Palaeoclimate reconstructions reveal a strong link between El Niño-Southern Oscillation and Tropical Pacific mean state

Deglacial increase of seasonal temperature variability in the tropical ocean

The relatively stable Holocene climate was preceded by a pronounced event of abrupt warming in the Northern Hemisphere, the termination of the Younger Dryas (YD) cold period1,2. Although this transition has been intensively studied, its imprint on low-latitude ocean temperature is still controversial and its effects on sub-annual to decadal climate variability remain poorly understood1,3,4. Sea surface temperature (SST) variability at these timescales in the tropical Atlantic is expected to intensify under current and future global warming and has considerable consequences for environmental conditions in Africa and South America, and for tropical Pacific climate5–8. Here we present a 100-µm-resolution record obtained by mass spectrometry imaging (MSI) of long-chain alkenones in sediments from the Cariaco Basin9–11 and find that annually averaged SST remained stable during the transition into the Holocene. However, seasonality increased more than twofold and approached modern values of 1.6 °C, probably driven by the position and/or annual range of the Intertropical Convergence Zone (ITCZ). We further observe that interannual variability intensified during the early Holocene. Our results demonstrate that sub-decadal-scale SST variability in the tropical Atlantic is sensitive to abrupt changes in climate background, such as those witnessed during the most recent glacial to interglacial transition.

Thermal coupling of the Indo-Pacific warm pool and Southern Ocean over the past 30,000 years

The role of the tropical Pacific Ocean and its linkages to the southern hemisphere during the last deglacial warming remain highly controversial. Here we explore the evolution of Pacific horizontal and vertical thermal gradients over the past 30 kyr by compiling 340 sea surface and 7 subsurface temperature records, as well as one new ocean heat content record. Our records reveal that La Niña-like conditions dominated during the deglaciation as a result of the more intense warming in the western Pacific warm pool. Both the subsurface temperature and ocean heat content in the warm pool rose earlier than the sea surface temperature, and in phase with South Pacific subsurface temperature and orbital precession, implying that heat exchange between the tropical upper water column and the extratropical Southern Ocean facilitated faster warming in the western Pacific. Our study underscores the key role of the thermal coupling between the warm pool and the Southern Ocean and its relevance for future global warming.

The mechanism of the last deglacial global warming is key for future climate. Here, the authors shed light on the pivotal role of the thermal coupling between the western Pacific warm pool and the Southern Ocean.

Modulation of late Pleistocene ENSO strength by the tropical Pacific thermocline

The El Niño Southern Oscillation (ENSO) is highly dependent on coupled atmosphere-ocean interactions and feedbacks, suggesting a tight relationship between ENSO strength and background climate conditions. However, the extent to which background climate state determines ENSO behavior remains in question. Here we present reconstructions of total variability and El Niño amplitude from individual foraminifera distributions at discrete time intervals over the past ~285,000 years across varying atmospheric CO₂ levels, global ice volume and sea level, and orbital insolation forcing. Our results show a strong correlation between eastern tropical Pacific Ocean mixed-layer thickness and both El Niño amplitude and central Pacific variability. This ENSO-thermocline relationship implicates upwelling feedbacks as the major factor controlling ENSO strength on millennial time scales. The primacy of the upwelling feedback in shaping ENSO behavior across many different background states suggests accurate quantification and modeling of this feedback is essential for predicting ENSO’s behavior under future climate conditions.

How the El Niño Southern Oscillation depends on the background conditions is not well known. Here, the authors present individual foraminifera distributions which show that central Pacific variability is related to the warmth and depth of the thermocline across varying climate background conditions over the past ~285,000 years.

Pacific warm pool subsurface heat sequestration modulated Walker circulation and ENSO activity during the Holocene

Dynamics driving the El Niño-Southern Oscillation (ENSO) over longer-than-interannual time scales are poorly understood. Here, we compile thermocline temperature records of the Indo-Pacific warm pool over the past 25,000 years, which reveal a major warming in the Early Holocene and a secondary warming in the Middle Holocene. We suggest that the first thermocline warming corresponds to heat transport of southern Pacific shallow overturning circulation driven by June (austral winter) insolation maximum. The second thermocline warming follows equatorial September insolation maximum, which may have caused a steeper west-east upper-ocean thermal gradient and an intensified Walker circulation in the equatorial Pacific. We propose that the warm pool thermocline warming ultimately reduced the interannual ENSO activity in the Early to Middle Holocene. Thus, a substantially increased oceanic heat content of the warm pool, acting as a negative feedback for ENSO in the past, may play its role in the ongoing global warming.

Deciphering the Variability in Mg/Ca and Stable Oxygen Isotopes of Individual Foraminifera

Foraminifera are commonly used in paleoclimate reconstructions as they occur throughout the world's oceans and are often abundantly preserved in the sediments. Traditionally, foraminifera-based proxies like δ18O and Mg/Ca are analyzed on pooled specimens of a single species. Analysis of single specimens of foraminifera allows reconstructing climate variability on timescales related to El Niño-Southern Oscillation or seasonality. However, quantitative calibrations between the statistics of individual foraminifera analyses (IFA) and climate variability are still missing. We performed Mg/Ca and δ18O measurements on single specimens from core top sediments from different settings to better understand the signal recorded by individual foraminifera. We used three species of planktic foraminifera (Globigerinoidesruber (s.s.), T. sacculifer, and N. dutertrei) from the Indo-Pacific Warm Pool and one species (G. ruber (pink)) from the Gulf of Mexico. Mean values for the different species of Mg/Ca versus calculated δ18O temperatures agree with published calibration equations. IFA statistics (both mean and standard deviation) of Mg/Ca and δ18O between the different sites show a strong relationship indicating that both proxies are influenced by a common factor, most likely temperature variations during calcification. This strongly supports the use of IFA to reconstruct climate variability. However, our combined IFA data for the different species only show a weak relationship to seasonal and interannual temperature changes, especially when seasonal variability increases at a location. This suggests that the season and depth habitat of the foraminifera strongly affect IFA variability, such that ecology needs to be considered when reconstructing past climate variability.

Evolution and forcing mechanisms of El Niño over the past 21,000 years

The El Niño Southern Oscillation (ENSO) is Earth's dominant source of interannual climate variability, but its response to global warming remains highly uncertain. To improve our understanding of ENSO's sensitivity to external climate forcing, it is paramount to determine its past behaviour by using palaeoclimate data and model simulations. Palaeoclimate records show that ENSO has varied considerably since the Last Glacial Maximum (21,000 years ago), and some data sets suggest a gradual intensification of ENSO over the past ∼6,000 years. Previous attempts to simulate the transient evolution of ENSO have relied on simplified models or snapshot experiments. Here we analyse a series of transient Coupled General Circulation Model simulations forced by changes in greenhouse gasses, orbital forcing, the meltwater discharge and the ice-sheet history throughout the past 21,000 years. Consistent with most palaeo-ENSO reconstructions, our model simulates an orbitally induced strengthening of ENSO during the Holocene epoch, which is caused by increasing positive ocean-atmosphere feedbacks. During the early deglaciation, ENSO characteristics change drastically in response to meltwater discharges and the resulting changes in the Atlantic Meridional Overturning Circulation and equatorial annual cycle. Increasing deglacial atmospheric CO2 concentrations tend to weaken ENSO, whereas retreating glacial ice sheets intensify ENSO. The complex evolution of forcings and ENSO feedbacks and the uncertainties in the reconstruction further highlight the challenge and opportunity for constraining future ENSO responses.

Reduced El Niño-Southern Oscillation during the Last Glacial Maximum

El Niño-Southern Oscillation (ENSO) is a major source of global interannual variability, but its response to climate change is uncertain. Paleoclimate records from the Last Glacial Maximum (LGM) provide insight into ENSO behavior when global boundary conditions (ice sheet extent, atmospheric partial pressure of CO2) were different from those today. In this work, we reconstruct LGM temperature variability at equatorial Pacific sites using measurements of individual planktonic foraminifera shells. A deep equatorial thermocline altered the dynamics in the eastern equatorial cold tongue, resulting in reduced ENSO variability during the LGM compared to the Late Holocene. These results suggest that ENSO was not tied directly to the east-west temperature gradient, as previously suggested. Rather, the thermocline of the eastern equatorial Pacific played a decisive role in the ENSO response to LGM climate.