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Hyung-Gyu Lim: Research

Updated in March 24th 2022

The El Niño–Southern Oscillation (ENSO) strongly influences phytoplankton in the tropical Pacific, with El Niño conditions suppressing productivity in the equatorial Pacific (EP) and placing nutritional stresses on marine ecosystems. The Geophysical Fluid Dynamics Laboratory’s (GFDL) Earth System Model version 4.1 (ESM4.1) captures observed ENSO-chlorophyll patterns (r=0.57) much better than GFDL’s previous ESM2M (r=0.23). Most notably, the observed post-El Niño “chlorophyll rebound” is substantially improved in ESM4.1 (r=0.52). We find that an anomalous increase in iron propagation from western Pacific (WP) subsurface to the cold tongue via the equatorial undercurrent (EUC) and subsequent post-El Niño surfacing, unresolved in ESM2M, is the primary driver of chlorophyll rebound. We also find that this chlorophyll rebound is augmented by high post-El Niño dust-iron deposition anomalies in the eastern EP. This post-El Niño chlorophyll rebound provides a previously unrecognized source of marine ecosystem resilience independent from the La Niña that sometimes follows.

GFDL Research Highlight

It has been suggested that the freshwater flux due to the recent melting of the Antarctic ice- sheet/shelf will suppress ventilation in the Southern Ocean. In this study, we performed idealized earth system simulations to examine the impacts of Antarctic meltwater on surface phytoplankton biomass in the Antarctic Ocean. The enhanced stratification due to the meltwater leads to a decrease in the surface nitrate concentration but an increase in the surface dissolved iron concentration. These changes are associated with the reduced upwelling of nitrate-rich deep water and the trapped iron exported from the terrestrial sediment. Because of the limited iron availability in the Southern Ocean, the trapped iron in surface water enhances the chlorophyll concentration in the open ocean. However, in the marginal sea along the Antarctic coastline where the iron is relatively sufficient, a nitrate reduction induces a chlorophyll decrease, indicating a regime shift from iron-limited to nitrate-limited conditions.

Antarctic marine biological variability modulates climate systems via the biological pump. However, the knowledge of biological response in the Southern Ocean to climate variability still has been lack of understanding owing to limited ocean color data in the high latitude region. We investigated the surface chlorophyll concentration responses to the Southern annular mode (SAM) in the marginal sea of the Southern ocean using satellite observation and reanalysis data focusing on the austral summer. The positive phase of SAM is associated with enhanced and poleward‐shifted westerly winds, leading to physical and biogeochemical responses over the Southern ocean. Our result indicates that chlorophyll has strong zonally asymmetric responses to SAM owing to different limiting factors of phytoplankton growth per region. For the positive SAM phase, chlorophyll tends to increase in the western Amundsen–Ross Sea but decreases in the D’Urville Sea. It is suggested that the distinct limiting factors are associated with the seasonal variability of sea ice and upwelling per region.

Human activities such as fossil fuel combustion, land-use change, nitrogen (N) fertilizer use, emission of livestock, and waste excretion accelerate the transformation of reactive N and its impact on the marine environment. This study elucidates that anthropogenic N fluxes (ANFs) from atmospheric and river deposition exacerbate Arctic warming and sea ice loss via physical–biological feedback. The impact of physical–biological feedback is quantified through a suite of experiments using a coupled climate–ocean–biogeochemical model (GFDL-CM2.1-TOPAZ) by prescribing the prein- dustrial and contemporary amounts of riverine and atmospheric N fluxes into the Arctic Ocean. The experiment forced by ANFs represents the increase in ocean N inventory and chlorophyll concentrations in present and projected future Arctic Ocean relative to the experiment forced by preindustrial N flux inputs. The enhanced chlorophyll concentrations by ANFs reinforce shortwave attenuation in the upper ocean, generating additional warming in the Arctic Ocean. The strongest responses are simulated in the Eurasian shelf seas (Kara, Barents, and Laptev Seas; 658–908N, 208–1608E) due to increased N fluxes, where the annual mean surface temperature increase by 12% and the annual mean sea ice concentration decrease by 17% relative to the future projection, forced by preindustrial N inputs.

In recent decades, greenhouse warming has accelerated the melting of Antarctic glaciers, which discharges freshwater into the Southern Ocean and therefore reduces the surface density. Surface freshening in the Southern Ocean induces cooling and sea ice expansion on the surface, such that it could delay global warming and further lead to a northward shift of the Intertropical Convergence Zone (ITCZ). Here, we examine the distinct regional impacts of Antarctic meltwater forcing over the globe by analyzing experiments with and without meltwater forcing. For example, the Antarctic meltwater forcing induces a global cooling but leads to regional warming in East Asia. We find that Antarctic meltwater forcing leads to reduced convection in the Western North Pacific (WNP) due to the northward shift of the ITCZ and an overall cooling in the tropics. This circulation change in WNP induces regional warming in East Asia via atmospheric teleconnection.

It has been shown that the interaction between marine phytoplankton and climate systems may intensify Arctic warming in the future via shortwave heating associated with increased spring chlorophyll bloom. However, the changes of chlorophyll variability and its impact on the Arctic future climate are uncomprehended. Lim et al. (Clim Dyn. s00382-018-4450-6, 2018a) (Part I) suggested that two nonlinear rectifications of chlorophyll variability play cooling role in present-day climate. In this study, we suggest that the decreased interannual chlorophyll variability may amplify Arctic surface warming (+ 10% in both regions) and sea ice melting (− 13% and − 10%) in Kara-Barents Seas and East Siberian- Chukchi Seas in boreal winter, respectively. Projections of earth system models show a future decrease in chlorophyll both mean concentration and interannual variability via sea ice melting and intensified surface-water stratification in summer. We found that suggested two nonlinear processes in Part I will be reduced by about 31% and 20% in the future, respectively, because the sea ice and chlorophyll variabilities, which control the amplitudes of nonlinear rectifications, are projected to decrease in the future climate. The Arctic warming is consequently enhanced by the weakening of the cooling effects of the nonlinear rectifications. Thus, this additional biological warming will contribute to future Arctic warming. This study sug- gests that effects of the mean chlorophyll and its variability should be considered to the sensitivity of Arctic warming via biogeophysical feedback processes in future projections using earth system models.

The limiting factors of the Arctic chlorophyll is seasonally dependent. In spring time light availability significantly induces chlorophyll spring bloom (pre-bloom). But, the nutrient availability induces summer chlorophyll summer bloom (post-bloom). Because, the vertical mixing of summer ocean is seasonally weakened by seasonally reduced sea ice concentration and extended open ocean, that environment of the Arctic Ocean can absorb more shortwave radiation and in turn the surface warming and ocean stratifications. The summer nutrient inventory is, therefore, significantly depleted. This lack of nutrient condition in summer can be enhanced by wind-driven mixing, that observed by Ardyna et al. 2014 argued that the frequency of summer post bloom is increased in recent decades. However, in the future climate projections by CMIP5 ESMs are expected to decreased summer chlorophyll by enhanced stratification by sea ice melting (Cabre et al. 2015).

The seasonally-dependent ice-phytoplankton coupling leads Arctic climate sensitivity. We identified that in addition to the effect of the mean chlorophyll change, an interannual chlorophyll variability substantially influences the Arctic mean climate state, even though the mean chlorophyll remains the same. We found that two nonlinear rectifications of chlorophyll variability induced Arctic cooling. One was due to the effect of a nonlinear shortwave heating term, which was induced by the positive ice–phytoplankton covariability in the boreal summer. The other was due to a cooling effect by rectification of a nonlinear function of the shortwave absorption rate, which reduced the shortwave absorption rate by interannually varying chlorophyll. In the Coupled Model Intercomparison Project, earth system models that included biogeophysical feedback simulated a colder Arctic condition than models without a biogeophysical feedback. This result suggests a possible mechanism in understanding how chlorophyll variability interacts with the Arctic climate system and its impact on the Arctic mean climate state.

In the tropical Pacific, it has been suggested that phytoplankton induces SST response in observation and model. The warming effect of phytoplankton is almost similar with diverse studies. However, responses of SST are now controversial depending on the model.

In case of GFDL CM2.1, there has been tropical Pacific cold SST bias . In accordance with this physical bias, chlorophyll concentration is also overestimated in the eastern tropical Pacific. So far, chlorophyll effect in GFDL CM2.1 shows cooling effects. In this point, we may think that physical bias is partly attributed by biological bias.

Also, physiology, cold surface, may induce tropical ocean upwelling. Because of this, chlorophyll bias is partly related with enhanced replenishment of nutrients. There has been two-way feedback between physical and biological systems.

The physical or biological corrections would be mutual supplementation or co-operative effects in Earth system model.

In order to examine the threshold of the volcanic forcing that leads to the El Niño-like warming, we analyze a millennium ERIK simulation (AD 1000–1850) forced by three external forcings including greenhouse gases, solar forc- ing and volcanic eruptions using the ECHO-G coupled climate model. It is found that there exists a threshold of the volcanic forcing above 15 W/m2 to lead the El Niño-like warming in the climate model. When the volcanic forcing is above this threshold forcing, then the intensity of the Inter-tropical Con- vergence Zone (ITCZ) is weakened and its position is shifted to the south. This might be associated with the processes of less evaporation in the subtropical cloudless region by a cool- ing due to the reduction of net surface shortwave radiation. Concurrently, a weakening of ITCZ is associated with a weak- ening of the trade winds and the subsequent Bjerknes feedback causes El Niño-like warming. Therefore, El Niño-like warm- ing events can occur when volcanic eruption is above thresh- old forcing, implying that there exists a certain level of radia- tive forcing change which is capable of changing the state of tropical Pacific sea surface temperature. The last millennium simulation of Paleoclimate Modeling Intercomparison Project Phase 3 climate models also indicates that there may exist a threshold forcing to lead the El Niño-like warming, which has been also discussed in the present study.

Due to the dramatic increase in the global mean surface temperature (GMST) during the twentieth century, the climate science community has endeavored to deter- mine which mechanisms are responsible for global warm- ing. By analyzing a millennium simulation (the period of 1000–1990 AD) of a global climate model and global cli- mate proxy network dataset, we estimate the contribution of solar and greenhouse gas forcings on the increase in GMST during the present warm period (1891–1990 AD). Linear regression analysis reveals that both solar and greenhouse gas forcing considerably explain the increase in global mean temperature during the present warm period, respectively, in the global climate model. Using the global climate proxy network dataset, on the other hand, statistical approach suggests that the contribution of greenhouse gas forcing is slightly larger than that of solar forcing to the increase in global mean temperature during the present warm period. Overall, our result indicates that the solar forcing as well as the anthropogenic greenhouse gas forc- ing plays an important role to increase the global mean temperature during the present warm period.