Understanding the regional hydrological response to varying CO2 concentration is critical for cost-bene t analysis of mitigation and adaptation polices in the near future. To characterize summer monsoon rainfall change in East Asia due to a change in the CO2 pathway, we used the Community Earth System Model (CESM) with 28 ensemble members in which the CO2 concentration increases at a rate of 1% per year until its quadrupling peak, i.e., 1,468 ppm (ramp-up period), followed by a decrease of 1% per year until the present-day climate conditions, i.e., 367 ppm (ramp-down period). Although the CO2 concentration change is symmetric in time, the rainfall response is not symmetric. The amount of summer rainfall in East Asia is much larger during a ramp-down period than during a ramp-up period when the two periods of the same CO2 concentration are compared. This asymmetrical rainfall response is mainly due to an enhanced El Niño-like warming pattern as well as an increase in the meridional sea surface temperature gradient in the western North Paci c during a ramp-down period. These sea surface temperature patterns enhance the atmospheric teleconnections to East Asia and the local meridional circulations around East Asia, resulting in more rainfall over East Asia during the ramp-down period. This result implies that the removal of CO2 does not guarantee the return of regional rainfall to the previous climate state with the same CO2 concentration.
The hydrological cycle has a significant impact on human activities and ecosystems, so understanding its mechanisms with respect to a changing climate is essential. In particular, a more detailed understanding of hydrological cycle response to transient climate change is required for successful adaptation and mitigation policies. In this study, we exploit large ensemble model experiments using the Community Earth System Model version 1.2.2 (CESM1) in which CO2 concentrations increase steadily and then decrease along the same path. Our results show that precipitation changes in the CO2 increasing and decreasing phases are nearly symmetrical over land but asymmetric over oceans. After CO2 concentrations peak, the ocean continues to uptake heat from the atmosphere, which is a key process leading the hydrological cycle’s contrasting response over land and ocean. The symmetrical hydrological cycle response over land involves a complex interplay between rapid responses to CO2 and slower responses to ensuing warming. Therefore, the surface energy constraints lead to the contrasting hydrological response over land and ocean to CO2 forcing that needs to be verified and considered in climate change mitigation and adaption actions.
The frequency and duration of marine heatwaves have been increasing with ocean warming due to climate change. In particular, the Northeast Pacific has experienced intense and extensive marine heatwaves since the late 1990s – characteristically called “the Blob”. Here, an investigation of satellite-derived and reanalysis data supported by idealized coupled model experiments show that Arctic warming plays an important role in the increase in Northeast Pacific marine heatwave days during boreal summers. Strong Arctic warming has acted to change the atmospheric circulation pattern over the Northeast Pacific and reduce the low-level cloud fraction from late spring to early summer. We show that the enhancement of solar radiative heat fluxes and reduced latent heat loss over a relatively large area has favored an increase in sea surface temperatures and marine heatwave days. An idealized model experiment performed here, designed to isolate the impact of Arctic warming, supports this hypothesis. The projected changes of Arctic climate on the occurrence of marine heatwaves should be considered in climate change adaptation and mitigation plans.
The northwestern part of the East/Japan Sea (EJS) is a region with large sea surface temperature (SST) variability and is known as a hotspot of marine heatwaves (MHW) stress for marine environments that peaked in boreal winter (January-February-March). This could have profound impacts on the marine ecosystems over the EJS. Here, we used a set of high-resolution satellite and reanalysis products to systematically analyze the spatiotemporal SST variations and examine their linkage to a large-scale mode of climate variability, such as the Arctic Oscillation (AO). The results show that AO-related wind forcing modulates the SST variability over the EJS via the oceanic dynamic adjustment processes. In particular, the abnormally warm SSTs in the northwestern part of the EJS are driven by the anomalous anticyclonic eddy-like circulation and Ekman downwelling during a positive AO phase. This physical linkage between a positive AO and the abnormally warm SST could be conducive to MHW occurrences in the EJS as in the extremely positive AO event during the winter of 2020. These results have implications that the MHW occurrences in the EJS could be amplified by natural climate variability along with long-term SST warming.
A composite analysis was conducted on the reanalysis dataset for 1979–2016, along with an idealized model experiment to show that the relationship between the East Asian jet stream (EAJS) and the East Asian winter monsoon (EAWM) is nonstationary. The relationship between EAWM and the EAJS weakened during the late 1990s. This decadal change in the EAJS–EAWM relationship was mainly due to a change in the secondary circulation across the EAJS between two contrasting periods, induced by the northward shift of the EAJS. A possible mechanism associated with the decadal change in meridional displacement of the EAJS is proposed. The enhanced convective activity in the western tropical Pacific after the late 1990s results in stronger Hadley circulation that could have contributed to the northward displacement of the Hadley circulation boundary latitude. Subsequently, this leads to the northward shift of the EAJS. Therefore, it is necessary to define a new EAJS index to account for the EAWM variability based on the change in the oceanic and atmospheric mean state across the late 1990s.
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