Spatiotemporal characteristics of seasonal to multidecadal variability of pCO2 and air‐sea CO2 fluxes in the equatorial Pacific Ocean
Authors: Valsala Vinu K. , Roxy Mathew Koll , Ashok Karumuri , Murtugudde Raghu 
Published: December 2014

Seasonal, interannual, and multidecadal variability of seawater pCO2 and air‐sea CO2 fluxes in the equatorial Pacific Ocean for the past 45 years (1961–2005) are examined using a suite of experiments performed with an offline biogeochemical model driven by reanalysis ocean products. The processes we focus on are: (a) the evolution of seasonal cycle of pCO2 and air‐sea CO2 fluxes during the dominant interannual mode in the equatorial Pacific, i.e., the El Niño‐Southern Oscillation (ENSO), (b) its spatiotemporal characteristics, (c) the combined and individual effects of wind and ocean dynamics on pCO2 and CO2 flux variability and their relation to canonical (eastern Pacific) and central Pacific (Modoki) ENSOs and (d) the multidecadal variability of carbon dynamics in the equatorial Pacific and its association with the Pacific Decadal Oscillations (PDO). The simulated mean and seasonal cycle of pCO2 and CO2 fluxes are comparable with the observational estimates and with other model results. A new analysis methodology based on the traditional Empirical Orthogonal Functions (EOF) applied over a time‐time domain is employed to elucidate the dominant mode of interannual variability of pCO2 and air‐sea CO2 fluxes at each longitude in the equatorial Pacific. The results show that the dominant interannual variability of CO2 fluxes in the equatorial Pacific (averaged over 5°N–10°S) coevolves with that of ENSO. Generally a reduced CO2 source in the central‐to‐eastern equatorial Pacific evident during June–July of the El Niño year (Year:0) peaks through September of Year:0 to February of Year:+1 and recovers to a normal source thereafter. In the region between 160°W and 110°W, the canonical El Niño controls the dominant variability of CO2 fluxes (mean and RMS of anomaly from 1961 to 2005 is 0.43±0.12 PgC yr−1). However, in the western (160°E–160°W) and far eastern (110°W–90°W) equatorial Pacific, CO2 flux variability is dominantly influenced by the El Niño‐Modoki (0.3±0.06 and 0.11±0.04 PgC yr−1, respectively). On the other hand, the interannual variability of pCO2 is correlated with the canonical El Niño mostly to the east of 140°W and with El Niño‐Modoki to the west of 140°W. Decoupling of CO2 flux and pCO2 variability at various locations in the equatorial Pacific is attributable to the differences in the combined and individual effects of ocean dynamics and winds associated with these two types of ENSO. A multidecadal variability in the equatorial Pacific sea‐air CO2 fluxes and pCO2 exhibits a positive phase during the 1960s, a negative phase during the 1980s, and then positive again by the 2000s. Within the ocean, the dissolved inorganic carbon (DIC) anomalies are traceable to the northern Pacific via thermocline pathways at decadal timescales. The multidecadal variability of equatorial Pacific CO2 fluxes and pCO2 are determined by the phases of the PDO and the corresponding scale of the El Niño‐Modoki variability, whereas canonical El Niño's contribution is to mainly determine the variability at interannual timescales. This study segregates the impacts of different types of ENSOs on the equatorial Pacific carbon cycle and sets the framework for analysing its spatiotemporal variability under global warming.