Western Tropical Pacific Ocean Research Articles

Dynamic features of near-inertial oscillations in the Northwestern Pacific derived from mooring observations from 2015 to 2018

Near-inertial oscillation is an important physical process transferring surface wind energy into deep ocean. We investigated the near-inertial kinetic energy (NIKE) variability using acoustic Doppler current profiler measurements from a mooring array deployed in the tropical western Pacific Ocean along 130°E at 8.5°N, 11°N, 12.6°N, 15°N, and 17.5°N from September 2015 to January 2018. Spatial features, decay timescales, and significant seasonal variability of the observed NIKE were described. At the mooring sites of 17.5°N, 15°N, and 12.6°N, the NIKE peaks occurred in boreal autumn and the NIKE troughs were observed in boreal spring. By contrast, the NIKE at 11°N and 8.5°N showed peaks in winter and troughs in summer. Tropical cyclones and strong wind events played an important role in the emergence of high-NIKE events and explained the seasonality and latitudinal characteristics of the observed NIKE.

Impacts of the Indian Ocean Dipole on Sea Level and Gyre Circulation of the Western Tropical Pacific Ocean

AbstractInterannual variabilities of sea level and upper-ocean gyre circulation of the western tropical Pacific Ocean (WTPO) have been predominantly attributed to El Niño–Southern Oscillation (ENSO). The results of the present study put forward important modulation effects by the Indian Ocean dipole (IOD) mode. The observed sea level in the WTPO shows significant instantaneous and lagged correlations (around −0.60 and 0.40, respectively) with the IOD mode index (DMI). A composite of 14 “independent” IOD events for 1958–2017 shows negative sea level anomalies (SLAs) of 4–7 cm in the WTPO during positive IOD events and positive SLAs of 6–8 cm in the following year that are opposite in sign to the El Niño effect. The IOD impacts are reproduced by large-ensemble simulations of a climate model that generate respectively 430 and 519 positive and negative independent IOD events. A positive IOD induces westerly winds over the western and central tropical Pacific and causes negative SLAs through Ekman upwelling, and it facilitates the establishment of a La Niña condition in the following year that involves enhanced Pacific trade winds and causes positive SLAs in the WTPO. Ocean model experiments confirm that the IOD affects the WTPO sea level mainly through modulating the tropical Pacific winds. Variability of the Indonesian Throughflow (ITF) induced by IOD winds has a relatively weak effect on the WTPO. The IOD’s impacts on the major upper-ocean currents are also considerable, causing anomalies of 1–4 Sv (1 Sv ≡ 106 m3 s−1) in the South Equatorial Current (SEC) and North Equatorial Countercurrent (NECC) volume transports.

Sensitivity of Benthic Foraminifera to Carbon Flux in the Western Tropical Pacific Ocean

ABSTRACT We investigated benthic foraminiferal species as tracers for carbon export flux in the Indo-Pacific warm pool (International Ocean Discovery Program Expedition 363). In core tops, the distribution of lower bathyal and upper abyssal species can be separated into two distinct groups. Foraminifera belonging to the high carbon flux (>3.5 g C m−2 year−1), ‘warm’ (>3.5°C) group are Bolivina robusta, Bulimina aculeata, Globobulimina pacifica, Hoeglundina elegans, Laticarinina pauperata, and Cibicidoides pachyderma. The lower carbon flux, ‘cold’ group includes Oridorsalis umbontus, Uvigerina bifurcata, and Planulina wuellerstorfi. An index based on the percent ‘warm’ assemblage with respect to the total ‘warm’ plus ‘cold’ species in core-top samples correlates significantly with carbon flux (r = 0.91, P = 0.0007) and modern bottom water temperatures (r = 0.94, P = 0.0002). When applied to down-core species abundances based on core catchers spanning the late Miocene through Pleistocene, we observed that sites from the northwestern Australian margin show marked changes in the ‘warm’ index, suggesting a large paleoenvironmental signal in this dynamic region. At Papua New Guinea, down-core abundances of the ‘warm’ group are highest (>80%), consistent with high organic matter input via the Sepik River. At the deeper of the two sites, down-slope movement in this tectonically unstable region may have contributed to organic matter input. At Manus Basin, the ‘warm’ species abundances are also relatively high and covary with the percent abundance of Uvigerina proboscidea, providing further evidence for the use of this index as a tracer for carbon flux. Overall, this study contributes evidence for the relationship between benthic foraminiferal assemblages and carbon export flux in the Indo-Pacific warm pool, suggesting that the ‘warm’ index can be used as a tracer for paleoproductivity.

Intermediate Intraseasonal Variability in the Western Tropical Pacific Ocean: Meridional Distribution of Equatorial Rossby Waves Influenced by a Tilted Boundary

AbstractIntermediate-depth intraseasonal variability (ISV) at a 20–90-day period, as detected in velocity measurements from seven subsurface moorings in the tropical western Pacific, is interpreted in terms of equatorial Rossby waves. The moorings were deployed between 0° and 7.5°N along 142°E from September 2014 to October 2015. The strongest ISV energy at 1200 m occurs at 4.5°N. Peak energy at 4.5°N is also seen in an eddy-resolving global circulation model. An analysis of the model output identifies the source of the ISV as short equatorial Rossby waves with westward phase speed but southeastward and downward group velocity. Additionally, it is shown that a superposition of first three baroclinic modes is required to represent the ISV energy propagation. Further analysis using a 1.5-layer shallow water model suggests that the first meridional mode Rossby wave accounts for the specific meridional distribution of ISV in the western Pacific. The same model suggests that the tilted coastlines of Irian Jaya and Papua New Guinea, which lie to the south of the moorings, shift the location of the northern peak of meridional velocity oscillation from 3°N to near 4.5°N. The tilt of this boundary with respect to a purely zonal alignment therefore needs to be taken into account to explain this meridional shift of the peak. Calculation of the barotropic conversion rate indicates that the intraseasonal kinetic energy below 1000 m can be transferred into the mean flows, suggesting a possible forcing mechanism for intermediate-depth zonal jets.

Morphology and molecular phylogeny of three new deep-sea species of Chrysogorgia (Cnidaria, Octocorallia) from seamounts in the tropical Western Pacific Ocean.

Three new species of Chrysogorgia were discovered from seamounts in the tropical Western Pacific Ocean. Chrysogorgia dendritica sp. nov. and Chrysogorgia fragilis sp. nov. were collected from the Kocebu Guyot of the Magellan Seamount chain with the water depth of 1,821 m and 1,279–1,321 m, respectively, and Chrysogorgia gracilis sp. nov. was collected from a seamount adjacent to the Mariana Trench with the water depth of 298 m. They all belong to the Chrysogorgia “group A, Spiculosae” with rods distributed in body wall and tentacles, and differ from all congeners except C. abludo Pante & Watling, 2012 by having a tree-shaped colony (vs. bottlebrush-shaped, planar or biflabellate). Chrysogorgia dendritica sp. nov. is unique in having a monopodial stem, the 1/3L branching sequence and the amoeba-shaped sclerites (sclerites branched toward to many directions) at the body bases of polyps. Chrysogorgia fragilis sp. nov. is most similar to C. abludo, but differs by the regular 1/3L branching sequence and elongate flat scales in coenenchyme. Chrysogorgia gracilis sp. nov. is easily separated from congeners by the 1/4L branching sequence, the absence of sclerites in the basal body wall, and the very sparse scales in coenenchyme. Based on the phylogenetic and genetic distance analyses of mtMutS gene, all the available Chrysogorgia species were separated into two main groups: one includes C. binata, C. cf. stellata and C. chryseis, which have two or more fans emerging from a short main stem (bi- or multi-flabellate colony); the other one includes all the species with the branching patterns as a single ascending spiral (clockwise or counterclockwise, bottlebrush-shaped colony), a fan (planar colony) and a bush of branches perched on top of a long straight stem (tree-shaped colony). Additionally, the tree-shaped colony represents a new branching pattern in Chrysogorgia, and therefore we extend the generic diagnosis.

Virioplankton distribution in the tropical western Pacific Ocean in the vicinity of a seamount.

The shallow Caroline Seamount is located in the tropical western Pacific Ocean. Its summit is 57 m below the surface and penetrates the euphotic zone. Therefore, it is ideal for the study of the influence of seamount on plankton distribution. Here, virioplankton abundance and distribution were investigated by flow cytometry (FCM) in the Caroline Seamount in August and September 2017. The total abundance of virus‐like particles (VLP) was in the range of 0.64 × 106–18.77 × 106 particles/ml and the average was 5.37 ± 3.75 × 106 particles/ml. Three to four distinct viral subclusters with similar side scatter but different green fluorescence intensities were identified. Above the deep chlorophyll maximum (DCM), two medium fluorescence virus (MFV) subclusters were discriminated. Between the DCM and the deeper layers, only one MFV subcluster was resolved. In general, low fluorescence viruses (LFV) comprised the most abundant subclusters. In the 75–150 m water column, however, the MFV abundance was higher than the LFV abundance. High fluorescence viruses (HFV) constituted the least abundant subcluster throughout the entire water column. Virioplankton abundance was significantly enhanced at the seamount stations. Environmental factors including water temperature and nitrate concentration were the most correlated with the variation in virioplankton abundance at the seamount stations. Interactions between shallow seamounts and local currents can support large virus standing stocks, causing a so‐called indirect “seamount effect” on the virioplankton.

Depth‐Resolved Photochemical Lability of Dissolved Organic Matter in the Western Tropical Pacific Ocean

AbstractWater samples collected from various depths of the offshore South China and Philippine Seas were exposed to solar‐simulated radiation. Photomineralization of dissolved organic carbon (DOC) and photobleaching of chromophoric dissolved organic matter (CDOM) and its humic‐like fluorescent constituent (FDOM) were observed in all samples. Protein‐like FDOM was, however, either photo‐decomposed or photo‐produced, depending on the sample's depth. The photobleaching of CDOM and humic‐like FDOM was much faster in deep than in shallow water samples while photomineralization displayed a weaker vertical zonation. Prior‐irradiated deep water inoculated with surface‐water bacteria showed enhanced microbial DOC removal but CDOM production. Results from this study suggest that deep‐ocean CDOM and FDOM can barely survive photobleaching during one ocean mixing cycle, but photochemical turnover of the bio‐refractory deep DOC is considerably longer than its average radiocarbon age. Coupled photochemical‐microbial processes can not only remove part of the bio‐refractory deep DOM but also regenerate part of it during ocean overturning circulation.

Molecular diversity and spatial distribution of benthic foraminifera of the seamounts and adjacent abyssal plains in the tropical Western Pacific Ocean

Abstract Deep-sea systems are among the least understood ecosystems on Earth. Forming large proportions of deep-sea benthic communities, benthic foraminifera play important roles; however, their molecular diversity and distribution patterns in the deep-sea seamounts and abyssal plains of the tropical Western Pacific Ocean are poorly known. In this study, using next-generation sequencing, we investigated the molecular diversities and community compositions of benthic foraminifera of the tropical Western Pacific Ocean seamounts and adjacent abyssal plains, with water depth ranging between 263 and 5900 m. A region of the small subunit (SSU) rRNA gene (about 300 bp) was amplified with the foraminiferal-specific primers (s14F3-s17), and the PR2 database was selected as the reference database for sequence assignment. Monothalamiids were assigned most of the 1399 operational taxonomic units (OTUs) obtained. The foraminifera genetic novelty, with an average similarity of 91.33% to reference sequences in the deep sea, was extremely high. Comparing the different habitats, the foraminiferal diversities of the seamounts investigated were both higher than those of their adjacent abyssal plains. Clustering of results showed the foraminifera community composition of the Y3 Seamount to be most like that of its adjacent abyssal plain. Our study indicates that the foraminiferal diversity distribution patterns and community structures between the two seamounts differ, yet they resemble those of their adjacent abyssal plain areas. We infer the possibilities that seamounts are diversity hotspots, possessing particularly high benthic species richness, and that foraminifera spread from them to the adjacent abyssal plains. The results also revealed variations in foraminifera at different water depths and temperatures in seamounts and adjacent abyssal plains. Additionally, an extremely high genetic novelty of foraminifera was observed in these habitats, which points to a hotspot for the discovery of new foraminifera species in deep-sea systems.

Decadal Variation of the Kuroshio Intrusion Into the South China Sea During 1992–2016

AbstractDecadal variation of the Kuroshio intrusion into the South China Sea (KIS) during 1992–2016 is investigated using high‐resolution Hybrid Coordinate Ocean Model reanalysis data. The results indicate that both the yearly occurrence frequency of the Kuroshio intrusion and Luzon Strait transport show significant decadal variations and were generally high in approximately 1993, 2003–2004, and 2015–2016 and low in 1998–1999 and 2009–2010. The KIS is closely connected to the decadal oscillation of the meridional sea surface height anomaly (SSHA) gradient west of the Luzon Strait (LS). Further analyses reveal that large‐scale wind forcing in the western tropical Pacific Ocean is a major factor regulating decadal variation of the KIS, while local wind forcing has little impact. Cyclonic wind stress curl anomalies (WSCAs) over the western tropical Pacific Ocean induce negative SSHAs that propagate westward to the eastern coast of the Philippines via upwelling Rossby waves, weakening the Kuroshio transport and consequently enhancing the KIS. In addition, signals of negative SSHAs can propagate clockwise along the Philippines as coastal Kelvin waves and hence strengthen the northward SSHA gradient west of the LS, eventually leading to a stronger KIS. The situation is reversed when anticyclonic WSCAs prevail in the western tropical Pacific Ocean. The Philippines‐Taiwan Oscillation is linked to decadal WSCAs over the western tropical Pacific Ocean and hence regulates the decadal KIS.

Mixed-layer Heat Budget in Western and Eastern Tropical Pacific Ocean during El Niño Event in 2015/2016

Temporal variation of mixed-layer heat budget at two contrasting locations, namely, western Pacific (warm water pool) and eastern Pacific (cold tongue) during the extreme El Niño phenomenon in 2015/2016 is evaluated. Oceanic and atmospheric datasets, including sea surface temperature (SST), wind stress, shortwave radiation (SWR), longwave radiation, latent heat flux (LHF), and sensible heat flux are analyzed. A slight warming occurred in the eastern tropical Pacific associated with a positive SST anomaly, which reflected the weakening or reversal of the trade winds. Meanwhile, the western tropical Pacific exhibited a cooling tendency during the development phase of El Niño. Analysis of the mixed-layer heat budget shows that the net heat flux due to SWR and LHF significantly contributes to the warming of the eastern tropical Pacific. The contribution from horizontal advection was extremely small on both sides. The analysis shows that the residual term significantly contributes to cooling (warming) tendency observed in the western (eastern) tropical Pacific. This condition may suggest that residual process due to entrainment and diffusivity played an important role in the evolution of cooling (warming) process in the western (eastern) tropical Pacific.

Spatial and temporal differences in the reproductive traits of skipjack tuna Katsuwonus pelamis between the subtropical and temperate western Pacific Ocean

The reproductive traits of skipjack tuna (Katsuwonus pelamis) in the subtropical (10–25°N) and temperate (25–42°N) western Pacific Ocean were investigated to examine the geographical differences in spawning potential. In total, 91% of spawning capable female specimens appeared in waters with sea surface temperatures (SSTs) of over 24 °C. The length of the spawning season for females varied among sampling areas according to the seasonal fluctuations in SSTs, with shorter spawning seasons in the high latitudinal areas (north of 25°N). Mature males were observed during almost all sampling months in all sampling areas. The total spawning fraction during the spawning season in each sampling area ranged from 0.23 to 0.46. Seasonal fluctuations in the spawning fraction were observed in the temperate western Pacific Ocean. The mean relative batch fecundity differed significantly between sampling areas (p < 0.05) and ranged from 67.2 to 106.1 oocytes per gram of body less the ovary weight. The estimated mean relative batch fecundity tended to be lower in the temperate western Pacific Ocean than in the subtropical western Pacific Ocean and the tropical western and central Pacific Ocean. The reproductive traits observed in the present study indicated that the spawning grounds of skipjack tuna in the western Pacific Ocean were expanded in the temperate western Pacific Ocean in the boreal summer, and that the spawning potential per individual in these areas was inferior to those in the subtropical western Pacific Ocean and tropical western and central Pacific Ocean.

Role of Sea Surface Salinity Feedback in MJO Predictability: A Study with CFSv2

Abstract While previous studies suggested that salinity could feed back onto MJO variability via modulating upper ocean stratification and further on SST, there is no direct evidence yet proving (or disproving) the importance of this feedback in MJO evolution and its predictability. This study is an initial attempt to quantify the role of SSS feedback on MJO predictability, based on a “perfect model” framework with the CFSv2. Specifically, the SSS feedback is isolated by nudging model SSS to climatological states during forecasts. For comparison, two more experiments were done, one as a benchmark experiment by estimating MJO predictability in CFSv2 and another one for estimating the role of SST feedback. Analyses of these experiments indicate that SSS feedback exerts negligible influences on MJO predictability within the constraints of the model, in contrast to significant impacts from SST feedback. Further analysis showed that a lack of SSS influence in MJO predictability can be attributed to marginal changes in SST associated with the SSS nudging. However, there is a caveat to the conclusion about SSS feedback. Because the barrier layer (BL) acts as a “bridge” for possible SSS influences on SST over the tropical Indian and western Pacific oceans, its simulation in CFSv2 is further explored. Analyses indicate that, in spite of realistic simulations of the MJO and intraseasonal SSS variability in CFSv2, significant BL simulation biases are present in the tropical oceans, including too thin a climatological thickness, too small intraseasonal variations, and an unrealistic intraseasonal BL–SST relationship. Thus, our predictability experiments cannot reject the hypothesis that SSS does play a role in MJO predictability; it is possible that biases in CFSv2 influence its ability to capture such signals.

RETRACTED: Close Kin Proximity in Yellowfin Tuna (Thunnus albacares) as a Driver of Population Genetic Structure in the Tropical Western and Central Pacific Ocean

Recent studies argue for the presence of genetic population structure in yellowfin tuna (Thunnus albacares) in all oceans. However, the persistence of family groups has never been considered a viable mechanism of structure, nor has it been measured. We analyzed genetic similarity among 280 yellowfin tunas from seven population samples collected in the Western and Central Pacific Ocean (WCPO) using single nucleotide polymorphisms, and found population structure that was significantly explained by the presence of 96 individuals involved in 332 half or full sib dyads. We found significantly higher mean and median relatedness between individuals from the same sample groups, compared to individuals from different groups; and high relatedness between individuals caught at the same fish-aggregating device (FAD) than between those caught in the wider exclusive economic zone (EEZ). Alternatively, the EEZ of the Federated States of Micronesia may harbor exceptionally large numbers of close kin. We conclude that yellowfin directly school with related individuals through their first year, and at least demonstrate tightly overlapping regional fidelity as adults. These results may explain, to some extent, the patterns of population genetic structure recently observed in yellowfin tuna.

Environmental characteristics in three seamount areas of the Tropical Western Pacific Ocean: Focusing on nutrients

Nutrients distribution and influencing factors in three seamount areas named Y3, M2 and C4 of the Tropical Western Pacific Ocean (TWPO) were investigated. Nutrients showed obvious uplifts around the three seamounts, consistent with the uplifts of isotherms and isohalines, indicating the existence of a bottom-up process of nutrients. Meanwhile, compared with the stations away from seamount and the reference stations in the TWPO, nutrients concentrations around seamount were much higher. Among the three seamounts, the average nutrients concentrations were highest in Y3, while they were lowest in C4. Moreover, compared with the obvious nitrogen limitation in Y3 and M2, the N:P (13.5:1) and Si:N (6.1:1) were closed to the Redfield ratio. The current-seamount interaction was the determining influencing factor on nutrients distribution, causing the hydrology dynamic changes such as uplifts and Taylor column. Meanwhile, T and S also affected nutrients distribution, especially nutrients and T showed significant negative correlations.

Global multivariate point pattern models for rain type occurrence

We seek statistical methods to study the occurrence of multiple rain types observed by satellite on a global scale. The main scientific interests are to relate rainfall occurrence with various atmospheric state variables and to study the dependence between the occurrences of multiple types of rainfall (e.g. short-lived and intense vs. long-lived and weak; the heights of the rain clouds are also considered). Commonly in point process model literature, the spatial domain is assumed to be a small, and thus planar domain. We consider the log-Gaussian Cox Process (LGCP) models on the surface of a sphere and take advantage of cross-covariance models for spatial processes on a global scale to model the stochastic intensity function of the LGCP models. We present analysis results for rainfall observations from the TRMM satellite and atmospheric state variables from MERRA-2 reanalysis data over the tropical Eastern and Western Pacific Ocean, as well as over the entire tropical and subtropical ocean regions. Statistical inference is done through Monte Carlo likelihood approximation for LGCP models. We employ covariance approximation to deal with massive data.

A Coupled Global Atmosphere-Ocean Model for Air-Sea Exchange of Mercury: Insights into Wet Deposition and Atmospheric Redox Chemistry.

Air-sea exchange of mercury (Hg) is the largest flux between Earth system reservoirs. Global models simulate air-sea exchange based either on an atmospheric or ocean model simulation and treat the other media as a boundary condition. Here we develop a new modeling capability (NJUCPL) that couples GEOS-Chem (atmospheric model) and MITgcm (ocean model) at the native hourly model time step. The coupled model is evaluated against high-frequency simultaneous measurements of elemental mercury (Hg0) in both the atmosphere and surface ocean obtained during five published cruises in the Atlantic, Pacific, and Southern Oceans. Results indicate that the calculated global Hg net evasion flux is 12% higher for the online model than the offline model. We find that the coupled online model captures the spatial pattern of the observations; specifically, it improves the representation of peak seawater Hg0 (Hg0aq) concentration associated with enhanced precipitation in the intertropical convergence zone in both the Atlantic and the Pacific Oceans. We investigate the causes of the observed Hg0aq peaks with two sensitivity simulations and show that the high Hg0aq concentrations are associated with elevated convective cloud mass flux and bromine concentrations in the tropical upper troposphere. Observations of elevated Hg0aq concentrations in the western tropical Pacific Ocean merit further study involving BrO vertical distribution and cloud resolving models.

Archaeal community structure in sediments from a seamount in the Mariana Volcanic Arc

Seamounts are subsurface mountains in the ocean. Examination of the abundance and distribution of Archaea in seamount ecosystems may provide a better understanding of their ecological functions. Most studies of marine archaeal assemblages in seamount area have focused on hydrothermal vents or ferromanganese crusts. We investigated the archaeal communities from a seamount of the Mariana Volcanic Arc, in the tropical western Pacific Ocean by using high-throughput sequencing. Thaumarchaeota was dominant in the sediments of all sample stations. Community diversity and species richness were greatest at stations near the top of the seamount, and lowest at the deepest station. One sample station on the steep southeast slope that faced the Yap-Mariana trench had a unique composition of Archaea. In summary, depth has an important influence on archaeal community structure, and the geographic properties and sediment characteristics may explain the unique distribution patterns of Archaea in this seamount. This study provides a foundation for future research on Archaea in seamounts.

Climatic characteristics of East Asian tropical monsoon depressions

The climatic characteristics of 260 East Asian tropical monsoon depressions (EAMDs) are investigated using the ERA-Interim reanalysis dataset and a tracking dataset of global monsoon low-pressure systems. Most EAMDs form over the South China Sea (SCS) and the western tropical Pacific Ocean in July–October and have an average lifetime of 10 days. The vertical structures of EAMDs are usually upright or tilt slightly westward with height. The warm-over-cold thermal structure is a distinctive characteristic of EAMDs and two potential vorticity (PV) centers are related to the warm core in the upper level and the specific humidity center in the lower level, respectively. We divided the EAMDs into four groups: eastward-moving, westward-moving, turning, and northwestward-moving EAMDs. Most of the eastward-moving EAMDs form over the SCS in May and June, whereas the westward-moving EAMDs form over both the SCS and the western Pacific Ocean in July–October. The turning and northwestward-moving EAMDs are mainly generated over the western Pacific Ocean and have longer lifetimes. The structures of the eastward-moving and turning EAMDs show common characteristics in each stage. Their vertical structures change from upright in the developing and peak stages to northeast tilting with height in the attenuating stage, especially for the specific humidity. By contrast, the structures of westward- and northwestward-moving EAMDs show little change during their lifetime. They are symmetrical relative to the vertical axis of the EAMDs over their whole lifetime and only vary in strength.

Detection and attribution of upper-tropospheric warming over the tropical western Pacific

Both model and satellite observations suggest that the warming in the tropical upper troposphere is more pronounced than in the lower troposphere. This study describes the tropical upper-tropospheric-warming (UTW) in the ECMWF Reanalysis Interim dataset, and reveals radiative and non-radiative processes contributing to the long-term change in UTW. The maximum intensity of the UTW is found between 250 and 400 hPa with a warming rate about 3K/decade over the tropical Atlantic and western Pacific oceans. The warming is more prominent from boreal winter to early spring than the rest of the year. In the tropical western Pacific, where the warmest ocean water resides, the long-term change in the annual mean UTW is mainly contributed by the variations of atmospheric deep convection and the oceanic dynamical process and heat storage (Ocn) with the latter playing a more important role. The UTW caused by the deep convection is mainly through the latent heat release associated with enhanced upward motion and condensation. The UTW related to Ocn is mainly a result of enhanced upward long-wave radiation from the ocean surface due to the large heat storage in the thermocline. The variations in Ocn mainly contribute to the UTW from winter to early spring, while the changes in deep convection play a more essential role from summer to fall. Also shown by the differences in deep convection, a strengthening of the Walker circulation since the early 2000s is also found in the reanalysis, which is most apparent in boreal summer and fall over the tropical Pacific, facilitating the deep convection to dominate.

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

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