Tropical Western North Pacific Research Articles

Modulations of El Niño-Southern oscillation on tropical cyclone activity affecting Taiwan

Tropical cyclone (TC) activity affecting Taiwan and El Niño-Southern Oscillation (ENSO) exhibit systematic relationships in the July–September season. El Niño events can induce more [EN(more TC)] or fewer [EN(fewer TC)] TCs affecting Taiwan, but fewer TCs are dominantly caused by La Niña events [LN(fewer TC)]. In the EN(more TC) type, a meridionally stratified circulation pattern exists consisting of a westward-extending anomalous cyclone in the south over the western North Pacific (WNP) and anomalous anticyclonic shears in the north. Consequently, TC genesis tends to increase in the tropical WNP south of 20oN and decrease to the north. Increases of northward and northwestward TC movement from the tropical WNP toward the subtropical northwestern WNP result in more TCs affecting Taiwan. Variability features of TC activity and large-scale oceanic-atmospheric patterns in the LN(fewer TC) type are generally opposite to those of the EN(more TC) type. In the EN(fewer TC) type, an anomalous cyclone elongates northwestward from the tropical WNP toward Taiwan and Japan. TC genesis increases in the tropical WNP east of 150°E. These TCs exhibit enhanced northward movement. In the 120o-150°E region, suppressed TC formation is followed by reduced westward TC movement from the WNP toward Taiwan, leading to fewer TCs affecting Taiwan. For atmospheric circulation variability in the WNP, both the monsoon trough (MT) and western Pacific subtropical high (WPSH) weaken in the LN(fewer TC) type, but intensify in the EN(more TC) type. A significant southeastward intensification of the MT and an eastward retreat of the WPSH occur in the EN(fewer TC) type.

Concentrations of transparent exopolymer particles (TEPs) and their role in the carbon export in the South China Sea and western tropical North Pacific

The role of TEPs in the carbon cycle remains inadequately understood in oligotrophic tropical oceans. This study investigates TEP concentrations, distributions, sinking behavior and fluxes in the oligotrophic South China Sea (SCS) and western tropical North Pacific (WTNP). The results suggested that TEPs levels were relatively low [< 60 μg Xeq. L−1 (μg xanthan gum equivalent per liter)] in both regions, and they were higher in the epipelagic layer than in deeper layers. TEP concentrations correlated significantly positively with Chl a and picophytoplankton biomass, and TEP-associated carbon contributed significantly to particulate organic carbon (POC) pool in the SCS and WTNP. The sinking flux of TEPs constituted a mean of 61% of the total POC flux in the SCS and 46% in the WTNP, highlighting their important role in carbon export in these areas. Generally, this study should provide good insight into the role TEPs play in the carbon cycle in oligotrophic tropical oceans.

Mean, Annual, and Interannual Circulation and Volume Transport in the Western Tropical North Pacific From the Western Pacific Ocean State Estimates (WPOSE)

AbstractInterannual volume transport anomalies in the far western tropical North Pacific were calculated using the Western Pacific Ocean State Estimates (WPOSE; 2009–2017; 115°E−170°E, 15°S–27°N) to assess the magnitude, phase, and vertical structure of volume transport, and the relationship of interannual transport to the El Niño‐Southern Oscillation phenomena. The mean velocity and thermohaline structure for the WPOSE were verified using Argo climatology and autonomous glider observations. Transport was calculated for both an upper layer (surface‐26 kg m−3) and a lower layer (26–27.3 kg m−3). Annual volume transport anomalies were small north of 8°N but varied by 50% of the mean volume transport between the equator and 8°N. Interannual anomalies exceeded annual anomalies throughout the region. Volume transports of the North Equatorial Current, the Mindanao Current, and the South Equatorial Current/New Guinea Coastal Undercurrent increased beginning in September 2014, leading to a large increase in transport in the North Equatorial Counter Current before the mature phase of El Niño in 2015/2016. The increase in transport was related to the meridional gradient in thermocline depth centered at 5°N at the southern part of the Mindanao Dome, where extreme shoaling of the thermocline took place. Lower‐layer volume transports were not always in phase with those of the upper‐layer and made considerable contributions to total transport variability.

On the north equatorial current spatiotemporal modes and responses in the western boundary currents

The North Equatorial Current (NEC) and western boundary currents (WBCs) of the Kuroshio Current (KC) and the Mindanao Current (MC) characterize the circulation pattern and water mass transports in the tropical western North Pacific. We utilized the validated three-dimensional and time-dependent China Sea Multi-scale Ocean Modeling System to investigate the variability of the WBCs and the physical processes linked with the upstream NEC and associated with the circulation. The latitude-dependent KC and MC strengthen downstream from the NEC bifurcation (NB) and are maintained by the northern and southern parts of the NEC, respectively. By effectively isolating the NEC spatiotemporal modes representing its intensity, meridional migration, and bifurcation location, we investigate the responses of the WBCs to the characteristics in an integrated NEC-WBCs current system. The NEC intensity peaks in spring and summer, while its bifurcation location strongly correlates with its migration. The southward (northward) NEC migration during early spring (late autumn) feeds more NEC waters to the MC (KC). Yet, the simultaneous southward (northward) shifting NB distributes more NEC waters to the KC (MC) and offset the contribution of the migration. Thus, the intensity of the KC (MC) is primarily controlled by the NEC intensity, reaching the maximum during spring (summer). The specific water masses such as the North Pacific Tropical Water and the Equatorial Surface Water, are bounded by the time-dependent NB, distributing northward and southward respectively through the WBCs. By resolving spatiotemporal modes in NEC-KC-MC, the study provides a new understanding to the WBCs in the NEC-WBCs current system.

Asian rainfall anomaly patterns associated with interannual variations of early and peak summer rainfall over the Indochina Peninsula

AbstractThis study investigates the rainfall anomaly patterns over Asia associated with interannual variations of early summer (May–June) and peak summer (July–August) Indochina Peninsula (ICP) rainfall during 1979–2016. It is found that the early and peak summer rainfall variation in the ICP displays an out‐of‐phase relation to that in central Asia and central China, respectively. Analysis reveals that the out‐of‐phase early summer rainfall variation between the ICP and central Asia tends to occur in ENSO decaying years and the out‐of‐phase peak summer rainfall variation between the ICP and central China tends to occur in ENSO developing years. The early summer out‐of‐phase rainfall anomaly pattern between the ICP and central Asia forms due to a Rossby wave type response to anomalous heating extending from the ICP to northeast India, which is attributed to a combined influence of same sign sea surface temperature (SST) anomalies in the equatorial eastern Pacific and tropical southwestern Indian Ocean and opposite sign SST anomalies in the tropical western North Pacific. The peak summer out‐of‐phase rainfall variation between the ICP and central China forms due to a meridional atmospheric circulation pattern over East Asia and the western North Pacific, which is resulted from the combined impacts of opposite SST anomalies in the equatorial central Pacific and tropical southeastern Indian Ocean. Further analysis indicates that the out‐of‐phase rainfall variations between the ICP and central Asia in early summer and between the ICP and central China in peak summer may occur without anomalous tropical Indo‐Pacific SST forcing.

Asymmetric Changes in Intraseasonal Oscillation Intensity over the Tropical Western North Pacific in El Niño and La Niña Developing Summers

Abstract The intraseasonal oscillations (ISOs) over the tropical western North Pacific (WNP) modulate atmospheric convection and heating and affect weather and climate in remote regions through atmospheric teleconnection. The present study unravels the fact that the ISO intensity increase over the tropical WNP in El Niño developing summers is larger, with the center located eastward, compared with the decrease in La Niña developing summers. The asymmetric ISO intensity changes are attributed to the eastward shift of regions of anomalous low-level westerly winds, ascent, easterly shear of zonal winds, and large moisture in El Niño developing summers and westward shift of regions of opposite anomalies in La Niña developing summers, respectively. The asymmetric atmospheric mean anomalies, in turn, are due to the westward shift of anomalous cooling in La Niña developing summers compared to anomalous warming in El Niño developing summers. The 10–20- and 30–60-day ISOs show different patterns of intensity variations due to their different source regions and propagation paths. Atmospheric model simulations confirm the asymmetric response of boreal summer ISO intensity over the tropical WNP to El Niño and La Niña events and the role of asymmetric atmospheric background field changes. Sensitivity experiments illustrate that asymmetric changes in the ISO intensity and atmospheric background fields over the tropical WNP are due to their asymmetric response to opposite tropical central-eastern Pacific SST anomalies. The asymmetry in tropical central-eastern Pacific SST anomalies in El Niño and La Niña events has a small impact on asymmetric ISO intensity changes over the tropical WNP.

Severe Cold Winters in East Asia Linked to First Winter of La Niña Events and in North America Linked to Second Winter

AbstractLa Niña often persists for two years, unlike El Niño. Five single‐year and 8 multi‐year La Niña events occurred over 71 boreal winters (1950/51–2020/21). Atmospheric reanalysis data show a marked increase in diabatic heating over the tropical northwestern Pacific (TNWP) in first winters of La Niña events in addition to a decrease in diabatic heating over the tropical eastern Pacific, common to first and second winters. Linear baroclinic model experiments indicate that combinations of the heating form different atmospheric teleconnections in the mid‐latitudes; first winters with the increased heating over the TNWP form teleconnection across the North Pacific, leading to cold winters in Japan. In second winters with the reduced heating over the TNWP (∼50% of first winters), teleconnection shifts east, resulting in severe cold winters in northwestern North America, whereas there is no impact on Japan.

Boreal Summer Negative Correlation Relationship Between Interannual SST and Precipitation Anomalies in the Tropical and Subtropical Western North Pacific

AbstractWestern North Pacific (WNP) is a key region for ENSO development and its effect on East Asian summer monsoon. Different from most other tropical oceanic areas, sea surface temperature (SST) and precipitation anomalies over the WNP are negatively correlated in boreal summer, indicating the ocean is mainly driven by the atmosphere rather than the opposite over there. Two interannual variability modes of such negative correlation relationship are revealed with SVD method, called tropical and subtropical WNP air‐sea coupled modes (WNP‐TCM and WNP‐STCM) in terms of their locations accordingly. Their core regions are both consistent with the near‐zero line of low‐level zonal wind, corresponding to the Northwest Pacific monsoon trough and the west ridge of WNP subtropical high, respectively. In WNP‐TCM, precipitation anomaly is caused by local vorticity anomaly, which is the result of Matsuno‐Gill response to SST anomalies with the reversed signs in central equatorial Pacific and eastern equatorial Indian Ocean. In WNP‐STCM, it is produced by local divergence anomaly, which is remotely forced by SST anomalies with the same signs in equatorial Pacific and Indian Ocean through anomalous Hadley circulation. SST anomalies in the two modes are both controlled by ocean thermal process, especially the solar radiation anomaly. The key factors determining such unique WNP air‐sea interaction are the weak latent heat flux and horizontal SST advection anomalies, which are both closely related to the near‐zero low‐level zonal wind. In addition, WNP‐TCM often happens in ENSO developing summer whereas WNP‐STCM usually appears in ENSO decaying summer.

What Determine the Performance of the ENSO‐East Asian Winter Monsoon Relationship in CMIP6 Models?

AbstractThis study evaluates the relationship between El Niño‐Southern Oscillation (ENSO) and the East Asian winter monsoon (EAWM) in 26 Coupled Model Intercomparison Project Phase 6 (CMIP6) models. Of particular concern is the plausible factors for the model's capability of simulating the ENSO‐EAWM relationship. Results show that the model's ability of simulating the ENSO‐EAWM relationship is more dependent upon the longitudinal extension of ENSO‐related equatorial Pacific sea surface temperature (SST) anomalies than the amplitude of the equatorial central‐eastern Pacific SST anomalies. The influence of the amplitude of ENSO on the simulation of the ENSO‐EAWM relationship depends on the westward extension of ENSO‐related equatorial Pacific SST anomalies. Another factor for the model's ability of simulating the ENSO‐EAWM relationship is the SST anomalies in the tropical western North Pacific (WNP). A westward extension of the equatorial Pacific SST anomalies shifts the west branch of anomalous Walker circulation too far westward, which causes westward displaced anomalous ascending (descending) motion around the Philippines through modulating regional meridional vertical circulation in El Niño (La Niña) years. The weak SST anomalies in the tropical WNP lead to the failure of inducing anomalous lower‐level anticyclone (cyclone) over the Philippine Sea through a Rossby wave response in El Niño (La Niña) years. The accompanying weak anomalous lower‐level southwesterly (northeasterly) winds along the west flank of the anomalous anticyclone (cyclone) account for the weak ENSO‐EAWM relationship.

Nutrient regulation of biological nitrogen fixation across the tropical western North Pacific.

Nitrogen fixation is critical for the biological productivity of the ocean, but clear mechanistic controls on this process remain elusive. Here, we investigate the abundance, activity, and drivers of nitrogen-fixing diazotrophs across the tropical western North Pacific. We find a basin-scale coherence of diazotroph abundances and N2 fixation rates with the supply ratio of iron:nitrogen to the upper ocean. Across a threshold of increasing supply ratios, the abundance of nifH genes and N2 fixation rates increased, phosphate concentrations decreased, and bioassay experiments demonstrated evidence for N2 fixation switching from iron to phosphate limitation. In the northern South China Sea, supply ratios were hypothesized to fall around this critical threshold and bioassay experiments suggested colimitation by both iron and phosphate. Our results provide evidence for iron:nitrogen supply ratios being the most important factor in regulating the distribution of N2 fixation across the tropical ocean.

An Interdecadal Change in the Influence of ENSO on the Spring Tibetan Plateau Snow-Cover Variability in the Early 2000s

AbstractEl Niño–Southern Oscillation (ENSO) and the Tibetan Plateau snow cover are important factors in interannual climate variability. The relationship between ENSO and the Tibetan Plateau snow variation is still undetermined. While some studies suggested that ENSO is a key factor of changes in snow cover over the Tibetan Plateau, other studies noted independence between the two. The present study revealed a prominent interdecadal change in the relationship between ENSO and the spring Tibetan Plateau snow-cover variation in the early 2000s. There is a significant positive correlation between ENSO and the spring Tibetan Plateau snow-cover variation in the period 1988–2003, but an obvious negative relationship is detected in the period 2004–19. The interdecadal change in the ENSO–snow relationship is related to the distinct pathway of ENSO influence on the spring Tibetan Plateau snow-cover variation during the two periods. In the period 1988–2003, ENSO induces anomalous convection over the tropical western North Pacific that in turn causes atmospheric circulation and moisture anomalies over the Tibetan Plateau. The resultant winter snow anomalies over the central-eastern Tibetan Plateau persist to the following spring. In the period 2004–19, ENSO induces North Atlantic sea surface temperature (SST) anomalies in winter that are maintained to the following spring. The North Atlantic SST anomalies then stimulate the atmospheric circulation anomalies extending to the Tibetan Plateau that induce snow-cover anomalies there in spring. The different processes of ENSO influence lead to opposite anomalies of spring snow cover over the Tibetan Plateau in the two periods.

An Extreme Tropical Cyclone Silence in the Western North Pacific in July 2020

ABSTRACT The western North Pacific (WNP) tropical cyclone (TC) number in July 2020 hit a record low since 1949, with no TC formation throughout July. It is shown that this record-breaking TC event can be largely attributed to the extremely strong lower-level anticyclone and subsidence over the TC-genesis-prone region (GPR), concurrent with an intensified western Pacific subtropical high (WPSH) there. This configuration is closely linked to both the air–sea interaction in the tropical Indo-western Pacific and the vigorous wave activities over the mid-high latitude Eurasia. The warming western Indian Ocean and the resultant active convection excited an anomalous lower-level anticyclone and descending atmospheric Kelvin wave over the GPR, together with a strengthened WPSH. Accompanied with this monthly background field, long-lasting Madden-Julian Oscillation was confined west of 90°E, which intensified the proposed anticyclonic vorticity and decreased the moisture condition in the South China Sea and tropical WNP. Meanwhile, two active wave fluxes prevailed across the jet streams over the mid-high latitude Eurasia with a quasi-barotropic cyclonic abnormity dominating from Northeast China to Japan. The resultant positive potential vorticity on its south excited the anomalous local ascending around 35°N, which favoured the upper-level convergence in the tropical WNP and reinforced the local subsidence through modulating the meridional circulation. This extratropical impact further aggravated the suppressed circulation condition for TC formation and increased the chance that the extreme tropical cyclone silence would happen.

The habitat-modifying red alga Ramicrusta on Pacific reefs: A new generic record for the Tropical Northwestern Pacific and the description of four new species from Guam.

The genus Ramicrusta (order Peyssonneliales) is a new record for Micronesia, with range expansions of Ramicrusta fujiiana and R. lateralis to Guam. In addition, four species (Ramicrusta adjoulanensis, R. asanitensis, R. labtasiensis, and R. taogamensis) are newly described from Guam using molecular and anatomical characters. Ramicrusta lateralis specimens from Guam share most anatomical features with the holotype description from Vanuatu, but the plants from Guam are more tightly adherent, rigid, and robust than those of Vanuatu. Ramicrusta adjoulanensis possesses a well-developed epithallus with frequent cell fusions, secondary pit connections, and lacking hair bases or trichocytes, similar to Ramicrusta bonairensis. Ramicrusta adjoulanensis differs from other Ramicrusta species in having occasionally free margins and being attached by frequently produced, relatively long rhizoids (75–100 μm long). Ramicrusta asanitensis shares features with many other species, but the thickness of the crust (upwards of 2 mm thick), heavy calcification in the epithallus, and the extent of secondary, tertiary, and quaternary growth, differentiate it from other Ramicrusta species. Ramicrusta labtasiensis shares features with its close relative Ramicrusta lateralis but possesses frequent, robust, and relatively long rhizoids (75–95 μm long) throughout its entire undersurface. Ramicrusta taogamensis resembles its close relative Ramicrusta appressa but is primarily distinguished by its generally well-developed epithallus with occasional secondary pit connections and cell fusions. The six species reported here make Guam equal to Vanuatu in currently having the highest known species richness of Ramicrusta in the world.

Attribution of the seasonality of atmospheric heating changes over the western tropical Pacific with a focus on the spring season

Atmospheric diabatic heating, a major driving force of atmospheric circulation over the tropics, is strongly confined to the tropical western North Pacific (TWNP) region, with the global warmest sea surface temperature (SST). The changes in diabatic heating over the TWNP, which exert great impacts on the global climate system, have recently exhibited a noticeable seasonal dependence with a remarkable increase in boreal spring. In this study, we applied observations, reanalysis data, and numerical experiments to investigate the causes of the seasonality in heating changes. Results show that in boreal spring convection is more sensitive to the TWNP SST, leading to a more significant enhancement of deep convection, although the increase in the SST is nearly the same as that in the other seasons. In the non-spring seasons, the enhanced convection due to increased local SST is suppressed by the anomalous anticyclonic wind shear over the TWNP, generated by the easterly wind anomalies induced by the tropical Indian Ocean (TIO) warming via the Kevin waves. However, the TIO warming does not show any suppressing effect in spring because it is much weaker than that in the other seasons and thus the warming itself cannot induce sufficient convective heating anomalies to excite the Kelvin waves.

Modulation of centennial-scale hydroclimate variations in the middle Yangtze River Valley by the East Asian-Pacific pattern and ENSO over the past two millennia

Meteorological observations reveal an inverse relationship of summer monsoon rainfall between the tropical western North Pacific (WNP) and middle Yangtze River Valley (mid-YRV) on interannual and intraseasonal timescales. This seesaw-like pattern, named the East Asian-Pacific (EAP) pattern, is one of the major teleconnections that affect summer precipitation in the mid-YRV. However, due to the scarcity of long and continuous instrumental data, the behavior and mechanism of the EAP pattern on a long timescale are poorly understood. Here, we present a composite high-resolution summer monsoon rainfall record from 230 BC to 1810 AD, based on 1221 δ18O measurements and 22 230Th dates from two stalagmites (EB1 and EB5) in Shenqi Cave over the mid-YRV. Our new record shows that monsoon rainfall in the mid-YRV varies at a dominant periodicity of ∼100 years, probably linked to solar activity and ENSO variability. The multi-centennial fluctuations in the EB record, broadly consistent with the North Hemisphere temperature reconstruction, display strong similarities to two stalagmite δ18O records from nearby Heshang and Luoshui caves. To represent the integrated hydroclimate variations over the mid-YRV, we extracted the first principal component (PC1) of these three cave δ18O records by using the principal component analysis. We reveal a striking inverse relationship of wet/dry conditions between the mid-YRV and tropical WNP, suggesting that changes in the EAP pattern have an important impact on centennial-scale monsoon precipitation changes. Furthermore, we compile monsoon rainfall records from the mid-YRV and WNP to reconstruct a new EAP index. This EAP index exhibits a strong correlation with the ENSO proxy, confirming that the ENSO variability may play a key role in modulating the monsoon rainfall in the mid-YRV through the EAP teleconnection.

Changing Impact of ENSO Events on the Following Summer Rainfall in Eastern China since the 1950s

AbstractEl Niño–Southern Oscillation (ENSO) events, which generally mature in winter, profoundly affect the following summer rainfall in eastern China (ECSR), but such an impact can change significantly with decadal background. This study examines how the impact has changed since the 1950s by running correlation and regression analyses. It is found that ENSO’s impact on ECSR has undergone two decadal shifts, one in the late 1970s and the other in the 1990s. Sequentially, three distinct ENSO-induced ECSR anomaly patterns are categorized, which exhibit both robust and changeable sides. The robust side manifests generally as more precipitation in the Yangtze River basin affected by the anomalous tropical western North Pacific anticyclone (WNPAC) in the post–El Niño summer. The changeable side is reflected in the more variable ENSO-induced rainfall anomalies north of the Yangtze River, due to the different ENSO-induced East Asian midlatitude circulation anomalies. Meanwhile, the El Niño–induced drought in South China has been enhanced since the late 1970s with the intensification of the anomalous WNPAC. ENSO’s changing impact on the ECSR stems from the changes of ENSO-induced tropical and midlatitude circulation anomalies over East Asia, which are associated with different zonal (from the tropical Pacific to the Indian Ocean) and meridional (from the tropical Pacific to the midlatitude North Pacific) teleconnections of ENSO-induced SST anomalies. The former affects the intensity and location of the anomalous WNPAC by affecting the Indian Ocean capacitor effect and convection anomalies over the tropical Indo-western Pacific. The latter modulates the ocean-to-atmosphere feedback in the midlatitude North Pacific, contributes to different local geopotential anomaly sources, and then directly or indirectly through the Rossby wave train affects the East Asian midlatitude circulation.

Atmospheric and Surface Seawater CO2 Measurements on R/V ISABU in the Western North Pacific in the Summer of 2018

The ocean takes up approximately 24% of anthropogenic carbon dioxide (CO&lt;sub&gt;2&lt;/sub&gt;) emitted into the atmosphere in a year. The oceanic CO&lt;sub&gt;2&lt;/sub&gt; uptake shows regional and seasonal differences depending on physical and chemical characteristics of seawater and biological activities (such as CO&lt;sub&gt;2&lt;/sub&gt; fixation). In the tropical Western North Pacific, the surface water temperature is high, the supply of deep water is limited, and tropical cyclones usually pass in summer. We investigated atmospheric and sea surface CO&lt;sub&gt;2&lt;/sub&gt; concentrations in this area using the continuous underway pCO2 measuring system equipped on the Research Vessel ISABU of Korea Institute of Ocean Science and Technology for about 21 days from August 29 to September 19, 2018. During the cruise, 9,367 CO&lt;sub&gt;2&lt;/sub&gt; data were obtained from this measuring system with temperature, salinity, and GPS information. Higher CO&lt;sub&gt;2&lt;/sub&gt; concentrations of the surface seawater than those of the atmosphere were observed in the areas of 22°N-23.5°N and 29°N-35°N where CO&lt;sub&gt;2&lt;/sub&gt; was emitted into the atmosphere, while most of the areas between 17.5°N and 20.5°N were sinks for the atmospheric CO&lt;sub&gt;2&lt;/sub&gt;. This dataset can be used for future research on the distribution of partial pressure of carbon dioxide over the global ocean surface.

A Contrast of Recent Changing Tendencies in Genesis Productivity of Tropical Cloud Clusters over the Western North Pacific in May and October

Tropical cloud clusters (TCCs) are embryos of tropical cyclones (TCs) and may have the potential to develop into TCs. The genesis productivity (GP) of TCCs is used to quantify the proportion of TCCs that can evolve into TCs. Recent studies have revealed a decrease in GP of western North Pacific (WNP) TCCs during the extended boreal summer (July–October) since 1998. Here, we show that the changing tendencies in GP of WNP TCCs have obvious seasonality. Although most months could see recent decreases in GP of WNP TCCs, with October experiencing the strongest decreasing trend, May is the only month with a significant recent increasing trend. The opposite changing tendencies in May and October could be attributed to different changes in low-level atmospheric circulation anomalies triggered by different sea surface temperature (SST) configurations across the tropical oceans. In May, stronger SST warming in the tropical western Pacific could prompt increased anomalous westerlies associated with anomalous cyclonic circulation, accompanied by the weakening of the WNP subtropical high and the strengthening of the WNP monsoon. Such changes in background atmospheric circulations could favor the enhancement of atmospheric eddy kinetic energy and barotropic energy conversions, resulting in a recent intensified GP of WNP TCCs in May. In October, stronger SST warming in the tropical Atlantic and Indian Oceans contributed to anomalous easterlies over the tropical WNP associated with anomalous anticyclonic circulation, giving rise to the suppressed atmospheric eddy kinetic energy and recent weakened GP of WNP TCCs. These results highlight the seasonality in recent changing tendencies in the GP of WNP TCCs and associated large-scale atmospheric-oceanic conditions.

Size-Fractionated Biogenic Silica Standing Stocks and Carbon Biomass in the Western Tropical North Pacific: Evidence for the Ecological Importance of Pico-Sized Plankton in Oligotrophic Gyres

Biogenic silica (bSi) standing stocks and carbon (C) biomass of small plankton are rarely studied together in previous analyses, especially in oligotrophic gyres. Within the oligotrophic western tropical North Pacific, based on size-fractionated bSi and biovolume-derived C analyses in three size fractions (i.e., 0.2–2; 2–20; &amp;gt;20 μm), we observed that picophytoplankton (&amp;lt;2 μm) contributed a measurable and significant proportion of both bSi standing stocks and C biomass. The estimated contributions of pico-sized fraction to total bSi standing stocks and living C biomass averaged 66 and 49%, respectively, indicating the ecological importance of small plankton in the Si and C cycles in oligotrophic areas. In contrast, the average contributions of large diatoms (i.e., cells &amp;gt;2 μm) to total bSi standing stocks and living C biomass were 9 and 16%, respectively, suggesting that the role of diatoms in marine Si and C cycles may have been overestimated in previous analyses. Due to the overwhelming predominance of picocyanobacteria in the oligotrophic western tropical North Pacific, their contributions to total bSi stocks and C biomass were quantitatively important and accounted for more of the bSi and C associated with living cells than did diatoms. In addition, water temperature and light intensity were likely the key determinants of the variations in size-fractionated bSi standing stocks and living C biomass, but not nutrient availability. Collectively, these findings encourage a reconsideration of the previously underestimated role of small plankton in understanding the Si and C cycles in the ocean, and may provide insights into the interpretations of disproportionate budgets of Si and C in oligotrophic oceans.

Decision-Tree-Based Classification of Lifetime Maximum Intensity of Tropical Cyclones in the Tropical Western North Pacific

The National Typhoon Center of the Korea Meteorological Administration developed a statistical–dynamical typhoon intensity prediction model for the western North Pacific, the CSTIPS-DAT, using a track-pattern clustering technique. The model led to significant improvements in the prediction of the intensity of tropical cyclones (TCs). However, relatively large errors have been found in a cluster located in the tropical western North Pacific (TWNP), mainly because of the large predictand variance. In this study, a decision-tree algorithm was employed to reduce the predictand variance for TCs in the TWNP. The tree predicts the likelihood of a TC reaching a maximum lifetime intensity greater than 70 knots at its genesis. The developed four rules suggest that the pre-existing ocean thermal structures along the track and the latitude of a TC’s position play significant roles in the determination of its intensity. The developed decision-tree classification exhibited 90.0% and 80.5% accuracy in the training and test periods, respectively. These results suggest that intensity prediction with the CSTIPS-DAT can be further improved by developing independent statistical models for TC groups classified by the present algorithm.