Contrasting interannual atmospheric CO2 variabilities and their terrestrial mechanisms for two types of El Niños

This is the final version. Available on open access from EGU via the DOI in this recordData availability. The monthly atmospheric CO2 concentration is from NOAA/ESRL (https://www.esrl.noaa.gov/gmd/ ccgg/trends/index.html). The Niño3.4 index is from ERSST4 (http://www.cpc.ncep.noaa.gov/data/indices/ersst4.nino.mth. 81-10.ascii). Temperature and precipitation are from CRUNCEPv6 (ftp://nacp.ornl.gov/synthesis/2009/frescati/temp/land_ use_change/original/readme.htm). TRENDY v4 data are available from Stephen Sitch (s.a.sitch@exeter.ac.uk) upon your reasonable request.El Niño has two different flavors, eastern Pacific (EP) and central Pacific (CP) El Niños, with different global teleconnections. However, their different impacts on the interannual carbon cycle variability remain unclear. Here we compared the behaviors of interannual atmospheric CO2 variability and analyzed their terrestrial mechanisms during these two types of El Niños, based on the Mauna Loa (MLO) CO2 growth rate (CGR) and the Dynamic Global Vegetation Model's (DGVM) historical simulations. The composite analysis showed that evolution of the MLO CGR anomaly during EP and CP El Niños had three clear differences: (1) negative or neutral precursors in the boreal spring during an El Niño developing year (denoted as yr0), (2) strong or weak amplitudes, and (3) durations of the peak from December (yr0) to April during an El Niño decaying year (denoted as yr1) compared to October (yr0) to January (yr1) for a CP El Niño, respectively. The global land-atmosphere carbon flux (FTA) simulated by multi-models was able to capture the essentials of these characteristics. We further found that the gross primary productivity (GPP) over the tropics and the extratropical Southern Hemisphere (Trop+SH) generally dominated the global FTA variations during both El Niño types. Regional analysis showed that during EP El Niño events significant anomalous carbon uptake caused by increased precipitation and colder temperatures, corresponding to the negative precursor, occurred between 30°S and 20°N from January (yr0) to June (yr0). The strongest anomalous carbon releases, largely due to the reduced GPP induced by low precipitation and warm temperatures, occurred between the equator and 20°N from February (yr1) to August (yr1). In contrast, during CP El Niño events, clear carbon releases existed between 10°N and 20°S from September (yr0) to September (yr1), resulting from the widespread dry and warm climate conditions. Different spatial patterns of land temperatures and precipitation in different seasons associated with EP and CP El Niños accounted for the evolutionary characteristics of GPP, terrestrial ecosystem respiration (TER), and the resultant FTA. Understanding these different behaviors of interannual atmospheric CO2 variability, along with their terrestrial mechanisms during EP and CP El Niños, is important because the CP El Niño occurrence rate might increase under global warming.This study was supported by the National Key R&D Program of China (grant no. 2016YFA0600204 and no. 2017YFB0504000), the Natural Science Foundation of Jiangsu Province, China (grant no. BK20160625), and the National Natural Science Foundation of China (grant no. 41605039). Andrew Wiltshire was supported by the Joint UK BEIS/Defra Met Office Hadley Centre Climate Programme (GA01101)