Western Equatorial Pacific Ocean Research Articles

Seasonality of the intraseasonal variability in the upper equatorial western Pacific Ocean currents

Seasonality of the intraseasonal variability in the upper equatorial western Pacific Ocean currents

Different responses of plankton community to mesoscale eddies in the western equatorial Pacific Ocean

The western equatorial Pacific Ocean exhibits a complex pattern of currents and mesoscale eddies. However, the impacts of dynamic hydrological systems on plankton community remain uncertain due to the low resolution of field observation. In this study, we conducted a comparative study involving regional high-resolution field observations, satellite observations, and the numerical model. Our aim was to clarify the spatial distribution characteristics and underlying factors influencing phytoplankton communities in the western equatorial Pacific Ocean under different currents and mesoscale eddies. The combination of field observations and numerical model suggested that the Mindanao Eddy (cyclonic eddy) exhibited a significantly higher total chlorophyll a concentration than the North Equatorial Current, and the contributions of diatom increased as well. In the subtropical gyre, although the intensity of a transient cyclonic eddy was weak, it still lifted the subsurface phytoplankton up and increased their chlorophyll a concentration. In the Halmahera Eddy (anti-cyclonic eddy), however, Prochlorococcus and haptophytes were the dominant groups, with lower total chlorophyll a compared to the surrounding North Equatorial Counter Current. Generally, the distributions of these phytoplankton communities were primarily influenced by variations in temperature and nutrient concentrations. Interestingly, this study further revealed that both cyclonic and anticyclonic eddies enhanced the grazing rate of microzooplankton through the increasing of prey concentration and temperature, respectively. These factors driven by different eddies affect phytoplankton communities directly or indirectly through bottom-up and top-down controls, and demonstrate the impact of different mesoscale eddies on phytoplankton community in the western equatorial Pacific Ocean, which is a region of great significance to both the regional and global ecosystems.

On the Consistency of Ice Cloud Optical Models for Spaceborne Remote Sensing Applications and Broadband Radiative Transfer Simulations

AbstractAqua satellite Moderate Resolution Imaging Spectroradiometer (MODIS) 1‐km observations are collocated with Clouds and the Earth's Radiant Energy System (CERES) fields of view taken during July 2008 afternoon satellite passes over the equatorial western Pacific Ocean. Radiation simulations are compared with collocated CERES observations to better understand the sensitivity of computed fluxes to two ice cloud broadband radiation parameterization schemes and inferred ice cloud characteristics. In particular, the radiation computational schemes and ice cloud property retrievals are based on two respective ice particle models, the MODIS Collection 6 (MC6) aggregate model and a more microphysically consistent two‐habit model (THM). The simulation results show that both MC6 and THM overestimate the shortwave (SW) and longwave (LW) cloud radiative effects at the top of the atmosphere, as compared to the CERES observations; the difference between the MC6 and THM‐based ice cloud retrievals is too small to compensate for the differences between the two model‐based radiation schemes. Therefore, the present finding suggests that broadband radiative simulations are more sensitive to the radiation parameterization scheme than to the input cloud properties retrieved using the corresponding ice cloud particle optical property model.

Rainfall and Convection in ERA5 and MERRA-2 over the Northern Equatorial Western Pacific during PISTON

Abstract This study evaluates rainfall, cloudiness, and related fields in the European Centre for Medium-Range Weather Forecasts fifth-generation climate reanalysis (ERA5) and the National Aeronautics and Space Administration’s Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), gridded global reanalysis products against observations from the Office of Naval Research’s Propagation of Intraseasonal Tropical Oscillations (PISTON) field campaign. We focus on the first PISTON cruise, which took place from August to October 2018 in the northern equatorial western Pacific Ocean. We find biases in the mean surface heat and radiative fluxes consistent with observed biases in high and low cloud fraction and convective activity in the reanalyses. Biases in the high, middle, and low cloud fraction are also consistent with the biases in the thermodynamic profiles, with positive biases in upper-level humidity associated with excessive high cloud in both products, whereas negative biases in humidity above the boundary layer are associated with too few low and middle clouds and increased static stability. ERA5 exhibits a profile that is more top-heavy than that of MERRA-2 during periods dominated by MCSs and stronger upward motion during rainy periods, consistent with higher total rainfall in this product during PISTON. The coarser grid size in MERRA-2 relative to ERA5 and the fact that MERRA-2 did not assimilate PISTON data likely both contribute to the overall larger biases seen in MERRA-2. The observed biases in the reanalyses during PISTON have also been seen in comparisons of these products with satellite data, suggesting that the results of this study are more broadly applicable.

Zonal Temperature Gradients in the Tropical Free Troposphere

Abstract The horizontal temperature gradients in the tropical free troposphere are generally assumed to be weak. We show with ERA5 data that substantial zonal virtual temperature (Tυ) gradients persist climatologically in the tropical free troposphere and investigate their causes. The gradients change seasonally: Tυ at 500 hPa over the equatorial western Pacific Ocean (EWP) is usually much warmer (up to 3 K) than that over the equatorial central Pacific Ocean (ECP) during December–February (DJF), while the temperature differences between EWP and ECP are much smaller during June–August (JJA). During DJF, Tυ gradients over the Pacific prevail throughout the entire free troposphere, especially in the upper troposphere near 300 hPa. We find that the associated hydrostatic pressure gradients are mainly balanced by the nonlinear terms in the momentum equation, in particular via zonal wind advection. Strong zonal winds occur near the equator in boreal winter, transporting zonal momentum so as to balance the pressure gradient force. The zonal winds are due to large-scale equatorial waves, excited by a heating pattern that is relatively symmetric about the equator. In boreal summer, the large-scale equatorial waves are less active in the Pacific region due to a more asymmetric temperature pattern, so the zonal momentum advection and Tυ gradients are both much weaker. The results point to an important role of the nonlinear terms in the tropical balanced dynamics, stressing the need for an improved theoretical understanding and modeling framework of the tropical atmosphere that includes these nonlinear terms, or their net effect.

Understanding the Role of Biogenic Magnetite in Geomagnetic Paleointensity Recording: Insights From Ontong Java Plateau Sediments

AbstractMarine sediments can preserve continuous paleomagnetic intensity records. Different magnetic minerals may acquire remanent magnetizations differently, so that compositional variations of magnetic mineral assemblages in sediments may hinder extraction of reliable relative paleointensity (RPI) records. To better understand this issue, we conducted a paleo‐ and rock magnetic study of a sediment core from the Ontong Java Plateau, western equatorial Pacific Ocean. RPI estimated by normalizing natural remanent magnetization (NRM) with anhysteretic remanent magnetization (ARM) decreases downcore with an inverse correlation with the ratio of ARM susceptibility to saturation isothermal remanent magnetization. This relationship indicates that the RPI signal weakens as the proportion of biogenic magnetite increases. The NRM–ARM demagnetization diagrams obtained have concave‐down curvature. These observations indicate that the RPI recording efficiency of the biogenic component is lower than that of the terrigenous component if we assume that the magnetizations of the high‐ and low‐coercivity windows are carried dominantly by biogenic and terrigenous components, respectively. This assumption is supported by first‐order reversal curve measurements, transmission electron microscope observations, low‐temperature measurements, and extraction of silicate‐hosted magnetic inclusions from the sediments. Previous studies have suggested that the RPI recording efficiency of biogenic magnetite is higher than that of the terrigenous component, which disagrees with our results. Different concentrations of silicate‐hosted magnetic inclusions in different sedimentary environments might explain this contradiction. We conclude that biogenic magnetite contributes to RPI records with lower efficiency than unprotected terrigenous magnetic minerals in the studied sediments. Changing biogenic magnetite proportions distort ARM‐normalized RPI estimations.

Reduction of Equatorial Obduction by Atmospheric Intraseasonal Oscillations in the Western and Central Pacific Ocean

AbstractObduction is the large‐scale, irreversible upward water transport from the permanent pycnocline to the surface mixed layer. Using Hybrid Coordinate Ocean Model (HYCOM), this study explores the impact of atmospheric intraseasonal oscillations (ISOs) on the Pacific equatorial obduction that plays a fundamental role in regulating the tropical Pacific climate. Parallel HYCOM experiments forced by atmospheric fields with and without ISO variability are compared to assess the effect of ISOs on the ocean. The results suggest that the ISOs can reduce the annual obduction rate of the western and central equatorial Pacific Ocean (WCEPO; 150°E−160°W, 5°S–5°N) by ∼12%, and their impact on the eastern Pacific is rather limited. The ISOs cause prominent intraseasonal variability of mixed layer depth (MLD) in the WCEPO primarily through surface wind forcing, which in turn affects the two key processes controlling the annual obduction rate. First, the intraseasonal MLD deepening events narrow down the total time windows for obduction by ∼30% and thereby give rise to the decrease in obduction rate; second, it leads to high‐frequency entrainment events and enhances the entrainment rate by ∼18%. The increased entrainment rate is overwhelmed by the reduced time windows for effective entrainment, and the net effect of ISOs is to reduce the obduction rate and modify its seasonal cycle. This work highlights the importance of atmospheric ISOs in the equatorial ocean dynamics, with implications for tropical surface temperature and climate variabilities and biogeochemical cycle.

Sedimentary CaCO3 Accumulation in the Deep West Pacific Ocean

Distribution of calcium carbonate (CaCO3) in marine sediment has been studied over the last century, and influence by multiple factors with regard to dissolution and dilution of sedimentary CaCO3 has long been established. There is still lack of quantification on the influence of those factors, so it remains elusive to determine which specific process is driving the down-core variation of CaCO3 content (wtCaCO3%) records. Here, based on a newly compiled CaCO3 data set and a carbonate model, depth-profiles of sedimentary wtCaCO3% from the West Pacific Ocean can be well illustrated, and influence from different factors on their distribution features can be quantified. The deep ocean circulation is found to largely shape the inter-basin disparity in sedimentary wtCaCO3% distribution between the equatorial regions (e.g., the Western Equatorial Pacific Ocean and the Central Pacific Ocean) and the north–west regions (the Philippine Sea and the Northwest Pacific Ocean) in our study region. Moreover, the slow carbonate dissolution rate in the deep Central Pacific Ocean guarantees better accumulation of CaCO3 at depth compared to that in other regions. However, enhanced dilution by non-carbonate materials of sedimentary CaCO3 on a topographic complex can potentially obstruct the dissolution profiles constituted by sedimentary wtCaCO3% in the pelagic ocean. The aforementioned assertion suggests that changes of wtCaCO3% accumulation in marine sediment in the West Pacific Ocean can be used to dictate past changes of the deep ocean circulation (2,500 to 3,000 m) in this area but constraint on the non-carbonate flux, especially on the topographic complex, should be necessary.

Importance of High‐Frequency (≤30‐day) Wind Variability to the Annual Climatology of the Surface Latent Heat Flux Inferred From the Global Tropical Moored Buoy Array

AbstractPrevious studies conducted based on reanalysis data sets have suggested that high‐frequency (≤30‐day) wind variability has a significant impact on the climatology of surface latent heat flux (LHF). The present study quantifies the impact of high‐frequency winds using field measurements derived from the global tropical moored buoy array (GTMBA) and, in turn, evaluates the performance of existing reanalysis products on capturing the role of these high‐frequency winds. The buoy observations demonstrate that high‐frequency winds at the n‐day (n = 1, 2, …, 30) timescale make a positive contribution to the annual climatology of LHF by nonlinearly enhancing the mean surface wind speed. Furthermore, this contribution increases as the timescale decreases, with maxima occurring at the 1‐day (subdaily) timescale. The subdaily winds contribute 4.6 W m−2, on average over all GTMBA sites. The largest contribution exceeds 10.0 W m−2 over the equatorial western Pacific Ocean. Hourly reanalysis products underestimate the contribution of the high‐frequency winds at timescales shorter than about a week, especially at the subdaily timescale, which is underestimated by approximately 50–60%. This finding suggests that an improvement in the performance of subdaily wind variability is needed to improve the fidelity of next‐generation reanalysis products.

Performance of Cloud 3D Solvers in Ice Cloud Shortwave Radiation Closure Over the Equatorial Western Pacific Ocean

AbstractFor retrieving cloud optical properties from satellite images or computing these properties from climate model output, computationally efficient treatments of cloud horizontal inhomogeneity include the Monte Carlo Independent Column Approximation (McICA) and the Tripleclouds method. Computationally efficient treatment of cloud horizontal radiation exchanges includes the SPeedy Algorithm for Radiative TrAnsfer through CloUd Sides (SPARTACUS). As a test to derive properties from satellite images, we collocate Moderate Resolution Imaging Spectroradiometer (MODIS) cloud retrievals with near‐nadir Cloud and the Earth's Radiant Energy System (CERES) footprints in July 2008 over an equatorial western Pacific Ocean region to compare the performance of the McICA, Tripleclouds, and SPARTACUS solvers to the conventional plane‐parallel homogeneous (PPH) treatment. PPH overestimates cloud albedo, and the three solvers effectively reduce overestimation with root mean square error of shortwave upwelling irradiance decreasing between 15.72 and 18.53 W m−2, or about 22%–25%. Although cloud top variability does not get fed into the simulations, all three solvers also reduce the effect of cloud top variability on cloud albedo. Entrapment (energy reflected downward from clouds) and horizontal radiation transfer have opposite effects on the SPARTACUS cloud albedo simulation. The net effect depends on the cloud vertical extent, the unawareness of which limits the performance of the SPARTACUS solver.

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.

Sensitivity of radiative flux simulations to ice cloud parameterization over the equatorial western Pacific Ocean region

AbstractPrevious studies suggest explanations of the observed cancellation of shortwave (SW) and longwave (LW) cloud radiative effects (CREs) at the top of the atmosphere (TOA) over tropical oceans where deep convection prevails, such as interactions among cloud microphysics, radiation, and dynamics. However, simulations based on general circulation models (GCMs) show disagreement in terms of the net (SW + LW) CREs over tropical deep convective ocean regions. One of the GCM uncertainty sources is the parameterization of ice cloud bulk optical properties. In this study, a combination of active and passive satellite daytime cloud retrievals is used to study the sensitivity of radiation flux calculations to ice cloud parameterization over the equatorial western Pacific Ocean region. Three ice cloud schemes are tested. The first is a widely used scheme that assumes hexagonal column ice particles. The second scheme treats ice particles as aggregates of surface-roughened hexagonal columns. The third scheme best matches the cloud ice mass-dimension relation in the cloud microphysics scheme by assuming a mixture of two ice particle habits. The results show that the hexagonal-column-based scheme has the weakest SW CRE but strongest LW CRE among the three. In addition, cloud optical thickness and effective radius are used to cluster cold-top single-layer ice clouds into three types, which resemble thin cirrus, detrained anvil clouds, and deep convective cores, respectively. In agreement with previous studies, cloud SW heating overwhelms LW cooling in the upper portion of anvil-like clouds.

Volume Transport Variability in the Western Equatorial Pacific and its Relations to Halmahera Throughflow

This study investigates the coherency of volume transport between Halmahera throughflow and current major system in the western equatorial Pacific Ocean (Mindanao Current – MC, New Guinea Coastal/Under Current – NGCC/NGCUC, and North Equatorial Counter Current – NECC). The validated daily ocean general circulation model datasets of INDESO (2010-2014) were used in this study. The results showed that the estimated average transport volume was 25.6 Sv flowing southward through MC, 34.5 Sv flowing eastward through NECC, 18.3 Sv flowing northwestward through NGCC/NGCUC, and 2.5 Sv flowing southward through the Halmahera Sea. The variability of volume transport was dominated by intraseasonal, semiannual, and annual time-scales. The increased transport of NECC corresponded to the intensification of MC and NGCC/NGCUC transports. NGCC/ NGCUC significantly controlled the South Pacific water inflow into the Halmahera Sea because of the positively high correlation between NGCC/NGCUC transport and Halmahera throughflow transport.

The Characteristics of Northeast China Cold Vortex With Different Active Paths in June and Their Relationship With Precipitation and Pre-SST

In this study, a K-means clustering (KMC) method was used to identify the paths of the Northeast China (NEC) Cold Vortex (NCCV). The NCCV was divided into four types according to the identified active paths: (1) Eastward movement type (EM); (2) Southeastward long-distance movement type (SLM); (3) Eastward short-distance movement type (ESM); and (4) Southward short-distance movement type (SSM). The characteristics of the four types of the NCCV, along with their impacts on the precipitation during early summer in NEC, were studied. The results showed that the KMC method can effectively divide the NCCV events into four different types. The maintaining days of these four types of the NCCV were found to have obvious interannual and interdecadal variation features. For example, the maintaining days of the EM and ESM types were mainly characterized by interannual variability, while the SLM and SSM types have the obvious 10–13a interdecadal variation along with interannual variability. In terms of the spatial distributions and impacts on precipitation, the EM type was found to appear in the majority of the areas located in NEC, the SLM type mainly occurred in the northwestern region of NEC and the highest rain center was located in the south-central portion, while the ESM type and SSM type were observed precipitation only appear in a small portion of the northeastern region. In addition, it is also observed the distribution of the sea-surface temperature (SST) anomalies had close relationship with the formation of these four types of the NCCV. The tripole distributions of the SST anomalies in the Atlantic Ocean corresponded to the EM type of the NCCV, the positive anomalies of SST in the eastern equatorial Pacific Ocean and negative anomalies in the western equatorial Pacific corresponded to the SLM type, the positive SSTs in the Northwest Pacific correspond to the ESM type, while negative anomalies SST in the western equatorial Pacific Ocean corresponded to the SSM type of the NCCV.

A three-dimensional gravest empirical mode determined from hydrographic observations in the western equatorial Pacific Ocean

A spatially varying non-stationary gravest empirical mode (GEM) field is constructed by incorporating the calendar month as an extra parameter to infer the subsurface thermohaline structure from the Pressure-sensor equipped Inverted Echo Sounder (PIES) observations in the western equatorial Pacific Ocean. This monthly GEM (MGEM) overcomes the application limitation of traditional GEM in the equatorial ocean due to the poor vertical coherence thermohaline structure there. The MGEM can capture 77% of the thermohaline variance for the upper 500 dbar, which is only 62% by the traditional GEM method. The most significant improvement of MGEM is for the deep ocean between 500 and 1900 dbar, where only 12% of the thermohaline variance can be captured by the GEM, but improved to 44% by the MGEM method. Based on the MGEM, a 24-month volume transport time series is estimated from observations by three PIESs deployed across the nascent North Equatorial Countercurrent (NECC) at 4°N, 6°N, and 8°N along the 130°E transect during 2014–2016. By establishing an empirical relationship between the 24 month-long volume transport and the synchronous altimeter data, a 24 year-long time series of NECC volume transport during 1993–2016 are constructed, and the seasonal and interannual variations of the NECC are investigated. The nascent NECC between 4°N-6°N was found to have quite strong seasonal variations with the strongest transport of 23 Sv occurring in February and the weakest transport of 11 Sv occurring in September. It is strengthened during the developing phase of El Niños, and is weakened during La Niñas. The 2015/16 El Niño exerts a significant impact on the nascent NECC with the amplitude comparable to that of the 1997/98 event, which even obscures the NECC's seasonality. • Pressure-sensor equipped Inverted Echo Sounder (PIES) observations in the western equatorial Pacific Ocean for the first time. • Development of a 3-dimensional GEM field with the third axis being time-of-year (MGEM field). • Significant improvement of MGEM in capturing thermohaline variance of deep ocean between 500 and 1900 dbar. • The nascent NECC has strong seasonal variations with its transport being doubled in winter compared with that in fall.

Anomalous Upper-Ocean Circulation of the Western Equatorial Pacific following El Niño Events

AbstractMooring measurements at ~140°E in the western equatorial Pacific Ocean documented greatly intensified eastward subsurface currents, which largely represent the nascent Equatorial Undercurrent, to ~67 cm s−1 in boreal summer of 2016. The eastward currents occupied the entire upper 500 m while the westward surface currents nearly disappeared. Historical in situ data observed similar variations after most El Niño events. Further analysis combining satellite and reanalysis data reveals that the eastward currents observed at ~140°E are a component of an anomalous counterclockwise circulation straddling the equator, with westward current anomalies retroflecting near the western boundary and feeding southeastward current anomalies along the New Guinea coast. A 1.5-layer reduced-gravity ocean model is able to crudely reproduce these variations, and a hierarchy of sensitivity experiments is performed to understand the underlying dynamics. The anomalous circulation is largely the delayed ocean response to equatorial wind anomalies over the central-to-eastern Pacific basin emerging in the mature stage of El Niño. Downwelling Rossby waves are generated by the reflection of equatorial Kelvin waves and easterly winds in the eastern Pacific. Upon reaching the western Pacific, the southern lobes of Rossby waves encounter the slanted New Guinea island and deflect to the equator, establishing a local sea surface height maximum and leading to the detour of westward currents flowing from the Pacific interior. Additional experiments with edited western boundary geometry confirm the importance of topography in regulating the structure of this cross-equatorial anomalous circulation.

Cladocopium infistulum sp. nov. (Dinophyceae), a thermally tolerant dinoflagellate symbiotic with giant clams from the western Pacific Ocean

ABSTRACT Members of the genus Cladocopium are the most ecologically dominant and geographically widespread group of mutualistic dinoflagellates. The addition of species descriptions to this newly erected genus should stimulate scientific investigations and communication about the biology of these important symbionts. Here we combine morphology and genetic evidence with ecology to formally describe Cladocopium infistulum sp. nov. An emended description for C. goreaui is also provided. Both are among the few species of Cladocopium capable of growing in culture medium. The cells of C. infistulum are significantly smaller in size and differ in amphiesmal plate tabulation (morphology of the motile cell) from C. goreaui. Both appear to lack knob ornamentation typical of the elongate amphiesmal vesicle (EAV) in other Symbiodiniaceae. Phylogenetic analyses of nucleotide sequences from nuclear ribosomal, chloroplast, and mitochondrial genes differentiate C. infistulum from other Cladocopium spp. This new species is ecologically specialised, and known to occur only at high densities in digestive diverticula–a complex system of capillary-like hollow tubes that ramify the siphonal mantles of giant clams (Tridacnidae). Clams living in predominantly warm and shallow inshore reefal environments harbour this symbiont; and its biogeographic range appears restricted to the equatorial western Pacific Ocean. This new species description emphasises the need to formally recognise the high species diversity and corresponding physiological and ecological breadth that encompass Cladocopium.

Observations of Flow Separation and Mixing around the Northern Palau Island/Ridge

AbstractTurbulent mixing adjacent to the Velasco Reef and Kyushu–Palau Ridge, off northern Palau in the western equatorial Pacific Ocean, is examined using shipboard and moored observations. The study focuses on a 9-day-long, ship-based microstructure and velocity survey, conducted in November–December 2016. Several sections (9–15 km in length) of microstructure, hydrographic, and velocity fields were acquired over and around the reef, where water depths ranged from 50 to 3000 m. Microstructure profiles were collected while steaming slowly either toward or away from the reef, and underway current surveys were conducted along quasi-rectangular boxes with side lengths of 5–10 km. Near the reef, both tidal and subtidal motions were important, while subtidal motions were stronger away from the reef. Vertical shears of currents and mixing were stronger on the northern and eastern flanks of the reef than on the western flanks. High turbulent kinetic energy dissipation rates, 10−6–10−4W kg−1, and large values of eddy diffusivities, 10−4–10−2m2s−1, with strong turbulent heat fluxes, 100–500 W m−2, were found. Currents flowing along the eastern side separated at the northern tip of the reef and generated submesoscale cyclonic vorticity of about 2–4 times the planetary vorticity. The analysis suggests that a torque, imparted by the turbulent bottom stress, generated the cyclonic vorticity at the northern boundary. The northern reef is associated with high vertical transports resulting from both submesoscale flow convergences and energetic mixing. Even though the area around Palau represents a small footprint of the ocean, vertical velocities and mixing rates are several orders magnitude larger than in the open ocean.

Equatorial Pacific seawater pCO2 variability since the last glacial period

The ocean may have played a central role in the atmospheric pCO2 rise during the last deglaciation. However, evidence on where carbon was exchanged between the ocean and the atmosphere in this period is still lacking, hampering our understanding of global carbon cycle on glacial–interglacial timescales. Here we report a new surface seawater pCO2 reconstruction for the western equatorial Pacific Ocean based on boron isotope analysis—a seawater pCO2 proxy—using two species of near-surface dwelling foraminifera from the same marine sediment core. The results indicate that the region remained a modest CO2 sink throughout the last deglaciation.

Regional Characteristics of Interannual Variability of Summer Rainfall in the Maritime Continent and Their Related Anomalous Circulation Patterns

AbstractUsing the NCEP–NCAR reanalysis and Global Precipitation Climatology Project monthly rainfall, we have investigated the regional features of interannual variations of rainfall in the Maritime Continent (MC) and their related anomalous atmospheric circulation patterns during boreal summer by employing the rotated empirical orthogonal function (REOF) analysis. Our results demonstrate that the rainfall variabilities in the MC are of very striking regional characteristics. The MC is divided into four independent subregions on the basis of the leading REOF modes; these subregions are located in central-eastern Indonesia (subregion I), the oceanic area to the west of Indonesia (subregion II+V), the part of the warm pool in the equatorial western Pacific Ocean (subregion III), and Guam (subregion IV+VI).The anomalous precipitation in different subregions exhibits different variation periodicities, which are associated with different circulation patterns as a result of atmospheric response to different sea surface temperature anomaly (SSTA) patterns in the tropical Indo-Pacific sector. It is found that rainfall anomalies in subregion I are induced by the Pacific ENSO, whereas those in subregion II+V are dominated by a triple SSTA pattern with positive correlations in the MC and negative correlation centers in the tropical Pacific and tropical Indian Ocean. Rainfall anomalies in subregion III mainly resulted from an SSTA pattern with negative correlations in the eastern MC and positive correlations in the western equatorial Pacific east of the MC. A horseshoe SSTA pattern in the central Pacific is found to affect the precipitation anomalies in subregion IV+VI. All of the results of this study are helpful for us to better understand both the climate variations in the MC and monsoon variations in East Asia.