Impact of Changing Winds on the Mauna Loa CO2 Seasonal Cycle in Relation to the Pacific Decadal Oscillation. Issue 13 (1st July 2022)

• Record Type: Journal Article
• Authors: Jin, Yuming; Keeling, Ralph F.; Rödenbeck, Christian; Patra, Prabir K.; Piper, Stephen C.; Schwartzman, Armin
• Is Part Of: Journal of geophysical research. Volume 127:Issue 13(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-07-01
• Subjects: Large‐scale circulation change -- Hadley cell expansion -- Land biogeochemistry -- Empirical orthogonal functions of winds; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2021JD035892
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: Long‐term measurements at the Mauna Loa Observatory (MLO) show that the CO2 seasonal cycle amplitude (SCA) increased from 1959 to 2019 at an overall rate of 0.22 ± $\pm $ 0.034 ppm decade −1 while also varying on interannual to decadal time scales. These SCA changes are a signature of changes in land ecological CO2 fluxes as well as shifting winds. Simulations with the TM3 tracer transport model and CO2 fluxes from the Jena CarboScope CO2 Inversion suggest that shifting winds alone have contributed to a decrease in SCA of −0.10 ± $\pm $ 0.022 ppm decade −1 from 1959 to 2019, partly offsetting the observed long‐term SCA increase associated with enhanced ecosystem net primary production. According to these simulations and MIROC‐ACTM simulations, the shorter‐term variability of MLO SCA is nearly equally driven by varying ecological CO2 fluxes (49%) and varying winds (51%). We also show that the MLO SCA is strongly correlated with the Pacific Decadal Oscillation (PDO) due to varying winds, as well as with a closely related wind index (U‐PDO). Since 1980, 44% of the wind‐driven SCA decrease has been tied to a secular trend in the U‐PDO, which is associated with a progressive weakening of westerly winds at 700 mbar over the central Pacific from 20°N to 40°N. Similar impacts of varying winds on the SCA are seen in simulations at other low‐latitude Pacific stations, illustrating the difficulty of constraining trend and variability of land CO2 fluxes using observations from low latitudes due to the complexity of circulation changes. Plain Language Summary: The CO2 seasonal amplitude is an indicator of the growing‐season productivity of land ecosystems. CO2 observation at Mauna Loa Observatory (MLO) showed an increasing amplitude from 1959 to 2019, probably due to significantly enhanced productivity over the Eurasia temperate and boreal forest. In this study, we show that about a third of this observed amplitude increase at MLO was offset by long‐term circulation changes, which isolated the air at low latitudes from the influence of northern high‐latitude ecosystems. This wind‐driven impact is also important at other low‐latitude Pacific stations. The CO2 seasonal amplitude at MLO also has considerable short‐term variability (on the scale of multiple years to decades), roughly half of which is regulated by the changing winds, while the remaining half is driven by the variability of ecological CO2 fluxes. We show that the wind influence is closely associated with a climate index known as the Pacific Decadal Oscillation. Our study indicates that the attribution of amplitude changes at low‐latitudes stations to ecosystem changes is complicated by wind shifts. Key Points: The increasing CO2 seasonal amplitude observed at the Mauna Loa Observatory since 1959 has been significantly offset by circulation changes Variability of CO2 seasonal cycles at Mauna Loa is partly regulated by circulation changes associated with the Pacific Decadal Oscillation Circulation changes complicate using low‐latitudes CO2 observation to constrain changing land carbon cycle