A study on interaction between the tropical Pacific mean states and El Niño-Southern Oscillation

Abstract

The El Niño-Southern Oscillation (ENSO) is the strongest interannual variability, characterized by oscillations between rising (El Niño) and falling (La Niña) sea surface temperatures (SST) in the equatorial Pacific. ENSO directly affects the distribution of rainfall in the tropics and can have a strong influence on weather around the world, though the atmospheric teleconnection. ENSO interacts with the mean state of the tropical Pacific Ocean, which exhibits variability not only on interannual but also on decadal time scales. Therefore, the global influence of ENSO on precipitation, temperature, ecological and socioeconomic factors can change from interannual to decadal timescales. Therefore, understanding the interaction between tropical mean state and ENSO is crucial for accurate seasonal and future climate prediction, and has implications for both climatic conditions and socio- economic factors. The variability of ENSO in response to tropical Pacific decadal variability (TPDV) is known as decadal modulation of ENSO. In Chapter Ⅱ, I examined the physical processes that control the decadal modulation of the ENSO amplitude, based on two long-term simulations using state-of-the-art coupled global climate models. This study suggests that this phenomenon can be attributed mainly to thermocline feedback changes, particularly changes in the oceanic response to zonal wind stress. In addition, two critical features of the background state are found to contribute significantly to changes in the equatorial thermocline feedback: 1) the subtropical–tropical cells and 2) ocean stratification. It is suggested that weak (strong) background subtropical meridional overturning circulation partly contributes to regulating the narrower (wider) meridional scales of the sea surface temperature and the associated zonal wind stress anomalies. The more stratified the ocean, the stronger ocean responses to a given wind stress forcing, which affects the ENSO amplitude. In contrast, the decadal modulation of ENSO may also induce the TPDV via a nonlinear rectification effect. In Chapter Ⅲ, I comprehensively analyzed to understand the relationships between TPDV and ENSO, based on the 32 long-term simulations of the state-of-the-art coupled GCMs, focusing in particular on the point that ENSO asymmetry is one of strong drivers of TPDV. The first Empirical Orthogonal Function (EOF) mode for the 11-year moving SST in the coupled models is commonly characterized by El Niño-like decadal variability with Bjerknes air–sea interaction. However, the second EOF mode can be separated into two groups: 1) some models have a zonal dipole SST pattern, and 2) other models are characterized by a meridional dipole pattern. I found that models with the zonal dipole pattern in the second mode tend to simulate strong ENSO amplitude and asymmetry compared with those of the other models. Also, the residual patterns, which are defined as the summation of El Niño and La Niña SST composite anomalies, are quite similar to the decadal dipole pattern, which suggests that ENSO residuals can cause the decadal dipole pattern. I found that decadal modulation of ENSO variability in these models strongly depends on the phase of the dipole decadal variability and also the nonlinear dynamic heating terms are directly rectified into the mean state. The decadal changes in ENSO residual correspond well with the decadal changes in the dipole pattern. In addition to inherent natural internal variability, the tropical mean state also changes under external forcing, such as increased concentrations of carbon dioxide (CO2). In Chapter Ⅳ, I analyzed the relevance of ENSO to the change in the mean state of the tropics under a variety of CO2 pathways. In the first part of Chapter Ⅳ, I investigated the future changes in the characteristics of ENSO under three different CO2 emission rates using the Community Earth System Model version 2 (CESM2). ENSO SST variability decreases abruptly in high CO2 emission scenarios, after reaching a certain threshold. The weakening ENSO SST variability results from a non-directional change in the thermohaline feedback, disrupting the balance between thermocline feedback and thermal damping. The non-directional change in thermocline feedback is caused by a weakening of the wind stress response to El Niño. Enhanced atmospheric stability under global warming reduces energy transfer to the lower atmosphere due to weakened convective momentum transport, resulting in a reduced response of wind stress despite an increased wind response to El Niño. Two critical features of the wind changes were found to contribute significantly to the reduced wind stress response in the central Pacific: 1) climatological wind 2) wind response to El Niño. The wind stress response is strengthened by an amplified wind response to El Niño. In contrast, the weakened trade winds, caused by El Niño-like warming, play a role in reducing the wind stress response. These results suggest that there may be abrupt changes in ENSO over the next 20-30 years that deviate from our anticipated changes. Recently, there has been a need to project the climate changes in CO2 reduction scenarios as well as CO2 increase scenarios. The 2015 Paris Climate Agreement set a target of limiting the global temperature increase to 1.5°C, which required a reduction in CO2 emissions. From this perspective, in the last part of Chapter Ⅳ, I analyzed the changes in the characteristic of ENSO when the increased CO2 returns to its current level using the CESM1. The deep ocean, a vast thermal reservoir, absorbs excess heat under greenhouse warming, which ultimately regulates the Earth& surface climate. Even if CO2 emissions are successfully reduced, the stored heat will gradually be released, maintaining a particular pattern of ocean warming. Here, I show that deep ocean warming will lead to El Niño-like ocean warming and resultant increased precipitation in the tropical eastern Pacific with southward shift of Intertropical convergence zone. These changes in the tropical Pacific mean state cause an eastward shift of the ENSO, leading to an increase in the Eastern Pacific (EP) El Niño. In addition, the deep ocean-induced mean state change is expected to lead to more frequent convective extreme El Niño events, with 40 to 80% increase compared to the present climate. This study suggests that current or future global warming will affect us in various ways for much longer than we thought. This thesis sheds light on the complex interactions of ENSO with tropical Pacific mean state. From an internal variability perspective, I demonstrated the nonlinear rectification impact of ENSO and the effect of the Pacific mean state on ENSO by quantitatively assessing the ENSO feedback. In addition, I suggested the underlying mechanism for the change in ENSO in response to changes in tropical Pacific mean state under an external forcing. These results can have implications for climate predictions, adaptation strategies, and the understanding of how ENSO dynamics may change in the context of ongoing climate change.