9326/5/3/035201
- Khain AP (2009) Notes on state-of-the-art investigations of aerosol effects on precipitation: a critical review. Environ Res Lett 4 015004. doi: 10.1088/1748-9326/4/1/015004
- Ma J, Xie S-P, Kosaka Y (2011) Mechanisms for tropical tropospheric circulation change in response to global warming. J Clim (in press)Google Scholar
- Meehl GA et al (2007) Global climate projections. In: Solomon S et al (eds) Climate change 2007: the physical science basis. Cambridge University Press, Cambridge, pp 747–845Google Scholar
- Neelin JD (2007) Moist dynamics of tropical convection zones in monsoons, teleconnections and global warming. In: Schneider T, Sobel A (eds) The global circulation of the atmosphere. Princeton University Press, Princeton, p 385Google Scholar
- Simmons A, Uppala S, Dee D, Kobayashi S (2007) ERA-interim: new ECMWF reanalysis products from 1989 onwards. ECMWF Newsletter No. 110Google Scholar
- Trenberth KE, Fasullo JT (2009) Global warming due to increasing absorbed solar radiation. Geophys Res Lett. doi: 10.1029/2009GL037527
- Trenberth KE, Fasullo JT, Mackaro J (2011) Atmospheric moisture transports from ocean to land and global energy flows in reanalyses. J Clim (in press)Google Scholar
- Webster PJ, Yang S (1992) Monsoon and Enso: selectively interactive systems. Q J R Meteorol Soc 118:507, 877–926. doi: 10.1002/qj.49711850705