Western Pacific Warm Pool Research Articles

Weak linkage of winter surface air temperature over Northeast Asia with East Asian winter monsoon during 1993–2003

A colder winter over Northeast Asia (NEA) has traditionally been attributed to the stronger East Asian winter monsoon (EAWM) on interannual timescale. However, the present work proposes this robust linkage becomes weak during 1993–2003, along with the changes of the underlying condition in the last three decades. During 1981–1992 and 2004–2016, the loss of autumn Arctic sea ice condensation (SIC) leads to the stratospheric warming and the polar vortex weakening, which results in the enhancement of the Siberian High (SH) and the colder surface air temperature (SAT) over NEA. In particular, the persisting anomalies of snow cover near the Mongolian Plateau from autumn to winter could amplify the influences of Arctic SIC on the EAWM during 1981–1992. However, during 1993–2003 both the EAWM and the SAT over NEA are modulated by the tropical SSTAs in the western Pacific warm pool, corresponding to a weak association between the EAWM and the SAT over NEA. These tropical warm SSTAs could excite a meridional teleconnection over East Asia to generate a barotropic anticyclonic anomaly in situ. It not only weakens the SH and EAWM but also decreases the SAT over NEA. Therefore, the predictor of winter SAT over NEA is altered from the cryosphere to the tropical SSTAs in this episode. The changes of interdecadal background may contribute to the distinct interannual relationship between EAWM and winter SAT over NEA. This will increase the uncertainty in seasonal forecast on the winter climate over East Asia.

Simulation Study of Cloud Properties Affected by Heterogeneous Nucleation Using the GRAPES_SCM during the TWP-ICE Campaign

This study used the Global/Regional Assimilation and PrEdiction System Single-Column Model (GRAPES_SCM) to simulate monsoon precipitation with deep convective cloud and associated cirrus during the Tropical Warm Pool International Cloud Experiment (TWP-ICE), especially during the active and suppressed monsoon periods. Four cases with different heterogeneous nucleation parameterizations were simulated by using the ensemble method. All simulations clearly separated the active and suppressed monsoon periods, and they reproduced the major characteristics of monsoonal cloud such as the total cloud hydrometeor mixing ratio distribution, and precipitation and radiation properties. The results showed that the number concentration production rate of different heterogeneous nucleation parameterizations varied substantially under the given temperature and water vapor mixing ratio. However, ice formation and precipitation during the monsoon period were affected only slightly by the different heterogeneous nucleation parameterizations. This study also captured clear competition between different ice formation processes.

Investigating ENSO and WPWP modulated typhoon variability in the South China Sea during the mid–late Holocene using sedimentological evidence from southeastern Hainan Island, China

The link between tropical cyclone (TC) activity in the South China Sea (SCS) and global climate change is commonly debated and there is a clear need for long-term geological records of TC activity if we are to clarify this connection. Multi-millennial, high-resolution paleo-storm records from the SCS are rare in the region and this causes difficulties for those exploring potential climate drivers of TC variability over centennial to millennial timescales and reduces our ability to fully assess the risks associated with future TC activity. This paper presents an age-constrained, mud-dominated sedimentary sequence from a coastal lagoon on Hainan Island, China. Multiproxy analyses incorporating chronological, lithological, sedimentological, and geochemical evidence were used to infer storm deposits preserved within the sequence and to reconstruct a time series of storm activity in the SCS during the mid-to-late Holocene. This long sedimentary record shows that TCs were highly active over the periods 5500 to 3500 and 1700 to 0 cal yrs. BP, and these periods contrast with a relatively quiet period 3500–1700 cal yrs. BP. An apparent inverse correlation between TC reconstructions from the SCS and those from the Korean Peninsula and Japan implies an oscillating pattern across the SCS and western North Pacific (WNP) over centennial to millennial timescales. A comparison between the sedimentary and paleoclimatic records implies that the El Niño–Southern Oscillation was not the only mechanism responsible for typhoon variability over the past 7500 cal yrs. BP, suggesting that other factors such as the thermal state of the western Pacific warm pool likely also had a strong influence on both SCS and WNP TC variability.

Temperature Evolution of the Indo‐Pacific Warm Pool Over the Holocene and the Last Deglaciation

AbstractThe Indo‐Pacific Warm Pool (IPWP) contains the warmest surface ocean waters on our planet making it a major source of heat and moisture to the atmosphere. Changes in the extent and position of the IPWP likely impacted the tropical and global climate in the past and may also do in the future. With the aim to put recent ocean changes into a longer temporal context, we present new paleoceanographic sea surface temperature reconstructions from the heart of the Western Pacific Warm Pool, which is the warmest region within the IPWP, across the last 17,000 years. To provide an improved spatial and temporal regional context we use new and published sea surface temperature records from the IPWP and update previous compilation efforts (Linsley et al., 2010, https://doi.org/10.1038/ngeo1920). We similarly conclude that the IPWP was warmer in the early Holocene compared to the late Holocene. However, with the new data we are able to argue against a western displacement/expansion of the IPWP and suggest a warmer southern IPWP in the early Holocene. We explore the potential drivers of the Holocene IPWP cooling and propose a combination of processes including changes in the monsoonal winds associated with the position of the rain belt, the tropical Pacific mean climate, and upper water column mixing. The proposed climatic processes differentially impacted the IPWP subregions resulting in spatially diverse trends. Additionally, the late deglacial section of the records mostly show a gradual IPWP warming similar in structure to the atmospheric CO2 and/or Antarctica rising temperatures.

Different vertical distribution of zooplankton community between North Pacific Subtropical Gyre and Western Pacific Warm Pool: its implication to carbon flux

The mesozooplankton in both epipelagic and mesopelagic zones is essentially important for the study of ecosystem and biological carbon pump. Previous studies showed that the diel vertical migration (DVM) pattern of mesozooplankton varied among ecosystems. However, that pattern was largely unknown in the Western Pacific Warm Pool (WPWP). The vertical distribution, DVM and community structure of mesozooplankton from the surface to 1 000 m were compared at Stas JL7K (WPWP) and MA (North Pacific Subtropical Gyre, NPSG). Two sites showed similarly low biomass in both epipelagic and mesopelagic zones, which were in accordance with oligotrophic conditions of these two ecosystems. Stronger DVM (night/day ratio) was found at JL7K (1.31) than that at MA (1.09) on surface 0–100 m, and an obvious night increase of mesopelagic biomass was observed at JL7K, which was probably due to migrators from bathypelagic zone. Active carbon flux by DVM of zooplankton was estimated to be 0.23 mmol/(m2·d) at JL7K and 0.16 mmol/(m2·d) at MA. The community structure analysis showed that calanoid copepods, cnidarians and appendicularians were the main contributors to DVM of mesozooplankton at both sites. We also compared the present result with previous studies of the two ecosystems, and suggested that the DVM of mesozooplankton was more homogeneous within the WPWP and more variable within the NPSG, though both ecosystems showed typically extremely oligotrophic conditions. The different diel vertical migration strength of mesozooplankton between NPSG and WPWP implied different efficiency of carbon pump in these two ecosystems.

Millennial- and centennial-scale variations in the upper-water temperature of Western Pacific Warm Pool during the last deglaciation

During the last deglaciation period, climate changes in the northern and southern high-latitude regions show a pattern of so-called “bipolar seesaw” on millennial scale. Previous research interpreted that the weakening of the Atlantic Meridional Overturning Circulation (AMOC) and the associated north-south migration of the Intertropical Convergence Zone (ITCZ) influenced the interhemispheric heat redistribution and thus climate state. However, such a hypothesis lacks the support from the western tropical Pacific, in which the deglacial millennial-scale changes is unclear as the published reconstructions usually conflict with one another. As the major oceanic heat source, the Western Pacific Warm Pool (WPWP) affects the global climate system, with its sea surface temperature (SST) change forcing the tropical atmospheric circulation and its thermocline dynamic modulating the upper-ocean heat content and thermal structure. This study explores the upper-water temperature changes during the transition from the Last Glacial Maximum (LGM) to the Holocene, i.e., the last deglaciation (~23−8 ka). Foraminifera are inferred to calcify in equilibrium with the seawater they live in, make it possible to reconstruct the environmental properties of corresponding water masses where certain foraminifera dwell. We analyzed the δ 18O and Mg/Ca of the mixed-layer dwelling planktonic foraminifera Globigerinoides ruber and the upper-thermocline dweller Pulleniatina obliquiloculata from sediment core MD01-2386 locating at the core of WPWP, and reconstructed decadal-resolved records of SST and thermocline water temperature (TWT) for the last deglaciation. The reconstructed results show that: (1) SST and TWT started initial warming onsets around 20−21 ka, following the initial decrease of precession parameter; (2) SST and TWT stopped warming and plateaued during the interval of Heinrich stadial 1 (HS-1) and Younger Dryas (YD; (3) TWT varied with larger amplitudes than SST. Further, we compared the SST and TWT of core MD01-2386 with the proxy-records of temperature and precipitation of the WPWP and the model-output of El Nino variability. The records of SST, TWT and the planktonic foraminifera δ 18O from the western equatorial Pacific are coherent with the Borneo stalagmite δ 18O, thus suggesting the coupled feedbacks among the thermocline dynamic, sea-surface heating and atmospheric convective activity over the heart of WPWP during the last deglaciation. While, contrasts exist between the records of near-equator region and off-equator area and thus suggests opposite changes in the atmospheric convective activity near the equator and the hydrological cycle change outside the equator. This finding may indicate a relative migration or intensity change of the atmospheric convection center, rather than a north-south shift of the rain belt. In addition, the planktonic foraminiferal proxy records demonstrate significant centennial-scale variabilities, mainly on the periodicity of 220−260 a. This cycle is coherent with the 10Be record of ice core and roughly coincident with the Suess cycle of solar activity. To sum up, the results of this study allow us to propose three main conclusions. (1) The upper water temperature records of core MD01-2386 show obvious millennial-scale changes, which are closely coupled with the upper water temperature and atmospheric convection activity in the western equatorial Pacific, indicating consistent changes of the ocean-atmosphere coupled system in the western equatorial Pacific on the millennial-scale of the last deglaciation. (2) On the millennium scale in the last deglaciation, opposite changes in the air-sea feedbacks exist between the near-equator and off-equator areas of the warm pool, which may indicate relative changes of the atmospheric convevtion center but not a north-south shift of rain belt. (3) The planktonic foraminifera proxy records present significant cycling of 200−400 a centered at ~260 a and may reflect the linkage between centennial-scale solar activity and the upper water dynamics of the western equatorial Pacific.

Holocene changes in biome size and tropical cyclone activity around the Northern South China Sea

The South China Sea (SCS), characterized by a large continental shelf, is located at the edge of the Asian monsoon domain. In this study, two marine sediment cores from the northern SCS (NSCS) continental slope were investigated to construct composite vegetation and precipitation isotopic composition records based on the δ13C and δD values of plant-wax n-alkanes throughout the Holocene (last 11,200 years; i.e. 11.2 ka). The composite δ13Cwax record indicates an overall predominance of C3 vegetation over the last 11.2 ka. Before 8 ka BP, higher δ13Cwax values are attributed to preferential wax input from grassland and wetland biomes on the exposed continental shelf. After the inundation of the shelf by eustatic sea level rise until ca. 8 ka BP grassland and wetland biomes suffered a major size reduction and arboreal vegetation became better represented in the δ13Cwax record. The composite temperature corrected δDwax-T record suggests that moisture source variability drove precipitation isotopic composition changes during the Holocene. Lower δDwax-T values before 8.3 ka BP are interpreted as a larger moisture contribution by Pacific Ocean tropical cyclones, whereas higher δDwax-T values after 8.5 ka BP are interpreted as a larger moisture contribution from the Indian Ocean summer monsoon. Higher incidence of tropical cyclones in the NSCS during the Early Holocene was related to a temporary westward shift of the Western Pacific Warm Pool and enhanced insolation over the Northern Hemisphere. Both external and internal forcing mechanisms regulated moisture source changes in East Asia during the Holocene.

Contributions of SST Anomalies in the Indo-Pacific Ocean to the Interannual Variability of Tropical Cyclone Genesis Frequency over the Western North Pacific

AbstractThis study attempts to evaluate quantitatively the contributions of sea surface temperature (SST) anomalies in the Indo-Pacific Ocean to the interannual variability of tropical cyclone (TC) genesis frequency (TCGF) over the western North Pacific (WNP). Three SST factors in the Indo-Pacific Ocean are found to play key roles in modulating the interannual variability of WNP TCGF. They are summer SST anomaly in the east Indian Ocean (EIO), the summer El Niño–Southern Oscillation Modoki index (EMI), and the spring SST gradient (SSTG) between the southwestern Pacific and the western Pacific warm pool. Results show that the three factors together can explain 72% of the total variance of WNP TCGF in the typhoon season for the period 1980–2015. Among them, the spring SSTG and the summer EIO contribute predominantly to the interannual variability of TCGF, followed by the summer EMI, with respective contributions being 39%, 38%, and 23%. Further analysis shows that the summer EMI was affected significantly by the spring SSTG and thus had a relatively lower contribution to the TCGF than the spring SSTG. In addition, a statistical model is constructed to predict the WNP TCGF in the typhoon season by a combination of the May EIO and the spring SSTG. The new model can reproduce well the observed WNP TCGF and shows an overall better skill than the ECMWF Seasonal Forecasting System 5 (SEAS5) hindcasts. This statistical model provides a good tool for seasonal prediction of WNP TCGF.

Savanna in equatorial Borneo during the late Pleistocene

Equatorial Southeast Asia is a key region for global climate change. Here, the Indo-Pacific Warm Pool (IPWP) is a critical driver of atmospheric convection that plays a dominant role in global atmospheric circulation. However, fluctuating sea-levels during the Pleistocene produced the most drastic land-sea area changes on Earth, with the now-drowned continent of Sundaland being exposed as a contiguous landmass for most of the past 2 million years. How vegetation responded to changes in rainfall that resulted from changing shelf exposure and glacial boundary conditions in Sundaland remains poorly understood. Here we use the stable carbon isotope composition (δ13C) of bat guano and High Molecular Weight n-alkanes, from Saleh Cave in southern Borneo to demonstrate that open vegetation existed during much the past 40,000 yrs BP. This location is at the southern equatorial end of a hypothesized ‘savanna corridor’ and the results provide the strongest evidence yet for its existence. The corridor would have operated as a barrier to east-west dispersal of rainforest species, and a conduit for north-south dispersal of savanna species at times of lowered sea level, explaining many modern biogeographic patterns. The Saleh Cave record also exhibits a strong correspondence with insolation and sea surface temperatures of the IPWP, suggesting a strong sensitivity of vegetation to tropical climate change on glacial/interglacial timeframes.

Effect of El Ni\xf1o on the response ratio of Hadley circulation to different SST meridional structures

The role of El Niño in influencing the response of the Hadley circulation (HC) to different tropical sea surface temperature (SST) meridional structures is investigated over the period 1979–2016. The explained variance of the equatorially asymmetric principal mode of HC variability is enhanced during El Niño events that are characterized by equatorially symmetric El Niño SST anomalies. It is shown that the response of the HC to different SST meridional structures is greatly enhanced in El Niño events; this is because the SST over the Indo-Pacific warm pool (IPWP) shows inhomogeneous warming in El Niño events, with greater values in the southern IPWP. The asymmetric warming over the IPWP is opposite to the climatological meridional structure of the SST over the IPWP, which intensifies the equatorially symmetric variation of SST, and contributes to the enhanced response of the HC to SST. This point is further verified by the suppression of this enhanced response in El Niño events of the HC to SST when the effects of SST over the IPWP are removed, implying that the different warming amplitude within the IPWP during the El Niño events contributes to the enhanced response. The response in La Niña events is also explored, indicating a difference between El Niño and La Niña events. These results may help explain the corresponding variations of the HC during the El Niño events, and highlight the different influences of the El Niño and La Niña events on the response of the HC to SST.

Shelf exposure influence on Indo-Pacific Warm Pool climate for the last 450,000 years

The Indo-Pacific Warm Pool is the largest source of heat and rainfall on Earth, but the mechanisms driving long-term climate variability in this region remain uncertain. Some studies suggest that precessional variations in insolation exert a primary role; alternatively, sea level change may be more important, with exposure of the Sunda and Sahul shelves leading to weakened Walker circulation across the Indian Ocean during glacial periods. The limited number of paleoclimate records from the warm pool that extend beyond the Last Glacial Maximum makes it difficult to test these competing hypotheses. Here, we present a 450,000-year long reconstruction of sea surface temperatures, sub-surface temperatures, and regional vegetation from southern Sumatra. We show that the dominant signal in each record is the 100 ky glacial cycle. We find that both surface and sub-surface temperatures cool during glacial periods, but the sub-surface cools more, reflecting a shoaling of the thermocline. Southern Sumatra and western Java consistently exhibit increased C4 grass coverage and an enhanced dry season during glacial periods. The observed changes in the thermocline and rainfall seasonality are consistent with the shelf exposure mechanism, suggesting that sea level changes exert a prominent role in warm pool climate over glacial-interglacial timescales.

Sea-level, monsoonal, and anthropogenic impacts on the millennial-scale variability of siliciclastic sediment input into the western Philippine sea since 27 ka

Multiproxy records from Core MD06-3054 in the western Philippine Sea and published results from the nearby Core MD06-3052, which are both located at the northern margin of the Western Pacific Warm Pool (WPWP), provide precise reconstructions and evidence for the potential controls on continental erosion and weathering intensities in Luzon. These data also constrain sediment source-to-sink processes in the sea since 27 ka at the millennial scale. We demonstrate that the siliciclastic fraction is derived from Luzon volcanic rocks and eolian dust from mainland Asia. Stronger physical erosion and chemical weathering generally occurred during the last glaciation, Heinrich Event 1, the Younger Dryas, and the period from 10.3 to 11.3 ka. In contrast, the Bølling-Allerød was characterized by weaker physical erosion and chemical weathering. During the last glaciation, Heinrich Event 1, and the Younger Dryas, the lower sea level led to enhanced erosion and weathering of the exposed shelf sediments. In contrast, the weaker physical erosion and chemical weathering that occurred during the Bølling-Allerød and the Holocene, except after 2.1 ka, are closely correlated with the higher sea level. The stronger physical erosion and chemical weathering at 10.3–11.3 ka may be derived from the reworking of older sediments during the time of a strengthening summer monsoon. Furthermore, enhanced human activity since 2.1 ka has become the dominant control on erosion in Luzon. Rapid weathering changes in the WPWP during the last deglaciation highlight the high variability in local controls at low latitudes during these periods and the potential influence of controls at high latitudes.

Cool skin signals observed from Advanced Along-Track Scanning Radiometer (AATSR) and in situ SST measurements

Nighttime cool skin sea surface temperature (SST) signals, defined in this study as the differences between the SSTskin from the Advanced Along-Track Scanning Radiometer (AATSR) onboard Envisat satellite and in situ SSTs from drifting buoys and moorings, ΔT = SSTskin − SSTinsitu, are investigated on a global scale from July 2002 to April 2012. Global mean ΔT, averaged over the full study period, is −0.13 K, with most values falling between −0.1 and −0.2 K. The dominant role of wind speed on the ΔT is shown, with weaker winds usually corresponding to a cooler skin. The effect of air-sea temperature difference is also significant: warm skin (ΔT > 0 K) can be observed under large positive air-sea temperature differences. Other geophysical variables, such as the total column water vapor, in situ SST, and net heat flux, also affect ΔT, but to a lesser degree. Significant increase of ΔT size with SSTinsitu is observed when SSTinsitu is >28 °C. Tropical waters, such as the tropical Indian Ocean and the tropical warm pool (western Pacific and eastern Indian Ocean), are more frequently covered with a cool skin, largely due to the calm winds, very warm waters (especially for SSTinsitu >28 °C), and other environmental conditions supporting the development of large cool skin events. The ΔT seasonal pattern in the southern hemisphere is more regular, compared to the northern hemisphere. In both hemispheres, larger cool skin signals are seen during the local summer, mainly due to weaker winds. According to several previous cool skin models, higher winds tend to result in thinner cool skin layer depths, and hence in smaller ΔT amplitudes, regardless of stronger evaporation and heat loss. Given that wind is closely coupled with waves and turbulent mixing with wave breaking, the dependencies of ΔT on a few wave parameters are also investigated. A strong (moderate) dependency of ΔT on wave height (wave steepness) is identified, while the dependency of ΔT on wave breaking probability is less discernible.

Regional seasonality in the intraseasonal oscillation over the Indo-Pacific Oceans

The interlinked Eastern Indian Ocean (EIO) and Western Pacific Ocean (WPO), known as the Indo-Pacific warm pool, are highly active regions for intraseasonal oscillations (ISOs). Interestingly, distinct behaviors exist in ISO seasonality in these two basins. In the WPO, ISO intensity peaks in winter, decays rapidly starting from spring and reaches a minimum in summer. While in the EIO, ISO intensity exhibits a bimodal distribution, with a stronger peak in spring and a weaker one in autumn, followed by two troughs in summer and winter, respectively. Here, the regional ISO seasonality is understood in view of the regional differences in the background fields. For the bimodal ISO seasonality in the EIO, the increase from winter to spring is primarily due to elevated moisture content, the decrease from spring to summer is due to the decline in moisture and the meridional variation in vertical wind shear, and the increase from summer to autumn is mainly attributed to the meridional variation in vertical wind shear. In the WPO, the significant winter-summer contrast is mainly caused by change in moisture content.

A preliminary study on an upper ocean heat and salt content of the western Pacific warm pool region

On the basis of Argo profile data of the temperature and salinity from January 2001 to July 2014, the spatial distributions of an upper ocean heat content (OHC) and ocean salt content (OSC) of the western Pacific warm pool (WPWP) region and their seasonal and interannual variations are studied by a cyclostationary empirical orthogonal function (CSEOF) decomposition, a maximum entropy spectral analysis, and a correlation analysis. Probable reasons for variations are discussed. The results show the following. (1) The OHC variations in the subsurface layer of the WPWP are much greater than those in the surface layer. On the contrary, the OSC variations are mainly in the surface layer, while the subsurface layer varies little. (2) Compared with the OSC, the OHC of the WPWP region is more affected by El Nino-Southern Oscillation (ENSO) events. The CSEOF analysis shows that the OHC pattern in mode 1 has strong interannual oscillation, with eastern and western parts opposite in phase. The distribution of the OSC has a positive-negative-positive tripole pattern. Time series analysis shows that the OHC has three phase adjustments with the occurrence of ENSO events after 2007, while the OSC only had one such adjustment during the same period. Further analysis indicates that the OHC variations are mainly caused by ENSO events, local winds, and zonal currents, whereas the OSC variations are caused by much more complex reasons. Two of these, the zonal current and a freshwater flux, have a positive feedback on the OSC change in the WPWP region.

A new perspective of the 2014/15 failed El Ni\xf1o as seen from ocean salinity

This study investigates the 2014/15 failed El Niño using salinity from an ocean general circulation model. The results indicate that subsurface processes were especially strong in the summer of 2014 and they led to positive sea surface salinity anomalies in the central equatorial Pacific. The positive sea surface salinity anomalies induced a westward displacement of the sea surface salinity front that represents the eastern boundary of the western Pacific warm pool, preventing the warm surface water from shifting eastward as seen in a typical El Niño event. In the meantime, more salty water was transported equatorward by a strengthening subtropical cell in the South Pacific. The enhanced subsurface processes in the central equatorial Pacific conveyed the salinity anomalies of subtropical origin to the sea surface and were largely responsible for the sea surface salinity variability but had less impacts on sea surface temperature during the 2014/15 failed El Niño, suggesting some potential advantage of ocean salinity in the El Niño-Southern Oscillation prediction.

3500-year western Pacific storm record warns of additional storm activity in a warming warm pool

Frequent storm surges often cause catastrophic impacts on human lives and the global economy; however, these phenomena are not well understood. In this study, a regional storm reconstruction is performed based on a grain-size analysis and stratigraphic modelling of the accelerator mass spectrometry radiocarbon dates of benthic foraminifera from two neighbouring lagoon cores from Lingyang Reef in the Xisha Islands located in the northern South China Sea of the western Pacific. The dating results from the lagoon cores reveal a ~3500-year depositional history. Three different depositional units are recognized based on a time series of distinct grain-size variations that correspond to the following three stages of storm activity: intense and frequent storms from ~3500 to 3100 cal yr BP and ~1800 cal yr BP to present and weak and infrequent storms from ~3100 to 1800 cal yr BP. A high sedimentation rate remarked by reverse and chaos age is observed from ~2800 to 2600 cal yr BP in both cores, and it was likely associated with a tsunami event. In addition, grain-size variability may be associated with changes over time caused by the synchronous Asian monsoon and may also be correlated with climate records retrieved from the ice cores from Greenland; thus, this variability could indicate pervasive global climatic teleconnections. The overall temporal patterns of the isolated coral branches and shells from the sediment sequences are well correlated with the high sea surface temperatures in the western tropical Pacific. We suggest that increasing sea surface temperatures in the future may lead to more intense storm activity in the western Pacific warm pool as the planet warms.

Stable isotope record of middle Eocene summer monsoon and its instability in eastern China

Abstract Recent studies based on proxy records and paleoclimate simulations have suggested monsoon climate in Asia as early as the Eocene. However, good proxy record of Eocene monsoon is rare in eastern Asia where East Asia summer monsoon prevails. Here we study middle Eocene lacustrine deposits of the Shahejie Formation in the Bohai Bay Basin in eastern China to understand middle Eocene paleoclimate and its stability. Our study integrates new δ13C and δ18O data of authigenic lacustrine carbonates, lake water salinity proxy of Mg/Ca ratio, and published carbonate minerology, chemical weathering proxy data of CIA* and Ln(Al2O3/Na2O), and lithofacies interpretations from two cores to provide new understanding of paleoclimate elements. Our results show that in most part of the record, the authigenic calcite δ18O values in laminated mudstone-dominated lithofacies display frequent variations, up to 2.9‰, while Mg/Ca ratio and dolomite content remain low and nearly stable. These features suggest frequent variations of lake water level driven by changes in runoff amount because of changing intensity of summer monsoonal precipitation during the middle Eocene. The comparison of stable isotope variation with modern precipitation stable isotope record in the study area and seasonal precipitation of the laminated lithofacies both support the interpretation of seasonal climate. We interpret that the lake water and carbonate δ18O values decreased when the East Asia summer monsoon was strong and increased when the monsoon was weak. During two periods of the studied middle Eocene interval, the lake water experienced periodic intense evaporation, causing high Mg/Ca ratios and dolomite content, and high authigenic dolomite δ18O values up to 0‰. These periodic drying events likely reflect millennial-scale droughts that are common today to the East Asia summer monsoon because of warming in the Indo-Pacific Warm Pool. We infer that the frequent droughts in the two periods of the middle Eocene were resulted from very warm climate during the two periods. We conclude that the East Asia summer monsoon existed in eastern China during the middle Eocene, and the climate state was not stable.

Spatiotemporal Variability of Remote Sensing Ocean Net Primary Production and Major Forcing Factors in the Tropical Eastern Indian and Western Pacific Ocean

Based on widely used remote sensing ocean net primary production (NPP) datasets, the spatiotemporal variability of NPP is first analyzed over the tropical eastern Indian and western Pacific Ocean for the period 1998–2016 using the conventional empirical orthogonal function (EOF), the lead–lag correlation and the ensemble empirical mode decomposition (EEMD) technique. Barnett and Preisendorfer’s improved Canonical Correlation Analysis (BPCCA) is also applied to derive covariability patterns of NPP with major forcing factors of the chlorophyll a concentration (Chla), sea surface temperature (SST), sea level anomaly (SLA), ocean rainfall (Rain), sea surface wind (Wind), and current (CUR) under climate changes of El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). We find that: (1) The first two seasonal EOF modes capture significant temporal and meridional NPP variability differences, as NPP reaches peaks approximately three months later in the western Pacific Ocean than that of in the eastern Indian Ocean. (2) The second and third interannual EOF modes are closely related with ENSO with a two-month lag and synchronous with IOD, respectively, characterized by southwesterly positive anomaly centers during positive IOD years. (3) NPP presents different varying tendencies and similar multiscale oscillation patterns with interannual and interdecadal cycles of 2~3 years, 5~8 years, and 9~19 years in subregions of the Bay of Bengal, the South China Sea, the southeastern Indian Ocean, and the northwestern Pacific Ocean. (4) The NPP variability is strongly coupled with negative SST, SLA, and Rain anomalies, as well as positive Chla, Wind and CUR anomalies in general during El Niño/positive IOD years. The results reveal the diversity and complexity of large-scale biophysical interactions in the key Indo-Pacific Warm Pool region, which improves our understanding of ocean productivity, ecosystems, and carbon budgets.

Is Chinese stalagmite δ18O solely controlled by the Indian summer monsoon?

As a unique continental archive, speleothem has been widely used in reconstructing paleoclimate change. However, the interpretation of Chinese speleothems δ18O has remained a subject of debate. Recently, a Community Atmosphere Model version 3 (CAM3) study indicated that the stalagmite δ18O from eastern China reflected the Indian summer monsoon (ISM) intensity rather than the East Asian summer monsoon (EASM) intensity during Heinrich events. Here, we present a high-resolution speleothem δ18O record from Xianglong Cave in Shaanxi province, China, covering the period of 25.5–10.9 ka BP. The XL15 record shows similar variations with ice core record from Greenland and other climate records from China and India on millennial scale, including Heinrich 2 (H2), Heinrich 1 (H1), Bolling–Allerod (BA) and Younger Dryas (YD) events, supporting the connection between the Asian monsoon and northern high latitude climate. The δ18O amplitude of our record is larger than or similar to the stalagmite δ18O records from India during these events. In addition, differences of stalagmite δ18O in eastern China and the ISM region were observed on glacial-interglacial as well as decadal timescales. That means the ISM is not the sole controlling factor of Chinese stalagmite δ18O during Heinrich events. When subtracting the Indian stalagmite δ18O series from our XL15 record during H1 period, we found a significant negative correlation with sea surface temperature (SST) record of Western Pacific Warm Pool (WPWP). Consequently, our study suggests that the Chinese stalagmite δ18O is controlled by both the ISM and EASM on orbital-, millennial-, and decadal timescales.