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1.
Large volcanic eruptions can have major impacts on global climate, affecting both atmospheric and ocean circulation through changes in atmospheric chemical composition and optical properties. The residence time of volcanic aerosol from strong eruptions is roughly 2–3 y. Attention has consequently focused on their short-term impacts, whereas the long-term, ocean-mediated response has not been well studied. Most studies have focused on tropical eruptions; high-latitude eruptions have drawn less attention because their impacts are thought to be merely hemispheric rather than global. No study to date has investigated the long-term effects of high-latitude eruptions. Here, we use a climate model to show that large summer high-latitude eruptions in the Northern Hemisphere cause strong hemispheric cooling, which could induce an El Niño-like anomaly, in the equatorial Pacific during the first 8–9 mo after the start of the eruption. The hemispherically asymmetric cooling shifts the Intertropical Convergence Zone southward, triggering a weakening of the trade winds over the western and central equatorial Pacific that favors the development of an El Niño-like anomaly. In the model used here, the specified high-latitude eruption also leads to a strengthening of the Atlantic Meridional Overturning Circulation (AMOC) in the first 25 y after the eruption, followed by a weakening lasting at least 35 y. The long-lived changes in the AMOC strength also alter the variability of the El Niño–Southern Oscillation (ENSO).Proxy data (1, 2) suggest that the strong reduction of surface insolation over the tropics associated with tropical volcanic eruptions may increase the likelihood of the El Niño–Southern Oscillation (ENSO) and a consequent reduction of the zonal sea surface temperature (SST) gradient along the equatorial Pacific. Modeling studies do not yield consistent results and show both an El Niño-like (35) or La Niña-like (6, 7) anomalies following a tropical eruption. Recent studies have also suggested that volcanic eruptions can have a large imprint on ocean circulation, affecting the strength of the Atlantic Meridional Overturning Circulation (AMOC) (812) on 5- to 20-y timescales and inducing ocean heat content (OHC) anomalies (13, 14) that may persist for decades. However, this slow recovery has been questioned and may be an artifact of experimental design (15). Furthermore, all previous work on the climate impact of volcanic eruptions has focused on tropical volcanoes; no studies have addressed the potential effects of high-latitude eruptions on ENSO. Here, we use a coupled atmospheric–ocean–aerosol model [Norwegian Earth System Model: NorESM1-M (16, 17)] to identify the mechanisms by which high-latitude volcanic eruptions can impact ENSO behavior in both the short term (up to 2–3 y) and long term (approximately half-century), the latter being mediated by volcano-induced changes in ocean circulation.We simulate an extreme high-latitude multistage eruption starting on June 1st. We inject 100 Tg of SO2 and ash—as an analog for the ash injection—mostly into the upper-troposphere/lower stratosphere over a 4-mo period. The eruption is composed of eight injections, each lasting for 4 d and spaced out every 15 d (SI Appendix, Table S1). This experimental design was chosen as analog for one of the strongest high-latitude eruptions in historical time, the 1783 Laki eruption in Iceland. The simulated volcanic eruption starts from a specific year selected from a transient historical simulation (1850–2005). An ensemble of simulations (ENSv) is generated by slightly perturbing the initial conditions of the day of the eruption. In the same fashion, we generate an equivalent no-volcano ensemble (ENSnv) where the volcanic aerosol concentration is set to background conditions (SI Appendix). The climate perturbation induced by the volcanic eruption (Δv) can be simply expressed as Δv = STATEv – STATEnv, where STATEnv is the unperturbed climate state, and STATEv is the climate state induced by the eruption. To examine the short-term impact on ENSO, we analyze the simulations described by Pausata et al. (18) in which ENSnv and ENSv are composed of 20 pairs of simulation, each pair being integrated for 4 y. Here, we extend 10 of these pairs of simulations out to 60 y after the eruption to investigate its long-term impact on the AMOC, OHC, and the spatiotemporal properties of ENSO.  相似文献   
2.
We investigated the periodicity of Plasmodium vivax and P. falciparum incidence in time-series of malaria data (1990–2010) from three endemic regions in Venezuela. In particular, we determined whether disease epidemics were related to local climate variability and regional climate anomalies such as the El Niño Southern Oscillation (ENSO). Malaria periodicity was found to exhibit unique features in each studied region. Significant multi-annual cycles of 2- to about 6-year periods were identified. The inter-annual variability of malaria cases was coherent with that of SSTs (ENSO), mainly at temporal scales within the 3–6 year periods. Additionally, malaria cases were intensified approximately 1 year after an El Niño event, a pattern that highlights the role of climate inter-annual variability in the epidemic patterns. Rainfall mediated the effect of ENSO on malaria locally. Particularly, rains from the last phase of the season had a critical role in the temporal dynamics of Plasmodium. The malaria–climate relationship was complex and transient, varying in strength with the region and species. By identifying temporal cycles of malaria we have made a first step in predicting high-risk years in Venezuela. Our findings emphasize the importance of analyzing high-resolution spatial–temporal data to better understand malaria transmission dynamics.  相似文献   
3.
Tropical maritime precipitation affects global atmospheric circulation, influencing storm tracks and the size and location of subtropical deserts. Paleoclimate evidence suggests centuries-long changes in rainfall in the tropical Pacific over the past 2,000 y, but these remain poorly characterized across most of the ocean where long, continuous proxy records capable of resolving decadal-to-centennial climate changes are still virtually nonexistent despite substantial efforts to develop them. Here we apply a new climate proxy based on paired hydrogen isotope ratios from microalgal and mangrove-derived sedimentary lipids in the Galápagos to reconstruct maritime precipitation changes during the Common Era. We show that increased rainfall during the Little Ice Age (LIA) (∼1400–1850 CE) was likely caused by a southward migration of the Intertropical Convergence Zone (ITCZ), and that this shift occurred later than previously recognized, coeval with dynamically linked precipitation changes in South America and the western tropical Pacific. Before the LIA, we show that drier conditions at the onset of the Medieval Warm Period (∼800–1300 CE) and wetter conditions ca. 2 ka were caused by changes in the El Niño/Southern Oscillation (ENSO). Collectively, the large natural variations in tropical rainfall we detect, each linked to a multicentury perturbation of either ENSO-like variability or the ITCZ, imply a high sensitivity of tropical Pacific rainfall to climate forcings.Tropical Pacific precipitation patterns have a profound impact on global climate, and changes are projected far outside the region of origin (1). Coherent understanding of these climate dynamics is therefore critical for understanding when and how the distribution and intensity of global precipitation patterns have changed in the past and will change in the future, with far-reaching implications for managing the demand for freshwater resources in major population and agricultural centers in the tropics and midlatitudes. Tropical Pacific precipitation is largely dominated by zonally asymmetric variability associated with El Niño/Southern Oscillation (ENSO) and the zonally symmetric annual north–south migration of the Intertropical Convergence Zone (ITCZ). The extensive geographic footprint and intensity of these phenomena suggests that capturing their evolution in the paleoclimate record and within Earth system models should be straightforward, but in practice these targets have proven elusive. Perennial problems persist in simulating realistic ITCZ and ENSO dynamics, even in the latest generation of state-of-the-art climate models (2, 3). Paleoclimate records should theoretically be able to help constrain some of these dynamics, but it is challenging to distinguish between ITCZ- and ENSO-driven changes in a record from a single location because rainfall alone is influenced by both phenomena (48). Networks of paleoclimate records can help to resolve these issues, but proxy archives that are within the core ITCZ and ENSO regions (i.e., at sea level in the tropical Pacific) and have both the temporal resolution and duration to record the decadal to centennial changes that are of greatest societal relevance have been difficult to obtain.The Galápagos archipelago in the eastern equatorial Pacific is in a key center of action for ENSO and is ideally located for testing hypotheses regarding changes in the southern extent of annual ITCZ migration (7, 9). Precipitation is highly variable on seasonal and interannual timescales and correlated with the Niño 1–4 indices (Fig. 1 and Fig. S1) as well as with the multivariate ENSO index, local sea surface temperature, and the isotopic composition of precipitation (4, 5, 10). El Niño events bring heavy rain to the Galápagos, and changes in their intensity or recurrence interval manifest in the local precipitation record, but with increased sensitivity to eastern Pacific, as opposed to central Pacific, El Niño events (Fig. S1) (4). The islands are at the modern southernmost extent of annual ITCZ migration, and changes in its maximum southerly range bring large increases in annual rainfall (Fig. 1).Open in a separate windowFig. 1.Precipitation data and base map for Isabela lakes. (A) Average wet- (February–April) and (B) dry-season (August–October) precipitation (1997–2008; NASA GPCP). Locations: Galápagos (star), Cariaco Basin (diamond), Lake Pumacocha (square), Makassar Straight (circle). (C) Niño1+2 precipitation anomaly shows influence of eastern Pacific El Niño and La Niña events on Galápagos precipitation. Anomalies in figure are the difference between years where Niño 1+2 > 1 °C and years where Niño 1+2 < 1 °C (dataset from A and B). Comparison between Niño 1+2 and Niño 3.4 is shown in Fig. S1. (D) Map shows locations of Isabela Island lakes within the Galápagos archipelago (Map data: Google, DigitalGlobe) and coring locations in each lake (Diablas, red; Verdes, green; Escondida, blue). Bnb, Bainbridge Crater; EJ, El Junco Lake; Is, Isabela lakes.Open in a separate windowFig. S1.Comparison between Niño 1+2 and Niño 3.4 precipitation anomalies. (A) Niño 1+2 precipitation anomaly as shown in Fig. 1C, or the difference between years where Niño 1+2 > 1 °C and years where Niño 1+2 < 1 °C (dataset from Fig. 1 A and B). (B) As in A, but for Niño 3.4 index. Differences highlight the enhanced sensitivity of the Galápagos to eastern Pacific El Niño events but also demonstrate that the region is sensitive to ENSO activity as defined by both indices.The δ2H value of tropical precipitation as it falls and is temporarily sequestered in lakes (δ2HWater) reflects its transport history, making δ2HWater values an excellent hydroclimate proxy (11). Photoautotrophic organisms use hydrogen from these waters to synthesize lipids, transforming δ2HWater values into lipid δ2H values (δ2HLipid) that are preserved in sediments over geologic time. However, δ2HLipid values are offset from δ2HWater values by isotopic fractionation that occurs during biosynthesis and that varies by organism, and in response to environmental conditions such as salinity (12). This complicates efforts to apply δ2HLipid values as paleoclimate proxies in coastal sediments where salinity varies over time and common biomarkers are synthesized by a wide variety of organisms. Mangrove trees and cyanobacteria living in these locations access common source water, but salinity has an opposing effect on 2H/1H fractionation expressed in their lipids (1315). In phytoplankton 2H/1H fractionation decreases by 0.7–2.0‰ per unit increase in salinity (14, 15), whereas in mangroves it increases by 0.7–1.7‰ (13, 16). Combining these calibrations with measured algal and mangrove δ2HLipid values provides a method to simultaneously and quantitatively reconstruct salinity and δ2HWater values. This approach circumvents shortcomings that have previously hindered even qualitative application of δ2HLipid values in these environments and opens the door for widespread application of this technique in the high-accumulation-rate saline coastal lakes that are common in the tropics. This newly developed paired biomarker approach was applied to sediments collected from three coastal saline ponds on Isabela Island in the Galápagos archipelago (Fig. 1) to reconstruct salinity and δ2HWater values spanning the past 2,000 y of the Common Era.  相似文献   
4.
目的调查分析既往及正在执行的厄尔尼诺-南方涛动现象(El Ni1o-Southern Oscillation,ENSO)与流行性感冒关联的相关研究,并评价ENSO对流行性感冒相关指标(如流感流行高峰的出现时间、流感样病例报告人数等)的影响,为进一步开展天气及气象因素影响流感病毒传播研究提供依据和建议。方法计算机检索MEDLINE,EMBASE,Science Direct,HEED、中国生物医学文献数据库(CBMdisc)、万方学位论文全文数据库,查找1988~2016年10月已发表的关于ENSO与流行性感冒联系的文献,并运用循证医学方法进行文献数据提取和分析。结果共检索到78篇文献,其中符合纳入标准的已发表文献10篇。二次文献研究显示,出现ENSO与流感大流行的发生,流感高峰的出现时间,流感样病例就诊数及重症死亡数都有较大关联,且El Ni1o和La Ni1a现象对流感相关指标的影响也不尽相同。结论从纳入研究的已发表文献分析结果来看,ENSO与流行性感冒相关指标之间有较强的相关性。  相似文献   
5.
The most important driver of climate variability is the El Niño Southern Oscillation, which can trigger disasters in various parts of the globe. Despite its importance, conventional forecasting is still limited to 6 mo ahead. Recently, we developed an approach based on network analysis, which allows projection of an El Niño event about 1 y ahead. Here we show that our method correctly predicted the absence of El Niño events in 2012 and 2013 and now announce that our approach indicated (in September 2013 already) the return of El Niño in late 2014 with a 3-in-4 likelihood. We also discuss the relevance of the next El Niño to the question of global warming and the present hiatus in the global mean surface temperature.  相似文献   
6.
Although anomalous episodic warming of the eastern equatorial Pacific, dubbed El Niño by Peruvian fishermen, has major (and occasionally devastating) impacts around the globe, robust forecasting is still limited to about 6 mo ahead. A significant extension of the prewarning time would be instrumental for avoiding some of the worst damages such as harvest failures in developing countries. Here we introduce a unique avenue toward El Niño prediction based on network methods, inspecting emerging teleconnections. Our approach starts from the evidence that a large-scale cooperative mode—linking the El Niño basin (equatorial Pacific corridor) and the rest of the ocean—builds up in the calendar year before the warming event. On this basis, we can develop an efficient 12-mo forecasting scheme, i.e., achieve some doubling of the early-warning period. Our method is based on high-quality observational data available since 1950 and yields hit rates above 0.5, whereas false-alarm rates are below 0.1.  相似文献   
7.
The responses of tropical forests to environmental change are critical uncertainties in predicting the future impacts of climate change. The positive phase of the 2015–2016 El Niño Southern Oscillation resulted in unprecedented heat and low precipitation in the tropics with substantial impacts on the global carbon cycle. The role of African tropical forests is uncertain as their responses to short-term drought and temperature anomalies have yet to be determined using on-the-ground measurements. African tropical forests may be particularly sensitive because they exist in relatively dry conditions compared with Amazonian or Asian forests, or they may be more resistant because of an abundance of drought-adapted species. Here, we report responses of structurally intact old-growth lowland tropical forests inventoried within the African Tropical Rainforest Observatory Network (AfriTRON). We use 100 long-term inventory plots from six countries each measured at least twice prior to and once following the 2015–2016 El Niño event. These plots experienced the highest temperatures and driest conditions on record. The record temperature did not significantly reduce carbon gains from tree growth or significantly increase carbon losses from tree mortality, but the record drought did significantly decrease net carbon uptake. Overall, the long-term biomass increase of these forests was reduced due to the El Niño event, but these plots remained a live biomass carbon sink (0.51 ± 0.40 Mg C ha−1 y−1) despite extreme environmental conditions. Our analyses, while limited to African tropical forests, suggest they may be more resistant to climatic extremes than Amazonian and Asian forests.

Tropical forests are a critical component of the global carbon cycle because they are extensive (1), carbon dense (2), and highly productive (3). Therefore, consistent impacts on these forests can have global consequences. Their global importance is seen via atmospheric measurements of CO2, showing a near-neutral exchange of carbon across the terrestrial tropics; hence, the large carbon losses from deforestation and degradation are offset by the significant carbon uptake from intact tropical forests and tropical forest regrowth (4). Independently, ground observations of structurally intact old-growth tropical forests also show this uptake, with forest biomass carbon increasing across remaining African (5, 6), Amazonian (7), and Asian (8) forests. Yet, unlike in Amazonia (9, 10) and Asia (8), the impact of a severe drought or a drought and high-temperature event in African tropical forests has never been documented using ground data.High temperatures test the physiological tolerance of tropical trees. Above optimal temperatures, plants reduce their carbon uptake (11). This includes closing stomata to avoid water loss, reducing internal CO2 concentrations, and reducing carbon assimilation in the leaf. Higher temperatures increase vapor pressure deficits (12) and alongside reduced precipitation, increase the chance of hydraulic failure (13). Individually or in combination, these impacts can slow growth and may eventually kill trees (14), although tropical seedling growth can increase with experimental warming (15). As well as reduced carbon uptake, plants use more carbon under higher temperatures; respiration rates tend to increase with short-term increases in temperature both at the leaf (16) and forest stand (17) scales, again reducing tree growth and potentially leading to tree death via carbon starvation (18). Recent analyses of tropical forest plot data showed increased temperatures over the prior 5 y were associated with lower levels of carbon uptake from tree growth and higher levels of carbon loss from tree mortality (6). Furthermore, biome-wide spatial analyses suggest the existence of a temperature threshold above which carbon uptake from tree growth declines rapidly (19). Thus, with high temperature anomalies, we expect reduced tree growth and increased tree mortality.Drought also impacts trees as water deficits can slow tree growth and if of sufficient strength or duration, can kill trees, either via hydraulic failure or carbon starvation. Hydraulic failure of the xylem has been found across species and biomes in response to drought, while carbon starvation has been documented in some locations including one tropical site (20). Inventory plot observations before, during, and after droughts show the impacts of drought in Asia and Amazonia. In Asia, the 1997 to 1998 El Niño temporarily halted the carbon sink in live biomass in Bornean forests by increasing tree mortality (8, 21). In Amazonia, severe droughts in 2005 and 2010 elevated biomass mortality and in 2010, also significantly reduced tree growth (9, 10). The Amazon biomass carbon sink was reversed by the 2005 drought, and while it rapidly recovered, it is weaker since 2005 (7), potentially due to high-temperature impacts (6). However, while the impacts of short-term drought in their long-term context have been investigated in Amazonia and Asia, in Africa we so far lack any ground-based assessment of large-scale drought impacts due to a paucity of observations.Although the broad responses of African tropical forests to temperature and drought anomalies might be hypothesized from first principles and the responses of other continents, there are considerable uncertainties. On the one hand, there are grounds for expecting African forests to be especially vulnerable. African forests are already remarkably dry compared with Amazonian and Asian tropical forests, with almost 90% receiving <2,000 mm y−1 precipitation (22), the approximate amount necessary to maintain photosynthesis at high levels throughout the year (23). This low rainfall suggests African tropical forests may already be close to their physiological and ecological limits. Additionally, the lower temperatures African forests tend to experience—as many are situated at slightly higher altitude than forests in Amazonia—could result in limited species tolerace of high temperatures. African forests are also much less species rich than forests in Amazonia and Asia (2, 24), with a relative lack of species in high-temperature African forests (25), and this lower diversity could conceivably drive lower resistance to climate anomalies (26).Alternatively, the relatively dry conditions of African tropical forests may, perhaps counterintuitively, confer drought resistance. African climate has oscillated between wetter conditions in interglacial periods and cooler and drier conditions in glacial periods (27), so the African pool of species present today may be more drought tolerant because some of the most mesic-adapted biodiversity has been lost over time (28, 29). Drier African tropical forest tree diversity is similar to that of the Amazon or Asia, but tree diversity does not increase with shorter dry seasons in Africa as it does in Amazonia (25), suggesting that most wet-adapted species have been lost and either the dry-adapted species remained or these lineages have diversified more, potentially conferring drought resistance. Indeed, a 40-y drought in West Africa led to an increased abundance of deciduous species in tropical forests in Ghana (30, 31). The relatively cool conditions of African tropical forests might also imply resistance as these forests are further from a potential high-temperature threshold that may limit photosynthesis. Overall, African tropical forests could plausibly be more or less vulnerable to temperature and drought anomalies than Amazonian tropical forests.Understanding how intact African forests respond to climate anomalies is vital, not least because they have been providing a substantial long-term carbon sink, reducing the rate and magnitude of climate change (5, 6). The impacts of environmental change on African tropical forests are also important because of unique aspects of their structure. African forests typically have high aboveground biomass and so, high carbon storage per unit area—on average, one-third more than Amazon forests (2, 32, 33). African forests are composed of a smaller number of stems, ∼425 ha−1 (≥100-mm diameter), compared with ∼600 ha−1 in Amazonia and Asia (32) and so, are more dominated by large trees. Hence, even small decreases in growth of the large dominant trees or modest increases in the mortality of these trees could lead to large carbon stock reductions and a loss of the live biomass carbon sink.The 2015–2016 El Niño event provided a first opportunity to assess the impact of high temperatures and strong water deficits on the ∼450 Mha (6) of African tropical forests. While three very strong El Niño events have occurred in the last 50 y (1982 to 1983, 1997 to 1998, and 2015 to 2016), only the latter occurred after a network of long-term inventory plots had been established in Africa and was poised to capture an El Niño event (5). At the onset of the 2015–2016 El Niño, we organized a specific “emergency” six-nation remeasurement program to capture the impact of the climate anomaly on African tropical forests. We therefore combine climate data with measurements from 100 African Tropical Rainforest Observatory Network (AfriTRON) long-term inventory plots that were remeasured to capture the 2015–2016 El Niño event to address the following questions. 1) Did African tropical forests experience unprecedented temperature anomalies in the 2015–2016 El Niño? 2) Did African tropical forests experience unprecedented drought in the 2015–2016 El Niño? 3) Which climate anomalies drove forest responses to the 2015–2016 El Niño? 4) What were the overall impacts on the monitored old-growth structurally intact tropical forests?  相似文献   
8.
The interannual variation in malaria cases in Colombia between 1960 and 1992 shows a close association with a periodic climatic phenomenon known as El Niño Southern Oscilation (ENSO). Compared with other years, malaria cases increased by 17.3% during a Niño year and by 35.1% in the post Niño year. The annual total number of malaria cases is also strongly correlated ( r = 0.62, P < 0.001) with sea surface temperature (SST) anomalies in the eastern equatorial Pacific, a principal parameter of ENSO. The strong relation between malaria and ENSO in colombia can be used to predict high and low-risk years for malaria with sufficient time to mobilize resources to reduce the impact of epidemics. In view of the current El Niño conditions, we anticipate an increase in malaria cases in Colombia in 1998. Further studies to elucidate the mechanisms which underlie the association are required. As Colombia has a wide range of climatic conditions, regional studies relating climate and vector ecology to malaria incidence may further improve an ENSO-based early warning system. Predicting malaria risk associated with ENSO and related climate variables may also serve as a short-term analogue for predicting longer-term effects posed by global climate change.  相似文献   
9.
The El Niño−Southern Oscillation (ENSO) phenomenon, the most pronounced feature of internally generated climate variability, occurs on interannual timescales and impacts the global climate system through an interaction with the annual cycle. The tight coupling between ENSO and the annual cycle is particularly pronounced over the tropical Western Pacific. Here we show that this nonlinear interaction results in a frequency cascade in the atmospheric circulation, which is characterized by deterministic high-frequency variability on near-annual and subannual timescales. Through climate model experiments and observational analysis, it is documented that a substantial fraction of the anomalous Northwest Pacific anticyclone variability, which is the main atmospheric link between ENSO and the East Asian Monsoon system, can be explained by these interactions and is thus deterministic and potentially predictable.The El Niño−Southern Oscillation (ENSO) phenomenon is a coupled air−sea mode, and its irregular occurring extreme phases El Niño and La Niña alternate on timescales of several years (18). The global atmospheric response to the corresponding eastern tropical Pacific sea surface temperature (SST) anomalies (SSTA) causes large disruptions in weather, ecosystems, and human society (3, 5, 9).One of the main properties of ENSO is its synchronization with the annual cycle: El Niño events tend to grow during boreal summer and fall and terminate quite rapidly in late boreal winter (918). The underlying dynamics of this seasonal pacemaking can be understood in terms of the El Niño/annual cycle combination mode (C-mode) concept (19), which interprets the Western Pacific wind response during the growth and termination phase of El Niño events as a seasonally modulated interannual phenomenon. This response includes a weakening of the equatorial wind anomalies, which causes the rapid termination of El Niño events after boreal winter and thus contributes to the seasonal synchronization of ENSO (17). Mathematically, the modulation corresponds to a product between the interannual ENSO phenomenon (ENSO frequency: fE) and the annual cycle (annual frequency: 1 y-1), which generates near-annual frequencies at periods of  ~  10 mo (1 + fE) and  ~  15 mo (1 − fE) (19).In nature, a wide variety of nonlinear processes exist in the climate system. Atmospheric examples include convection and low-level moisture advection (19). An example for a quadratic nonlinearity is the dissipation of momentum in the planetary boundary layer, which includes a product between ENSO (E) and the annual cycle (A) due to the windspeed nonlinearity: vE⋅ vA (17, 19). In the frequency domain, this product results in the near-annual sum (1 + fE) and difference (1 − fE) tones (19). The commonly used Niño 3.4 (N3.4) SSTA index (details in SI Appendix, SI Materials and Methods) exhibits most power at interannual frequencies (Fig. 1A). In contrast, the near-annual combination tones (1 ± fE) are the defining characteristic of the C-mode (Fig. 1B).Open in a separate windowFig. 1.Schematic for the ENSO (E) and combination mode (ExA) anomalous surface circulation pattern and corresponding spectral characteristics. (A) Power spectral density for the normalized N3.4 index of the Hadley Centre Sea Ice and Sea Surface Temperature data set version 1 (HadISST1) 1958–2013 SSTA using the Welch method. (B) As in A but for the theoretical quadratic combination mode (ExA). (C) Regression coefficient of the normalized N3.4 index and the anomalous JRA-55 surface stream function for the same period (ENSO response pattern). (D) Regression coefficient of the normalized combination mode (ExA) index and the anomalous JRA-55 surface stream function (combination mode response pattern). Areas where the anomalous circulation regression coefficient is significant above the 95% confidence level are nonstippled.Physically, the dominant near-annual combination mode comprises a meridionally antisymmetric circulation pattern (Fig. 1D). It features a strong cyclonic circulation in the South Pacific Convergence Zone, with a much weaker counterpart cyclone in the Northern Hemisphere Central Pacific. The most pronounced feature of the C-mode circulation pattern is the anomalous low-level Northwest Pacific anticyclone (NWP-AC). This important large-scale atmospheric feature links ENSO impacts to the Asian Monsoon systems (2025) by shifting rainfall patterns (SI Appendix, Fig. S1B), and it drives sea level changes in the tropical Western Pacific that impact coastal systems (26). It has been demonstrated using spectral analysis methods and numerical model experiments that the C-mode is predominantly caused by nonlinear atmospheric interactions between ENSO and the warm pool annual cycle (19, 20). Local and remote thermodynamic air−sea coupling amplify the signal but are not the main drivers for the phase transition of the C-mode and its associated local phenomena (e.g., the NWP-AC) (20).Even though ENSO and the C-mode are not independent, their patterns and spectral characteristics are fundamentally different, which has important implications when assessing the amplitude and timing of their regional climate impacts (Fig. 1). Here we set out to study the role of nonlinear interactions between ENSO and the annual cycle (10) in the context of C-mode dynamics. Such nonlinearities can, in principle, generate a suite of higher-order combination modes, which would contribute to the high-frequency variability of the atmosphere—in a deterministic and predictable way.  相似文献   
10.
Assessing temporal variability in extreme rainfall events before the historical era is complicated by the sparsity of long-term “direct” storm proxies. Here we present a 2,200-y-long, accurate, and precisely dated record of cave flooding events from the northwest Australian tropics that we interpret, based on an integrated analysis of meteorological data and sediment layers within stalagmites, as representing a proxy for extreme rainfall events derived primarily from tropical cyclones (TCs) and secondarily from the regional summer monsoon. This time series reveals substantial multicentennial variability in extreme rainfall, with elevated occurrence rates characterizing the twentieth century, 850–1450 CE (Common Era), and 50–400 CE; reduced activity marks 1450–1650 CE and 500–850 CE. These trends are similar to reconstructed numbers of TCs in the North Atlantic and Caribbean basins, and they form temporal and spatial patterns best explained by secular changes in the dominant mode of the El Niño/Southern Oscillation (ENSO), the primary driver of modern TC variability. We thus attribute long-term shifts in cyclogenesis in both the central Australian and North Atlantic sectors over the past two millennia to entrenched El Niño or La Niña states of the tropical Pacific. The influence of ENSO on monsoon precipitation in this region of northwest Australia is muted, but ENSO-driven changes to the monsoon may have complemented changes to TC activity.Two primary components of tropical precipitation—monsoons and tropical cyclones (TCs)—are capable of producing high volumes of rainfall in short periods of time (extreme rainfall events) that lead to flooding. Because both systems respond to changes in atmospheric and sea surface conditions (1, 2), it is imperative that we understand their sensitivities to climate change. For example, over recent decades, warming of the oceans has driven increases in the mean latitude (3) and energy released by TCs (4). These storms (e.g., hurricanes, typhoons, tropical storms, and tropical depressions) can produce enormous economic and societal disruptions but also represent important components of low-latitude hydroclimate (5) and ocean heat budgets (6). Monsoon reconstructions spanning the last several millennia have been developed using a variety of proxies (710), including stalagmites (1113), but reconstructing past TC activity is generally more difficult. In most of the world’s ocean basins, accurate counts of TCs are limited to the start of the satellite era (since 1970 CE), an interval too short to capture changes occurring over multidecadal to centennial time scales. Therefore, as a complement to the historical record, sedimentological analyses of storm-sensitive sites have formed the basis of TC reconstructions, primarily in and around the North Atlantic and Caribbean basins (1419), that largely focus on near-coastal sequences, including beach ridges, overwash deposits, and shallow marine sediments. Together, these studies have revealed that North Atlantic and Caribbean TC activity varied substantially over the past several centuries to millennia, with multicentennial shifts attributed to a range of factors including atmospheric dynamics in the North Atlantic, North African rainfall, and El Niño/Southern Oscillation (ENSO).Today, ENSO represents a dominant control of interannual TC activity at a global scale through its influences on surface ocean temperature gradients and atmospheric circulation (2023). However, no record has clearly demonstrated the link between ENSO and prehistoric TCs in the tropical Pacific, Indian, or Australian regions, leaving unanswered questions about the sensitivity of cyclogenesis to ENSO before the modern era. This issue is of particular concern given modeling results that predict changes in ENSO behavior may accompany anthropogenic warming of the atmosphere (24, 25). Fully assessing the sensitivity of TCs to changes in climate requires high-resolution and precisely dated paleostorm reconstructions from multiple basins spanning periods beyond those available in observational data, a goal that has largely proven elusive. Few such records unambiguously derived from TCs have been identified, particularly in the western Pacific and Indo-Pacific (20, 2630).  相似文献   
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