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Correcting the Double-ITCZ Bias Dials down Future Precipitation over Mediterranean Climate Regions in North Hemisphere

Presentation Date
Wednesday, December 16, 2020 at 8:50pm



The semi-arid Mediterranean climate regions feature low annual precipitation amount but outstanding wet winter and dry summer, distinct from most other regions on the Earth. This stark discrepancy between temperature and precipitation peaks makes the winter precipitation more important for managing water resources and crop production in these regions. In response to warming, climate models project a wetting US Southwest (USSW) and drying Mediterranean basin (MED) during winter. However, both projections remain hugely uncertain. Here we identify significant and mechanistic relationships between the present-day double Intertropical Convergence Zone (ITCZ) bias and projected USSW and MED precipitation changes. Models with excessive double-ITCZ biases tend to exaggerate the increased precipitation over USSW and understate the decreased precipitation over MED in the warmer future. The physical processes underpinning the connection are established through the analysis of CMIP5 model simulations and perturbation experiments using a dry atmosphere model. Over USSW, models with larger double-ITCZ biases tend to produce enhanced precipitation changes over tropical central Pacific as a “wet-get-wetter” response, which will strengthen subtropical Pacific jet and deepen Aleutian low, leading to a wetter USSW under warming. The larger double-ITCZ bias in models is also related with weaker mean-state Atlantic Meridional Overturning Circulation (AMOC). Interestingly, the weaker mean AMOC is found to be corresponding to weaker slowdown of the AMOC under warming, which will cause stronger surface warming in northern North Atlantic and negative North Atlantic Oscillation, leading to a wetter MED. Therefore, the tropical pathway is the key to connecting the double-ITCZ bias to the USSW precipitation changes, while the high latitude pathway is more important for the MED region. Constraining the present-day simulated double-ITCZ with the observed value, the projected precipitation is reduced by about 36% and 31% over USSW and MED, respectively. Dialing down the future precipitation has crucial implications for these regions already under severe water stress.

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