Understanding elevated CO concentrations in East Asia relative to the global mean during boreal spring on the slow and interannual timescales.

It is important to examine the physical processes that regulate current CO concentrations in East Asia to understand the global carbon cycle. To do this, we begin by defining the difference between East Asian and global CO concentrations (East Asian CO concentration minus global CO concentration), which is referred to as East Asian local CO concentration (i.e., EA_LCO). Then, we examine the physical processes associated with the variability of EA_LCO during boreal spring (March-April-May) on the slow and interannual timescales. Our results indicate that there are two key factors leading to elevated CO concentrations in East Asia relative to the global mean during boreal spring; one is higher emissions in East Asia, which mostly explains the increasing in EA_LCO on the slow timescales. The other is a cool sea surface temperature (SST) in the eastern tropical Pacific (La-Nina-like SST), which is associated with an interannual higher CO concentration in East Asia than the global mean. Enhanced convective activity in the western tropical Pacific, which is associated with a La-Nina-like SST forcing, induces low-pressure circulation in the western North Pacific with northerly winds, leading to suppressed precipitation and cool surface temperature in East Asia. Subsequently, those suppress vegetation growth as well as gross primary product, resulting in relatively high CO concentrations in East Asia compared to the global mean.