Huang-Hsiung Hsu

This study investigates how deforestation in the Maritime Continent (MC) modulates extratropical atmospheric circulation over the North Pacific during different phases of El Niño–Southern Oscillation (ENSO). By analyzing ensemble idealized deforestation experiments, the research reveals a distinct modulation effect based on the ENSO phase. During El Niño, an elongated subtropical jet stream acts as a waveguide, confining deforestation-induced atmospheric waves, which leads to a flattened Aleutian low over the North Pacific and reduced impacts on North America. Conversely, during La Niña, the contracted subtropical jet allows these waves to propagate toward higher latitudes, where deforestation in the MC triggers a Pacific–North American (PNA)-like pattern and causing significant warming in North America. Linear baroclinic model experiments highlight the sensitivity of the extratropical atmospheric response to MC deforestation, particularly over New Guinea, emphasizing the regional dependency of this sensitivity. These findings suggest that deforestation in the MC may have broader impacts on higher latitudes, particularly during La Niña years or under future global warming scenarios that could favor prolonged or multiyear La Niña conditions. Significance Statement: This study uncovers the significant influence of Maritime Continent (MC) deforestation on extratropical atmospheric circulation, particularly over the North Pacific, with effects that vary depending on the phase of El Niño–Southern Oscillation (ENSO). By demonstrating how deforestation-induced atmospheric waves interact differently with the atmospheric jet streams during El Niño and La Niña phases, the research highlights the potential for MC deforestation to drive substantial climatic changes in higher latitudes, especially under La Niña conditions. These findings underscore the broader implications of regional land-use changes on global climate patterns, offering critical insights for future climate projections and mitigation strategies.

A two‐phase recovery of the Atlantic Meridional Overturning Circulation (AMOC) is revealed following the abrupt cessation of freshwater forcing (FWF) input at the end of the Younger Dryas in the TraCE‐21K‐II simulation. This staged AMOC reactivation is attributed to changes in sea surface salinity (SSS) pattern in the North Atlantic Ocean. The first recovery phase is mainly due to the termination of FWF; however, the increasing SSS through hemispheric‐scale northward transport of salty water is partially suppressed by melting water from high‐latitude regions, resulting in a subsequent oscillation in the AMOC strength. The second recovery phase is primarily linked to the sea ice retreat in the Greenland–Iceland–Norwegian Seas, which reinitiates the SSS transport through the Norwegian Current. Exploring such model characteristics in simulating the surface oceanic processes may improve our understanding of the AMOC's response to FWF.

The global sea surface warming pattern emerged since the early 1980s is characterized by a boomerang-shape warming in the western Pacific, the basin-wide warming in the Indian Ocean north of 30°S, and a triple-stripe warming in the North Atlantic. This pattern can be obtained with or without El Niño/La Niña signals, indicating the independence of El Niño/La Niña, and is the leading EOF with the El Niño/La Niña signals removed. A negative phase of this pattern started emerging in the early 1980s, switched to positive phase in the 1990s, and has been becoming more prominent for the past few years.CMIP models have been found to have difficulty simulating observed global sea surface temperature (SST) trend, especially the cooling trend in the tropical eastern Pacific. However, the cooling trend in the tropical eastern Pacific in the past four decades is statistically insignificant in our trend analysis adopting a more stringent signal detection method (namely, the False Discovery Rate, FDR). By applying the same trend detection and EOF approach to the simulated SST in the historical simulations of forty CMIP6 models by removing El Niño/La Niña signals, we detected in the ensemble mean SST a trend pattern closely resembling the observed, which also changes from negative to positive phases in the late 1990s and continues becoming more positive into 2014. Whereas each model has slightly different performance in simulating this trend pattern, the ensemble time series of corresponding trend pattern in each model correctly reflects the emergence and enhancement of the warming pattern in the past four decades. However, this model ability seems to be masked by the large fluctuations of El Niño/La Niña, an intrinsic climate mode contributing large internal variability to the global domain, and its temporal fluctuations cannot be synchronized in the coupled models in the historical experiments, which are strongly driven by continuously increasing radiative effect of greenhouse gases concentration. On the other hand, The models seem to be capable of simulating the emergence of the global ocean warming pattern in response to the prescribed increasing greenhouse gas concentration.

The Lorenz Energy Cycle (LEC) in the Taiwan Earth System Model Version 1 (TaiESM1) historical simulation is calculated and compared with ERA5 to evaluate the model performance from the thermodynamic aspect. The future change of LEC is accessed by comparing the SSP5‐8.5 and historical simulations in TaiESM1. TaiESM1 reasonably simulates the global mean, seasonal cycle, and spatial patterns of the energy reservoirs with larger values in the mean energy components and smaller in the eddy energy components. The energy cycle in TaiESM1 is about 35%–45% stronger than ERA5, except from December to February. The impact of global warming on the LEC is different at the vertical levels. The influence of meridional temperature gradient change is the dominant factor in the intensity of the energy cycle, and the change in static stability only contributes to the lower troposphere. Lifting the tropopause in the tropics increases the meridional temperature gradient and produces more zonal mean potential energy (P_M) in the upper troposphere. P_M is the primary driver of the LEC and leads to a more active energy cycle in the upper troposphere. As the tropical tropospheric depth increases and the mid‐latitude eddy activities become more active, more (less) energy is stored in the upper (lower) troposphere, and the energy conversion processes tend to become stronger (weaker) in the upper (lower) troposphere.

This study explores how future SST warming in remote ocean basins may affect the western North Pacific (WNP) wet season climate by applying a high-resolution atmospheric general circulation model to conduct a series of numerical experiments. A marked precipitation and tropical cyclone (TC) activity reduction, as well as enhanced anticyclonic circulation, in the WNP is projected in AMIP experiments forced by SST change in a future warming scenario. The sensitivity experiments reveal that various SST warming phenomena (e.g., in the global SST warming pattern, the tropical ocean belt, the Indian Ocean, the tropical Atlantic, and the subtropical northeast Pacific) and the increase in greenhouse gas concentration could weaken the precipitation, TC activity, and circulation. By contrast, the SST warming in the WNP and eastern equatorial Pacific has opposite and mixed effects, respectively, and tends to weakly offset the dominant influences of remote ocean warming. These results indicate that the WNP, being the epicenter of the global teleconnection of divergent and rotational flow, is susceptible to the influence of the SST warming in remote ocean basins. The remote forcing as projected in future scenarios would overwhelm the enhancing effect of local SST warming and weaken the circulation, convection, and TC activity in the WNP. These findings further the understanding of how the decreased precipitation and enhanced subtropical high in the WNP may be easily triggered by remote SST warming as revealed in the AMIP-type simulations. How this effect would be affected by air–sea coupling needs further investigation.

This study explores how future SST warming in remote ocean basins may affect the western North Pacific (WNP) wet season climate by applying a high-resolution atmospheric general circulation model to conduct a series of numerical experiments. A marked precipitation and tropical cyclone (TC) activity reduction, as well as enhanced anticyclonic circulation, in the WNP is projected in AMIP experiments forced by SST change in a future warming scenario. The sensitivity experiments reveal that various SST warming phenomena (e.g., in the global SST warming pattern, the tropical ocean belt, the Indian Ocean, the tropical Atlantic, and the subtropical northeast Pacific) and the increase in greenhouse gas concentration could weaken the precipitation, TC activity, and circulation. By contrast, the SST warming in the WNP and eastern equatorial Pacific has opposite and mixed effects, respectively, and tends to weakly offset the dominant influences of remote ocean warming. These results indicate that the WNP, being the epicenter of the global teleconnection of divergent and rotational flow, is susceptible to the influence of the SST warming in remote ocean basins. The remote forcing as projected in future scenarios would overwhelm the enhancing effect of local SST warming and weaken the circulation, convection, and TC activity in the WNP. These findings further the understanding of how the decreased precipitation and enhanced subtropical high in the WNP may be easily triggered by remote SST warming as revealed in the AMIP-type simulations. How this effect would be affected by air–sea coupling needs further investigation.

This study uses the Community Atmosphere Model 5.3 coupled to a 1-D ocean model to investigate the effects of intraseasonal sea surface temperature (SST) feedback frequency on Madden–Julian oscillation (MJO) simulations with intervals at 30 min and 1, 3, 6, 12, 18, 24, and 30 d. The large-scale nature of the MJO in simulations remains intact with decreasing feedback frequency, although it becomes increasingly unrealistic in both structure and amplitude, until 1 per 30 d when the intraseasonal fluctuations are overwhelmingly dominated by unorganized small-scale perturbations in both atmosphere and ocean, as well as at the atmosphere–ocean interface where heat and energy are rigorously exchanged. The main conclusion is that the less frequent the SST feedback, the more unrealistic the simulations. Our results suggest that more spontaneous atmosphere–ocean interaction (e.g., ocean response once every time step to every 3 d in this study) with high vertical resolution in the ocean model is a key to the realistic simulation of the MJO and should be properly implemented in climate models.