Western Pacific Warm Pool Research Articles

High‐resolution upper Pliocene to Pleistocene calcareous nannofossil biostratigraphy in Ocean Drilling Program Hole 1146A in the South China Sea

AbstractWe established a high‐resolution calcareous nannofossil biostratigraphy for the late Pliocene–Pleistocene by analyzing a 242 m‐thick, continuous sedimentary succession from Ocean Drilling Program Site 1146, Hole A, in the South China Sea (SCS). A total of 14 calcareous nannofossil datums were detected in the SCS succession. They are, in descending order: first occurrence (FO) of Emiliania huxleyi, last occurrence (LO) of Pseudoemiliania lacunosa, LO of Reticulofenestra asanoi, FO of Gephyrocapsa parallela, FO of R. asanoi, LO of large Gephyrocapsa spp., FO of large G. spp., FO of Gephyrocapsa oceanica, FO of Gephyrocapsa caribbeanica, LO of Calcidiscus macintyrei, LO of Discoaster brouweri, LO of Discoaster pentaradiatus, LO of Discoaster surculus, and LO of Discoaster tamalis. The FO of E. huxleyi was not precisely detected due to poor preservation and dissolution of nannofossils in the underlying strata. We refined the previous calcareous nannofossil biostratigraphy in the SCS by identifying Gephyrocapsa species and four evolutionary extinction events of the genus Discoaster. The proposed calcareous nannofossil biostratigraphy correlates with those reported in other terrestrial and marine areas/sites and global benthic foraminiferal δ18O records. The age–depth curves based on nannofossil biostratigraphy indicate a significant increase in the sedimentation rates at the LO of R. asanoi (0.91–0.85 Ma). The timing of this increase corresponds to reef expansion in the Ryukyu Islands linked to a stepwise increase in Kuroshio Current intensity. This timing is broadly coeval with a sea surface temperature increase of ∼2 °C in the northwestern Pacific due to expansion of the Western Pacific Warm Pool towards the north and south subtropical regions. This can be explained by increased weathering and erosion of terrestrial areas in glacial periods and increased rainfall causing higher sediment transport in interglacial periods, which were both linked to Middle Pleistocene Transition‐related climatic changes.

Why Indian summer monsoon circulation indices? Fidelity in representing rainfall variability and teleconnections

AbstractWith the recognition that the circulation is strongly coupled with the Indian summer monsoon rainfall (ISMR), a number of large‐scale circulation indices have been proposed to represent the Indian summer monsoon to aid diagnosis of teleconnections with ISMR and evaluation of simulations by climate models. The circulation indices, however, correlate poorly with ISMR raising doubts on the coupling between the two. In this study, we identify reasons behind the poor correlations and estimate a potential limit on this correlation between circulation indices and ISMR. One mechanistic reason for poor correlations between circulation indices and ISMR is that the period and variance explained by the oscillatory modes of circulation indices differ significantly from those of ISMR leading to their phases also differing between modes of circulation indices and corresponding modes of ISMR. Further, the wind anomaly patterns associated with wind indices project strongly on the climatological monsoon circulation unlike that associated with the ISMR, another reason for their poor correlation with ISMR. As a result, all circulation indices underestimate the negative correlation between June–September sea‐surface temperature (JJAS SST) over the Niño3.4 and ISMR and fail to simulate the lead–lag relationship between the two. A conservative estimate of the interannual variance explained by coupling of the JJAS rainfall over continental India and winds in the neighbourhood is strong, explaining about 55% variance distributed in three leading coupled modes. Therefore, the effectiveness of any circulation indices in explaining the ISMR variance is potentially limited to that associated with the dominant mode (∼30%). The local response of winds to deep convection over the waters of the Indian Ocean and western Pacific warm pool leads to a strong increasing trend in some of the circulation indices and adds “noise” modes contributing to the de‐correlation between circulation indices and ISMR.

Variability in upper-ocean salinity stratification in the tropical Pacific Ocean

Using a gridded array for real-time geostrophic oceanography (Argo) program float dataset, the features of upper-ocean salinity stratification in the tropical Pacific Ocean are studied. The salinity component of the squared Brunt-Vaisala frequency N2 (N 2 ) is used to represent salinity stratification. Layer-max N 2 (LMN), defined as the N 2 maximum over the upper 300 m depth, and halocline depth (HD), defined as the depth where the N 2 maximum is located, are used to specifically describe the intensity of salinity stratification. Salinity stratification in the Topical Pacific Ocean has both spatial and temporal variability. Over the western and eastern equatorial Pacific, the LMN has a large magnitude with a shallow HD, and both have completely opposite distributions outside of the equatorial region. An obvious seasonal cycle in the LMN occurs in the north side of eastern equatorial Pacific and freshwater flux forcing dominates the seasonal variations, followed by subsurface forcing. At the eastern edge of the western Pacific warm pool around the dateline, significant interannual variation of salinity stratification occurs and is closely related to the El Nino Southern Oscillation event. When an El Nino event occurs, the precipitation anomaly freshens sea surface and the thermocline shoaling induced by the westerly wind anomaly lifts salty water upward, together contribute to the positive salinity stratification anomaly over the eastern edge of the warm pool. The interannual variations in ocean stratification can slightly affect the propagation of first baroclinic gravity waves.

Mechanisms of the decadal variability of monsoon rainfall in the southern Tibetan Plateau

The Tibetan Plateau (TP), as a whole, has undergone a moistening process since the late 1990s. However, the southern Tibetan Plateau (STP) is an exception, where summer monsoon precipitation amount has decreased, and lakes have shrunk. The cause for the precipitation decrease is not clear yet. Here we show that the monsoon (June to September) mean precipitation changes in the STP from 1979 to 2018 features a decadal variation component with a peak of around 10 years that is superposed on an upward ‘trend’ from 1979 to 1998 and a downward ‘trend’ afterward. We find that the decadal variation of the STP precipitation is associated with a large-scale dipolar sea surface temperature (SST) pattern between the equatorial central Pacific and the Indo-Pacific warm pool. A wet STP corresponds to negative SST anomaly in the equatorial central Pacific and positive SST anomaly in the Indo-Pacific warm pool. This equatorial SST gradient in the western Pacific generates pronounced easterly anomalies and a dipolar rainfall anomaly (i.e. a positive rainfall anomaly over the Maritime Continent and a negative anomaly in the equatorial western and central Pacific). Due to less precipitation over the equatorial western Pacific, the suppressed heat source appears to excite an anomalous anticyclonic band along 15–20° N extending from the Philippine Sea to the Bay of Bengal by emanating westward propagating descending transient Rossby waves. The low-level anticyclonic circulation over the Bay of Bengal further enhances northward moisture transport toward the STP and promote upward motion in the STP through changing local meridional circulation. Besides, the linearized atmospheric general circulation model experiments demonstrate that the dipole heating source can generate a high-pressure zone under the control of anticyclone over the western Pacific, which can extend westward to the Indian monsoon region.

Pairwise‐rotated EOFs of global cloud cover and their linkages to sea surface temperature

AbstractSea surface temperature (SST) could significantly affect the dynamic and thermodynamic conditions of the atmospheric circulation and consequently the cloud variations. Here we use several different satellite records to extract the spatial‐time modes of total cloud cover (TCC) by employing a pairwise rotation of Empirical Orthogonal Function analysis. The results show that the first two principal oscillation modes of TCC are closely associated with the Central Pacific El Niño Southern Oscillation (CP ENSO) and Eastern Pacific (EP) ENSO during the 1980s–2000s, while the ENSO‐like mode of TCC can provide an evident contribution to the TCC change during the 2000s–2010s. In CP El Niño, the cloud vertical structure decomposed from CloudSat observations shows an increase of cloud occurrence frequency near the equator and around 40°, and a decrease around 10° in both hemispheres, suggesting a symmetric tightening of Hadley cell (HC). In addition, cloud occurrence frequency increases around 180°, which is accompanied by an eastward shift of Walker circulation (WC). In EP El Niño, TCC increases (decreases) over the Equatorial Eastern Pacific (Western Pacific warm pool), and decreases asymmetrically over the subtropical Pacific Ocean, indicating a weakening of WC and an asymmetric tightening of HC, respectively. The different responses of circulation and clouds to CP and EP El Niño highlight the nonlinearity of El Niño SST forcing. We also construct a trend mode of TCC to investigate cloud long‐term responses to SST warming by transferring the linear trends of the rest modes to a specific mode. The principal components (PCs) of TCC trend modes are strongly correlated with global‐mean SST (GSST) with correlation a coefficient of about 0.60 during the 1980s–2000s and 0.45 during the 2000s–2010s, suggesting a continued influence of global SST warming on TCC. The global TCC change is mainly influenced by the combined effects of Atlantic Multi‐decadal Oscillation (AMO), Pacific Decadal Oscillation (PDO) and Indian Ocean Dipole (IOD). The variation about the trend mode of TCC is closely associated with PDO and IOD.

Miocene Sea Surface Dynamics in the Western Equatorial Pacific Based on Calcareous Nannofossil Records

Calcareous nannofossil in the sediments from Ocean Drilling Program (ODP) Hole 806B in the Ontong Java Plateau have been examined to reconstruct surface water condition in the western equatorial Pacific (WEP) throughout the Miocene. By using quantitative techniques, 107 sediment samples have been prepared into nannofossil slides and observed under microscope. Changes in Reticulofenestra size variations, Discoaster relative abundance and nannofossil accumulation rates (NAR) suggest that WEP surface water was dynamic. During early to middle Miocene from 18 to 14 Ma, the surface water was warm and depleted in nutrient with deep thermocline due to warm period of the Middle Miocene Climatic Optimum. Temperature cooled, nutrient increased and thermocline shoaled at 14 Ma during the East Antarctic Ice Sheet Expansion (EAIE). Surface water warmed and thermocline deepened after 13.2 Ma as the western Pacific warm pool (WPWP) began to established. At 9 Ma, surface water suddenly changed into cooler and nutrient rich condition. The collapse of ocean stratification coincided with the onset of Asian Monsoon Intensification. Warm surface water and deep thermocline occurred again in the latest Miocene as the brief return of the WPWP during 7–6 Ma.

Initiation of the Western Pacific Warm Pool at the Middle Miocene Climate Transition?

AbstractAcross the middle Miocene, Earth's climate underwent a major cooling and expansion of the Antarctic ice sheet. However, the associated response and development of the tropical climate system is not fully understood, in part because this is influenced by both global climate and also low‐latitude tectonic gateways and paleoceanography. Here we use combined δ18O and Mg/Ca of planktic foraminifera to reconstruct the thermal history and changes in hydrology from the Indo‐Pacific region from 16.5 to 11.5 Ma. During the warmth of the early middle Miocene, our records indicate a dynamic ocean‐atmosphere system in the Indo‐Pacific region, with episodes of saltier and warmer tropical surface waters associated with high pCO2 and retreat of the Antarctic ice sheet. We show that across the Middle Miocene Climate Transition (MMCT) surface ocean temperatures in the Indo‐Pacific cooled by ~2°C, synchronous with the advance of the Antarctic ice sheet. The associated cooling in the Southern Ocean appears to have started earlier and was stronger. Further, we show that western Pacific Ocean warmed and eastern tropical Indian Ocean freshened following the MMCT, likely caused by the constriction of the Indonesian Seaway and reduced connectivity between the Pacific and Indian Oceans following Antarctic glaciation. The MMCT therefore represented a key phase in the evolution of the West Pacific Warm Pool and associated tropical climate dynamics.

Unraveling Glacial Hydroclimate in the Indo‐Pacific Warm Pool: Perspectives From Water Isotopes

AbstractThe Indo‐Pacific Warm Pool (IPWP) is home to the warmest sea surface temperatures in the world oceans, favoring strong tropospheric convection and heavy rainfall. The mechanisms controlling long‐term change in the region's hydroclimate are still uncertain. Here, we present a 450,000‐year record of precipitation δD from southern Sumatra that records a consistent pattern of glacial isotopic enrichment and interglacial depletion. We synthesize existing paleo‐indicators of precipitation δD and δ18O in the IPWP and compare results with water isotope‐enabled climate simulations of the Last Glacial Maximum (LGM). The simulations show glacial isotopic enrichment over the eastern Indian Ocean extending into the southern IPWP and isotopic depletion over Southeast Asia, the west Pacific, and Australia. The pattern of simulated LGM isotopic change agrees generally well with our proxy synthesis. We conclude that reorganization of regional circulation under glacial conditions controls precipitation isotope variability in the IPWP: Low‐level tropospheric convergence dominates the signal in the north/east, whereas divergence controls the response in the south/west. Additional sensitivity simulations suggest that the LGM ice sheets and the associated lowering in sea level, rather than decreased greenhouse gases, are responsible for the distinctive spatial pattern in glacial changes of precipitation isotopes and hydroclimate across the IPWP.

Initiation of the Western Pacific Warm Pool at the Middle Miocene Climate Transition?

Initiation of the Western Pacific Warm Pool at the Middle Miocene Climate Transition?

Relationships Between Jianghuai Meiyu Anomaly and the Collaborative Evolution of Wave Trains in the Upper and Lower Troposphere in Mid-July of 2020

Based on the ECMWF reanalysis dataset (ERA5), SST , OLR and the daily precipitation data, the relationship between the Meiyu anomaly in the Yangtze-Huaihe river basin (YHB) and the collaborative evolution of wave trains in the upper and lower troposphere in the mid-July of 2020 is analyzed. The results show that the key circulation background for the long-lasting Meiyu season in the summer of 2020 is the coexistence and collaborative evolution of the Silk-Road (SR) wave train in the upper troposphere and the – + Pacific-Japan (PJ) wave train in the middle-lower troposphere, which is in favor of the movement towards each other of the South Asia high (SAH) and the western Pacific subtropical high (WPSH), resulting in the upper-level divergence over the YHB where is the right side of the jet entrance area. The WPSH strengthens and extends westward, transporting abundant water vapor to the YHB from the Western Pacific and South China Sea. Meanwhile, the meridional circulation develops at the middle and high latitudes, which is in favor of cold-dry air and warm-moist air strongly interact over the YHB, leading to the long-lasting Meiyu season. The coordinative evolution of the SR and PJ wave trains not only affects the persistence and intensity of precipitation in the YHB, but also can be regarded as a precursor signal for the occurrence of persistent heavy rainfall to some extent. The wave-activity flux dispersing eastward along the upper-level jet is beneficial to the development and maintenance of the SR wave train, and the poleward dispersing wave-activity flux is related to the development and maintenance of the PJ wave train. Additionally, along with the weakening El Nino in 2019, the anomalous convection caused by the warm SST in the Western Pacific warm pool is conducive to the formation and maintenance of the PJ wave train. The abnormal convection caused by the warm SST in the Indian Ocean and the east coast of the tropical Atlantic may affect the SR wave train. These are also one of the reasons for the long Meiyu season in 2020.

Automated analysis of foraminifera fossil records by image classification using a convolutional neural network

Abstract. Manual identification of foraminiferal morphospecies or morphotypes under stereo microscopes is time consuming for micropalaeontologists and not possible for nonspecialists. Therefore, a long-term goal has been to automate this process to improve its efficiency and repeatability. Recent advances in computation hardware have seen deep convolutional neural networks emerge as the state-of-the-art technique for image-based automated classification. Here, we describe a method for classifying large foraminifera image sets using convolutional neural networks. Construction of the classifier is demonstrated on the publicly available Endless Forams image set with a best accuracy of approximately 90 %. A complete automatic analysis is performed for benthic species dated to the last deglacial period for a sediment core from the north-eastern Pacific and for planktonic species dated from the present until 180 000 years ago in a core from the western Pacific warm pool. The relative abundances from automatic counting based on more than 500 000 images compare favourably with manual counting, showing the same signal dynamics. Our workflow opens the way to automated palaeoceanographic reconstruction based on computer image analysis and is freely available for use.

Factors Regulating the Multidecadal Changes in MJO Amplitude over the Twentieth Century

AbstractThis study examined multidecadal changes in the amplitude of the boreal-winter Madden–Julian oscillation (MJO) over the twentieth century using two century-long reanalysis datasets (20CR and ERA-20C). Both revealed reasonable MJO variability compared to other state-of-the-art reanalysis datasets. We detected pronounced multidecadal variations along with an increasing trend in MJO amplitude during the period 1900–2009 in both datasets, although this linear trend was less significant in the reconstructed MJO index proposed by Oliver and Thompson. The two twentieth-century reanalysis datasets and the Oliver–Thompson MJO index consistently showed the intensified amplitude of MJO precipitation and circulation in the later decades (1970–99) compared to the earlier decades (1920–49). The most significant enhancement of MJO precipitation in the later decades appeared over the western Pacific warm pool. To understand the mechanisms controlling the changes in western Pacific MJO precipitation amplitude over the twentieth century, we diagnosed the moisture budget equation. The enhanced MJO precipitation variability in the later decades mainly came from increased moisture associated with a strengthened low-level convergence anomaly working on background mean moisture []. Further diagnosis showed that the effect of anomalous circulation (∇ ⋅ V′) change on the MJO precipitation amplitude change over the twentieth century was about an order larger than that of mean moisture () change, different from the mechanisms (i.e., increased gradient of ) responsible for the intensified MJO precipitation amplitude under future warmer climate. The enhanced MJO circulation anomalies during 1970–99 may be caused by an enhanced diabatic heating anomaly, offset partly by the increased mean static stability.

Variability of Diurnal Sea Surface Temperature during Short Term and High SST Event in the Western Equatorial Pacific as Revealed by Satellite Data

Near-surface diurnal warming is an important process in the climate system, driving exchanges of water vapor and heat between the ocean and the atmosphere. The occurrence of the hot event (HE) is associated with the high diurnal sea surface temperature amplitude (δSST), which is defined as the difference between daily maximum and minimum sea surface temperature (SST). However, previous studies still show some inconsistency for the area of HE occurrence and high δSST. The present study produces global δSST data based on the SST, sea surface wind data derived from microwave radiometers, and solar radiation data obtained from visible/infrared radiometers. The value of δSSTs are estimated and validated over tropical oceans and then used for investigating HE in the western equatorial Pacific. A three-way error analysis was conducted using in situ mooring buoy arrays and geostationary SST measurements by the Himawari-8 and Geostationary Operational Environmental Satellite (GOES). The standard deviation error of daily and 10-day validation is around 0.3 °C and 0.14–0.19 °C, respectively. Our case study in the western Pacific (from 110°E to 150°W) shows that the area of HE occurrence coincided well with the area of high δSST. Climatological analysis shows that the collocated area between high occurrence rate of HE and high δSST, which coincides with the western Pacific warm pool region in all seasons. Thus, this study provides more persuasive evidence of the relation between HE occurrence and high δSST.

Coral-based proxy calibrations constrain ENSO-driven sea surface temperature and salinity gradients in the Western Pacific Warm Pool

Constraining past variability in ocean conditions in the Western Pacific Warm Pool (WPWP) and examining how it has been influenced by the El-Niño Southern Oscillation (ENSO) is critical to predicting how these systems may change in the future. To characterize the spatiotemporal variability of the WPWP and ENSO during the past three decades, we analyzed climate proxies using coral cores sampled from Porites spp. from Kosrae Island (KOS) and Woleai Atoll (WOL) in the Federated States of Micronesia. Coral skeleton samples drilled along the major growth axis were analyzed for oxygen isotopes (δ18Oc) and trace element ratios (Sr/Ca), used to reconstruct sea surface salinity and temperature (SSS and SST). Pseudocoral δ18O time series (δ18Opseudo) were calculated from gridded instrumental observations and compared to δ18Oc, followed by fine-tuning using coral Sr/Ca and gridded SST, to produce age models for each coral. The thermal component of δ18Oc was removed using Sr/Ca for SST, to derive δ18O of seawater (δ18Osw), a proxy for SSS. The Sr/Ca, and δ18Osw records were compared to instrumental SST and SSS to test their fidelity as regional climate recorders. We found both sites display significant Sr/Ca-SST calibrations at monthly and interannual (dry season, wet season, mean annual) timescales. At each site, δ18Osw also exhibited significant calibrations to SSS across the same timescales. The difference between normalized dry season SST (Sr/Ca) anomalies from KOS and WOL generates a zonal SST gradient (KOSWOLSST), capturing the east-west WPWP migration observed during ENSO events. Similarly, the average of normalized dry season δ18Osw anomalies from both sites produces an SSS index (KOSWOLSSS) reflecting the regional hydrological changes. Both proxy indices, KOSWOLSST and KOSWOLSSS, are significantly correlated to regional ENSO indices. These calibration results highlight the potential for extending the climate record, revealing spatial hydrological gradients within the WPWP and ENSO variability back to the end of the Little Ice Age.

Impact of Indo-Pacific warm pool Hadley circulation on the seasonal forecast performance for summer precipitation over the western North Pacific

The western North Pacific summer climate affects the densely populated East Asian countries, while seasonal forecasting over these regions remains challenging for dynamical models. This study evaluates the effect of the spring Indo-Pacific warm pool (IPWP) Hadley circulation on the forecast performance for summer western North Pacific (WNP) precipitation in the Met Office Global Seasonal Forecast System (GloSea5). GloSea5 skillfully predicts IPWP Hadley circulation, but has moderate forecast skill for summer WNP precipitation. Compared to observations, the significant relationship between the spring IPWP Hadley circulation and the summer WNP precipitation is overestimated in most hindcast members and in the ensemble mean. Furthermore, we confirmed that the forecast ensemble members with a stronger such regional circulation–precipitation relationship have better forecast performance for the summer WNP precipitation, suggesting the importance of such a relationship for the seasonal forecast of WNP precipitation. These results also imply the need to further investigate other important factors for the WNP precipitation, the effects of which may be suppressed by the overestimated regional circulation–precipitation relationship in GloSea5.

Impact of the Indo-Pacific Warm Pool on the Hadley, Walker, and Monsoon Circulations

The Indo-Pacific warm pool (IPWP) is enclosed by a 28 °C isotherm and plays a vital role in controlling atmospheric circulations. However, the effects of changes in regional warm pool sea surface temperatures (SSTs) remain unexplored. We divided the IPWP into the Indian and Pacific sectors and distinguished their responses to natural variability and global warming. Furthermore, we examined the impacts of the interannual variability (IAV) in warm pool SST on the tropical Hadley, Walker, and monsoon circulations. The Hadley circulation was affected by warm pool SST warming, i.e., warmer SSTs over the warm pool strengthened the upward branch of Hadley circulation, whereas the downward branch was respectively weakened and strengthened in the Northern and Southern Hemispheres. Walker circulation was strengthened (weakened) in the warming (natural) mode. Consequently, the Walker circulation is weakened since the natural variability of warm pool SST plays a more dominant role rather than the warming trend of SSTs over the warm pool. Furthermore, our analysis displays that warm pool warming has little impact on the monsoon circulation. Our findings highlight the different roles of the IAV of warm pool regions in each tropical circulation as part of the warming trend and natural variability.

Cooperative effects of tropical Pacific and Atlantic SST forcing in southern China winter precipitation variability

This study investigates the variability of winter precipitation over southern China (SC) and the impacts of sea surface temperature (SST) forcing in the tropical Pacific and Atlantic Oceans. The SC winter precipitation displays two major spatial modes: a same-sign mode with the maximum center located in southeastern China, and a north–south dipole mode with opposite variations between the lower reach of the Yangtze River and the southeastern coast of China. Both modes are associated with the variation of the western North Pacific (WNP) anticyclone. The same-sign mode is associated with canonical El Nino through the Pacific-East Asian teleconnection in the lower troposphere. The north–south dipole mode is subject to the impact of the western Pacific warm pool associated with the central Pacific El Nino and equatorial Atlantic forcing. The influence of equatorial Atlantic warming is through an upper-tropospheric zonal wave pattern from the tropical Atlantic to the mid-latitudes of East Asia. Both the tropical Pacific and Atlantic oceanic forcing induce a north–south dipole pattern of winter precipitation anomalies over SC by affecting the northward extension of the WNP anticyclone. Moreover, the observed statistical relationship between the dipole mode and the equatorial Atlantic SST anomalies is present in hindcasts/forecasts of the NCEP Climate Forecast System version 2 (CFSv2), confirming a potential source of the predictability of the north–south dipole-like variation of the winter precipitation in SC.

The evolution of the Kuroshio Current over the last 5 million years since the Pliocene: Evidence from planktonic foraminiferal faunas

Meridional heat transport of the western Pacific boundary current (the Kuroshio Current) is one of the key factors in global climate change. This current is important because it controls the temperature gradient between low latitudes and the North Pacific and so significantly influences mid-latitude atmosphere-ocean interactions. Here we reconstruct changes in hydrological conditions within the mid-latitude mainstream of the Kuroshio Current based on faunal analysis of planktonic foraminifera in core DSDP 296 from the Northwest Pacific Ocean. This approach enabled us to deduce evolutionary processes within the Kuroshio Current since the Pliocene. A total of 57 species in the coarser section (> 150 µm) were identified; results indicate that planktonic foraminiferal faunal evolution has mainly been characterized by three major stages, the first of which comprised mixed-layer warm-water species of Globigerinoides ruber which first appeared between 3.5 and 2.7 Ma and then gradually increased in content. Percentages of another warm-water species of G. conglobatus also gradually increased in number over this interval. Variations in warm-water species indicate a gradual rise in sea surface temperature (SST) and imply initiation of Kuroshio Current impact on the Northwest Pacific Ocean since at least 3.5 Ma. Secondly, over the period between 2.7 and 2.0 Ma, thermocline species of Globigerina calida , Neogloboquadrina humersa , Neogloboquadrina dutertrei , and Pulleniatina obliquiloculata started to appear in the section. This fauna was dominated by G. ruber as well as increasing G. conglobatus contents. These features imply a further rise in SST and its gradually enhanced influence on thermocline water, suggesting strengthening of the Kuroshio Current since 2.7 Ma. Thirdly, between 2.0 Ma and present, increasing contents of thermocline species (i.e., G. calida , N. dutertrei and P. obliquiloculata ) indicate a gradual rise in seawater temperature at this depth and also imply more intensive Kuroshio Current during this period. On the basis of comparative records from cores ODP 806 and DSDP 292 from the low latitude Western Pacific, we propose that initiation of the impact of the Kuroshio Current in the Northwest Pacific and it subsequent stepwise intensifications since 3.5 Ma can be closely related to the closure and restriction of the Indonesian and Central American seaways as well as variations in the Western Pacific Warm Pool (WPWP) and equatorial Pacific region.

Spatio-Temporal Variations of Water Vapor Budget over the Tibetan Plateau in Summer and Its Relationship with the Indo-Pacific Warm Pool

The water vapor budget (WVB) over the Tibetan Plateau (TP) is closely related to the large-scale atmospheric moisture transportation of the surrounding mainland and oceans, especially for the Indo-Pacific warm pool (IPWP). However, the procession linkage between the WVBs over the TP and its inner basins and IPWP has not been sufficiently elucidated. In this study, the relationship between the summer WVB over the TP and the IPWP was quantitatively investigated using reanalysis datasets and satellite-observed sea surface temperature (SST). The results show that: (1) the mean total summer vapor budget (WVBt) over the TP in the period of 1979–2018 was 72.5 × 106 kg s−1. Additionally, for the 13 basins within the TP, the summer WVB has decreased from southeast to northwest; the Yarlung Zangbo River Basin had the highest WVB (33.7%), followed by the Upper Yangtze River Basin, Ganges River Basin and Qiangtang Plateau. (2) For the past several decades, the WVBt over the TP has experienced an increasing trend (3.81 × 106 kg s−1 decade−1), although the southern boundary budget (WVBs) contributed the most and is most closely related with the WVBt, while the eastern boundary budget (WVBe) experienced a decreasing trend (4.21 × 106 kg s−1 decade−1) which was almost equal to the interdecadal variations of the WVBt. (3) For the IPWP, we defined a new warm pool index of surface latent heat flux (WPI-slhf), and found that an increasing WPI-slhf would cause an anticyclone anomaly in the equatorial western Indian Ocean (near 70° E), resulting in the increased advent of water vapor to the TP. (4) On the interdecadal scale, the correlation coefficients of the variation of the summer WVBt over the TP with the WPI-slhf and Indian Ocean Dipole (IOD) signal were 0.86 and 0.85, respectively (significant at the 0.05% level). Therefore, the warming and the increasing slhf of the IPWP would significantly contribute to the increasing WVB of the TP in recent decades.

Thermocline dynamics in the northwestern tropical Pacific over the past 700 kyr

Variations in thermoclinal conditions play an important role in the evolution of the Western Pacific Warm Pool (WPWP) and in connecting the tropical and extratropical regions of the Pacific Ocean. However, secular thermoclinal changes in the northwestern part of the WPWP and their forcing mechanisms are not yet well understood, particularly on orbital timescales. In this study, we report thermoclinal variation at Site MD06-3047 B in the western Philippine Sea over the last 700 kyr. Our data show that variations in thermocline temperature and salinity in the study area were correlated with changes in Earth’s obliquity, with warmer and saltier (colder and fresher) thermocline waters corresponding to high (low) Earth-axial obliquity. At high obliquity, increases in thermocline temperature and salinity may have been linked to advection of warm North Pacific tropical water. In the study area, a lower (higher) vertical temperature gradient in the upper water column suggests a deeper (shallower) thermocline during glacials (interglacials). The main factor leading to thermoclinal deepening during glacials was strengthening of the East Asian winter monsoon (EAWM), and more La Niña-like conditions may have contributed to concurrent subsurface warming and thus thermoclinal deepening. In-phase thermoclinal deepening in the northern and peri-equatorial southern WPWP during glacial stages may have been triggered by changes in glacial boundary conditions, such as increased ice sheet thickness, through intensification of the EAWM and strengthening of the tropical Trade Winds.