Western Pacific Warm Pool Research Articles

TROPOMI tropospheric ozone column data: geophysical assessment and comparison to ozonesondes, GOME-2B and OMI

Abstract. Ozone in the troposphere affects humans and ecosystems as a pollutant and as a greenhouse gas. Observing, understanding and modelling this dual role, as well as monitoring effects of international regulations on air quality and climate change, however, challenge measurement systems to operate at opposite ends of the spatio-temporal scale ladder. Aboard the ESA/EU Copernicus Sentinel-5 Precursor (S5P) satellite launched in October 2017, the TROPOspheric Monitoring Instrument (TROPOMI) aspires to take the next leap forward by measuring ozone and its precursors at unprecedented horizontal resolution until at least the mid-2020s. In this work, we assess the quality of TROPOMI's first release (V01.01.05–08) of tropical tropospheric ozone column (TrOC) data. Derived with the convective cloud differential (CCD) method, TROPOMI daily TrOC data represent the 3 d moving mean ozone column between the surface and 270 hPa under clear-sky conditions gridded at 0.5∘ latitude by 1∘ longitude resolution. Comparisons to almost 2 years of co-located SHADOZ ozonesonde and satellite data (Aura OMI and MetOp-B GOME-2) conclude to TROPOMI biases between −0.1 and +2.3 DU (<+13 %) when averaged over the tropical belt. The field of the bias is essentially uniform in space (deviations <1 DU) and stable in time at the 1.5–2.5 DU level. However, the record is still fairly short, and continued monitoring will be key to clarify whether observed patterns and stability persist, alter behaviour or disappear. Biases are partially due to TROPOMI and the reference data records themselves, but they can also be linked to systematic effects of the non-perfect co-locations. Random uncertainty due to co-location mismatch contributes considerably to the 2.6–4.6 DU (∼14 %–23 %) statistical dispersion observed in the difference time series. We circumvent part of this problem by employing the triple co-location analysis technique and infer that TROPOMI single-measurement precision is better than 1.5–2.5 DU (∼8 %–13 %), in line with uncertainty estimates reported in the data files. Hence, the TROPOMI precision is judged to be 20 %–25 % better than for its predecessors OMI and GOME-2B, while sampling at 4 times better spatial resolution and almost 2 times better temporal resolution. Using TROPOMI tropospheric ozone columns at maximal resolution nevertheless requires consideration of correlated errors at small scales of up to 5 DU due to the inevitable interplay of satellite orbit and cloud coverage. Two particular types of sampling error are investigated, and we suggest how these can be identified or remedied. Our study confirms that major known geophysical patterns and signals of the tropical tropospheric ozone field are imprinted in TROPOMI's 2-year data record. These include the permanent zonal wave-one pattern, the pervasive annual and semiannual cycles, the high levels of ozone due to biomass burning around the Atlantic basin, and enhanced convective activity cycles associated with the Madden–Julian Oscillation over the Indo-Pacific warm pool. TROPOMI's combination of higher precision and higher resolution reveals details of these patterns and the processes involved, at considerably smaller spatial and temporal scales and with more complete coverage than contemporary satellite sounders. If the accuracy of future TROPOMI data proves to remain stable with time, these hold great potential to be included in Climate Data Records, as well as serve as a travelling standard to interconnect the upcoming constellation of air quality satellites in geostationary and low Earth orbits.

Precession cycles of the El Ni\xf1o/Southern oscillation-like system controlled by Pacific upper-ocean stratification

Modern observations have presented linkages between subsurface waters of the western Pacific warm pool and both El Niño/Southern Oscillation-related and extratropic-controlled upper-ocean stratification on interannual timescales. Moreover, studies have showed that such controls may operate on orbital cycles, although the details remain unclear. Here we present paired temperature and salinity reconstructions for the surface and thermocline waters in the central western Pacific warm pool over the past 360,000 years, as well as transit modeling results from an Earth system model. Our results show that variations in subsurface temperature and salinity in the western Pacific warm pool have consistently correlated with the shallow meridional overturning cell over the past four glacial-interglacial cycles, and they vary on eccentricity and precession cycles. The shallow meridional overturning cell regulates subsurface waters of the western Pacific warm pool by changing subtropical surface water density and thus equatorial upper-ocean stratification, acting as an El Niño/Southern Oscillation-like process in the precession band. Therefore, the western Pacific warm pool is critical in connecting the austral shallow meridional overturning cell to the Earth’s climate system on orbital timescales.

The Variability of Summer Atmospheric Water Cycle over the Tibetan Plateau and Its Response to the Indo-Pacific Warm Pool

The Tibetan Plateau (TP), atmosphere, and Indo-Pacific warm pool (IPWP) together constitute a regional land–atmosphere–ocean water vapor transport system. This study uses remote sensing data, reanalysis data, and observational data to explore the spatiotemporal variations of the summer atmospheric water cycle over the TP and its possible response to the air-sea interaction in the IPWP during the period 1958–2019. The results reveal that the atmospheric water cycle process over the TP presented an interannual and interdecadal strengthening trend. The climatic precipitation recycle ratio (PRR) over the TP was 18%, and the stronger the evapotranspiration, the higher the PRR. On the interdecadal scale, the change in evapotranspiration has a significant negative correlation with the Pacific Decadal Oscillation (PDO) index. The variability of the water vapor transport (WVT) over the TP was controlled by the dynamic and thermal conditions inside the plateau and the external air-sea interaction processes of the IPWP. When the summer monsoon over the TP was strong, there was an anomalous cyclonic WVT, which increased the water vapor budget (WVB) over the TP. The central and eastern tropical Pacific, the maritime continent and the western Indian Ocean together constituted the triple Sea Surface Temperature (SST) anomaly, which enhanced the convective activity over the IPWP and induced a significant easterly wind anomaly in the middle and lower troposphere, and then generated pronounced easterly WVT anomalies from the tropical Pacific to the maritime continent and the Bay of Bengal. Affected by the air-sea changes in the IPWP, the combined effects of the upstream strengthening and the downstream weakening in the water vapor transport process, directly and indirectly, increased the water vapor transport and budget of TP.

An Investigation on Seasonal and Diurnal Cycles of TOA Shortwave Radiations from DSCOVR/EPIC, CERES, MERRA-2, and ERA5

Reflected shortwave (SW) solar radiations at the top of atmosphere from Clouds and the Earth’s Radiant Energy System (CERES), Modern Era-Retrospective analysis for Research and Applications version 2 (MERRA-2), and ECMWF Reanalysis 5th Generation (ERA5) are examined to better understand their differences in spatial and temporal variations (seasonal and diurnal cycle timescale) with respect to the observations from the Earth Polychromatic Imaging Camera (EPIC) on the Deep Space Climate Observatory (DSCOVR) satellite. Comparisons between two reanalyses (MERRA-2 and ERA5) and EPIC reveal that MERRA-2 has a generally larger deviation from EPIC than ERA5 in terms of the SW radiance and diurnal variability in all seasons, which can be attributed to larger cloud biases in MERRA-2. MERRA-2 produces more ice/liquid water content than ERA5 over the tropical warm pool, leading to positive SW biases in cloud and radiance, while both reanalyses underestimate the observed SW radiance from EPIC in the stratus-topped region off the western coast of US/Mexico in the boreal summer. Himalaya/Tibet region in the boreal spring/summer and the midlatitude Southern Hemisphere in the boreal winter are the regions where MERRA-2 and ERA5 deviate largely from EPIC, but their deviations have the opposite sign. Vertical structures of cloud ice/liquid water content explain reasonably well these contrasting differences between the two reanalyses. As two independent observations, CERES and EPIC agree well with each other in terms of the SW radiance maps, showing 2–3% mean absolute errors over the tropical midlatitudes. The CERES-EPIC consistency further confirms that the reanalyses still have challenges in representing the SW flux and its global distribution. In the CERES-EPIC observation differences, CERES slightly overestimates the diurnal cycle (as a function of local solar time) of the observed EPIC irradiance in the morning and underestimates it in the afternoon, while the opposite is the case in the reanalyses.

Diurnal Variation in Surface Latent Heat Flux and the Effect of Diurnal Variability on the Climatological Latent Heat Flux over the Tropical Oceans

AbstractDiurnal variation in surface latent heat flux (LHF) and the effects of diurnal variations in LHF-related variables on the climatological LHF are examined using observations from the Global Tropical Moored Buoy Array. The estimated amplitude of the climatological diurnal LHF over the Indo-Pacific warm pool and the equatorial Pacific and Atlantic cold tongues is remarkable, with maximum values exceeding 20.0 W m−2. Diurnal variability of sea surface skin temperature (SSTskin) is the primary contributor to the diurnal LHF amplitude. Because the diurnal SSTskin amplitude has an inverse relationship with surface wind speed over the tropical oceans, an inverse spatial pattern between the diurnal LHF amplitude and surface wind speed results. Resolving diurnal variations in the SSTskin and wind improves the estimate of the climatological LHF by properly capturing the daytime SSTskin and daily mean wind speed, respectively. The diurnal SSTskin-associated contribution is large over the warm pool and equatorial cold tongues where low wind speeds tend to cause strong diurnal SSTskin warming, while the magnitude associated with the diurnal winds is large over the highly dynamic environment of the intertropical convergence zone. The total diurnal contribution is about 9.0 W m−2 on average over the buoy sites. There appears to be a power function (linear) relationship between the diurnal SSTskin-associated (wind-associated) contribution and surface mean wind speed (wind speed enhancement from diurnal variability). The total contribution from diurnal variability can be estimated accurately from high-frequency surface wind measurements using these relationships.

Evolution of Sea Surface Hydrology Along the Western Australian Margin Over the Past 450 kyr

AbstractWe collected a suite of core top samples during R/V Sonne Cruise SO257 in May 2017 along the southwestern front of the Indo‐Pacific Warm Pool (IPWP) to monitor the variability of Southern Hemisphere tropical and subtropical sea surface hydrology and to assess temperature and salinity reconstructions with data sets reflecting conditions in the post‐monsoonal season. In our core top samples, a steep increase in planktic δ18O, associated with a decrease in sea surface temperature (SST), indicates that the southwestern front of the IPWP is located between 23° and 24°S during austral fall. We additionally reconstructed SST, sea surface salinity ,and δ18O seawater (δ18Osw) over the last 450 kyr in two sediment successions located within and beyond the monsoonal rain belt. Our records show that SST was highly coherent and phase‐locked with atmospheric pCO2 during the last 450 kyr. The regional differences in the δ18Osw records reveal that the Western Australian Margin north of 15°S remained seasonally under the influence of IPWP water masses, even during glacials. The temporal variability in upper ocean hydrology along the Western Australian Margin is not directly coupled to local monsoonal precipitation, but is strongly affected by advective mixing of Indonesian Throughflow derived water masses.

Possible influence of the warm pool ITCZ on compound climate extremes during the boreal summer

In a globally changing climate, there is a growing concern for understanding and predicting compound climate extremes. However, the relationship of compound climate extremes with each other has been mostly analyzed in isolation and/or on regional scales. Little attention has been paid to their simultaneous occurrence and compound impacts worldwide. Here we demonstrate that the compound climate extremes in the Northern Hemisphere during the boreal summer are interconnected from the tropics to the Arctic. This connection originates from the interannual variations of the Indo-Pacific warm pool’s intertropical convergence zone (ITCZ). We demonstrate that the warm pool ITCZ (WPI) convection possibly influences three major teleconnection patterns (i.e. zonal, meridional, and circumglobal) where compound climate extremes occur along the wave train excited by the WPI convection. Most notably, the WPI can sufficiently explain climate variabilities in the North Atlantic region, which influences the occurrence of compound climate extremes in many parts of Europe and North America. Our findings advance the understanding of the interannual global/regional variability of climate extremes and are potentially valuable for predicting seasonal high-impact climate extremes.

The effect of the variability of wind forcing on ENSO simulation in an OGCM: case of canonical and protracted event

This study is an attempt to understand the onset and the evolution of canonical (typical length ~ 18–24 months; CE) and protracted El Niño (> greater than 3 years; PE) compared to the normal state (NS) in an ocean model. Indo-Pacific warm pool indicates higher sea surface temperature (SST) before the onset of strong CE compared to the NS and PE. The ocean model (MOM5.1.0) used in the study shows a systematic SST bias in the Indo-Pacific Ocean with warmer (cooler) temperatures in the western (eastern) Pacific during NS, CE, and PE exhibiting La Niña–like conditions. The model also exhibits deeper thermocline depth in the western equatorial Pacific Ocean during PE and CE than NS, indicating higher heat content values. Despite higher heat content in the western Pacific before the onset of El Niño, the difference in the variability of surface wind forcing during the preceding months determines the type of El Niño. The difference in surface wind forcing among the NS, PE, and CE states without altering the ocean state can modify the subsurface propagation in the equatorial Pacific Ocean. A change in longitudinal extent of upwelling Kelvin waves from western Pacific towards eastern Pacific along with the change in surface wind forcing decides the fate of El Niño. Based on the results of model experiments for 1948–2009, observed features of the recent protracted El Niño of 2014–2016 appear to be a blend of PE and CE in terms of ocean dynamics and surface wind forcing.

High-resolution marine heatwave mapping in Australasian waters using Himawari-8 SST and SSTAARS data

Marine heatwaves (MHWs) have significant ecological and economic impact at local and regional scales. Consequently, there is a pressing need to map the fine-scale temporal and spatial patterns of MHWs, for both historical and near real-time events. Satellite remote sensing of Sea Surface Temperature (SST) provides fundamental data for the mapping of MHWs. This study used high-resolution Himawari-8 SST and the Sea Surface Temperature Atlas of the Australian Regional Seas (SSTAARS) data, which have a spatial resolution of ~2 km, to map recent and near real-time MHW events in waters around Australasia. The high-resolution MHWs mapping identified two broad areas of MHW hotspots that occurred between August 2015 and February 2019. The Tropical Warm Pool region (including the Great Barrier Reef and part of the Coral Sea) was affected by strong and prolonged MHW conditions for the greater part of 2016. The unusually strong 2015–16 El Niño event was believed to be the primary underlying driver for the MHWs, and the air-sea heat flux was likely the key local process controlling the heat budget. The south-east of the study area (including Australia's south-east coast, the Tasman Sea and New Zealand's east coast) suffered severe MHWs in 2015–16, 2017–18 and 2018–19. The 2015–16 summer Bass Strait and East Tasmania MHW event was likely due to the anomalous heat transported by the East Australia Current. The air-sea heat flux, however, was likely an important local driver for the MHW events in the Tasman Sea during the 2016 autumn, the 2017–18 summer and the 2018–19 summer. The anomalous heat flux into ocean was attributed to potential underlying climate drivers including ENSO teleconnections, positive SAM and their interaction. This mapping has not only enhanced our understanding of the spatio-temporal characteristics of several previously documented MHWs, but also identified and mapped several previously undocumented MHWs. Two case studies in the Beagle Marine Park located in north-eastern Bass Strait and the Elizabeth and Middleton Reefs situated ~550 km east of the Australian coast within the Lord Howe Marine Park proved the value of Himawari-8 SST and SSTAARS data in mapping fine details of MHWs in a small area, which could not be captured by the broad-scale DOISST data. During the 2017–18 summer event, the case studies revealed much stronger MHW influence in the shallow waters east of the Beagle Marine Park where most of the important rocky reef habitats exist. During the same event, the case studies also confirmed more severe MHW impact on the Elizabeth Reef than the Middleton Reef which resulted in bleaching to selective coral species on the Elizabeth Reef. The near-real time MHWs mapping showed that both the GBR and the Coral Sea marine parks were experiencing MHW conditions in early March 2020, with most affected areas having strong MHW class.

Variation of Hourly Extreme Precipitation in the Three Gorges Reservoir Region, China, from the Observation Record

Extreme hourly precipitation is amongst the most prominent driving factors of flash floods and geological disasters. Based on the hourly precipitation data of 35 stations in the Three Gorges Reservoir Region (TGRR) from 1998 to 2020, we analyzed the spatiotemporal variation characteristics of hourly extreme precipitation indexes. The selected indicators included the frequency, intensity, period, annual maximum, trend of hourly heavy precipitation (20–50 mm/h) and hourly extreme heavy precipitation (≥50 mm/h) in the TGRR. Closely related climatic factors such as the Western Pacific Subtropical High Intensity (WPSHI) were also discussed. The results showed that in 2010–2020, the cumulative frequency of heavy precipitation magnitude between 25 and 40 mm/h slightly increased, while the corresponding frequency for magnitudes ≥50 mm/h decreased. In summer, the frequency of both heavy and extreme heavy precipitation increased in June and decreased in August, indicating a shift of extreme events to an earlier time in the flood season. The cumulative frequency of heavy precipitation in July had a period of about 7a, and that of extreme heavy precipitation had a period of 3a. The annual average intensity of heavy precipitation and extreme heavy precipitation in the TGRR was 28.9 mm/h and 61.4 mm/h per station, respectively, and both fluctuated and insignificantly decreased from 1998 to 2020. The annual maximum hourly precipitation center in the TGRR moved downstream from west to northeast. The frequency of heavy precipitation was relatively small along the main stream of the river valley. Both the frequency and total amount of heavy precipitation in southeast of the TGRR were significantly higher than those in other regions. Heavy precipitation in the majority of stations with high elevation (higher than 500 m) showed a decreasing trend. The cumulative frequency of precipitation with an intensity of 20–50 mm/h was closely correlated with the Western Hemisphere Warm Pool (WHWP) Index in February and the WPSHI Index in January, and especially, the abnormal large annual frequency (top 20%) showed strong correlation with the two indexes, implying highly predictable factors for extreme events. The frequency of precipitation intensity above 50 mm/h was correlated with the Western Pacific Warm Pool (WPWP) Area Index in January and the WPWP Intensity Index in November of last year. The research results provide a strong and refined factual basis for the assessment and prediction of extreme precipitation, and for disaster prevention and mitigation, in the TGRR.

Enhanced North Pacific subtropical gyre circulation during the late Holocene

The North Pacific Subtropical Gyre circulation redistributes heat from the Western Pacific Warm Pool towards the mid- to high-latitude North Pacific. However, the driving mechanisms of this circulation and how it changed over the Holocene remain poorly understood. Here, we present alkenone-based sea surface temperature reconstructions along the Kuroshio, California and Alaska currents that cover the past ~7,000 years. These and other paleorecords collectively demonstrate a coherent intensification of the boundary currents, and thereby the basin-scale subtropical gyre circulation, since ~3,000–4,000 years ago. Such enhanced circulation during the late Holocene appears to have resulted from a long-term southward migration of the Intertropical Convergence Zone, associated with Holocene ocean cooling. Our results imply that the North Pacific Subtropical Gyre circulation could be weakened under future global warming.

Impacts of Precipitation–Evaporation–Salinity coupling on upper ocean stratification and momentum over the tropical pacific prior to onset of the 2018 El Niño

Precipitation affects upper ocean salinity and temperature over a wide range of temporal and spatial scales. Although the effects of freshwater on the diurnal mixing and formation of barrier layer have been shown in previous studies, the impacts of freshwater from the large-scale precipitation associated with the Madden–Julian Oscillation (MJO) on the ocean from the western to central-eastern Pacific are not well understood. This study investigates the coupling of precipitation, evaporation, and salinity (P–E–S) and its impact on the upper ocean stratification and momentum on time scales from days to weeks (intraseasonal) and beyond. The Unified Wave INterface – Coupled Model (UWIN-CM) is used to simulate multiscale air–sea interactions during January–July 2018, including multiple MJO events leading up to the onset of the 2018–2019 El Niño. The atmospheric component of the UWIN-CM is constrained by reanalysis and reinitialized daily, while the ocean component evolves continuously and is fully coupled with the atmosphere. The initial and lateral boundary conditions for the atmosphere and ocean are from the ECMWF ERA5 and HYCOM global analysis, respectively. The UWIN-CM simulation captures the surface winds, rainfall, and the evolution of the upper ocean compared with in situ and satellite observations. To determine the effect of P–E–S coupling, the UWIN-CM simulation with full P–E–S coupling is compared with a model experiment in which the P–E input to the ocean is set to zero (NoPE). The difference between the two increases with time and becomes particularly large during the 3rd MJO event in April 2018. Two key results emerge. First, heavy precipitation and strong winds from the MJO events result in a locally fresher, warmer western Pacific warm pool with a shallower mixed layer and thicker barrier layer. Second, the fresh, warm water mass extends into the central-eastern Pacific from mid-April to late May with up to 0.5 m s−1 eastward currents. A well-defined salinity/density front, propagating eastward at about 0.3 m s−1, is located at the leading edge of the warm pool. The P–E–S coupling plays an important role in the eastward extension of the warm pool prior to the onset of El Niño in July 2018.

A Decade of Eastern Tropical Indian Ocean Observation Network (TIOON)

AbstractAs an important part of the Indo-Pacific warm pool, the Indian Ocean has great significance for research on the Asian monsoon system and global climate change. From the 1960s onward, several international and regional programs have led to important new insights into the Indian Ocean. The eastern Tropical Indian Ocean Observation Network (TIOON) was established in 2010. The TIOON consists of two parts: large-scope observations and moored measurements. Large-scope observations are performed by the eastern Tropical Indian Ocean Comprehensive Experiment Cruise (TIO-CEC). Moored measurements are executed by the TIOON mooring array and the hydrological meteorological buoy. By 2019, 10 successful TIO-CEC voyages had been accomplished, making this mission the most comprehensive scientific investigation in China. The TIO-CEC voyages have collected temperature/salinity profiles, GPS radiosonde profiles, and other observations in the Indian Ocean. To supplement the existing buoy array in the Indian Ocean, an enhanced TIOON mooring array consisting of eight subthermocline acoustic Doppler current profiler (ADCP) moorings, was established since 2013. The TIOON mooring equipped with both upward-looking and downward-looking WHLS75K ADCP provide valuable current monitoring information to depth of 1,000 m in the Indian Ocean. To improve air–sea interaction monitoring, two real-time hydrological–meteorological buoys were deployed in 2019 and 2020 in the equatorial Indian Ocean. A better understanding of the Indian Ocean requires continuous and long-term observations. The TIOON program and other aspiring field investigation programs will be promoted in the future.

Deglacial trends in Indo-Pacific warm pool hydroclimate in an isotope-enabled Earth system model and implications for isotope-based paleoclimate reconstructions

The Indo-Pacific Warm Pool (IPWP) is the largest source of atmospheric heating and moisture on Earth, energizing the global moisture and energy budgets and controlling global ocean-atmosphere circulation. However, the mechanisms driving orbital-scale changes in hydroclimate and proxy records of precipitation isotopic composition remain poorly known. Here, we use the isotope-enabled Transient Climate Evolution (iTRACE) experiment to investigate long-term hydroclimate and precipitation isotope changes in the IPWP during the last deglaciation, and their response to different climate forcings (sea level and ice sheet, greenhouse gases, orbital forcing, and meltwater flux). The simulations suggest land-sea configuration as the main factor driving long-term hydroclimate and precipitation isotope changes. The exposure of NW Australian shelf (before 14 ka) excited Bjerknes feedbacks across the equatorial Indian Ocean (IO), leading to a warm/wet western IO and cool/dry eastern IO dipole pattern, with overall drying and more enriched precipitation isotopic compositions over the Maritime Continent. However, the exposed Sunda Shelf and the main body of Sahul Shelf (before 12 ka) experienced locally dry conditions but more depleted precipitation isotopic compositions. Greenhouse gases and orbital forcing contribute to a weaker hydroclimate dipole pattern that opposes the effects of NW Australian shelf exposure. Different regions within the IPWP have different simulated sensitivities to these forcings. The heterogeneous responses of precipitation to different forcings across the Maritime Continent and location-dependent relationships between precipitation and its isotopic composition result from a variety of regional climatological processes and may explain the heterogeneity of isotopic records of hydroclimate around the IPWP.

A Climatological Study of the Mechanisms Controlling the Seasonal Meridional Migration of the Atlantic Warm Pool in an OGCM

The tropical Atlantic Warm Pool is one of the main drivers of the marine intertropical convergence zone and the associated coastal Northeast Brazilian and West-African monsoons. Its meridional displacement is driven by the solar cycle, modulated by the atmosphere and ocean interactions, whose nature and respective proportions are still poorly understood. This paper presents a climatological study of the upper ocean and lower atmosphere contributions to the warm pool seasonal migration, using an Ocean General Circulation Model (OGCM). First, we provide quantitative, albeit simple, pieces of evidence on how the large amplitude of migration in the west, compared to the east, is mainly due to the strong east–west contrast of the background meridional SST gradient intensities, which is maintained by equatorial and eastern tropical upwellings. Our main results consist first in identifying a diagnostic equation for the migration speed of the two meridional boundary isotherms of the Warm Pool, expressed in terms of the various mixed-layer heat fluxes. We then evidence and quantify how, in general, the migration is forced by air–sea fluxes, and damped by ocean circulation. However, remarkable controls by the ocean are identified in some specific regions. In particular, in the northwestern part of the Warm Pool, characterized by a large temperature inversion area, the boreal spring northward movement speed depends on the restitution of the solar heating by the thermocline. Additionally, over the southern part of the Warm Pool, our study quantifies the key role of the equatorial upwelling, which, depending on the longitude, significantly accelerates or slows down the summer poleward migration.

Tropical Western Pacific Hydrology During the Last 6,000 Years Based on Wildfire Charcoal Records From Borneo

AbstractThe forcing of multicentennial climate variability in the Western Pacific Warm Pool (WPWP) region is not fully understood. Here, we generated 6‐kyr‐long continuous charcoal records from five peatlands in Borneo. Every several hundred years, peaks of charcoal influx were identified in the composite record, indicating that the peatlands repeatedly experienced dry conditions and wildfires. Major fire events were identified at ∼5.4, 4.7, 4.4, 3.7, 3.2, 2.7, 2.4, 2.2, 1.7, 1.1, 0.6, and 0.3 ka. Most of these coincided with the maxima of Borneo speleothem δ18O and occurred in the high solar activity periods following the solar minima. This suggests that the higher solar activity decreased rainfall, increasing dryness and wildfire frequency. This result challenges the hypothesis that high solar activity intensifies atmospheric convection in the WPWP area.

Decadal and Multidecadal Variability in ERSSTv5 Global SST during 1879–2018

AbstractDecadal and multidecadal variability in the ERSSTv5 global SST dataset are studied in terms of implicit fast (noise) and slow (signal) processes that affect variability on decadal time scales. Using a new method that better estimates the fast, or noise, component of decadal variability, estimates of the modes of variability in the slow component are possible. The fast component of decadal variability has a leading fast mode, which explains 62% of the variance, and it is shown that this fast variability, or decadal climate noise, is well represented by any of the indices associated with intradecadal or interannual variability in the tropical Pacific Ocean. Three slow modes are identified, representing 69% of the slow multidecadal variance, after removing the radiative forcing trend. These modes are shown to be related to variability in the Atlantic multidecadal oscillation (AMO) and SST multidecadal variability in the central western Pacific and in the Indian Ocean gyre region, respectively. The first and third slow modes represent two phases of a propagating mode with a period of about 80 years. The second slow mode represents multidecadal variability of the western Pacific warm pool, which is less robust than the other two and shown to be weakly related to the AMO with a lag of about 30 years; fast variability in this region is related to the leading fast mode. Three regions of significant slow variability are identified south of Australia, south of Africa, and near the Drake Passage in association with the Antarctic Circumpolar Current.

Central Asian Precipitation Shaped by the Tropical Pacific Decadal Variability and the Atlantic Multidecadal Variability

AbstractKnown as one of the largest semiarid to arid regions in the world, central Asia and its economy and ecosystem are highly sensitivity to the changes in precipitation. The observed precipitation and related hydrographic characteristics have exhibited robust decadal variations in the past decades, but the reason remains unknown. Using the pacemaker experiments of the Community Earth System Model (CESM1.2), we find that the tropical Pacific decadal variability (TPDV) and the Atlantic multidecadal variability (AMV) are the main drivers of the interdecadal variations in central Asian precipitation during 1955–2004. Both the decadal-scale warming of the tropical Pacific and North Atlantic are favorable for wetter conditions over central Asia. The positive TPDV is accompanied with high sea level pressure (SLP) over the Indo–western Pacific warm pool. Southwesterly winds along the northwestern flank of the high SLP can transport more moisture to southeastern central Asia. The warm AMV can excite a circumglobal teleconnection (CGT) pattern. A trough node of the CGT to the west of central Asia drives an anomalous ascending motion and increased precipitation over this region. The results based on the CESM model are further demonstrated by the pacemaker experiments of MRI-ESM2-0. Based on the observational TPDV and AMV indices, we reasonably reconstruct the historical precipitation over central Asia. Our results provide hints for the decadal prediction of precipitation over central Asia.

A Common Base Mode of Asian Summer Monsoon Variability across Timescales

AbstractThe summer intraseasonal oscillation (ISO) is characterized by a northward-moving rainband in the Indo–western Pacific warm pool region. The physical origin of the ISO is not fully understood, as it is masked by strong interaction of convection and circulation. This study examines intraseasonal to interannual variability during June–August over the Indo–western Pacific warm pool region. The results show that the tropical northwest Pacific anomalous anticyclone (NWP-AAC) is a fundamental mode on both intraseasonal and interannual time scales, destabilized by the monsoon mean state, specifically through barotropic energy conversion and convective feedback in the low-level confluence between the monsoon westerlies and easterly trade winds. On the interannual time scale, the NWP-AAC shows a biennial tendency, reversing phase from the summer of El Niño to the summer that follows; the AAC in post–El Niño summer is excited indirectly through sea surface temperature anomalies in the Indo–NWP. On the intraseasonal time scale, the column-integrated moisture advection causes the NWP-AAC-related convection to propagate northward. Our results provide a unifying view of multiscale Asian summer monsoon variability, with important implications for subseasonal to seasonal prediction.

An Overview of Oligocene to Recent Sediments of the Western Pacific Warm Pool (WPWP) (International Ocean Discovery Program-IODP Exp. 363) Using Warm and Cool Foraminiferal Species

We use the excellent sediment recovery of International Ocean Discovery Program (IODP) Exp. 363, in the Western Pacific Warm Pool (WPWP) to assess down-core variations in the abundance of warm versus cool benthic foraminiferal species through a warm benthic foraminifers (WBF) curve. The total percentage of the “warm” shallower species group (Laticarinina pauperata, Cibicidoides kullenbergi, C. robertsonianus, Cibicidoides sp., Hoeglundina elegans, and Bulimina aculeata) and of the “cool” species group from deep waters (Pyrgo murrhina, Planulina wuellerstorfi, Uvigerina peregrina, and Globobulimina hoeglundi, Hopkinsina pacifica) at all sites is used to assess paleo temporal and spatial variations in preservation and marine temperature. Our study sites span water depths ranging from 875 m to 3421 m and our results indicate that well-preserved living and fossil foraminifera characterize mudline and core sediments at all water depths attesting the wide environmental tolerance of these species to temperature and pressure. Using magneto-and biostratigraphy datum, these sediments are of Oligocene age. Our low-resolution study showed that with the exception of core 1486B which the linear tendency of warmer species is toward cool sediments in old times, all of them show that older sediments indicate warmer periods than today, which is expected from Miocene to Recent. Our results provide evidence for the preservation potential of deeply buried sediments, which has implications on climate reconstructions based on the population dynamics of calcareous benthic foraminifera.