Pacific Warm Pool Research Articles

Moored Observations of Transport in the Solomon Sea

AbstractThe Solomon Sea is a marginal sea in the western Pacific warm pool that contains the South Pacific low latitude western boundary currents. These low latitude western boundary currents chiefly exit the Solomon Sea through three channels (Vitiaz Strait, St. George's Channel, and Solomon Strait) and serve as the primary source water for the Equatorial Undercurrent. Simulations have shown that transport partitioning between the straits determines the water mass structure of the Equatorial Undercurrent, but the relative contributions of transport through each strait have not been observed before. As part of the Southwest Pacific Ocean Circulation and Climate Experiment, an array of moorings was deployed simultaneously in the three outflow channels of the Solomon Sea from July 2012 until March 2014 to resolve transport and water properties in each strait. Above deep isopycnals (σ0 ≤ 27.5), Vitiaz and Solomon Straits account for 54.2% and 36.2% of the mean transport, respectively, with the remaining 9.6% exiting through St. George's Channel. The strongest subinertial transport variability is observed in Solomon Strait and dominates total Solomon Sea transport variability, and a significant fraction of this variability is at intraseasonal time scales. Finally, a previously unobserved deep current at 1,500‐m depth is found to enter the Solomon Sea through Solomon Strait, with a deployment‐mean transport of 4.6 Sv (Sv ≡106 m3/s).

Nonlinear response of Northern Hemisphere stratospheric polar vortex to the Indo\u2013Pacific warm pool (IPWP) Ni\xf1o

Variations in tropical sea surface temperatures (SST) have pronounced impacts on the stratospheric polar vortex, with the role of El Niño being the focus of much research interest. However, the Indo–Pacific warm pool (IPWP), which is the warmest body of seawater in the world, has received less attention. The IPWP has been warming in recent years. This paper presents for the first time the remarkable nonlinearity in Northern Hemisphere (NH) stratospheric circulation and temperature response to IPWP warming (the so-called IPWP Niño) in boreal winter. The magnitude of NH stratospheric vortex weakening is strong and significant in case of moderate IPWP Niño, but is weak and insignificant in strong IPWP Niño case. This phenomenon is robust in both the historical simulations and observations. An idealized model experiments forced with linear varying SST forcing in the IPWP region isolate the nonlinearities arising from IPWP Niño strength. Westward extension of precipitation into the Maritime Continent drives attenuation and westward shift of extratropical waves during strong IPWP Niño events. Linear wave interference analysis reveals this leads to weak interference between the climatological and anomalous stationary waves and thereby a weak response of the stratospheric vortex. These findings imply a distinct stratospheric vortex response to the IPWP Niño, and provide extended implications for the surface climate in the NH.

Extracting historic cyclone data from coastal dune deposits in eastern Hainan Island, China

Understanding long-term, large cyclone activity is critical for coastal management and disaster prevention and mitigation, particularly with projected rising sea level and increasing cyclone frequency and the intensity. Here we present sedimentary evidence (cyclone overwash deposits, beachrock boulders, and cyclone-induced clastic deposits) of historic large cyclone events from coastal dune deposits on eastern Hainan Island, China. Based on luminescence dating and sedimentological analysis, eight historic tropical cyclone events (at 1095 ± 90 BCE, 900–1000 BCE, 975 ± 50 CE, 1720 ± 20 CE, 1740 ± 35 CE, 1790 ± 25 CE, 1850 ± 15 CE, and 1895 ± 10 CE) have been identified in this region. The elevation of the termination point of cyclone overwash deposit layers that represent the paleo-cyclone minimum wave run-up stage, in combination with knowledge of present-day cyclone events, was applied to assess the intensity of these extreme events. The intensity of these cyclone events was likely greater than that of cyclones in the local instrumental records. Furthermore, these large cyclone events correspond to historic records of devastating cyclones. Through comparison with regional and global climate records, we infer that the variability of large cyclones in the South China Sea was probably modulated by the El Niño-Southern Oscillation, western Pacific warm pool, and local sea surface temperatures over the past 3400 yr. These results provide an improved understanding of long-term, extreme cyclone events and their response to climate change, thereby contributing to regional planning and coastal defense strategies.

Reconstructing organic matter sources and rain rates in the southern West Pacific Warm Pool during the transition from the deglaciation period to early Holocene

Transient features in the organic carbon content of deep-sea sediment cores resulting from changes in the flux and/or quality of sedimentary organic matter are not uncommon. We examined the geochemical characteristics of sediments retrieved with a gravity corer from the northwestern Solomon Sea (3908 m water depth), southern West Pacific Warm Pool (WPWP). δ13C and δ15N of sedimentary organic matter, together with TOC/TN data suggest that the organic material is characterized by a mixture of marine and terrestrial origins with a higher contribution from marine algae. The data were analyzed with an inverse non-steady-state reaction-transport model to examine the variability and magnitude of particulate organic carbon (POC) flux to the seafloor during the transition between the deglaciation period and early Holocene. Measured POC content and porewater NO3−, NH4+, DIC and SO42− concentrations were used to constrain the model. Hindcast results revealed that POC flux decreased from 75 to 37.5 μg cm−2 yr−1 during the deglaciation–early Holocene transition. The rate of POC degradation in the present setting is slightly lower compared to that in the pre-Holocene setting. The synchronous decline in the relative contribution of terrestrial organic matter input and rapid sea level rise during this transition suggest that sea level, rather than surface productivity, is the dominant factor controlling the POC deposition flux in the Solomon Sea. This is conceivable because the sampling site is proximal to high-relief islands with high rainfall, a well-developed submarine canyon system and narrow and steep continental margins. Consequently, we suggest that deep-water basins in proximity to similar high-relief mountainous islands in the tropical Pacific may represent important sinks for terrestrial organic material, especially during sea level lowstands.

Eddy induced SST variation and heat transport in the western North Pacific Ocean

Vigorous mesoscale eddies and significant sea surface temperature (SST) variations are found in the northern edge of the Pacific warm pool that features large SST gradient. However, the relations between SST gradient, eddies, and SST variations in this region remain unexplored. In this study, by combining multi-altimeter sea surface height (SSH), satellite microwave SST observations and a mesoscale eddy dataset, we investigated the two-dimensional structures of SSTa (SST anomalies) in two subareas, which have different eddy characteristics and can be discriminated in the intensity of background SST gradient. The eddy characteristics of this region, focusing on their concentration to 21°N, are also described. We found that eddies can result in two distinct patterns of eddy-induced SST anomalies, with two different horizontal processes respectively. One is a monopole pattern that is caused by elevation/depression of the isopycnals, and another is a dipole pattern caused by eddy’s rotation and stirring of the background SST field. In addition, contributions of both parts to the total SSTa and the SSTa variations were evaluated. The intensity of surrounding SST gradient plays an important role in shaping the SSTa structure: when SST gradient is larger, the eddy-associated SSTa pattern organizes more dipole. The distinct annual cycle in two components of SSTa is associated with the seasonal modulation of the warm pool’s horizontal structure.

Weak linkage of winter surface air temperature over Northeast Asia with East Asian winter monsoon during 1993–2003

A colder winter over Northeast Asia (NEA) has traditionally been attributed to the stronger East Asian winter monsoon (EAWM) on interannual timescale. However, the present work proposes this robust linkage becomes weak during 1993–2003, along with the changes of the underlying condition in the last three decades. During 1981–1992 and 2004–2016, the loss of autumn Arctic sea ice condensation (SIC) leads to the stratospheric warming and the polar vortex weakening, which results in the enhancement of the Siberian High (SH) and the colder surface air temperature (SAT) over NEA. In particular, the persisting anomalies of snow cover near the Mongolian Plateau from autumn to winter could amplify the influences of Arctic SIC on the EAWM during 1981–1992. However, during 1993–2003 both the EAWM and the SAT over NEA are modulated by the tropical SSTAs in the western Pacific warm pool, corresponding to a weak association between the EAWM and the SAT over NEA. These tropical warm SSTAs could excite a meridional teleconnection over East Asia to generate a barotropic anticyclonic anomaly in situ. It not only weakens the SH and EAWM but also decreases the SAT over NEA. Therefore, the predictor of winter SAT over NEA is altered from the cryosphere to the tropical SSTAs in this episode. The changes of interdecadal background may contribute to the distinct interannual relationship between EAWM and winter SAT over NEA. This will increase the uncertainty in seasonal forecast on the winter climate over East Asia.

Investigating ENSO and WPWP modulated typhoon variability in the South China Sea during the mid–late Holocene using sedimentological evidence from southeastern Hainan Island, China

The link between tropical cyclone (TC) activity in the South China Sea (SCS) and global climate change is commonly debated and there is a clear need for long-term geological records of TC activity if we are to clarify this connection. Multi-millennial, high-resolution paleo-storm records from the SCS are rare in the region and this causes difficulties for those exploring potential climate drivers of TC variability over centennial to millennial timescales and reduces our ability to fully assess the risks associated with future TC activity. This paper presents an age-constrained, mud-dominated sedimentary sequence from a coastal lagoon on Hainan Island, China. Multiproxy analyses incorporating chronological, lithological, sedimentological, and geochemical evidence were used to infer storm deposits preserved within the sequence and to reconstruct a time series of storm activity in the SCS during the mid-to-late Holocene. This long sedimentary record shows that TCs were highly active over the periods 5500 to 3500 and 1700 to 0 cal yrs. BP, and these periods contrast with a relatively quiet period 3500–1700 cal yrs. BP. An apparent inverse correlation between TC reconstructions from the SCS and those from the Korean Peninsula and Japan implies an oscillating pattern across the SCS and western North Pacific (WNP) over centennial to millennial timescales. A comparison between the sedimentary and paleoclimatic records implies that the El Niño–Southern Oscillation was not the only mechanism responsible for typhoon variability over the past 7500 cal yrs. BP, suggesting that other factors such as the thermal state of the western Pacific warm pool likely also had a strong influence on both SCS and WNP TC variability.

The sensitivity of climatological SST to slab ocean model thickness

A pronounced tropical cooling (> 4 °C) and high-latitude warming in the annual mean sea surface temperature (SST) climatology is found in a numerical experiment conducted with a coupled model consisting of an atmospheric general circulation model (AGCM) coupled to a slab ocean model (SOM) in which the time-independent SOM thickness is reduced by a factor of two. The results suggest that biases in the ocean mixed layer depth could be contributing to SST biases in coupled atmosphere–ocean general circulation models. These changes in annual mean SST are noteworthy since in simple climate models the SOM thickness controls the amplitude and phase of the SST annual cycle, but it plays no role in determining the annual mean SST. Results from the numerical experiment indicate that halving of the SOM thickness not only changes annual cycle amplitude but results in asymmetrical changes in the annual cycle that rectify onto annual mean SST. The changed SOM thickness is found to primarily affect the surface net solar flux and latent heat flux components. However, due to the SOM equilibrium energy budget constraint, the annual mean net surface heat flux does not change, and the solar and latent heat fluxes changes compensate for each other. There is no such constraint on the annual cycle of heat fluxes, where the solar and latent components act to reinforce each other over much of the ocean, with the notable exception of the tropic Pacific warm pool region. We investigate the influence of the amplitude of the SST annual cycle on the annual mean SST through the net surface heat flux and its components using a two-step approach. First, two prescribed SST AGCM simulations are carried out, both with the annual mean SST of the full-thickness SOM coupled model simulation: one forced with the SST annual cycle of the full-thickness SOM coupled simulation, and the other forced with the SST annual cycle of the half-thickness SOM coupled simulation. Second, the impacts of these changes in the annual cycle on the annual mean SST are assessed in a full-thickness AGCM-SOM coupled simulation forced by the net, annual-mean, surface heat flux changes produced by the SST annual cycle changes in the first step. It is found that these net surface heat flux changes can largely reproduce the annual mean SST sensitivity to SOM thickness found in the coupled SOM thickness halving experiment. The effects on the annual mean heat flux components and annual mean SST from changes in the SOM thickness in an idealized limiting case are also considered.

Temperature Evolution of the Indo‐Pacific Warm Pool Over the Holocene and the Last Deglaciation

AbstractThe Indo‐Pacific Warm Pool (IPWP) contains the warmest surface ocean waters on our planet making it a major source of heat and moisture to the atmosphere. Changes in the extent and position of the IPWP likely impacted the tropical and global climate in the past and may also do in the future. With the aim to put recent ocean changes into a longer temporal context, we present new paleoceanographic sea surface temperature reconstructions from the heart of the Western Pacific Warm Pool, which is the warmest region within the IPWP, across the last 17,000 years. To provide an improved spatial and temporal regional context we use new and published sea surface temperature records from the IPWP and update previous compilation efforts (Linsley et al., 2010, https://doi.org/10.1038/ngeo1920). We similarly conclude that the IPWP was warmer in the early Holocene compared to the late Holocene. However, with the new data we are able to argue against a western displacement/expansion of the IPWP and suggest a warmer southern IPWP in the early Holocene. We explore the potential drivers of the Holocene IPWP cooling and propose a combination of processes including changes in the monsoonal winds associated with the position of the rain belt, the tropical Pacific mean climate, and upper water column mixing. The proposed climatic processes differentially impacted the IPWP subregions resulting in spatially diverse trends. Additionally, the late deglacial section of the records mostly show a gradual IPWP warming similar in structure to the atmospheric CO2 and/or Antarctica rising temperatures.

Detecting Change in the Indonesian Seas

1. Introduction The Indonesian seas play a fundamental role in the coupled ocean and climate system with the Indonesian Throughflow (ITF) providing the only tropical pathway connecting the global oceans. Pacific warm pool waters passing through the Indonesian seas are cooled and freshened by strong air-sea fluxes and mixing from internal tides to form a unique water mass that can be tracked across the Indian Ocean basin and beyond. The Indonesian seas lie at the climatological center of the atmospheric deep convection associated with the ascending branch of the Walker Circulation. Regional SST variations cause changes in the surface winds that can shift the center of atmospheric deep convection, subsequently altering the precipitation and ocean circulation patterns within the entire Indo-Pacific region. Recent multi-decadal changes in the wind and buoyancy forcing over the tropical Indo-Pacific have directly affected the vertical profile, strength, and the heat and freshwater transports of the ITF. These changes influence the large-scale sea level, SST, precipitation and wind patterns. Observing long-term changes in mass, heat and freshwater within the Indonesian seas is central to understanding the variability and predictability of the global coupled climate system. Although substantial progress has been made over the past decade in measuring and modelling the physical and biogeochemical variability within the Indonesian seas, large uncertainties remain. A comprehensive strategy is needed for measuring the temporal and spatial scales of variability that govern the various water mass transport streams of the ITF, its connection with the circulation and heat and freshwater inventories and associated air-sea fluxes of the regional and global oceans. This white paper puts forward the design of an observational array using multi-platforms combined with high-resolution models aimed at increasing our quantitative understanding of water mass transformation rates and advection within the Indonesian seas and their impacts on the air-sea climate system.

Different vertical distribution of zooplankton community between North Pacific Subtropical Gyre and Western Pacific Warm Pool: its implication to carbon flux

The mesozooplankton in both epipelagic and mesopelagic zones is essentially important for the study of ecosystem and biological carbon pump. Previous studies showed that the diel vertical migration (DVM) pattern of mesozooplankton varied among ecosystems. However, that pattern was largely unknown in the Western Pacific Warm Pool (WPWP). The vertical distribution, DVM and community structure of mesozooplankton from the surface to 1 000 m were compared at Stas JL7K (WPWP) and MA (North Pacific Subtropical Gyre, NPSG). Two sites showed similarly low biomass in both epipelagic and mesopelagic zones, which were in accordance with oligotrophic conditions of these two ecosystems. Stronger DVM (night/day ratio) was found at JL7K (1.31) than that at MA (1.09) on surface 0–100 m, and an obvious night increase of mesopelagic biomass was observed at JL7K, which was probably due to migrators from bathypelagic zone. Active carbon flux by DVM of zooplankton was estimated to be 0.23 mmol/(m2·d) at JL7K and 0.16 mmol/(m2·d) at MA. The community structure analysis showed that calanoid copepods, cnidarians and appendicularians were the main contributors to DVM of mesozooplankton at both sites. We also compared the present result with previous studies of the two ecosystems, and suggested that the DVM of mesozooplankton was more homogeneous within the WPWP and more variable within the NPSG, though both ecosystems showed typically extremely oligotrophic conditions. The different diel vertical migration strength of mesozooplankton between NPSG and WPWP implied different efficiency of carbon pump in these two ecosystems.

Millennial- and centennial-scale variations in the upper-water temperature of Western Pacific Warm Pool during the last deglaciation

During the last deglaciation period, climate changes in the northern and southern high-latitude regions show a pattern of so-called “bipolar seesaw” on millennial scale. Previous research interpreted that the weakening of the Atlantic Meridional Overturning Circulation (AMOC) and the associated north-south migration of the Intertropical Convergence Zone (ITCZ) influenced the interhemispheric heat redistribution and thus climate state. However, such a hypothesis lacks the support from the western tropical Pacific, in which the deglacial millennial-scale changes is unclear as the published reconstructions usually conflict with one another. As the major oceanic heat source, the Western Pacific Warm Pool (WPWP) affects the global climate system, with its sea surface temperature (SST) change forcing the tropical atmospheric circulation and its thermocline dynamic modulating the upper-ocean heat content and thermal structure. This study explores the upper-water temperature changes during the transition from the Last Glacial Maximum (LGM) to the Holocene, i.e., the last deglaciation (~23−8 ka). Foraminifera are inferred to calcify in equilibrium with the seawater they live in, make it possible to reconstruct the environmental properties of corresponding water masses where certain foraminifera dwell. We analyzed the δ 18O and Mg/Ca of the mixed-layer dwelling planktonic foraminifera Globigerinoides ruber and the upper-thermocline dweller Pulleniatina obliquiloculata from sediment core MD01-2386 locating at the core of WPWP, and reconstructed decadal-resolved records of SST and thermocline water temperature (TWT) for the last deglaciation. The reconstructed results show that: (1) SST and TWT started initial warming onsets around 20−21 ka, following the initial decrease of precession parameter; (2) SST and TWT stopped warming and plateaued during the interval of Heinrich stadial 1 (HS-1) and Younger Dryas (YD; (3) TWT varied with larger amplitudes than SST. Further, we compared the SST and TWT of core MD01-2386 with the proxy-records of temperature and precipitation of the WPWP and the model-output of El Nino variability. The records of SST, TWT and the planktonic foraminifera δ 18O from the western equatorial Pacific are coherent with the Borneo stalagmite δ 18O, thus suggesting the coupled feedbacks among the thermocline dynamic, sea-surface heating and atmospheric convective activity over the heart of WPWP during the last deglaciation. While, contrasts exist between the records of near-equator region and off-equator area and thus suggests opposite changes in the atmospheric convective activity near the equator and the hydrological cycle change outside the equator. This finding may indicate a relative migration or intensity change of the atmospheric convection center, rather than a north-south shift of the rain belt. In addition, the planktonic foraminiferal proxy records demonstrate significant centennial-scale variabilities, mainly on the periodicity of 220−260 a. This cycle is coherent with the 10Be record of ice core and roughly coincident with the Suess cycle of solar activity. To sum up, the results of this study allow us to propose three main conclusions. (1) The upper water temperature records of core MD01-2386 show obvious millennial-scale changes, which are closely coupled with the upper water temperature and atmospheric convection activity in the western equatorial Pacific, indicating consistent changes of the ocean-atmosphere coupled system in the western equatorial Pacific on the millennial-scale of the last deglaciation. (2) On the millennium scale in the last deglaciation, opposite changes in the air-sea feedbacks exist between the near-equator and off-equator areas of the warm pool, which may indicate relative changes of the atmospheric convevtion center but not a north-south shift of rain belt. (3) The planktonic foraminifera proxy records present significant cycling of 200−400 a centered at ~260 a and may reflect the linkage between centennial-scale solar activity and the upper water dynamics of the western equatorial Pacific.

Holocene changes in biome size and tropical cyclone activity around the Northern South China Sea

The South China Sea (SCS), characterized by a large continental shelf, is located at the edge of the Asian monsoon domain. In this study, two marine sediment cores from the northern SCS (NSCS) continental slope were investigated to construct composite vegetation and precipitation isotopic composition records based on the δ13C and δD values of plant-wax n-alkanes throughout the Holocene (last 11,200 years; i.e. 11.2 ka). The composite δ13Cwax record indicates an overall predominance of C3 vegetation over the last 11.2 ka. Before 8 ka BP, higher δ13Cwax values are attributed to preferential wax input from grassland and wetland biomes on the exposed continental shelf. After the inundation of the shelf by eustatic sea level rise until ca. 8 ka BP grassland and wetland biomes suffered a major size reduction and arboreal vegetation became better represented in the δ13Cwax record. The composite temperature corrected δDwax-T record suggests that moisture source variability drove precipitation isotopic composition changes during the Holocene. Lower δDwax-T values before 8.3 ka BP are interpreted as a larger moisture contribution by Pacific Ocean tropical cyclones, whereas higher δDwax-T values after 8.5 ka BP are interpreted as a larger moisture contribution from the Indian Ocean summer monsoon. Higher incidence of tropical cyclones in the NSCS during the Early Holocene was related to a temporary westward shift of the Western Pacific Warm Pool and enhanced insolation over the Northern Hemisphere. Both external and internal forcing mechanisms regulated moisture source changes in East Asia during the Holocene.

Contributions of SST Anomalies in the Indo-Pacific Ocean to the Interannual Variability of Tropical Cyclone Genesis Frequency over the Western North Pacific

AbstractThis study attempts to evaluate quantitatively the contributions of sea surface temperature (SST) anomalies in the Indo-Pacific Ocean to the interannual variability of tropical cyclone (TC) genesis frequency (TCGF) over the western North Pacific (WNP). Three SST factors in the Indo-Pacific Ocean are found to play key roles in modulating the interannual variability of WNP TCGF. They are summer SST anomaly in the east Indian Ocean (EIO), the summer El Niño–Southern Oscillation Modoki index (EMI), and the spring SST gradient (SSTG) between the southwestern Pacific and the western Pacific warm pool. Results show that the three factors together can explain 72% of the total variance of WNP TCGF in the typhoon season for the period 1980–2015. Among them, the spring SSTG and the summer EIO contribute predominantly to the interannual variability of TCGF, followed by the summer EMI, with respective contributions being 39%, 38%, and 23%. Further analysis shows that the summer EMI was affected significantly by the spring SSTG and thus had a relatively lower contribution to the TCGF than the spring SSTG. In addition, a statistical model is constructed to predict the WNP TCGF in the typhoon season by a combination of the May EIO and the spring SSTG. The new model can reproduce well the observed WNP TCGF and shows an overall better skill than the ECMWF Seasonal Forecasting System 5 (SEAS5) hindcasts. This statistical model provides a good tool for seasonal prediction of WNP TCGF.

The Madden‐Julian Oscillation in an Aquaplanet‐Like General Circulation Model With and Without Continents

AbstractWe have conducted experiments with the Community Atmospheric Model version 5.1 configured as an aquaplanet, with the sun at March equinox, in order to study how the model Madden‐Julian oscillation (MJO) responds to (i) the introduction of continents and (ii) realistic sea surface temperature (SST) distribution. Model results are compared with ERA‐Interim reanalysis and analyzed in terms of the moist static energy (MSE) budget to study the growth and propagation of MJO. With idealized zonally symmetric SST, the aquaplanet model produces a double Intertropical Convergence Zone (ITCZ) and an MJO‐like mode with variance at intraseasonal (30‐ to 96‐day) periods and zonal wave number 1. When we introduce continents with realistic orography and interactive surface temperature, soil moisture, and albedo, the variance of MJO is mainly confined to zonal wave numbers 1 and 2 but reduced by a factor of 3 due to weaker boundary layer moisture convergence. With prescribed climatological January SST boundary condition (which includes Indo‐west Pacific warm pool) in the presence of continents, model MJO variance is enhanced and distributed across zonal wave numbers 1 to 5, in closer agreement with observation. Thus, the presence of land by itself is not enough, but realistic SST distribution is necessary for improved space‐time characteristics of model MJO. Both in simulations and ERA‐Interim data, meridional and vertical advection of MSE promotes eastward movement of MJO. In the model experiments, meridional advection of low‐level MSE anomaly is most significant in the vicinity of the ITCZ, indicating the importance of processes that determine the location of (single or double) ITCZ.

Stochastic Parameterization of Subgrid-Scale Velocity Enhancement of Sea Surface Fluxes

Abstract Subgrid-scale (SGS) velocity variations result in gridscale sea surface flux enhancements that must be parameterized in weather and climate models. Traditional parameterizations are deterministic in that they assign a unique value of the SGS velocity flux enhancement to any given configuration of the resolved state. In this study, we assess the statistics of SGS velocity flux enhancement over a range of averaging scales (as a proxy for varying model resolution) through systematic coarse-graining of a convection-permitting atmospheric model simulation over the Indian Ocean and west Pacific warm pool. Conditioning the statistics of the SGS velocity flux enhancement on 1) the fluxes associated with the resolved winds and 2) the precipitation rate, we find that the lack of a separation between “resolved” and “unresolved” scales results in a distribution of flux enhancements for each configuration of the resolved state. That is, the SGS velocity flux enhancement should be represented stochastically rather than deterministically. The spatial and temporal statistics of the SGS velocity flux enhancement are investigated by using basic descriptive statistics and through a fit to an anisotropic space–time covariance structure. Potential spatial inhomogeneities of the statistics of the SGS velocity flux enhancement are investigated through regional analysis, although because of the relatively short duration of the simulation (9 days) distinguishing true inhomogeneity from sampling variability is difficult. Perspectives for the implementation of such a stochastic parameterization in weather and climate models are discussed.

Seasonal variations in size and intensity of the Indo-western Pacific warm pool in different sectors

The present study investigates the roles of solar radiation and monsoon wind in annual and semiannual variations of size and intensity of the Indo-western Pacific warm pool. The Indo-western Pacific warm pool is divided into four sectors with the equator and 100°E (north of the equator) and 135°E (south of the equator) longitude as the demarcations. These sectors display different seasonal variations in size and intensity and different roles of surface heat flux and oceanic processes. The size of the North Indian Ocean warm pool displays a semiannual variation with two peaks in May and October, while that in the other three sectors shows an annual variation. These features are attributable to a dominant role of solar radiation in the four sectors and an additional effect of the Asian monsoon in the North Indian Ocean sector. The intensity of the warm pool displays semianunal variations, with two peaks in May and August in the North Indian Ocean sector, April and November in the South Indian Ocean sector, May–June and September in the North Pacific Ocean sector and December and April in the South Pacific Ocean sector. The semiannual feature is associated with two effects of monsoon winds: wind-induced ocean advection and convection-related cloud-radiation changes. These two changes induce a decrease of sea surface temperature during the monsoon period, leading to the formation of semiannual variations in the size and intensity of the warm pool.

Sea-level, monsoonal, and anthropogenic impacts on the millennial-scale variability of siliciclastic sediment input into the western Philippine sea since 27 ka

Multiproxy records from Core MD06-3054 in the western Philippine Sea and published results from the nearby Core MD06-3052, which are both located at the northern margin of the Western Pacific Warm Pool (WPWP), provide precise reconstructions and evidence for the potential controls on continental erosion and weathering intensities in Luzon. These data also constrain sediment source-to-sink processes in the sea since 27 ka at the millennial scale. We demonstrate that the siliciclastic fraction is derived from Luzon volcanic rocks and eolian dust from mainland Asia. Stronger physical erosion and chemical weathering generally occurred during the last glaciation, Heinrich Event 1, the Younger Dryas, and the period from 10.3 to 11.3 ka. In contrast, the Bølling-Allerød was characterized by weaker physical erosion and chemical weathering. During the last glaciation, Heinrich Event 1, and the Younger Dryas, the lower sea level led to enhanced erosion and weathering of the exposed shelf sediments. In contrast, the weaker physical erosion and chemical weathering that occurred during the Bølling-Allerød and the Holocene, except after 2.1 ka, are closely correlated with the higher sea level. The stronger physical erosion and chemical weathering at 10.3–11.3 ka may be derived from the reworking of older sediments during the time of a strengthening summer monsoon. Furthermore, enhanced human activity since 2.1 ka has become the dominant control on erosion in Luzon. Rapid weathering changes in the WPWP during the last deglaciation highlight the high variability in local controls at low latitudes during these periods and the potential influence of controls at high latitudes.

On Relationships between Nonrecurving Western North Pacific Tropical Cyclones, the Madden–Julian Oscillation, and the East Asian Subtropical Jet

Abstract A 200-hPa zonal momentum budget is performed to examine the role that western North Pacific tropical cyclones (TCs) play in helping to organize intraseasonal extratropical circulation anomalies that occur with the Madden–Julian oscillation (MJO). Zonal wind is linearly decomposed into components that occur on MJO time scales (i.e., 20–100-day periods), as well as those that occur with lower and higher frequency. Dates during Northern Hemisphere fall that feature nonrecurving TCs within a search radius centered on a South China Sea grid point when the MJO is convectively active over the Maritime Continent and west Pacific warm pool are used to generate composites of relevant budget terms. These composites are then compared to others that are based on the full list of dates that feature a convectively active MJO in the same location during NH fall without regard for TC presence. Composite results highlight the primary momentum sources that guide the evolution of the NH extratropical zonal wind and associated mass field in each event set. TCs help to accelerate the East Asian subtropical jet that evolves with the MJO by modulating the high-frequency subtropical circulation over Southeast Asia. The phasing of this circulation with its underlying MJO time-scale component enables it to transfer momentum to the emerging subtropical jet. This momentum is integrated into the more slowly evolving flow and carried forward by other processes, which leads to the development of a westerly momentum surge along the subtropical jet that spans the length of the North Pacific Ocean.

A preliminary study on an upper ocean heat and salt content of the western Pacific warm pool region

On the basis of Argo profile data of the temperature and salinity from January 2001 to July 2014, the spatial distributions of an upper ocean heat content (OHC) and ocean salt content (OSC) of the western Pacific warm pool (WPWP) region and their seasonal and interannual variations are studied by a cyclostationary empirical orthogonal function (CSEOF) decomposition, a maximum entropy spectral analysis, and a correlation analysis. Probable reasons for variations are discussed. The results show the following. (1) The OHC variations in the subsurface layer of the WPWP are much greater than those in the surface layer. On the contrary, the OSC variations are mainly in the surface layer, while the subsurface layer varies little. (2) Compared with the OSC, the OHC of the WPWP region is more affected by El Nino-Southern Oscillation (ENSO) events. The CSEOF analysis shows that the OHC pattern in mode 1 has strong interannual oscillation, with eastern and western parts opposite in phase. The distribution of the OSC has a positive-negative-positive tripole pattern. Time series analysis shows that the OHC has three phase adjustments with the occurrence of ENSO events after 2007, while the OSC only had one such adjustment during the same period. Further analysis indicates that the OHC variations are mainly caused by ENSO events, local winds, and zonal currents, whereas the OSC variations are caused by much more complex reasons. Two of these, the zonal current and a freshwater flux, have a positive feedback on the OSC change in the WPWP region.