Abstract:

During extreme El Nino events, the Intertropical Convergence Zone moves to the normally cold and dry east equatorial Pacific, resulting in a nonlinear rainfall increase with sea surface temperature in the region. An arbitrary threshold value of boreal winter total rainfall (e.g., 5 mm d(-1)) in the east equatorial Pacific was used by previous studies to capture this feature. Under greenhouse warming, the frequency of extreme El Nino events is projected to increase, so is the mean east equatorial Pacific rainfall. Is the projected frequency increase a consequence of the mean rainfall increase? We show that the projection is not significantly influenced by the increased mean rainfall. Instead, the increased frequency accounts for approximately 50% of the mean rainfall increase. Using upward atmospheric vertical velocity for defining extreme El Nino reaffirms the conclusion that the increased frequency results from increased probability of atmospheric deep convection, as the eastern equatorial Pacific warms faster than the surrounding regions.

Plain Language Summary An extreme El Nino event can be identified by its unusually high rainfall over the eastern equatorial Pacific Ocean exceeding a threshold of 5mmd(-1). This identification is supported by upward atmospheric velocity in the region. Using these definitions and statistical analysis, we show that the projected increase in frequency of extreme El Nino in response to greenhouse warming is not merely a consequence of increasing trends in the mean rainfall. Rather, it is due to increased probability of atmospheric deep convection made conducive by the faster warming of the eastern equatorial Pacific Ocean than the surrounding regions.

KeyWords: PACIFIC; ENSO

Authors: Cai, WJ (Cai, Wenju)^{[ 1,2,3 ]} ; Wang, GJ (Wang, Guojian)^{[ 1,2,3 ]} ; Santoso, A (Santoso, Agus)^{[ 3,4 ]} ; Lin, XP (Lin, Xiaopei)^{[ 1,2 ]} ; Wu, LX (Wu, Lixin)^{[ 1,2 ]}

[ 1 ] Ocean Univ China, Phys Oceanog Lab, CIMST, Qingdao, Peoples R China

[ 2 ] Qingdao Natl Lab Marine Sci & Technol, Qingdao, Peoples R China

[ 3 ] CSIRO Oceans & Atmosphere, Ctr Southern Hemisphere Oceans Res, Hobart, Tas, Australia

[ 4 ] Univ New South Wales, Australian Res Council, Ctr Excellence Climate Syst Sci, Kensington, NSW, Australia