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We investigate the tropical Pacific annual cycle and the El Niño/Southern Oscillation (ENSO) in four mid‐Holocene simulations. Our results show that both ENSO variability and the amplitude of the annual cycle of the tropical Pacific cold tongue are reduced under mid‐Holocene forcing, along with a modified annual cycle in ENSO variance. The weakened annual cycle of the cold tongue is attributed to an ocean dynamical response to westerly wind anomalies in the western equatorial Pacific in boreal spring in addition to a thermodynamic response to local insolation changes in the eastern Pacific. The anomalous westerly winds in boreal spring excite an annual downwelling Kelvin wave that deepens the thermocline and propagates eastward along the equator, reaching the central and eastern equatorial Pacific during the development season of ENSO in boreal summer. Upon reaching the eastern Pacific, the downwelling Kelvin wave deepens the near‐surface thermocline, warming the surface ocean and weakening the local ocean‐atmosphere coupling critical to the growth of ENSO events. The westerly wind anomaly is associated with a shift in convection in the western Pacific driven by greater cooling of the Maritime Continent than western Pacific Ocean during the first half of the year (January to June) under tropical insolation forcing. By elucidating a common set of mechanisms responsible for a reduced cold tongue annual cycle and ENSO variability in a diverse range of mid‐Holocene simulations, this work yields important insight into the linkages between the tropical Pacific annual cycle and ENSO that are critical for understanding tropical Pacific climate variability. 

Abstract. The response of the hydrological cycle to anthropogenic climatechange, especially across the tropical oceans, remains poorly understood due to the scarcity of long instrumental temperature and hydrological records. Massive shallow-water corals are ideally suited to reconstructing past oceanic variability as they are widely distributed across the tropics,rapidly deposit calcium carbonate skeletons that continuously record ambient environmental conditions, and can be sampled at monthly to annualresolution. Climate reconstructions based on corals primarily use the stable oxygen isotope composition (δ18O), which acts as a proxy for sea surface temperature (SST), and the oxygen isotope composition ofseawater (δ18Osw), a measure of hydrological variability. Increasingly, coral δ18O time series are paired with time series of strontium-to-calcium ratios (Sr/Ca), a proxy for SST, from the same coral to quantify temperature and δ18Osw variabilitythrough time. To increase the utility of such reconstructions, we presentthe CoralHydro2k database, a compilation of published, peer-reviewed coral Sr/Ca and δ18O records from the Common Era (CE). The database contains 54 paired Sr/Ca–δ18O records and 125 unpaired Sr/Ca or δ18O records, with 88 % of these records providing data coverage from 1800 CE to the present. A quality-controlled set of metadata with standardized vocabulary and units accompanies each record, informing the useof the database. The CoralHydro2k database tracks large-scale temperatureand hydrological variability. As such, it is well-suited for investigationsof past climate variability, comparisons with climate model simulationsincluding isotope-enabled models, and application in paleodata-assimilation projects. The CoralHydro2k database is available in Linked Paleo Data (LiPD) format with serializations in MATLAB, R, and Python and can be downloaded from the NOAA National Center for Environmental Information's Paleoclimate Data Archive at https://doi.org/10.25921/yp94-v135 (Walter et al., 2022). 

Coral strontium‐to‐calcium ratios (Sr/Ca) provide quantitative estimates of past sea surface temperatures (SST) that allow for the reconstruction of changes in the mean state and climate variations, such as the El Nino‐Southern Oscillation, through time. However, coral Sr/Ca ratios are highly susceptible to diagenesis, which can impart artifacts of 1–2°C that are typically on par with the tropical climate signals of interest. Microscale sampling via Secondary Ion Mass Spectrometry (SIMS) for the sampling of primary skeletal material in altered fossil corals, providing much‐needed checks on fossil coral Sr/Ca‐based paleotemperature estimates. In this study, we employ a set modern and fossil corals from Palmyra Atoll, in the central tropical Pacific, to quantify the accuracy and reproducibility of SIMS Sr/Ca analyses relative to bulk Sr/Ca analyses. In three overlapping modern coral samples, we reproduce bulk Sr/Ca estimates within ±0.3% (1σ). We demonstrate high fidelity between 3‐month smoothed SIMS coral Sr/Ca timeseries and SST (R = −0.5 to −0.8;p < 0.5). For lightly‐altered sections of a young fossil coral from the early‐20th century, SIMS Sr/Ca timeseries reproduce bulk Sr/Ca timeseries, in line with our results from modern corals. Across a moderately‐altered section of the same fossil coral, where diagenesis yields bulk Sr/Ca estimates that are 0.6 mmol too high (roughly equivalent to −6°C artifacts in SST), SIMS Sr/Ca timeseries track instrumental SST timeseries. We conclude that 3–4 SIMS analyses per month of coral growth can provide a much‐needed quantitative check on the accuracy of fossil coral Sr/Ca‐derived estimates of paleotemperature, even in moderately altered samples.

 

Prospects for coral persistence through increasingly frequent and extended heatwaves seem bleak. Coral recovery from bleaching is only known to occur after temperatures return to normal, and mitigation of local stressors does not appear to augment coral survival. Capitalizing on a natural experiment in the equatorial Pacific, we track individual coral colonies at sites spanning a gradient of local anthropogenic disturbance through a tropical heatwave of unprecedented duration. Unexpectedly, some corals survived the event by recovering from bleaching while still at elevated temperatures. These corals initially had heat-sensitive algal symbiont communities, endured bleaching, and then recovered through proliferation of heat-tolerant symbionts. This pathway to survival only occurred in the absence of strong local stressors. In contrast, corals in highly disturbed areas were already dominated by heat-tolerant symbionts, and despite initially resisting bleaching, these corals had no survival advantage in one species and 3.3 times lower survival in the other. These unanticipated connections between disturbance, coral symbioses and heat stress resilience reveal multiple pathways to coral survival through future prolonged heatwaves.

 

Decadal climate variability and change affects nearly every aspect of our world, including weather, agriculture, ecosystems, and the economy. Predicting its expression is thus of critical importance on multiple fronts. Poweret al. review what is known about tropical Pacific decadal climate variability and change, the degree to which it can be simulated and predicted, and how we might improve our understanding of it. More accurate projections will require longer and more detailed instrumental and paleoclimate records, improved climate models, and better data assimilation methods. —HJS

 

Global surface temperatures during the twentieth century are characterized by multidecadal periods of accelerated or reduced warming, which are thought to be driven by Pacific decadal variability, specifically changes in trade‐wind strength. However, the relationship between trade‐wind strength and global surface warming remains poorly constrained due to the scarcity of instrumental wind observations. Previous work has shown that corals growing at Tarawa Atoll (1.3°N, 173°E) incorporate dissolved Mn flushed from lagoon sediments by El Niño‐related westerly wind events (WWEs), providing records of both westerly wind variability and trade‐wind strength (on decadal time scales). Here, we explore the utility of this novel coral Mn/Ca‐wind proxy at two nearby islands that also feature west‐facing lagoons. Short coral Mn/Ca records from Butaritari (3°N, 173°E) and Kiritimati (2°N, 157.5°W) track WWEs, albeit with some intercolony variability in the magnitude and timing of the signal. Variability in coral Mn/Ca signal intensity among records from Butaritari suggests that wind‐driven mixing of the sediment Mn reservoir may be finite and/or localized. At Kiritimati, a coral growing outside the lagoon shows higher Mn/Ca concentrations during the 1997/1998 El Niño event, suggesting that nearshore sediments may be an overlooked dissolved Mn reservoir. Taken together, these results highlight a need for additional studies of Mn reservoir variability within and across atolls that hold promise for recording WWEs. These results also suggest that Mn/Ca records from multiple coral colonies and sites are needed to generate robust coral‐based wind reconstructions, particularly from sites with unknown or complex Mn transport pathways.

 

The tropical Pacific influences climate patterns across the globe, yet robust constraints on decadal to centennial‐scale climate variations are difficult to extract from sparse instrumental observations in this region. Oxygen isotope (δ¹⁸O) records from long‐lived corals enable the quantitative reconstruction of tropical Pacific climate variability and trends over the twentieth century and beyond, but such corals are exceedingly rare. Here, we use multiple short coral δ¹⁸O records to create a coral ‘ensemble’ reconstruction of twentieth century climate in the central tropical Pacific. Ten U/Th‐dated fossil coral δ¹⁸O records from Kiritimati Island (2°N, 157°W) span 1891 CE to 2006 CE, with the younger samples enabling quantitative comparison to a large ensemble of modern coral records and instrumental sea surface temperature. A composite record constructed of modern and fossil Kiritimati coral δ¹⁸O records shows a shift toward warmer and fresher conditions from 1970 CE onward, consistent with previously published records in this region.