Hyo-Jin Park

The Indian Ocean Dipole (IOD), characterized by an interannual fluctuation in zonal dipole pattern of sea surface temperature anomalies along the tropical Indian Ocean, has a large socioeconomic impact on neighboring countries. Here, we investigate the decadal modulations of IOD variability over the last 123 years (1900–2022), by analyzing the observational reanalysis data in conjunction with a low-order IOD model that accounts for both stochastic forcing and the remote impact of ENSO. The observed decadal changes in IOD variability are primarily attributed to the local air–sea coupled feedback, and secondarily to ENSO. The local feedback during the late winter has intensified since the late-1970s due to the Indian Ocean warming, the suppressed westerly winter monsoon in the southeastern Indian Ocean, the shallowing of the mean thermocline in the southeastern Indian Ocean, and the decrease in mean upwelling in the western Indian Ocean. Each of these enhances the convective instability, anomalous evaporative cooling, and oceanic vertical thermal advections, respectively. Intensified local feedback increases the likelihood of the early onset of IOD events in the late winter. Additionally, ENSO, which has strengthened since the mid-twentieth century, has extended the peak phase of IOD into the late fall in recent decades.

The Indian Ocean Dipole (IOD) has been proposed to be a key driver of biological processes in the Indian Ocean (IO) in the present climate. Given the expected influence of global warming on both the properties of the IOD and the biogeochemistry within the IO, a key question arises: How will the relationship between the IOD and chlorophyll evolve in a warming climate? Here, utilizing simulations from the Coupled Model Intercomparison Project (CMIP) Phase 6 Earth System models, our findings reveal a notable intensification in the IOD-chlorophyll relationship under greenhouse warming. This intensification is linked to an increase in phytoplankton biomass during the June to November period of positive IOD years in the southeastern IO (SEIO). Interestingly, our analysis indicates a substantial rise in IOD-induced chlorophyll levels in a warming climate, despite a marked decrease in IOD-induced upwelling in the SEIO. The shallower thermocline leads to an increase in the mean nutrient concentration in the subsurface layer, thereby facilitating an enhanced anomalous nutrient supply to the surface layer, which contributes to increased phytoplankton biomass. Our study highlights the consequential effects of IOD on chlorophyll dynamics and underscores the need for improvement of Earth System Models to resolve our understanding of biophysical interactions in the IO in response to global warming.

Indian Ocean Dipole phenomenon (IOD) refers to a dominant zonal contrast pattern of sea surface temperature anomaly (SSTA) over tropical Indian Ocean (TIO) on interannual time scales. Its positive phase, characterized by anomalously warm western TIO and anomalously cold southeastern TIO, is usually stronger than its negative phase, namely a positively skewed IOD. Here, we investigate causes for the IOD asymmetry using a prototype IOD model, of which physical processes include both linear and nonlinear feedback processes, El Nino’s asymmetric impact, and a state-dependent noise. Parameters for the model were empirically obtained using various reanalysis SST data sets. The results reveal that the leading cause of IOD asymmetry without accounting seasonality is a local nonlinear process, and secondly the state-dependent noise, the direct effect by the positively skewed ENSO and its nonlinear teleconnection; the latter two have almost equal contribution. However, the contributions by each process are season dependent. For boreal summer, both local nonlinear feedback process and the state-dependent noise are major drivers of IOD asymmetry with negligible contribution from ENSO. The ENSO impacts become important in boreal fall, along with the other two processes.

The Indian Ocean Dipole/Zonal mode (IOD) is an interannual phenomenon over the tropical Indian Ocean, causing a pronounced impact worldwide. Here, we investigate the mechanism of the change in IOD characteristics in a CO₂ removal simulation for an earth system model (ESM). As the CO₂ concentration increases, the intensity of IOD tends to increase, but at high CO₂ concentrations, further increases decrease the IOD intensity. The minimum IOD amplitude was recorded during the early decrease in CO₂. First, we developed a conceptual model for IOD that is composed of local air-sea coupled feedback, delayed ocean dynamics, El Niño impact, and noise forcing. Then, by adopting ESM results into this simple IOD model, we revealed that the local air–sea coupled feedback is a major factor for changing IOD amplitude, while El Niño does not exert a change in IOD amplitude. The local air–sea coupled feedback including thermocline feedback, wind-evaporation feedback, and Ekman feedback is strongly modified by the air–sea coupling strength during progression of a global warming. Consequently, under the higher CO₂ concentrations, IOD amplitude is reduced due to the weakening of air-sea coupling over tropical Indian Ocean.

The concentration of surface ozone in East Asia is high due to strong solar radiation, but decreases in areas affected by summer monsoons. This study analyzes the summer surface ozone variations in East Asia using meteorological and atmospheric chemistry variables in 12 models participating in Chemistry-Climate Model Initiative (CCMI) for the period of 1979 to 2010. The concentration of 850 hPa ozone was identified two modes by Empirical Orthogonal Functions (EOF) analysis. The first mode is an increase in all regions over East Asia, mainly in eastern China. This mode was associated with downward wind, weak horizontal wind speed, increase in temperatures, decrease in precipitation. The second mode showed high ozone concentrations in eastern China and low in northern Japan. In eastern China, temperatures and precipitation are decreased, and shortwave radiation reaches the surface is increased. In addition, the concentration of nitrogen oxides and carbon monoxide and the net ozone production are increased. The second mode was highly correlated with El Nino-Southern Oscillation (ENSO) and western North Pacific subtropical high (WNPSH) indices and was found to be closely associated with East Asian summer monsoons. Acknowledgements: This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2019R1A2C1008549). We acknowledge the modeling groups for making their simulations available for this analysis, the joint WCRP SPARC/IGAC Chemistry–Climate Model Initiative (CCMI) for organizing and coordinating the model simulations and data analysis activity, and the British Atmospheric Data Centre (BADC) for collecting and archiving the CCMI model output.