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1. Investigating Past Thermocline Conditions in the Eastern Equatorial Pacific Since the Last Glacial Interval Using Oxygen Isotope Gradients from Shallow- and Deep-dwelling Planktonic Foraminifera

   Smith, S; McManus, J; Pallone, C (December 2023, Proceedings American Geophysical Union)

   McManus, J (Ed.)

   The El Niño Southern Oscillation (ENSO) is a climate variation that occurs in the Eastern Equatorial Pacific (EEP) ocean, which influences the position of the thermocline today. The ENSO cycle is split into three states: a normal state, an El Niño state, and a La Niña state. Each one is marked by different sea surface temperatures (SST). Under normal conditions, the trade winds push warm water westward, away from the coast of South America. This allows cool water to upwell the east and creates a zonal SST gradient. Every few years, the trade winds slow, preventing the flow of warm water. This increases the SST in the EEP and produces an El Niño. The winds can also strengthen and move more warm water westward. The heightened zonal SST gradient forms a La Niña. This project investigates past conditions in the EEP by reconstructing the mean position of the thermocline: a layer in the ocean where temperature rapidly changes with depth. In the modern ocean, the thermocline is shallower in the east, where SSTs are cool, and deeper in the west, where SSTs are warm. The more uniform SST gradient during an El Niño event flattens the zonal slope thermocline; the stronger gradient during a La Niña steepens it. The temperature proxy used to determine past thermocline positions is the isotopic composition of oxygen in foraminifera (δ18O). Foraminiferal δ18O increases with depth in the water column, as temperature decreases and density increases. Two species with contrasting depth-habitats were analyzed; Globigerinoides ruber, which lives near the surface above the thermocline, and Neogloboquadrina dutertrei, which lives in the lower thermocline. When the thermocline shifts, it changes their difference in δ18O. A smaller difference indicates a deeper thermocline and an El Niño-like state; a greater difference indicates a shallower thermocline and a La Niña-like state. The forams were collected from two deep-sea sediment cores. The first was Ocean Drilling Program (ODP) Leg 202 Site 1239 (0.67˚S, 82.08˚W, 1414 m) drilled in the east near the coast of Ecuador. The second was ODP Leg 138 Site 849 (0.10˚S, 110.31˚W, 3858 m) toward the west. Rather than identifying specific ENSO events, this method provides insight into the position of the thermocline and therefore the mean state of the EEP during the Holocene and last glacial period. 

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2. El Niño‐La Niña Asymmetries in the Changes of ENSO Complexities and Dynamics Since 1990

   https://doi.org/10.1029/2023GL106395  

   Li, Xumin; Yu, Jin‐Yi; Ding, Ruiqiang (March 2024, Geophysical Research Letters)

   Abstract In around 1990, significant shifts occurred in the spatial pattern and temporal evolution of the El Niño‐Southern Oscillation (ENSO), with these shifts showing asymmetry between El Niño and La Niña phases. El Niño transitioned from the Eastern Pacific (EP) to the Central Pacific (CP) type, while La Niña's multi‐year (MY) events increased. These changes correlated with shifts in ENSO dynamics. Before 1990, El Niño was influenced by the Tropical Pacific (TP) ENSO dynamic, shifting to the Subtropical Pacific (SP) ENSO dynamic afterward, altering its spatial pattern. La Niña was influenced by the SP ENSO dynamic both before and after 1990 and has maintained the CP type. The strengthened SP ENSO dynamic since 1990, accompanied by enhanced precipitation efficiency during La Niña, make it easier for La Niña to transition into MY events. In contrast, there is no observed increase in precipitation efficiency during El Niño. 

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3. The Influence of ENSO Diversity on Future Atlantic Tropical Cyclone Activity

   https://doi.org/10.1175/JCLI-D-23-0286.1  

   Mueller, Teryn J; Patricola, Christina M; Bercos-Hickey, Emily (August 2024, Journal of Climate)

   El Niño–Southern Oscillation (ENSO) influences seasonal Atlantic tropical cyclone (TC) activity by impacting environmental conditions important for TC genesis. However, the influence of future climate change on the teleconnection between ENSO and Atlantic TCs is uncertain, as climate change is expected to impact both ENSO and the mean climate state. We used the Weather Research and Forecasting Model on a tropical channel domain to simulate 5-member ensembles of Atlantic TC seasons in historical and future climates under different ENSO conditions. Experiments were forced with idealized sea surface temperature configurations based on the Community Earth System Model (CESM) Large Ensemble representing: a monthly varying climatology, eastern Pacific El Niño, central Pacific El Niño, and La Niña. The historical simulations produced fewer Atlantic TCs during eastern Pacific El Niño compared to central Pacific El Niño, consistent with observations and other modeling studies. For each ENSO state, the future simulations produced a similar teleconnection with Atlantic TCs as in the historical simulations. Specifically, La Niña continues to enhance Atlantic TC activity, and El Niño continues to suppress Atlantic TCs, with greater suppression during eastern Pacific El Niño compared to central Pacific El Niño. In addition, we found a decrease in the Atlantic TC frequency in the future relative to historical regardless of ENSO state, which was associated with a future increase in northern tropical Atlantic vertical wind shear and a future decrease in the zonal tropical Pacific sea surface temperature (SST) gradient, corresponding to a more El Niño–like mean climate state. Our results indicate that ENSO will remain useful for seasonal Atlantic TC prediction in the future. 

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4. Decadal Modulation of the ENSO–Indian Ocean Basin Warming Relationship during the Decaying Summer by the Interdecadal Pacific Oscillation

   https://doi.org/10.1175/JCLI-D-20-0457.1  

   Liu, Fangyu; Zhang, Wenjun; Jin, Fei-Fei; Hu, Suqiong (April 2021, Journal of Climate)

   null (Ed.)

   Abstract Many previous studies have shown that an Indian Ocean basin warming (IOBW) occurs usually during El Niño–Southern Oscillation (ENSO) decaying spring to summer seasons through modifying the equatorial zonal circulation. Decadal modulation associated with the interdecadal Pacific oscillation (IPO) is further investigated here to understand the nonstationary ENSO–IOBW relationship during ENSO decaying summer (July–September). During the positive IPO phase, significant warm sea surface temperature (SST) anomalies are observed over the tropical Indian Ocean in El Niño decaying summers and vice versa for La Niña events, while these patterns are not well detected in the negative IPO phase. Different decaying speeds of ENSO associated with the IPO phase, largely controlled by both zonal advective and thermocline feedbacks, are suggested to be mainly responsible for these different ENSO–IOBW relationships. In contrast to ENSO events in the negative IPO phase, the ones in the positive IPO phase display a slower decaying speed and delay their transitions both from a warm to a cold state and a cold to a warm state. The slower decay of El Niño and La Niña thereby helps to sustain the teleconnection forcing over the equatorial Indian Ocean and corresponding SST anomalies there can persist into summer. This IPO modulation of the ENSO–IOBW relationship carries important implications for the seasonal prediction of the Indian Ocean SST anomalies and associated summer climate anomalies. 

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5. A Positive Indian Ocean Dipole Leads to an Indian Ocean Basin Mode that Favors the Development of La Niña the Following Year

   https://doi.org/10.1029/2024GL112632  

   Wang, Jing; Zhang, Shouwen; Li, Yuanlong; Sprintall, Janet (April 2025, Geophysical Research Letters)

   Abstract Interactions among the El Niño‐Southern Oscillation, Indian Ocean Basin mode (IOB), and Indian Ocean Dipole (IOD) significantly impact global climate variability and seasonal predictions. Traditionally, positive IOD (pIOD) and IOB warming events are associated with El Niño, driven by its influence on the tropical Indian Ocean through Walker Circulation anomalies. Our findings enrich this framework, revealing that a pIOD without El Niño can independently trigger IOB warming, and both types of pIODs can induce La Niña events. While El Niño primarily forces IOB warming and subsequent La Niña development via the atmospheric bridge across the Maritime Continent, pIODs independent of El Niño influence IOB warming through oceanic dynamics, which further favors La Niña development in the following year. The NMEFC‐CESM model sensitivity experiments underscore the critical role of thermocline processes in this mechanism, dependent on the pIOD's temperature amplitude, offering vital insights for forecasting post‐IOD, IOB, and La Niña events. 

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