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Tropical Rainfall Change - Patterns and Mechanisms

PI: J. David Neelin
Institution: UCLA
Additional Investigators: Joyce Meyerson, Hui Su, Christopher Holloway, Chia Chou, Melissa Strausberg
Climate model global warming simulations predict large regional changes in tropical rainfall, including regions of drought. Qualitatively similar changes occur during El Nino interannual variability. Using an intermediate climate model, we have identified a mechanism that creates regional reductions in precipitation at the margins of convection zones during warming. In this ``upped-ante mechanism'', a warm troposphere increases the value of surface boundary layer moisture required for convection to occur. In regions of plentiful moisture supply, moisture simply rises to maintain precipitation, but this increases the moisture gradient relative to neighboring subsidence regions. Reductions in rainfall then result for those margins of convection zones that have strong inflow of air from the subsidence regions and less frequently meet the increased ``ante'' for convection. In the intermediate climate model simulations analyzed, the upped-ante mechanism is the leading cause of tropical drought in the global warming case and is dominant in certain El Nino drought regions. In the ``anomalous gross moist stability mechanism", gross moist stability is reduced due to increased ABL moisture. As a result, convection is enhanced and precipitation becomes heavier over convective regions. While the upped-ante mechanism induces negative precipitation anomalies over the margins of convective regions, the anomalous gross moist stability mechanism induces positive precipitation anomalies within convective regions. The importance of variation in gross moist stability, which is likely to differ among climate models, is suggested as a potential factor causing discrepancies in the predicted tropical regional precipitation changes.

We propose to evaluate the GCM output for the following relationships: tropical tropospheric temperature and moisture relations; moisture increase in convection zones relative to other regions; relationship of drought to convective margins with inflow and to moisture gradients; terms from the moist static energy (MSE) budget useful to pathway diagnostics; contribution of surface fluxes over ocean regions as a signature of ocean dynamical feedbacks compared to other MSE budget terms. These relationships will be evaluated for consistency with hypothesized mechanisms and for indications of other mechanisms that might be at work.

Detailed evaluation will center first on changes for 30 year averages in the late 21st century when the warming is well established and the signals are large. We will begin with the SRES A1B case (relative to control) and the 2xCO2 equilibrium simulations with mixed-layer oceans which provide a slightly simpler comparison case. Once we believe we have established a reasonable set of mechanisms and that the evaluation procedures are working, we will then apply to the other available runs and time periods. We will conduct a survey of precipitation changes over the available runs and time periods early on, to guesstimate if there will be suprises relative to A1B and add periods/runs to the initial evaluation if needed.

Based on preliminary assessment of data from the IPCC/DDC site and from ECHAM4, we anticipate that the mechanisms will differ more among models than among scenarios for a given model. We also have indications that the multi-model ensemble approach seen in IPCC AR3 has severe limitations for tropical rainfall anomalies. For instance, at least one model that predict rainfall increases in subsaharan Africa does so because in the model climatology the region is in the center, not the margin, of a strong convection zone. We postulate that it may be possible to constrain the model subset used for each region based on criteria for climatology We also have seen indications from AMIP2 data that the models behave quite differently, some rather imperfectly, in ENSO teleconnection response. Unfortunately, the AMIP2 data are not coordinated with AR4, although some models overlap. We will take a preliminary look at using ENSO response as a means of evaluating quality of model response for particular sensitive regions, such as eastern equatorial South America, although this part is less likely to be completed in the tight time frame for AR4 turnaround.

Fields to be used include all surface and top-of-atmosphere fluxes, surface precipitation, and atmospheric 3-D winds, moisture and temperature. While the data specifications did not include divergence or the divergent wind component nor certain moisture flux terms useful in MSE budgets, we will use the available data. We can then provide feedback to the WGCM on the errors introduced by post-calculation of such terms, and on additional fields that it would be desirable to have for future comparisons for diagnosing tropical moist processes.
  • Chou, C, and C.-A. Chen, 2010: Depth of convection and the weakening of tropical circulation in global warming. J. Climate. In press. Abstract. Full Article. Edit.
  • Chou, C, and J.-Y. Tu, 2008: Hemispherical asymmetry of tropical precipitation in ECHAM5/MPI_OM during El Nino and under global warming. J. Climate, 21, 1309-1332. Abstract. Full Article. Edit.
  • Chou, C., J.-Y. Tu, and P.-H. Tan, 2007: Asymmetry of tropical precipitation change under global warming. Geophy. Res. Lett., 34, 10.1029/2007GL030327. Abstract. Full Article. Edit.
  • Chou, C., J.D. Neelin, C.A. Chen, and J.Y. Tu, 2009: Evaluating the “rich-get-richer” mechanism in tropical precipitation change under global warming. J. Climate, 22, 1982-2005. Abstract. Full Article. Edit.
  • Neelin, J. D., M. Munnich, H. Su, J.E. Meyerson and C. Holloway, 2006: Tropical drying trends in global warming models and observations. Proc. Natl. Acad. Sci., 103, 6110-6115, doi:10.1073/pnas.0601798103. Abstract. Full Article. Edit.
  • Tan, P.H., C. Chou, and J.Y. Tu, 2008: Mechanisms of global warming impacts on robustness of tropical precipitation asymmetry. J. Climate, 21, 5585-5602. Abstract. Full Article. Edit.

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