Widely distributed proxy records indicate that the Medieval Climate Anomaly (MCA; ~900–1350 AD) was characterized by coherent shifts in large-scale Northern Hemisphere atmospheric circulation ...patterns. Although cooler sea surface temperatures in the central and eastern equatorial Pacific can explain some aspects of medieval circulation changes, they are not sufficient to account for other notable features, including widespread aridity through the Eurasian sub-tropics, stronger winter westerlies across the North Atlantic and Western Europe, and shifts in monsoon rainfall patterns across Africa and South Asia. We present results from a full-physics coupled climate model showing that a slight warming of the tropical Indian and western Pacific Oceans relative to the other tropical ocean basins can induce a broad range of the medieval circulation and climate changes indicated by proxy data, including many of those not explained by a cooler tropical Pacific alone. Important aspects of the results resemble those from previous simulations examining the climatic response to the rapid Indian Ocean warming during the late twentieth century, and to results from climate warming simulations—especially in indicating an expansion of the Northern Hemisphere Hadley circulation. Notably, the pattern of tropical Indo-Pacific sea surface temperature (SST) change responsible for producing the proxy-model similarity in our results agrees well with MCA-LIA SST differences obtained in a recent proxy-based climate field reconstruction. Though much remains unclear, our results indicate that the MCA was characterized by an enhanced zonal Indo-Pacific SST gradient with resulting changes in Northern Hemisphere tropical and extra-tropical circulation patterns and hydroclimate regimes, linkages that may explain the coherent regional climate shifts indicated by proxy records from across the planet. The findings provide new perspectives on the nature and possible causes of the MCA—a remarkable, yet incompletely understood episode of Late Holocene climatic change.
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.
Decadal fluctuations in salinity, nutrients, chlorophyll, a variety of zooplankton taxa, and fish stocks in the Northeast Pacific are often poorly correlated with the most widely‐used index of ...large‐scale climate variability in the region ‐ the Pacific Decadal Oscillation (PDO). We define a new pattern of climate change, the North Pacific Gyre Oscillation (NPGO) and show that its variability is significantly correlated with previously unexplained fluctuations of salinity, nutrients and chlorophyll. Fluctuations in the NPGO are driven by regional and basin‐scale variations in wind‐driven upwelling and horizontal advection – the fundamental processes controlling salinity and nutrient concentrations. Nutrient fluctuations drive concomitant changes in phytoplankton concentrations, and may force similar variability in higher trophic levels. The NPGO thus provides a strong indicator of fluctuations in the mechanisms driving planktonic ecosystem dynamics. The NPGO pattern extends beyond the North Pacific and is part of a global‐scale mode of climate variability that is evident in global sea level trends and sea surface temperature. Therefore the amplification of the NPGO variance found in observations and in global warming simulations implies that the NPGO may play an increasingly important role in forcing global‐scale decadal changes in marine ecosystems.
This review of late-Holocene palaeoclimatology represents the results from a PAGES/CLIVAR Intersection Panel meeting that took place in June 2006. The review is in three parts: the principal ...high-resolution proxy disciplines (trees, corals, ice cores and documentary evidence), emphasizing current issues in their use for climate reconstruction; the various approaches that have been adopted to combine multiple climate proxy records to provide estimates of past annual-to-decadal timescale Northern Hemisphere surface temperatures and other climate variables, such as large-scale circulation indices; and the forcing histories used in climate model simulations of the past millennium. We discuss the need to develop a framework through which current and new approaches to interpreting these proxy data may be rigorously assessed using pseudo-proxies derived from climate model runs, where the `answer' is known. The article concludes with a list of recommendations. First, more raw proxy data are required from the diverse disciplines and from more locations, as well as replication, for all proxy sources, of the basic raw measurements to improve absolute dating, and to better distinguish the proxy climate signal from noise. Second, more effort is required to improve the understanding of what individual proxies respond to, supported by more site measurements and process studies. These activities should also be mindful of the correlation structure of instrumental data, indicating which adjacent proxy records ought to be in agreement and which not. Third, large-scale climate reconstructions should be attempted using a wide variety of techniques, emphasizing those for which quantified errors can be estimated at specified timescales. Fourth, a greater use of climate model simulations is needed to guide the choice of reconstruction techniques (the pseudo-proxy concept) and possibly help determine where, given limited resources, future sampling should be concentrated.
Abstract
The tropical West Pacific hosts the warmest part of the surface ocean and has a considerable impact on the global climate system. Reconstructions of past temperature in this region can ...elucidate climate connections between the tropics and poles and the sensitivity of tropical temperature to greenhouse forcing. However, existing data are equivocal and reliable information from terrestrial archives is particularly sparse. Here we constrain the magnitude and timing of land temperature change in the tropical West Pacific across the last deglaciation using an exceptionally precise paleothermometer applied to a well-dated stalagmite from Northern Borneo. We show that the cave temperature increased by 4.4 ± 0.3 °C (2 SEM) from the Last Glacial Maximum to the Holocene, amounting to 3.6 ± 0.3 °C (2 SEM) when correcting for sea-level induced cave altitude change. The warming closely follows atmospheric CO
2
and Southern Hemisphere warming. This contrasts with hydroclimate, as reflected by drip water δ
18
O, which responds to Northern Hemisphere cooling events in the form of prominent drying, while temperature was rising. Our results thus show a close response of tropical temperature to greenhouse forcing, independent of shifts in the tropical circulation patterns.
Any assessment of future climate change requires knowledge of the full range of natural variability in the El Niño/Southern Oscillation (ENSO) phenomenon. Here we splice together fossil-coral oxygen ...isotopic records from Palmyra Island in the tropical Pacific Ocean to provide 30-150-year windows of tropical Pacific climate variability within the last 1,100 years. The records indicate mean climate conditions in the central tropical Pacific ranging from relatively cool and dry during the tenth century to increasingly warmer and wetter climate in the twentieth century. But the corals also document a broad range of ENSO behaviour that correlates poorly with these estimates of mean climate. The most intense ENSO activity within the reconstruction occurred during the mid-seventeenth century. Taken together, the coral data imply that the majority of ENSO variability over the last millennium may have arisen from dynamics internal to the ENSO system itself.
Records of atmospheric carbon dioxide concentration (Pco₂) and Antarctic temperature have revealed an intriguing change in the magnitude of interglacial warmth and Pco₂ at around 430,000 years ago ...(430 ka), but the global climate repercussions of this change remain elusive. Here, we present a stalagmite-based reconstruction of tropical West Pacific hydroclimate from 570 to 210 ka. The results suggest similar regional precipitation amounts across the four interglacials contained in the record, implying that tropical hydroclimate was insensitive to interglacial differences in Pco₂ and high-latitude temperature. In contrast, during glacial terminations, drying in the tropical West Pacific accompanied cooling events in northern high latitudes. Therefore, the tropical convective heat engine can either stabilize or amplify global climate change, depending on the nature of the climate forcing.
In tropical paleoclimate studies, paleo‐precipitation is often reconstructed from proxies via the “amount effect,” that is, the empirical inverse relationship between local precipitation amount (P) ...and the oxygen isotopic composition of precipitation (δ18OP). However, recent research has illustrated numerous microphysical and dynamical controls on δ18OP that do not necessarily covary with P, complicating the reconstruction of circulation features like the Intertropical Convergence Zone. Here we introduce a new conceptual and statistical model for δ18OP that better captures the physical foundations for δ18OP as a tracer of hydrological balance. We find that bulk precipitation microphysics and cloud type exert comparable influences on δ18OP. Moisture transport plays an important secondary role in regions of deep atmospheric convection such as the Intertropical Convergence Zone and Indo‐Pacific Warm Pool. Our findings help reconcile conflicting interpretations of Intertropical Convergence Zone excursions, and provide a firm physical grounding for more nuanced, accurate interpretations of past hydroclimate using water isotope proxies.
Plain Language Summary
The oxygen isotopic composition of tropical precipitation is a powerful tool for “fingerprinting” the history of evaporation, condensation, and transport that water was subjected to in the atmosphere before it reached the ground as precipitation. For this reason, the oxygen isotopic composition of precipitation is commonly employed as a water cycle tracer, both in modern‐day contexts and in geologic archives. Translating oxygen isotope ratios into metrics of circulation and climate is not always straightforward, however, due to the range of processes that can affect precipitation. Here we introduce a novel conceptual and statistical framework for interpreting oxygen isotope ratios in tropical precipitation by deconvolving its multiple competing influences. We find that the relative importance of each factor varies geographically. Moisture source is particularly important around the Indo‐Pacific Warm Pool, while cloud type exerts strong influence in regions where stratiform clouds are abundant. These results help to reconcile conflicting interpretations of how the Intertropical Convergence Zone and other key features of tropical circulation respond to climate forcings, which are critical questions for past climate reconstructions as well as future climate projections.
Key Points
We present a new conceptual framework and statistical model for the oxygen isotopic composition of precipitation at tropical stations
Cloud type is leading influence where stratiform rain is abundant; moisture transport plays key role along tropical rain belt perimeters
δ18OP and Precip are both water cycle integrators, but δ18OP is a more reliable proxy for large‐scale hydrological processes than Precip
Background
Convalescent plasma therapy for COVID‐19 relies on transfer of anti‐viral antibody from donors to recipients via plasma transfusion. The relationship between clinical characteristics and ...antibody response to COVID‐19 is not well defined. We investigated predictors of convalescent antibody production and quantified recipient antibody response in a convalescent plasma therapy clinical trial.
Methods
Multivariable analysis of clinical and serological parameters in 103 confirmed COVID‐19 convalescent plasma donors 28 days or more following symptom resolution was performed. Mixed‐effects regression models with piecewise linear trends were used to characterize serial antibody responses in 10 convalescent plasma recipients with severe COVID‐19.
Results
Donor antibody titres ranged from 0 to 1 : 3892 (anti‐receptor binding domain (RBD)) and 0 to 1 : 3289 (anti‐spike). Higher anti‐RBD and anti‐spike titres were associated with increased age, hospitalization for COVID‐19, fever and absence of myalgia (all P < 0.05). Fatigue was significantly associated with anti‐RBD (P = 0.03). In pairwise comparison amongst ABO blood types, AB donors had higher anti‐RBD and anti‐spike than O donors (P < 0.05). No toxicity was associated with plasma transfusion. Non‐ECMO recipient anti‐RBD antibody titre increased on average 31% per day during the first three days post‐transfusion (P = 0.01) and anti‐spike antibody titre by 40.3% (P = 0.02).
Conclusion
Advanced age, fever, absence of myalgia, fatigue, blood type and hospitalization were associated with higher convalescent antibody titre to COVID‐19. Despite variability in donor titre, 80% of convalescent plasma recipients showed significant increase in antibody levels post‐transfusion. A more complete understanding of the dose‐response effect of plasma transfusion amongst COVID‐19‐infected patients is needed.
PDF
