Origin of Inter-model uncertainty in ENSO amplitude change Shang-Ping Xie Scripps Institution of Oceanography, UCSD Zheng, X.T., S.-P. Xie, L.H. Lv, and Z.Q. Zhou, 2016: Intermodel uncertainty in ENSO amplitude change tied to Pacific ocean warming pattern. J. Climate, submitted.
ENSO response to global warming Change in ENSO SST variance is uncertain. IPCC AR5 Ch14 Figure 14.14: Standard deviation of Niño3 SST anomalies from CMIP5 model experiments. PI, 20C, RCP4.5, RCP8.5 indicate pre-industrial control experiments, 20th century experiments, and 21st century experiment from the RCP4.5 and RCP8.5. Open dot and solid black line indicate multi-model ensemble mean and median, respectively, and the cross mark is 20th observation, respectively. Thick bar and thin outer bar indicate 50% and 75% percentile ranges, respectively.
21st-20th century change in El Nino anomalies Power et al. (2013, Nature) Intensified convective anomalies during El Nino, without changes in SST anomalies. El Nino anomalies & change (color) δp
El Nino amplitude changes Increased variance in rainfall and zonal wind. Why? 21st to 20th century ratio of ENSO amplitude for RCP8.5: CMIP5 multi-model ensemble mean (bar) and inter-model variability (box-and-whisker).
Upper air temperature increase = tropical mean ocean warming Atmospheric instability = Relative SST: Ocean warming pattern T * = T T tropical-mean SST threshold increases with tropical mean SST Johnson & Xie (2010, Nat Geo)
21st-20th century difference in El Nino anomalies Power et al. (2013, Nature) Intensified convective anomalies during El Nino, without changes in SST anomalies. Eq. peak in mean warming reduces the barrier for atmospheric convection. T* Fig. 14.8 (IPCC AR5). Relative warming to tropical mean ~ change in SST threshold
Increased T * Niño3 and ENSO-induced rainfall under global warming 1910-2000 2006-2095 Increased relative SST and ENSO-induced rainfall in the eastern equatorial Pacific due to mean state of warming pattern.
Two representative models Intensified rainfall var.
Changes in mean state and ENSO amplitude are correlated. T* Niño3 > 0 Large relative mean Nino3 SST Intensified atmospheric feedback Increased SST amplitude Inter-model scatterplots between T * Niño3 ( o C) and the 21st to 20th century ratio of standard deviation for (a) P Niño3, and (c) T Niño3.
Summary El Nino rainfall anomalies intensify under global warming as mean warming pattern reduces barrier to convection in the eastern Pacific. Inter-model uncertainty in ENSO amplitude change is related to that in mean state relative warming. Convective feedback, which is tied to the warming pattern, determines ENSO amplitude change. T*
Increased rainfall & storminess in El Nino winter on U.S. west coast in warmer climate. Intensified ocean warming in eastern Pacific Z. Zhou et al. (2014, JC)
AMIP tests warming pattern effect on ENSO change AMIP experiments Number of models amip runs: observed SST (1979-2008) 11 amip4k runs: observed SST (1979-2008) + uniform warming (4K) 11 amipfuture runs: observed SST (1979-2008) + SST warming pattern 11
SST pattern effect: AMIP experiments SST pattern 21st/20th ratio of σ(p ) in Nino3 Uncertainty due to SST pattern dominates; SST pattern intensifies rainfall variance.
Is the La Nina cooling natural or forced? Three mechanisms for forced SST pattern in Eq. Pacific Ocean dynamic thermostad reduced warming in the east (visible in CMIP models in seasonality of the warming. Weakened Walker circulation enhanced warming in the east. Evaporative damping enhanced warming on the equator and in the east. In CMIP models The equatorial peak in surface warming is common. More models favor enhanced warming in the east. Need to develop observational constraints on relative importance of these mechanisms, e.g., by studying the hiatus.
The warmer-get-wetter (Xie et al. 2010, JC) Coupled model: change in SST (contour) and precipitation The wet-get-wetter pattern is realized in atmospheric response to uniform SST warming. What about in coupled simulations with patterned warming? 2K uniform SST warming: mean (contour) and change of precipitation Wet-get-wetter pattern