Western Pacific Warm Pool Research Articles

Warm pool ocean heat content regulates ocean-continent moisture transport.

The Indo-Pacific Warm Pool (IPWP) exerts a dominant role in global climate by releasing huge amounts of water vapour and latent heat to the atmosphere and modulating upper ocean heat content (OHC), which has been implicated in modern climate change1. The long-term variations of IPWP OHC and their effect on monsoonal hydroclimate are, however, not fully explored. Here, by combining geochemical proxies and transient climate simulations, we show that changes of IPWP upper (0-200 m) OHC over the past 360,000 years exhibit dominant precession and weaker obliquity cycles and follow changes in meridional insolation gradients, and that only 30%-40% of the deglacial increases are related to changes in ice volume. On the precessional band, higher upper OHC correlates with oxygen isotope enrichments in IPWP surface water and concomitant depletion in East Asian precipitation as recorded in Chinese speleothems. Using an isotope-enabled air-sea coupled model, we suggest that on precessional timescales, variations in IPWP upper OHC, more than surface temperature, act to amplify the ocean-continent hydrological cycle via the convergence of moisture and latent heat. From an energetic viewpoint, the coupling of upper OHC and monsoon variations, both coordinated by insolation changes on orbital timescales, is critical for regulating the global hydroclimate.

Interdecadal Changes of the MERRA-2 Incoming Surface Solar Radiation (SSR) and Evaluation against GEBA & BSRN Stations

This study assesses and evaluates the 40-year (1980–2019) Modern-Era Retrospective Analysis for Research and Applications v.2 (MERRA-2) surface solar radiation (SSR) as well as its interdecadal changes (Δ(SSR)) against high quality reference surface measurements from 1397 Global Energy Balance Archive (GEBA) and 73 Baseline Surface Radiation Network (BSRN) stations. The study is innovative since MERRA-2 (Δ(SSR)) has never been evaluated in the past, while the MERRA-2 SSR fluxes themselves have not been evaluated in such large spatial scale, which is global here, and temporal basis, which counts 40-years. Other novelties of the study are the use of the highest quality BSRN stations, done for the first time in such an evaluation, as well as the use of a greater number of reference-GEBA stations than in other studies. Moreover, the assessment and evaluation in this study are largely based on SSR anomalies, while being done in depth, at spatial scales ranging from the local to global/hemispherical, and separately for land and ocean areas, and at temporal scales spanning intervals from decadal sub-periods to 40 years. Overall, the MERRA-2 deseasonalized SSR anomalies correlate well with either GEBA (R equal to 0.61) and BSRN (R equal to 0.62). The percentage of agreement between the sign of computed GEBA and MERRA-2 Δ(SSR) is equal to 63.4% and the corresponding percentage for MERRA-2 and BSRN is 50%. According to MERRA-2, strong and statistically significant positive Δ(SSR) (Brightening) is found over Europe, Central Africa, Mongolia, Mexico, Brazil, Argentina and some parts of the tropical oceans. In contrast, large and statistically significant negative Δ(SSR) (Dimming) occurs over the western Tropical Warm Pool, India, Southern East China, Amazonia, stratocumulus covered areas and some parts of oceans. MERRA-2 yields a dimming equal to −0.158 ± 0.005 W/m2/year over the globe from 1980 to 2019. This 40-year dimming, which occurred in both hemispheres, more over ocean than continental areas (−0.195 ± 0.006 and −0.064 ± 0.006 W/m2/year, respectively), underwent decadal scale variations.

Intensified aridity over the Indo-Pacific Warm Pool controlled by ice-sheet expansion during the Last Glacial Maximum

The magnitude, direction and cause of precipitation changes across the Indo-Pacific Warm Pool (IPWP) during the Last Glacial Maximum (LGM) remain elusive. In particular, it is still inconclusive whether tropical or extra-tropical factors controlled such precipitation changes. Determining the spatio-temporal distribution of precipitation in the IPWP during the LGM is a valid strategy to address this issue, but the existing precipitation records are dominantly from maritime continents and marginal seas, with few data from pelagic oceans. In order to fill this gap, we analyzed the oxygen isotopic compositions of single Ethmodiscus rex diatom frustules (δ18OE. rex) from a sediment core (WPD-03) consisting of laminated diatom mats (LDMs) in the eastern Philippine Sea (EPS). δ18OE. rex was controlled mainly by sea-surface salinity variation and, thus, can reflect open-ocean precipitation changes across the IPWP. Our precipitation proxy records, in combination with existing published data, reveal spatial patterns of precipitation change that indicate overall drying across the IPWP during the LGM. Based on a comparison of paleoclimatic records with modeling results, we propose that extra-tropical factors (ice-sheet size) controlled precipitation variability in the IPWP during the LGM through a combination of zonal shifts of ENSO and meridional migration of the ITCZ. Strong aridity during the LGM prevented formation of a subsurface barrier layer and, hence, allowed accessing of sufficient nutrients to surface waters, stimulating blooms of E. rex and subsequent formation of LDMs in the IPWP. These findings suggest an important role for high-latitude climate in the tropical hydrological cycle during the LGM.

Plio-Pleistocene Planktic Foraminiferal Biochronology of ODP Site 762B, Exmouth Plateau, Southeast Indian Ocean

ABSTRACTThe Indonesian Throughflow region connects the tropical Pacific Ocean with the eastern Indian Ocean, and variability of the Throughflow during the Plio-Pleistocene has been related to major climate shifts at the global level. Planktic foraminiferal biostratigraphy integrated with magnetochronology provides a robust time framework for assigning age control to major paleoceanographic events. Understanding of the coupled histories of the El Niño Southern Oscillation, Western Pacific Warm Pool, Indonesian Throughflow, and the eastern Indian Ocean has greatly benefitted from the chronological framework provided by planktic foraminiferal biostratigraphies from these regions. Ocean Drilling Program (ODP) Hole 762 lies under the influence of the Leeuwin Current, originating from the Indonesian Throughflow. Multiproxy data have been collected from Hole 762B and other nearby sites for paleoceanographic interpretation. However, a detailed planktic foraminiferal biostratigraphy integrated with magnetochronology is not available from this site. We provide here the sequential order of planktic foraminiferal First Occurrence (FO) and Last Occurrence (LO) events, which allowed us to divide the examined section into seven biostratigraphic zones. The biostratigraphy was integrated with magnetostratigraphy using revised ages of magnetochrons, which yielded biochronological age estimates for planktic foraminiferal events. A major planktonic faunal turnover between 3.4 and 2.7 Ma is probably related to the onset of the Northern Hemisphere glaciation and related changes in the Indonesian Throughflow. We have compared our numerical age estimates with published ages from other parts of the world. The diachronism observed probably is related to the local oceanographic setting of ODP Hole 762B, which has been alternatively influenced by changing strengths of the warm Leeuwin Current and the cold west Australian Current. The biochronology established here will be useful for correlating paleoceanographic events in the region.

Large-scale atmospheric features favoring the tropical cyclone activity affecting the Guangdong–Hong Kong–Macao Greater Bay Area of China

This study analyzes the large-scale circulation associated with the tropical cyclones (TCs) affecting the Guangdong–Hong Kong–Macao Greater Bay Area (GBA) in southern China during the peak TC season (July–September) from 1980 to 2019. The results show that the frequency of the TCs affecting the GBA is closely tied to a meridional Rossby wave train from Southeast Asia to Northeast Asia, manifested by the important roles of the variability of the monsoon trough (MT) and the western North Pacific subtropical high (WNPSH). In the active GBA TC years, intensified and westward-shifted MT and northeastward-shrunk WNPSH cause anomalous southeasterly wind over the southeast coast of China, and hence more TCs move northwestward to the GBA. Furthermore, the cyclonic convergent conditions related to the tacit-and-mutual configurations of MT and WNPSH favor more TC genesis in the west of 140° E over the WNP via significant larger meridional shear vorticity of the zonal winds, ascending motions, and positive vorticity. Opposite features of wind anomalies and TC genesis are seen in the inactive GBA TC years. Both statistical and numerical analyses confirm that the meridional Rossby wave train is associated with a Matsuno–Gill response to the anomalous convective activity over the western Pacific warm pool, which is maintained by the ascending branches of the La Niña-type Walker circulation over the Pacific and the monsoon-enhanced Walker circulation over the Indian Ocean.

Atmospheric concentrations and sources of black carbon over tropical Australian waters

Black carbon (BC) aerosols significantly contribute to radiative budgets globally, however their actual contributions remain poorly constrained in many under-sampled ocean regions. The tropical waters north of Australia are a part of the Indo-Pacific warm pool, regarded as a heat engine of global climate, and are in proximity to large terrestrial sources of BC aerosols such as fossil fuel emissions, and biomass burning emissions from northern Australia. Despite this, measurements of marine aerosols, especially BC remain elusive, leading to large uncertainties and discrepancies in current chemistry-climate models for this region. Here, we report the first comprehensive measurements of aerosol properties collected over the tropical warm pool in Australian waters during a voyage in late 2019. The non-marine related aerosol emissions observed in the Arafura Sea region were more intense than in the Timor Sea marine region, as the Arafura Sea was subject to greater continental outflows. The median equivalent BC (eBC) concentration in the Arafura Sea (0.66 μg m−3) was slightly higher than that in the Timor Sea (0.49 μg m−3). Source apportionment modelling and back trajectory analysis and tracer studies consistently suggest fossil fuel combustion eBC (eBCff) was the dominant contributor to eBC across the entire voyage region, with biomass burning eBC (eBCbb) making significant additional contributions to eBC in the Arafura Sea. eBCff (possibly from ship emissions or oil and gas rigs and their associated activities) and cloud condensation nuclei (CCN) were robustly correlated in the Timor Sea data, whereas eBCbb positively correlated to CCN in the Arafura Sea, suggesting different sources and atmospheric processing pathways occurred in these two regions. This work demonstrates the substantial impact that fossil fuel and biomass burning emissions can have on the composition of aerosols and cloud processes in the remote tropical marine atmosphere, and their potentially significant contribution to the radiative balance of the rapidly warming Indo-Pacific warm pool.

Coupled Atmosphere–Ocean Variations on Timescales of Days Observed in the Western Tropical Pacific Warm Pool During Mid‐March 2020

AbstractFrom 11 to 12 March 2020, we observed atmosphere and ocean variations during a temporal warming of the western tropical Pacific warm pool and the associated active convection on the equator by the R/V Mirai and unmanned surface vehicles, known as Wave Gliders. The warming, which extended the warm pool eastward at a speed of approximately 2.9 m s−1, was found to be caused mainly by high sensitivity of the temperature of the thin mixed layer to shortwave and longwave radiation flux. During the warming, westward‐propagating inertia‐gravity wave disturbances in addition to intensified northerly wind intermittently enhanced turbulent latent heat release into the atmosphere (>150 W m−2), leading to heavy rainfall from midnight to morning, so‐called diurnal dancing, in the eastern region of the warm pool. The strong air–sea coupling was related to the zonal wind variation trapped near the sea level, which caused their convergences, also intensified atmospheric convection. Subsequently (13–14 March), due to the atmospheric density stratification and wind speed changes, atmospheric disturbances radiated upward and ceased to be coupled with the ocean.

Climatic and tectonic constraints on the Plio–Pleistocene evolution of the Indonesian Throughflow intermediate water recorded by benthic δ18O from IODP site U1482

The Indonesian Throughflow (ITF) is the unique low-latitude connector between the Pacific and Indian oceans. During the Plio–Pleistocene, the ITF may have played a significant role in the global climate associated with the tectonic constriction of the Indonesian gateway, which was possibly one of the factors spurring the Northern Hemisphere glaciation. Persistent constriction of the gateway would have restricted the ITF intermediate water (ITF-IW) with a high priority over the ITF upper waters. Due to limited availability of sediment archives, however, no investigations have been carried out on the Plio–Pleistocene evolution of the ITF-IW. In this study, we present our work on a total of 1368 sediment samples spanning the past ∼3.17–1.07 Ma from Site U1482, which was drilled during the International Ocean Discovery Program Expedition 363 in the Timor Passage, the main exit of the ITF to the Indian Ocean. Site U1482 is bathed by the ITF-IW, the evolution of which could thus be monitored by benthic foraminiferal shells preserved in the sediments of the site. Based on establishment of the first high-resolution (∼1.53 kyr) Plio–Pleistocene benthic δ18O stratigraphy within the ITF region, we recognized that, superimposed over the prominent 41-kyr glacial–interglacial cycles, the long-term change of Site U1482 benthic δ18O was punctuated at ∼2.51–2.43 Ma (Marine oxygen isotope stage (MIS)100/99–96/95) and at ∼1.6 Ma (MIS 56/55), followed by an interval of decreased glacial–interglacial amplitudes in the duration of ∼1.6–1.2 Ma. We propose that Site U1482 benthic δ18O was mainly driven by the global ice-sheet growth prior to ∼1.6 Ma, in addition to possibly influenced by the tectonic constriction of the Indonesian gateway. In contrast, the decreased glacial–interglacial amplitudes in the benthic δ18O during the interval of ∼1.6–1.2 Ma more likely indicate the regional signal carried by the ITF-IW, which is attributed to a freshening of the ITF surface waters that was subsequently transmitted to the intermediate depths via vertical mixing in the Banda Sea, caused by enhanced regional precipitation related to monsoonal and western Pacific warm pool conditions.

Heinrich summers

Millennial-scale climate oscillations of the last ice age registered in Greenland and Antarctic ice cores did not always vary in unison. A striking example is that the strongest Antarctic warming episodes occurred during Heinrich episodes in the North Atlantic region. Although the bipolar seesaw affords a possible explanation for such anti-phasing, it does not account for the equally striking observation that climate varied in unison between the hemispheres about half the time. Such phasing differences suggest the need for an alternative hypothesis in which the polar regions at times responded in unison to common forcing, and at other times left the impression of a bipolar seesaw. We posit that this impression arose from the effect of warmer-than-usual summers on continental ice sheets adjacent to the North Atlantic Ocean during each Heinrich episode. The relatively warm Heinrich summers produced discharges of meltwater and icebergs of sufficient volume to stimulate very cold winter conditions from widespread sea ice on a freshened ocean surface. The intervals between Heinrich episodes featured relaxation of sea-ice-induced winter severity from reduced summertime influx of meltwater and icebergs, indicating relatively cooler summer conditions. It is postulated that the causative variations in freshwater fluxes were driven by a climate signal most evident in Antarctic ice cores but also recognized in other paleoclimate records in both polar hemispheres. We suggest that this widespread signal arose from changes in the latitude and strength of the austral westerlies and the resulting effect on the western Pacific tropical warm pool, a mechanism dubbed the Zealandia Switch.

Thermal coupling of the Indo-Pacific warm pool and Southern Ocean over the past 30,000 years

The role of the tropical Pacific Ocean and its linkages to the southern hemisphere during the last deglacial warming remain highly controversial. Here we explore the evolution of Pacific horizontal and vertical thermal gradients over the past 30 kyr by compiling 340 sea surface and 7 subsurface temperature records, as well as one new ocean heat content record. Our records reveal that La Niña-like conditions dominated during the deglaciation as a result of the more intense warming in the western Pacific warm pool. Both the subsurface temperature and ocean heat content in the warm pool rose earlier than the sea surface temperature, and in phase with South Pacific subsurface temperature and orbital precession, implying that heat exchange between the tropical upper water column and the extratropical Southern Ocean facilitated faster warming in the western Pacific. Our study underscores the key role of the thermal coupling between the warm pool and the Southern Ocean and its relevance for future global warming.

Multidecadal Changes in Zonal Displacement of Tropical Pacific MJO Variability Modulated by North Atlantic SST

Abstract The Madden–Julian oscillation (MJO) exhibits the most vigorous activity over the Indo-Pacific warm pool during the boreal winter. In this study, we find the interbasin connection between the tropical Pacific MJO activity and North Atlantic sea surface temperature at the interdecadal time scale. Accompanied with the negative phase of the Atlantic multidecadal oscillation (AMO), the enhanced MJO variability (i.e., larger amplitude of 20–90-day convection) tends to extend eastward into the central tropical Pacific (20°S–0°, 170°E–150°W), while the vigorous MJO variability is confined to the Indo-Pacific warm pool during the positive phase of AMO. The eastward displacement of MJO convection into the central tropical Pacific during the negative AMO phase is supported by the increased moist static energy and low-level moisture tendencies there when the low-frequency background (>90 days) westerly transports the moisture perturbation [] and the MJO-associated easterly perturbation transports the background moisture [] toward the central tropical Pacific. This suggests that the interdecadal changes in zonal displacement of MJO activity over the tropical Pacific are closely linked with the impacts of AMO on the background flow and moisture condition over the central tropical Pacific. The results of coupled model sensitivity experiments and preindustrial simulations of phase 6 of the Coupled Model Intercomparison Project (CMIP6) support the observational findings.

Combined Effects of Tropical Indo-Pacific–Atlantic SST Anomalies on Record-Breaking Floods over Central-North China in September 2021

Abstract The rainy season typically terminates over the area of central northern China (CNC) in late August with the rapid retreat of the East Asian summer monsoon (EASM). However, the CNC suffered continuous heavy rainfall in September 2021. The monthly accumulative precipitation amount broke its historical record since 1961 and caused severe floods. The present study ascribes this extreme rainfall event to the westward inland intrusion of the western North Pacific subtropical high (WNPSH). Both case study and statistical analysis indicate that westward extension of the WNPSH is significantly positively correlated with convection over the Maritime Continent (MC) and northern Indian Peninsula (NIP) under the influence of sea surface temperature anomalies (SSTAs). Atmospheric general circulation model (AGCM) experiments validate the individual and combined effects of SSTAs in the tropical Indo-Pacific–Atlantic Oceans on WNPSH and CNC rainfall. In particular, the warm SSTAs in the Indo-Pacific warm pool increase the probability of extremely westward extension of the WNPSH and above-normal rainfall over the CNC. The cold SSTAs in the equatorial central-eastern Pacific enlarge the zonal SSTA gradient, strengthen the anomalous convection over the MC and NIP, and intensify the WNPSH and CNC rainfall anomalies. Meanwhile, the warmer tropical Atlantic anchors the anomalous WNPSH and CNC rainfall in their observed positions. A statistical model based on the tropical SSTAs in August can skillfully predict the westward extension of the WNPSH in September 2021, suggesting that the combined effects of tropical Indo-Pacific–Atlantic SSTAs in late summer possibly tend to prolong rainy season of the EASM until early autumn.

A Model of the Convectively Coupled Equatorial Rossby Wave over the Indo-Pacific Warm Pool

Abstract The convectively coupled equatorial Rossby (CCER) wave can significantly affect tropical and extratropical weather, yet its dynamics is not fully understood. Here, a linear two-layer model is proposed for the n = 1 CCER wave over the Indo-Pacific warm pool. The physical processes include moisture feedback (i.e., a prognostic moisture variable), cloud–radiation feedback, moist convection that depends on column moisture, effect of background zonal flow, and wind-induced surface flux exchange (WISHE) that links enhanced surface evaporation to low-level zonal westerly anomaly based on observation. The emerging CCER mode possesses many features consistent with the observations, including the horizontal structures, a broad range of frequency, and the amplification at both planetary and synoptic scales. This CCER mode can be viewed as a westward-propagating moisture mode, which is driven westward by the Doppler shifting effect of background easterly flow and the pre-moistening effect of WISHE. This CCER mode is destabilized by WISHE and background easterly shear. The WISHE shifts the enhanced convection into warm zone at planetary scales (wavenumbers 1–5), therefore, inducing planetary-scale instability through generating the eddy available potential energy (EAPE). The background easterly shear stimulates the interaction between the barotropic and baroclinic components of the circulation, amplifying the CCER wave at synoptic scales (wavenumbers 6–15) by increasing the EAPE generation through modifying the phase relation between low-level moisture convergence and temperature.

Spatio-Temporal Characteristics of the Indo-Pacific Warm Pool and the Corresponding Rain Pool

The greatest warm body in the world exists in the tropical oceans, which stimulates deep convection, resulting in abundant water vapor and precipitation in the tropical atmosphere. Using multiple SST datasets and related precipitation and atmospheric parameter data, this study examines multi-scale variabilities of the Indo-Pacific warm pool (IPWP) as well as the associated rain pool (IPRP). The results show that the IPWP and IPRP are spatially analogous and have significant increasing trends of intensity and coverage. Seasonal variations of the IPWP and IPRP are the strongest and almost coincident with each other. Our results also confirm previous findings that the most important interannual variations of the IPWP and IPRP are associated with various types of ENSO. The composite analysis reveals that the IPWP’s SST structure is linked to the ENSO-induced trade wind anomaly and that SST structural changes cause changes in the position and intensity of the ascending branch of the Walker circulation, which in turn drives changes in the position and intensity of the IPRP.

Changes in Extreme Precipitation on the Tibetan Plateau and Its Surroundings: Trends, Patterns, and Relationship with Ocean Oscillation Factors

The Tibetan Plateau is among the region’s most sensitive areas to global climate change. The observation data from 113 meteorological stations on the Tibetan Plateau and surrounding regions in China for 1971–2017 were used to analyze the periodic oscillations and trends in precipitation and extreme precipitation on multiple time scales to ensemble empirical mode decomposition. The relationship between extreme precipitation and sea-surface temperature (SST) anomalies was also explored. The results were as follows. (1) The timing of extreme-precipitation events in the highlands is consistent, with increased total precipitation and increased frequency, intensity, and extreme values of extreme precipitation. (2) Changes in temperature and precipitation are not completely synchronized. The total extreme precipitation, number of extreme-precipitation days, maximum single-day precipitation, and extreme single-day precipitation intensity all showed increases with fluctuations; the quasi-3-year oscillation contributes the most to the extreme precipitation. PRCPTOT is most strongly correlated with R10 and R95p. (3) The spatiotemporal patterns of the first and second empirical orthogonal function modes of the indices differed significantly and were not spatiotemporally uniform, but exhibited local clustering. (4) The Indian Ocean Warm Pool Strength and Western Pacific Warm Pool Strength indices were most highly correlated with each extreme-precipitation index, and the timings of the extreme-precipitation events lagged behind those of the SST anomalies. This study improves our understanding of extreme precipitation events in the context of climate warming and provides a basic analysis for the further assessment and prediction of extreme precipitation on the Tibetan Plateau and the surrounding ecologically fragile areas.

Seasonal aridity in the Indo-Pacific Warm Pool during the Late Glacial driven by El Niño-like conditions

Abstract. Island South-East Asia (ISEA) is a highly humid region that hosts the world's largest tropical peat deposits. Most of this peat accumulated only relatively recently during the Holocene, suggesting that the climate was drier and/or more seasonal during earlier times. Although there is evidence for savanna expansion and drier conditions during the Last Glacial Maximum (LGM, 21 ka BP), the mechanisms behind hydroclimatic changes during the ensuing deglacial period have received much less attention and are poorly understood. Here we use CESM1 climate model simulations to investigate the key drivers behind ISEA climate at the end of the Late Glacial (14.7–11.7 ka BP), with a focus on the last stadial of the Younger Dryas (12 ka BP). We further simulate the preceding Allerød (13 ka BP) interstadial climate and perform a sensitivity experiment to disentangle the climate impacts due to orbital forcing and Late Glacial boundary conditions against a slowdown of the Atlantic Meridional Overturning Circulation (AMOC). A transient simulation (TRACE) is used to track the climate seasonality and orbitally driven change over time during the deglaciation into the Holocene. In agreement with proxy evidence, CESM1 simulates overall drier conditions during the Younger Dryas and Allerød. More importantly, ISEA experienced extreme seasonal aridity, in stark contrast to the ever-wet modern climate. We identify that the simulated drying and enhanced seasonality in the Late Glacial is mainly the result of a combination of three factors: (1) large orbital insolation difference on the Northern Hemisphere (NH) between summer and winter, in contrast to the LGM and the present day, (2) a stronger (dry) East Asian winter monsoon caused by a larger meridional thermal gradient and (3) a major reorganization of the Indo-Pacific Walker Circulation with an inverted land-sea circulation and a complete breakdown of deep convection over ISEA in NH winters. The altered atmospheric circulation, sea surface temperature and sea level pressure patterns led to conditions resembling extreme El Niño events in the modern climate and a dissolution of the Intertropical Convergence Zone (ITCZ) over the region. From these results we infer that terrestrial cooling of ISEA and at least a seasonal reversal of land-sea circulation likely played a major role in delaying tropical peat formation until at least the onset of the Holocene period. Our results also suggest that centennial to millennial shifts in AMOC strength modifies the Pacific Ocean hydroclimate via alteration of the position of the ITCZ, and a modulation of the Pacific Walker Circulation. However, Late Glacial AMOC shifts are overall less important than hydroclimate changes due to orbital forcing and boundary condition changes relative to the modern climate.

Maintenance of Western North Pacific Anomalous Anticyclone in Boreal Summer by Wind-Induced Moist Enthalpy Advection Mechanism

Abstract The western North Pacific anomalous anticyclone (WNPAC) is the most important interannual circulation anomaly over the western Pacific warm pool. It can persist from boreal winter to the following summer and has great impacts on the East Asian monsoon. The maintenance of the WNPAC in boreal summer was explained by Ekman divergence anomalies over the western North Pacific (WNP) induced by the equatorial easterly Kelvin waves from the tropical Indian Ocean and the Maritime Continent. In this study, we propose a new mechanism for the maintenance of the WNPAC in the season, which we refer to as the “wind-induced moist enthalpy advection” (WIMEA) mechanism. Warm anomalies over the tropical Indian Ocean enhance local deep convection and thus excite atmospheric easterly Kelvin waves to the east. Climatological moist enthalpy decreases eastward over the WNP due to the local northward extension of the intertropical convergence zone (ITCZ) with the establishment of the WNP summer monsoon. Hence, the easterly anomalies transport low moist enthalpy (dry) air westward to the WNP. The induced negative moist enthalpy advection anomalies drive downward motion anomalies and thus suppress deep convection over the WNP under the constraint of the moist static energy budget balance. This moist teleconnection mechanism does not rely on Ekman processes. Numerical experiments using a dry linear baroclinic model indicate that the WIMEA and the conventional Kelvin wave–induced Ekman divergence mechanism have nearly equal relative contributions to the maintenance of the summer WNPAC, with the former mainly driven by the enhanced convection over the tropical Indian Ocean, while the latter is driven by that over the Maritime Continent.

Recent Decadal Weakening of the Summertime Rainfall Interannual Variability Over Yellow-Huaihe River Valley Attributable to the Western Pacific Cooling

This study focuses on the interdecadal transition of summer rainfall interannual variability over the Yellow-Huaihe River valley (YHRV). It is found that the interannual variability of summer rainfall over the YHRV becomes significantly weakened after the late 2000s. In the decade before the late 2000s (hereafter P1), the variance is 1.28 mm d−1 and the mean value of interannual variability is 1.49 mm d−1. In the decade after the late 2000s (P2), the variance is 0.35 mm d−1 and the mean value of interannual variability is 0.62 mm d−1. The variance and mean value of summer rainfall interannual variability have considerably decreased by 72.7% and 58.4% since the late 2000s, respectively. The reasons for the aforementioned interdecadal transition are explored. The results show that the interdecadal shift of the East Asian jet stream axis and the western Pacific subtropical high (WPSH) are two major factors leading to the interdecadal transition of summer rainfall interannual variability over the YHRV. The northward shift of the jet stream axis, which resembles the Pacific-Japan (PJ) pattern, suppresses the development of summer rainfall over the YHRV. The interannual oscillation of WPSH in P1 is more significant than that in P2, which is consistent with the weakened interannual variability of summer rainfall in P2. Further analysis reveals that the modulation factors responsible for the above changes are El Niño in its decaying phase in the equatorial central eastern Pacific and the warming in the Maritime continent and the western Pacific (WP) in P1. While in P2, the primary modulation factor is La Niña in its developing phase in the equatorial central eastern Pacific. As for the mechanism of the growth of summer rainfall in P1 positive years, the WP warm pool and El Niño in its decaying phase strengthen the Walker circulation in the tropical Pacific and induce Hadley circulation, whose ascending branch over the YHRV provides a favorable condition for the development of rainfall. At the same time, the northward propagation of Rossby waves in response to the above SSTA pattern suppresses convective activities over the Northwestern Pacific (NWP), which indirectly intensifies the WPSH. As a result, the PJ pattern is triggered and provides a favorable large-scale circulation condition for the growth of rainfall.

Western to Central Equatorial Pacific Planktic Foraminiferal Fluxes: Implication for the Relationship Between Their Assemblage and Warm Pool Migration from 1999 to 2002

ABSTRACT Time-series sediment trap experiments were carried out in the western to central equatorial Pacific (Sites MT3, MT4, MT5, and MT7) from 1999 to 2002, over the transitional time period from La Niña to El Niño conditions, in order to evaluate temporal variability in planktic foraminiferal fluxes and their assemblages. The foraminiferal test flux follows a trophic gradient from higher fluxes in the Equatorial Upwelling Region (EUR) and lower fluxes in the Western Pacific Warm Pool (WPWP) region. Globigerinoides ruber commonly dominates in all sites through the experimental period. Trilobatus sacculifer, Globigerinita glutinata, and Neogloboquadrina dutertrei occurred especially in the WPWP. In contrast, Globigerina bulloides and Pulleniatina obliquiloculata characterized the fauna in the EUR. A change in hydrologic conditions from La Niña to El Niño was documented along the sites during the sampled time interval. Simultaneous with an eastward advancement of the WPWP, the EUR retreated to the east. Rapid decreases in the fluxes of G. ruber, G. bulloides, and P. obliquiloculata were recognized immediately after the more oligotrophic WPWP conditions prevailed at Sites MT4, MT5, and MT7. Fluxes of the total planktic foraminifer assemblage increased both in the EUR and in the western side of the WPWP under full El Niño conditions during the second half of 2002. Such increases in test fluxes in the western side of the WPWP were mainly attributed to G. ruber and N. dutertrei, suggesting inputs of eutrophic waters from the northern coastal area of Papua New Guinea. Test fluxes under El Niño conditions in the WPWP were at the same level as those in the EUR, giving rise to strongly increased carbonate fluxes in the western equatorial Pacific.

Identifying Hydro‐Sensitive Coral δ18O Records for Improved High‐Resolution Temperature and Salinity Reconstructions

AbstractStable oxygen isotopic ratios in corals (δ18Ocoral) are commonly utilized to reconstruct climate variability beyond the limit of instrumental observations. These measurements provide constraints on past seawater temperature, due to the thermodynamics of isotopic fractionation, but also past salinity, as both salinity and seawater δ18O (δ18Osw) are similarly affected by precipitation/evaporation, advection, and other processes. We use historical observations, isotope‐enabled model simulations, and the PAGES Iso2k database to assess the potential of δ18Ocoral to provide information on past salinity. Using ‘‘pseudocorals’’ to represent δ18Ocoral as a function of observed or simulated temperature and salinity/δ18Osw, we find that δ18Osw contributes up to 89% of δ18Ocoral variability in the Western Pacific Warm Pool. Although uncertainty in the δ18Osw‐salinity relationship influences the inferred salinity variability, corals from these sites could provide valuable δ18Osw reconstructions. Coordinated in situ monitoring of salinity and δ18Osw is vital for improving estimates of hydroclimatic change.