Interactive effects of the El Niño-Southern Oscillation and Indian Ocean Dipole on the tropical net ecosystem productivity

Abstract

El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) can exert abiotic stresses on biota to influence regional terrestrial carbon cycling. Here, we investigated their interactive effects on tropical net ecosystem productivity (NEP) when ENSO and IOD simultaneously occur (mainly El Niño & positive IOD [pIOD] and La Niña & negative IOD [nIOD]), based on TRENDYv9 multi-model simulations. Results suggest that NEP anomalies were dominated by IOD over South America near 20°S and Africa south of the Equator, but by ENSO over India and northern South America, and controlled by their amplifying effects over eastern Africa, the Indo-China peninsula, and most of Australia during September-October-November (SON). In the following December-January-February (DJF), IOD legacy effects generally enhanced ENSO-induced NEP anomalies over most of South America, eastern and southern Africa, and Australia. During March-April-May (MAM), the influence of IOD gradually disappeared. Our analysis revealed the dominant role of gross primary productivity (GPP) in these NEP anomalies. We further found asymmetric effects of soil moisture and temperature on NEP anomalies, showing higher correlation coefficients with soil moisture in El Niño & pIOD, but basically with temperature in La Niña & nIOD. Additionally, considering the simultaneous extreme of pIOD and El Niño activity in 1997/98, we calculated their seasonal individual contributions to NEP anomalies. We found that the pIOD event had the greatest influence in SON. During SON in 1997, individual contributions of pIOD and El Niño to NEP caused a land carbon source of −0.34 ± 0.15 and −0.35 ± 0.32 PgC yr−1 over South America, a carbon sink and source of 0.5 ± 0.19 and −0.54 ± 0.14 PgC yr−1 over Africa, and source of −0.08 ± 0.25 and −0.42 ± 0.37 PgC yr−1 over Asia-Pacific, respectively. Understanding interactive effects of IOD and ENSO on regional carbon cycling is crucial due to the more frequent extreme IOD and ENSO events under future climate warming.