The Recharge Oscillator (RO) is a simple mathematical model of the El Niño Southern Oscillation (ENSO). In its original form, it is based on two ordinary differential equations that describe the evolution of equatorial Pacific sea surface temperature and oceanic heat content. These equations make use of physical principles that operate in nature: (i) the air-sea interaction loop known as the Bjerknes feedback, (ii) a delayed oceanic feedback arising from the slow oceanic response to near-equatorial winds, (iii) state-dependent stochastic forcing from intraseasonal wind variations known as westerly wind bursts (WWBs), and (iv) nonlinearities such as those related to deep atmospheric convection and oceanic advection. These elements can be combined in different levels of RO complexity. The RO reproduces ENSO key properties in observations and climate models: its amplitude, dominant timescale, seasonality, and warm/cold phases amplitude asymmetry. We discuss the RO in the context of timely research questions. First, the RO can be extended to account for ENSO pattern diversity (with events that either peak in the central or eastern Pacific). Second, the core RO hypothesis that ENSO is governed by tropical Pacific dynamics is discussed from the perspective of influences from other basins. Finally, we discuss the RO relevance for studying ENSO response to climate change, and underline that accounting for ENSO diversity, nonlinearities, and better links of RO parameters to the long term mean state are important research avenues. We end by proposing important RO-based research problems.

A document by Yechul Shin. Click on the document to view its contents.

Gayan Pathirana1,2, Kyung Min Noh1,3*, Dong-Geon Lee1, Huiji Lee1 and Jong-Seong Kug1*1Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, South Korea.2Department of Oceanography and Marine Geology, FMST, University of Ruhuna, Matara, Sri Lanka.3Department of Atmospheric Sciences, Yonsei University, Seoul, South Korea.*Corresponding author. Email: [email protected], [email protected]: Indian Ocean Dipole, Biophysical Interaction, Phytoplankton

In most emissions scenarios consistent with the temperature targets of the Paris Agreement, carbon dioxide removal (CDR) would be required to achieve net negative emissions. The efficiency of CDR depends on the behavior of the natural carbon reservoirs, land and ocean, that regulate atmospheric CO2 concentrations, but their change in response to negative emissions is highly uncertain. Here, we investigate the response of the terrestrial and oceanic carbon cycle to negative emissions based on an idealized emission-driven simulation using a state-of-the-art Earth system model. The terrestrial and oceanic carbon sinks become carbon sources ~30 years after the onset of negative emissions. Thereafter, although the atmospheric CO2 concentration returns to its initial level, the terrestrial ecosystem and the ocean continue to release carbon. The ocean recovers as a carbon sink within a few decades, while the terrestrial ecosystem remains a carbon source until the end of the simulation. The prolonged carbon emissions from the land are due to the delayed response of respiration to the earlier increase in terrestrial carbon uptake. As a result, the total carbon stock on land gradually decreases but still remains higher than its initial state. The ocean carbon inventory shows an irreversible change within a few centuries due to the accumulation of anthropogenic CO2 in the deep ocean, which would take more than centuries to be removed naturally.

A Lagrangian plankton model (LPM) is developed, in which the motion of a large number of Lagrangian particles, representing a plankton community, is calculated under the turbulence field simulated by large eddy simulation. A spring phytoplankton bloom is realized using the LPM, and the mechanism for its generation is investigated. Mixing by convective eddies during the night helps to maintain the uniform concentration of phytoplankton within the mixed layer, even if the daily mean surface heat flux is positive in spring. Accordingly, the spring bloom can be predicted by the critical depth hypothesis, if the mixing layer is used instead of the mixed layer. The shoaling of the mixing layer occurs immediately after the start of surface heating, but the shoaling of the mixed layer is delayed. A new criterion for the spring bloom is proposed, which predicts that spring blooms are more likely to occur at higher latitudes, even if the atmospheric forcing is the same. Furthermore, various statistics of Lagrangian particles, such as the vertical migration of plankton, the residence time of plankton within the euphotic zone, and the growth of plankton are investigated by taking advantage of the LPM.

Here, we use idealized climate model simulations to elucidate the governing processes for eastern African interannual hydroclimate and vegetation changes and their relationship to the El Niño-Southern Oscillation (ENSO). Our analysis focuses on Tanzania. In the absence of ENSO-induced sea surface temperature anomalies in the Tropical Indian Ocean, El Niño causes during its peak phase negative precipitation anomalies over Tanzania due to a weakening of the tropical-wide Walker circulation. Resulting drought conditions increase wildfires and decrease vegetation cover. Subsequent wetter La Niña conditions reverse the trend, causing a gradual 1-year-long recovery phase. The 2-year-long vegetation response in Tanzania can be explained as a double-integration of local rainfall, which originates from the seasonally-modulated ENSO Pacific-SST forcing (ENSO Combination mode). In the presence of interannual TIO SST forcing, the southeast African ENSO precipitation and vegetation responses are muted due to Indian Ocean warming and the resulting anomalous upward motion in the atmosphere.

In recent decades, Antarctic ice-sheet/shelf melting has been accelerated, releasing freshwater into the Southern Ocean. It has been suggested that the meltwater flux could lead to cooling in the Southern Hemisphere, which would retard global warming and further induce a northward shift of the Inter-Tropical Convergence Zone (ITCZ). In this study, we use experimental ensemble climate simulations to show that Antarctic meltwater forcing has distinct regional climate impacts over the globe, leading in particular to regional warming in East Asia. It is suggested that Antarctic meltwater forcing leads to a negative precipitation anomaly in the Western North Pacific (WNP) via cooling in the tropics and the northward shift of the ITCZ. This suppressed convection in WNP induces an anticyclonic flow over the North Pacific, which leads to regional warming in East Asia. This hypothesis is supported by analyses of inter-ensemble spread and long-term control simulations.

Numerous studies have demonstrated that the Pacific Meridional Mode (PMM) plays a vital role in determining El Niño-Southern Oscillation (ENSO) events in the following winter season. However, little attention has been given to significant differences among the spatial patterns of the PMM. Here we show that the PMM exhibits a large diversity in spatial patterns, leading to distinct impacts on ENSO. Based on objective clustering analysis, two distinct spatial patterns of the PMM are detected. Cluster 1 (C1) PMM exhibits a strong sea surface temperature dipole over the subtropical eastern Pacific and mid-latitude central Pacific whereas cluster 2 (C2) features a classic dipole over the subtropical eastern Pacific and equatorial cold tongue regions. We find that the C1 PMM is strongly linked to ENSO events while the C2 PMM has no statistically significant relations with following ENSO. This gives new implications for ENSO dynamics and predictions.