Pacific Warm Pool Research Articles

Revealing the vegetation, fire and human activities in the lowland of eastern Taiwan during Late Holocene

Late Holocene vegetation, climate and human impacts were investigated using palynologiacal and marco-charcoal data from lacustrine sediments from a lowland lake of eastern Taiwan. On the basis of sixteen 210Pb dates and six radiocarbon dates, a reliable stratigraphic chronology was established. The 2680 year records of vegetation history and fire frequency indicated five distinctive stages, reflecting the exhibits of climate changes and human activities. During 2680–2410 cal yr BP, the lake was shallow and the climate was relatively cool, according to the domination of wetland and upper montane forest pollen. The following period during 2410–1510 cal yr BP, the increase of low montane forest pollen and the overall high value of fire frequency indicated the warm and less humid climate. During 1510-560 cal yr BP, the low montane forest pollen remained high values, and the climate was warm and stable. During 560-130 cal BP, the low pollen concentration companying with high frequency of nearby taxa, Lagerstroemia inferred to increase in soil erosion. The period with intensive soil erosion can link to the occurrences of extremely precipitation. It may correlate to the frequent typhoons during the Little Ice Age, during which the reduction of the West Pacific Warm Pool and expansion of the Northwestern Pacific High enhanced westerly shift of typhoon track toward Taiwan and southern China. During last 130 cal yr BP, the strong impacts of human activities were indicated by the low arboreal pollen, high cultivated Poaceae, and more fire events. It is worth to note that the global temperate anomalies could be identified within this lowland lacustrine sediments, including the Medieval Warm Period and the Little Ice Age, based on the proxies of pollen-inferred temperature. Pollen signals including cultivated Poaceae pollen and pollen diversity index indicate significant human agriculture during the intervals of 2680–2410 cal yr BP, and during 1510 cal yr BP to modern time, which may be linked to the local environment changes, leading to the population variation in eastern lowland Taiwan.

Variability of the Indonesian Throughflow thermal profile over the last 25-kyr: A perspective from the southern Makassar Strait

Abstract The Indonesian Throughflow (ITF), the sole conduit between the Pacific and Indian Oceans, regulates the heat and fresh water budgets between these oceans and plays a key role as the ‘warm water pathway’ in the global thermohaline circulation. In this study, we reconstructed a 25-kyr history of the ITF thermocline, based on well-dated, high-resolution (~200 yrs) records of Pulleniatina obliquiloculata δ18O and Mg/Ca from Core SO217-18515 (3.63°S, 119.54°E; water depth: 688 m), retrieved from the southern Makassar Strait. We integrated these new records with data from offshore Luzon (MD98-2188; 14.82°N, 123.49°E; water depth: 730 m), from the center of the Western Pacific Warm Pool (WPWP) (3cBX; 8.02°N, 139.6°E; water depth: 2829 m) and from the Timor Sea (MD01-2378; 13.08°S, 121.79°E; water depth: 1783 m). Both sea surface and thermocline temperatures decreased and their difference (ΔT) increased in the southern Makassar Strait and Timor Sea between 11 and 2.5 ka, corresponding to enhanced El Nino activity and dominance of East Asian winter monsoon over summer monsoon. In contrast, sea surface and thermocline temperatures increased and ΔT decreased during the last ~2.5 kyrs concomitantly with declining El Nino activity and relative weakening of the East Asian winter monsoon. Our results additionally show that ice volume corrected seawater δ18O of both sea surface and thermocline in Cores SO217-18515 and MD01-2378 co-vary with Borneo stalagmite δ18O, a proxy of regional precipitation in the WPWP, thus indicating vertical mixing of upper ocean waters along the ITF pathway. We propose that changes in the state of the El Nino-Southern Oscillation and in East Asian monsoon related to regional precipitation exerted a major influence on the ITF thermocline profile during the Holocene and largely controlled changes in ITF thermocline temperature throughout the last 25 kyrs.

Representation of the ENSO Combination Mode and its Asymmetric SST Response in Different Resolutions of HadGEM3

Previous studies have revealed a combination mode (C-mode) occurring in the Indo-Pacific region, arising from nonlinear interactions between ENSO and the western Pacific warm pool annual cycle. This paper evaluates the simulation of this C-mode and its asymmetric SST response in HadGEM3 and its resolution sensitivity using three sets of simulations at horizontal resolutions of N96, N216 and N512. The results show that HadGEM3 can capture well the spatial pattern of the C-mode associated surface wind anomalies, as well as the asymmetric response of SST in the tropical Pacific, but it strongly overestimates the explained variability of the C-mode compared to the ENSO mode. The model with the three resolutions is able to reproduce the distinct spectral peaks of the C-mode at the near annual combination frequencies, but the performance in simulating the longer periods is not satisfactory, presumably due to the unrealistic simulation of the ENSO mode. Increasing the horizontal resolution can improve the consistency between atmospheric and oceanic representations of the C-mode, but not necessarily enhance the accuracy of C-mode simulation compared with observation.

Decadal variability and trends of oceanic barrier layers in tropical Pacific

Decadal variability and trends of the isothermal layer depth (ILD), mixed layer depth (MLD), and barrier layer thickness (BLT) were analyzed for the tropical Pacific during 1979–2015. The decadal variability of ILD, MLD, and BLT shows a close connection with the Pacific Decadal Oscillation (PDO). At PDO positive phase, the eastward shift of precipitation and weakened trade winds result in thinner BLT in western Pacific and thicker BLT in central and eastern Pacific. The situation is reversed at PDO negative phase. The differences in BLT can be up to 9–15 m. The spatial distributions of decadal trends of ILD and MLD are complex, but a thickening of BLT in the western tropical Pacific is clearly present. The raw trends of ILD, MLD, and BLT averaged in the tropical Pacific (30° N–30° S, 120° E–75° W) from 1979 to 2015 are 1.62, 1.20, and 0.51 m per decade, respectively. PDO can explain about 25% of the increasing trends of BLT, while El Nino-Southern Oscillation (ENSO) only explains about 1.7%. Global warming and/or variability at longer time scales is responsible for the remaining increasing trends. The BLT change is related to the warming and freshening of the western Pacific warm pool in recent decades. The ocean-atmosphere interactions about trade winds, wind-driven ocean circulation, temperature, and precipitation/evaporation are discussed.

Predictability of phases and magnitudes of natural decadal climate variability phenomena in CMIP5 experiments with the UKMO HadCM3, GFDL-CM2.1, NCAR-CCSM4, and MIROC5 global earth system models

Data from decadal hindcast experiments conducted under CMIP5 were used to assess the ability of CM2.1, HadCM3, MIROC5, and CCSM4 Earth System Models (ESMs) to hindcast sea-surface temperature (SST) indices of the Pacific Decadal Oscillation (PDO), the tropical Atlantic SST gradient (TAG) variability, and the West Pacific Warm Pool (WPWP) SST variability from 1961 to 2010. The ESMs were initialized at specific times with observed data to make 10- and 30-year hindcasts/forecasts. Deterministic and probabilistic skill estimates show predictability of detrended WPWP index to 5 years’ lead time and of non-detrended WPWP index to 10 years’ lead time. These estimates also show atypical skill dependence of PDO and TAG indices on lead times, with increasing skill in the middle to end of 10-year hindcasts. The skill of ESMs to hindcast an observed DCV index (signal skill) is surprisingly greater than the skill to hindcast their own DCV index (noise skill) at some lead times. All ESMs hindcast occurrence frequencies of positive and negative phases of the indices, and probabilities of same-phase transitions from one year to the next reasonably well. Four, major, low-latitude volcanic eruptions are associated with phase transitions of all observed and some of the ensemble-average hindcast indices. All ESMs’ WPWP index hindcasts respond correctly to all eruptions as do three observed PDO phase transitions. No one of the ESMs’ hindcasts of the TAG index responds correctly to these eruptions. Some of the ESMs hindcast correct phase transitions in the absence of eruptions also, implying that initializations with observed data are beneficial in predicting phase transitions. The skills of DCV indices’ phase prediction up to at least two years in advance can be used to inform societal impacts adaptation decisions in water resources management and agriculture. The Atlantic region’s responses in these ESMs appear to be fundamentally incorrect.

Twentieth Century Seawater δ18O Dynamics and Implications for Coral‐Based Climate Reconstruction

AbstractThe oxygen isotopic composition of tropical coral skeletons (δ18O) is a crucial source of information on past El Niño/Southern Oscillation behavior. Both temperature and the δ18O of the surrounding seawater (δ18Osw) affect coral δ18O; a linear proportionality with δ18Osw is often used to infer past salinity variations, but the degree to which dynamical influences on δ18Osw may affect that relationship is still unclear. Here we use the isotope‐enabled Regional Ocean Modeling System to investigate the dynamics of δ18Osw and salinity variations in different twentieth century climate regimes. The dominant modes of δ18Osw variability are the background trend and eastern/central Pacific El Niño, similar to salinity; likewise, budget analysis reveals a strong impact of ocean dynamics (both advection and vertical mixing/diffusion) on δ18Osw variations during El Niño and La Niña events. These dynamics lead to alterations in the δ18Osw:salinity relationship across the Pacific: the linear approximation is most accurate near the eastern edge of the western Pacific warm pool but with nonlinearities due to large δ18Osw excursions during El Niño. At other sites, the δ18Osw:salinity relationship has substantial scatter and explains relatively little δ18Osw variance. Strikingly, interannual variability can lead to site‐specific δ18Osw:salinity regression coefficient changes of up to 0.4‰/psu on multiyear time scales. This calls the reliability of coral‐based estimates of El Niño/Southern Oscillation extremes into question and highlights the need for increased dynamical understanding as well as sustained in situ observations of δ18Osw and salinity variability.

Cloud-Precipitation Parameters and Radiative Forcing of Warm Precipitating Cloud over the Tropical Pacific Ocean Based on TRMM Datasets and Radiative Transfer Model

An approach is proposed for combining observations from the Precipitation Radar (PR) and the Visible and Infrared Scanner (VIRS) onboard the TRMM (Topical Rainfall Measuring Mission) satellite to investigate the climatology of warm precipitating cloud (WPC) microphysical properties, such as cloud effective radius (Re), cloud optical depth (Tau), and liquid water path (LWP) in the tropical Pacific Ocean (20° S–20° N) from 1998 to 2012. The WPCs are captured by VIRS/PR and categorized into two extreme (light and heavy) rain rate types (EL-WPC, EH-WPC). Their radiative effects are also simulated by the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) radiative transfer model. The results indicate that total, EL-WPC and EH-WPC reach their highest occurrence frequencies of 22%, 1.6% and 2.0% in the North-west Pacific, Intertropical Convergence Zone (ITCZ) and South Pacific Convergence Zone (SPCZ), respectively. Most of the EL-WPC has higher ratio to total WPC in the Pacific warm pool with warmer sea-surface temperature (SST), while the higher ratio for EH-WPC is located in SPCZ associated with deep convection. WPC has an average Re of 15.6 μm, Tau of 20, and LWP of 200 g m−2. EL-WPC is a little larger average Re than EH-WPC, and larger Re is distributed with higher echo top height (H). Moreover, for EH-WPC, the increased Re by the collision-coalescence process in lower H (<3.5 km) generates a stronger rain rate. In addition, although the H of EH-WPC decreases along the increased brightness temperature at 10.8 μm (BT4), this is not obvious in EL-WPC possibly due to a certain echo height to generate a light precipitation. With an increased rain rate of WPC, Re becomes larger in EL-WPC and smaller in EH-WPC. EL-WPC induces a cooling of approximately −0.5 W m−2 for radiative forcing, which is −3.0 W m−2 less than the EH-WPC.

Coccolith Assemblages and Primary Productivity Variations in the Central Western Pacific Warm Pool Over the Last 380 kyr

Coccolith assemblages in two gravity cores (KX21-2 and KX12-1) from the central Western Pacific Warm Pool (WPWP) have been analyzed with SYRACO. The variations of nutricline and primary productivity (PP) have been reconstructed based on these assemblages. The results show that the coccolith assemblages were dominated by Florisphaera profunda, Gephyrocapsa and Emiliania huxleyi over the last 380 kyr. Variations of nutricline and primary productivity can be divided into three intervals. Interval I (about 380–300 kyr): PP was high and nutricline was shallow; Interval II (about 300–160 kyr): PP decreased dramatically for a short time after the acme of G. caribbeanica in Mid-Brunhes while nutricline became deeper; Interval III (about 160 kyr–present): PP fluctuated at low levels and nutricline was deep. Variations of each coccolith taxon and PP were highly correlated in the two cores, which means that the geological environment is similar in the two cores. Spectrum analysis is performed for all coccolith taxons and PP, and the 19-kyr cycle is the most prominent. It means that the production of coccolithophores in the WPWP is mainly controlled by precession.

Multidecadal Variability in the Subseasonal Peak of Low-Level Convergence over the Pacific Warm Pool

In the Western North Pacific (WNP), the atmospheric low-level convergence is one of the main factors that influences the genesis of tropical cyclones (TC). It has been observed that the timing of the seasonal maxima in the low-level convergence and TC genesis has shifted since the mid-1990s from mid-August to late-July, with this shift having also affected the number of TC. A multidecadal frequency of 20 years was revealed in the timing variation of the tropical intraseasonal oscillation (ISO) in the Western Pacific, in which a weak WNP low-level convergence in spring may trigger an advanced ISO phase in summer and vice versa. The present diagnostic analysis does not identify any prominent oceanic variations associated with these multidecadal variations in the summer ISO or in the spring setup of the ISO. The atmospheric circulation does show an anomaly, which suggests an intensified extension of the subtropical high. The possible mechanism may be related to stochastic low-frequency variability of the atmosphere, which acts to influence the seasonal evolution of the WNP low-level convergence.

Coral‐Derived Western Pacific Tropical Sea Surface Temperatures During the Last Millennium

AbstractReconstructions of ocean temperatures prior to the industrial era serve to constrain natural climate variability on decadal to centennial timescales, yet relatively few such observations are available from the west Pacific Warm Pool. Here we present multiple coral‐based sea surface temperature reconstructions from Yongle Atoll, in the South China Sea over the last ~1,250 years (762–2013 Common Era [CE]). Reconstructed coral Sr/Ca‐sea surface temperatures indicate that the “Little Ice Age (1711–1817 CE)” period was ~0.7°C cooler than the “Medieval Climate Anomaly (913‐1132 CE)” and that late 20th century warming of the western Pacific is likely unprecedented over the past millennium. Our findings suggest that the Western Pacific Warm Pool may have expanded (contracted) during the Medieval Climate Anomaly (Little Ice Age), leading to a strengthening (weakening) of the Asian summer monsoon, as recorded in Chinese stalagmites.

Impact of the Spring SST Gradient between the Tropical Indian Ocean and Western Pacific on Landfalling Tropical Cyclone Frequency in China

The present study identifies a significant influence of the sea surface temperature gradient (SSTG) between the tropical Indian Ocean (TIO; 15°S–15°N, 40°–90°E) and the western Pacific warm pool (WWP; 0°–15°N, 125°–155°E) in boreal spring on tropical cyclone (TC) landfall frequency in mainland China in boreal summer. During the period 1979–2015, a positive spring SSTG induces a zonal inter-basin circulation anomaly with lower-level convergence, mid-tropospheric ascendance and upper-level divergence over the west-central TIO, and the opposite situation over the WWP, which produces lower-level anomalous easterlies and upper-level anomalous westerlies between the TIO and WWP. This zonal circulation anomaly further warms the west-central TIO by driving warm water westward and cools the WWP by inducing local upwelling, which facilitates the persistence of the anomaly until the summer. Consequently, lower-level negative vorticity, strong vertical wind shear and lower-level anticyclonic anomalies prevail over most of the western North Pacific (WNP), which decreases the TC genesis frequency. Meanwhile, there is an anomalous mid-tropospheric anticyclone over the main WNP TC genesis region, meaning a westerly anomaly dominates over coastal regions of mainland China, which is unfavorable for steering TCs to make landfall in mainland China during summer. This implies that the spring SSTG may act as a potential indicator for TC landfall frequency in mainland China.

Influence of the Madden–Julian Oscillation and Caribbean Low-Level Jet on East Pacific Easterly Wave Dynamics

Abstract The east Pacific warm pool exhibits basic-state variability associated with the Madden–Julian oscillation (MJO) and Caribbean low-level jet (CLLJ), which affects the development of easterly waves (EWs). This study compares and contrasts composite changes in the background environment, eddy kinetic energy (EKE) budgets, and EW tracks during MJO and CLLJ events. While previous studies have shown that the MJO influences jet activity in the east Pacific, the influence of the MJO and CLLJ on the east Pacific and EWs is not synonymous. The CLLJ is a stronger modulator of the ITCZ than the MJO, while the MJO has a more expansive influence on the northeastern portion of the basin. Anomalous low-level westerly MJO and CLLJ periods are associated with favorable conditions for EW development paralleling the Central American coast, contrary to previous findings about the relationship of the CLLJ to EWs. Easterly MJO and CLLJ periods support enhanced ITCZ EW development, although the CLLJ is a greater modulator of EW tracks in this region, which is likely associated with stronger moisture and convection variations and their subsequent influence on the EKE budget. ITCZ EW growth during easterly MJO periods is more reliant on barotropic conversion than during strong CLLJ periods, when eddy available potential energy (EAPE)-to-EKE conversion associated with ITCZ convection is more important. Thus, the influence of these phenomena on east Pacific EWs should be considered distinct.

Impact of ENSO longitudinal position on teleconnections to the NAO

While significant improvements have been made in understanding how the El Nino–Southern Oscillation (ENSO) impacts both North American and Asian climate, its relationship with the North Atlantic Oscillation (NAO) remains less clear. Observations indicate that ENSO exhibits a highly complex relationship with the NAO-associated atmospheric circulation. One critical contribution to this ambiguous ENSO/NAO relationship originates from ENSO’s diversity in its spatial structure. In general, both eastern (EP) and central Pacific (CP) El Nino events tend to be accompanied by a negative NAO-like atmospheric response. However, for two different types of La Nina the NAO response is almost opposite. Thus, the NAO responses for the CP ENSO are mostly linear, while nonlinear NAO responses dominate for the EP ENSO. These contrasting extra-tropical atmospheric responses are mainly attributed to nonlinear air-sea interactions in the tropical eastern Pacific. The local atmospheric response to the CP ENSO sea surface temperature (SST) anomalies is highly linear since the air-sea action center is located within the Pacific warm pool, characterized by relatively high climatological SSTs. In contrast, the EP ENSO SST anomalies are located in an area of relatively low climatological SSTs in the eastern equatorial Pacific. Here only sufficiently high positive SST anomalies during EP El Nino events are able to overcome the SST threshold for deep convection, while hardly any anomalous convection is associated with EP La Nina SSTs that are below this threshold. This ENSO/NAO relationship has important implications for NAO seasonal prediction and places a higher requirement on models in reproducing the full diversity of ENSO.

The Effects of Deep Convection on Regional Temperature Structure in the Tropical Upper Troposphere and Lower Stratosphere

AbstractUnderstanding the impact of deep convection on the thermodynamic structure of the tropical upper troposphere and lower stratosphere (UTLS) is vital because convection and temperatures play an important role in regulating stratospheric water vapor through direct convective injection and in enhancing the presence of thin cirrus clouds, both of which play a significant role in the climate. This study quantifies the UTLS vertical temperature structure changes near deep convection over the Pacific Warm Pool and the Tropical Atlantic Continental and Oceanic region. The deep convection observed from the Tropical Rainfall Measuring Mission satellite are collocated with high vertical resolution temperature profiles from the COSMIC GPS Radio Occultation satellites along with ERA‐Interim reanalysis from 2007 to 2011. COSMIC and ERA‐Interim observe warm temperature anomalies (0.2 to 0.8 K) within 10–14 km, then transitioning to a layer of cool anomalies (−0.4 to −1.5 K) within 14–17 km. Above the cold‐point tropopause, warm anomalies (<1 K) are observed for oceanic convection, whereas cool anomalies are displayed for land convection within 17–20 km. The amplitude of temperature anomalies increases for deeper convection, marked by higher 20 dBZ radar echo top heights or colder infrared cloud top temperatures. COSMIC also observes enhanced UTLS diurnal temperature variations of about 0.2–0.3 K in both regions near deep convection. ERA‐Interim shows generally good agreement with COSMIC on the UTLS temperature anomalies near deep convection but displays larger differences above the tropopause, especially near land convection.

Sediment provenance in the western Pacific warm pool from the last glacial maximum to the early Holocene: Implications for ocean circulation and climatic change

Sedimentary records including clay minerals, major elements, rare earth elements (REE) and 14C ages, were analyzed in one core from the western Pacific warm pool (WPWP), in order to identify the sources of sediments in the warm pool region from the last glacial maximum (LGM) to the early Holocene. The results showed that the sedimentation rate at the studied core site has decreased continuously since the LGM. Both major elements and clay minerals experienced staged changes in the study area. In the studied core, most major elemental concentrations decreased prior to 22kyrBP but increased gradually after 13kyrBP. Sediment provenance based on clay minerals and REEs further revealed that sediments were likely a mixture of dominant New Guinea sediments and Asian dust. The South Equatorial Current was the main force transporting New Guinea particles, while the East Asian winter monsoon and the correlated North Equatorial Countercurrent might have loaded the Asian dust into the study area. Moreover, Heinrich events were well imprinted in the western Pacific, and another cold climatic event that occurred at approximately 21kyrBP was also recorded in the study area. These events were similarly documented in ice from Greenland but were undistinguished in ice from the Antarctic and stalagmites from the Hulu Cave, China. A frequency analysis of multi-proxies confirmed that sun irradiance essentially controlled the variation in sedimentary records in the western Pacific from the LGM to the early Holocene.

A 50-Year Sr/Ca Time Series from an Enclosed, Shallow-Water Guam Coral: In situ Monitoring and Extraction of a Temperature Trend, Annual Cycle, and ENSO and PDO Signals

Bell, T.; Lander, M.A.; Jenson, J.W.; Randall, R.H.; Partin, J.W., and Prouty, N.G., 2019. A 50-year Sr/Ca time series from an enclosed, shallow-water Guam coral: In situ monitoring and extraction of a temperature trend, annual cycle, and ENSO and PDO signals. Journal of Coastal Research, 35(2), 269–286. Coconut Creek (Florida), ISSN 0749-0208.Located on the northern edge of the West Pacific Warm Pool and having a developed economy and modern infrastructure, Guam is well positioned and equipped for obtaining natural records of the west Pacific maritime paleoclimate. This study was a proof of concept to explore whether useful climate proxy records might be obtained from coral at readily accessible, even if geochemically nonoptimal, coastal sites. A 50-year Sr/Ca record (1960–2010) was thus obtained from a shallow-water, near-shore Porites lutea colony at a recreational facility inside Guam's Apra Harbor and compared with local and regional meteorological records, including the El Niño–Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) indices. The accessibility of the site enabled documentation of relevant environmental variables for 16 months (September 2009–December 2010): seawater δ18O, pH, seawater cations, and nitrate. Time series of seawater δ18O, pH, and cations show evidence of freshwater input from direct rainfall and stream discharge into the harbor. An anomalously higher mean and variable concentrations of Ba suggest the presence of river-borne, fine-grained terrigenous sediment. Nevertheless, the Sr/Ca time series reproduces a long-term warming trend seen in historical records of local air temperature and regional sea-surface temperature (SST) and closely tracks the ENSO and PDO indices over the entire 50-year record. The consistency of the results with Guam's historical instrumental records, previous coral δ18O results from Guam obtained by others, and previous Sr/Ca proxy results for SST in similar environments elsewhere demonstrate that accessible near-shore sites—where environmental conditions can be monitored—can produce useful Sr/Ca records of local and regional climate phenomena.

Impacts of SST anomalies in the Indian-Pacific basin on Northwest Pacific tropical cyclone activities during three super El Niño years

This study investigated the impact of sea surface temperature (SST) in several important areas of the Indian-Pacific basin on tropical cyclone (TC) activity over the western North Pacific (WNP) during the developing years of three super El Nino events (1982, 1997, and 2015) based on observations and numerical simulations. During the super El Nino years, TC intensity was enhanced considerably, TC days increased, TC tracks mostly recurved along the coasts, and fewer TCs made landfall in China. These characteristics are similar to the strong ENSO-TC relationship but further above the climatological means than in strong El Nino years. It indicates that super El Nino events play a dominant role in the intensities and tracks of WNP TCs. However, there were clear differences in both numbers and positions of TC genesis among the different super El Nino years. These features could be attributed to the collective impact of SST anomalies (SSTAs) in the tropical central-eastern Pacific and East Indian Ocean (EIO) and the SST gradient (SSTG) between the southwestern Pacific and the western Pacific warm pool. During 2015, the EIO SSTA was extremely warm and the anomalous anticyclone in the western WNP was enhanced, resulting in fewer TCs than normal. In 1982, the EIO SSTA and spring SSTG showed negative anomalies, followed by an increased anomalous cyclone in the western WNP and equatorial vertical wind shear. This intensified the conversion of eddy kinetic energy from large-scale flows, favorable for the westward shift of TC genesis. Consequently, anomalous TC activities during the super El Nino years resulted mainly from combined SSTA impacts of different key areas over the Indian-Pacific basin.

Growth variability of juvenile skipjack tuna (Katsuwonus pelamis) in the western and central Pacific Ocean

We examined growth rates of juvenile skipjack tuna using otolith daily increment analysis to clarify geographic differences in early-life stage growth in the western and central Pacific Ocean (WCPO). We used a liner regression model to analyze the relationship between standard length (SL) and age (in days). The slope of growth model was significant difference between regions. There were rapid increases in the otolith mean daily increment width of juveniles at 7 days after hatching (DAH) and 10 DAH in the Western Pacific Warm Pool (WARM) and North Pacific Tropical Gyre area (NPTG), respectively. These periods correspond to the timing at which larvae complete their digestive-system development and start the piscivory. We found significant difference in logarithmic back-calculated SL between WARM and NPTG at 3 DAH (p < 0.01). Mean growth rates until 10 DAH, when the larvae metamorphose into juveniles, were positively correlated with sea surface temperature (r = 0.52). These results indicate that geographic difference in the growth of juvenile skipjack tuna caught in the WCPO in boreal winter cause in the larval stage, and that these differences are related to sea surface temperature.

Decadal Change of the Western North Pacific Summer Monsoon Break around 2002/03

Abstract A significant decadal change is detected in the break of the western North Pacific summer monsoon (WNPSM) around 2002/03. For the period 1979–2002, the monsoon break occurs in early August, accompanied by noticeable convection suppression over the ocean to the east of the Mariana Islands (10°–20°N, 140°–160°E). However, for the period 2003–11, the monsoon break there is delayed until mid-August. This decadal change is attributable to the differences in the evolution of the WNPSM. Over this break region, convection becomes weaker after its peak in late July for the former period, and the monsoon break appears in early August. In contrast, for the latter period, convection continues strengthening in late July and reaches its peak in early August, and the monsoon break is delayed until mid-August. The differences in the evolution of sea surface temperature (SST) in the western Pacific warm pool region are responsible for the decadal change in the evolution of the WNPSM. In contrast to the former period, for the latter period the southern extent of the warm pool is remarkably warmed, and tends to be higher than the northern extent in mid- and late July, which enhances atmospheric convection nearby but inhibits the development of convection over the northern extent through a local meridional circulation. As the SST in the northern extent continues warming and becomes higher than that in the southern extent, the convection over the northern extent reaches its maximum intensification in early August. The presented results highlight that the spatial pattern of SST changes can modulate the subseasonal evolution of the WNPSM.

Evaluating AMIP Skill in Simulating Interannual Variability over the Indo–Western Pacific

AbstractLocal correlation between sea surface temperature (SST) and rainfall is weak or even negative in summer over the Indo–western Pacific warm pool, a fact often taken as indicative of weak ocean feedback on the atmosphere. An Atmospheric Model Intercomparison Project (AMIP) simulation forced by monthly varying SSTs derived from a parallel coupled general circulation model (CGCM) run is used to evaluate AMIP skills in simulating interannual variability of rainfall. Local correlation of rainfall variability between AMIP and CGCM simulations is used as a direct metric of AMIP skill. This “perfect model” approach sidesteps the issue of model biases that complicates the traditional skill metric based on the correlation between AMIP and observations. Despite weak local SST–rainfall correlation, the AMIP–CGCM rainfall correlation exceeds a 95% significance level over most of the Indo–western Pacific warm pool, indicating the importance of remote (e.g., El Niño in the equatorial Pacific) rather than local SST forcing. Indeed, the AMIP successfully reproduces large-scale modes of rainfall variability over the Indo–western Pacific warm pool. Compared to the northwest Pacific east of the Philippines, the AMIP–CGCM rainfall correlation is low from the Bay of Bengal through the South China Sea, limited by internal variability of the atmosphere that is damped in CGCM by negative feedback from the ocean. Implications for evaluating AMIP skill in simulating observations are discussed.