Tropical North Pacific Research Articles

Factors of boreal summer latent heat flux variations over the tropical western North Pacific

Surface latent heat flux (LHF) is an important component in the heat exchange between the ocean and atmosphere over the tropical western North Pacific (WNP). The present study investigates the factors of seasonal mean LHF variations in boreal summer over the tropical WNP. Seasonal mean LHF is separated into two parts that are associated with low-frequency (> 90-day) and high-frequency (≤ 90-day) atmospheric variability, respectively. It is shown that low-frequency LHF variations are attributed to low-frequency surface wind and sea-air humidity difference, whereas high-frequency LHF variations are associated with both low-frequency surface wind speed and high-frequency wind intensity. A series of conceptual cases are constructed using different combinations of low- and high-frequency winds to inspect the respective effects of low-frequency wind and high-frequency wind amplitude to seasonal mean LHF variations. It is illustrated that high-frequency wind fluctuations contribute to seasonal high-frequency LHF only when their intensity exceeds the low-frequency wind speed under which there is seasonal accumulation of high-frequency LHF. When high-frequency wind intensity is smaller than the low-frequency wind speed, seasonal mean high-frequency LHF is negligible. Total seasonal mean LHF anomalies depend on relative contributions of low- and high-frequency atmospheric variations and have weak interannual variance over the tropical WNP due to cancellation of low- and high-frequency LHF anomalies.

A Logistic-growth-equation-based Intensity Prediction Scheme for Western North Pacific Tropical Cyclones

Accurate prediction of tropical cyclone (TC) intensity remains challenging due to complex physical processes involved in TC intensity changes. A 7-day TC intensity prediction scheme based on the logistic growth equation (LGE) for the western North Pacific (WNP) has been developed using the observed and reanalysis data. In the LGE, TC intensity change is determined by a growth term and a decaying term. These two terms comprise four free parameters, including the time-dependent growth rate and maximum potential intensity (MPI), and two constants. With 33 years of training samples, optimal predictors are selected first, and then the two constants are determined based on the least square method, making the regressed growth rate from the optimal predictors as close to the observed as possible. The growth rate is further estimated based on a step-wise regression (SWR) method and a machine learning (ML) method for the period 19822014. Using the LGE-based scheme, a total of 80 TCs during 20152017 are used to make independent forecasts. Results show that the root mean square errors of the LGE-based scheme are much smaller than those of the official intensity forecasts from the China Meteorological Administration (CMA), especially for TCs in the coastal regions of East Asia. Moreover, the scheme based on the ML shows better forecast skills than that based on the SWR. The new prediction scheme also exhibits strong potential for rapid intensification and weakening forecasts and an extension of the CMA current 5-day forecasts to 7-day forecasts.

The Springtime Western Pacific Pattern: Its Formation and Maintenance Mechanisms and Climate Impacts

AbstractBased on daily data from the Japanese 55-year Reanalysis (JRA-55) covering the spring seasons from 1958 to 2018, this study examines the formation mechanisms and climate impacts of the springtime western Pacific (WP) pattern as subseasonal climate variability over North Pacific. Results suggest that the springtime WP pattern arises from a weak dipole-like disturbance over North Pacific and disturbances over East Asia. The energetic analysis suggests that the baroclinic energy conversion acts as an important energy source to balance the available potential energy loss caused by transient eddies and diabatic heating and acts as a kinetic energy (KE) source for the WP pattern. For the feedback forcing by total transient eddies, it acts as a major KE source for the WP pattern before day 0 and acts as a strong KE sink after day 0. It turns out that the barotropic energy conversion makes only weak KE contribution to the WP pattern. Once the WP pattern forms, East Asia and North America experience strong surface air temperature anomalies of opposite signs, while strong sea surface temperature anomalies are found to occur over midlatitude and tropical North Pacific at the same time. Concurrently, the Pacific jet and the storm track shift northward or southward around their climatological position. In addition, a dipole-like shallow convective anomaly appears over midlatitude North Pacific, and a band of anomalous deep convection tends to occur in the tropics as the energy of the WP pattern propagates into the region.

Iron(II) in the world's oxygen deficient zones

Oxygen deficient zones (ODZs) are regions in the marine water column where the concentration of oxygen is very low or zero and respiration occurs using nitrate as the terminal electron acceptor. Under such conditions, iron (Fe) would be predicted to be in the +3 valence state, and its geochemistry would be largely unaffected by the absence of oxygen. In contrast, observations show that Fe(II) is a major and sometimes dominant oxidation state in ODZs, even in the presence of a substantial amount of excess nitrate. Fe(II) plays an important role in Fe transport in ODZs. They are generally enriched in Fe throughout the regions where nitrate reduction is occurring. Here, it is argued that Fe(II) maxima arise as a result of processes common to the Eastern Tropical North and South Pacific as well as the Arabian Sea. Recent work has revealed that lateral advection of Fe(II) from reducing sediments is perhaps the major source and it appears that physical processes that are still poorly understood act over a surprisingly narrow density range. Tracers including iodine and 228Ra have been extremely useful in constraining these processes. The findings reported here have important implications for Fe transport from the continental margin to the ocean interior, and for microbes involved in the nitrogen cycle that have high Fe requirements.

Multiple Modulating Processes for Intensive Tropical Cyclone Activity Affecting Taiwan in September 2016

September 2016 harbored record-breaking three tropical cyclones (TCs) affecting Taiwan within a month. Multiple modulating processes governing these three TC events and associated rainfall and intensification features are examined. September 2016 fell in a La Nina phase. For interannual variability, major warm sea surface temperature (SST) anomalies in September 2016 shifted eastward toward the tropical western North Pacific (WNP) compared to the Maritime Continent during other La Nina events. These SST anomalies induced strong convergence and convection anomalies facilitating TC formation. The TCs formed in the joint region between a northern anticyclonic anomaly over the northern WNP and a southern cyclonic anomaly extending from the tropical WNP toward Taiwan. They were steered by anomalous easterly/southeasterly flows northwestward toward an anomalous cyclonic center overlying Taiwan, leading to intensive TC activity affecting Taiwan. For intraseasonal variability, the three TCs of September 2016 were steered by anomalous easterly/southeasterly flows in the southern section of a 30–60-day anomalous anticyclone over the subtropical WNP northwestward toward a 30–60-day anomalous cyclone that lay over Taiwan. During these events, local rainfall in Taiwan was mainly affected by moisture convergence due to 3–10-day transient anomalies, rather than intraseasonal anomalies. For TC intensification processes, faster intensification was assisted by decreases in vertical wind shear and increases in upward motion, moisture convergence, upper-level divergence, and SST. Intensification processes were mainly affected by transient anomalies. Overall, interannual and intraseasonal anomalies modulated TC genesis and movement, while transient anomalies influenced local rainfall and intensification processes.

Meridional Migration of Eastern North Pacific Tropical Cyclogenesis: Joint Contribution of Interhemispheric Temperature Differential and ENSO

AbstractThis study finds that the meridional migration of tropical cyclogenesis (TCG) between June and November over the eastern North Pacific is mainly due to changes in the Hadley circulation and associated changes in the location of the intertropical convergence zone as modulated by ENSO and the interhemispheric temperature differential (ITD). These two factors are largely independent of each other and play significant and nearly equivalent roles in the meridional migration of TCG over the ENP from 1979 to 2018. Through the use of regression models, we find that the combination of these two factors has significant skill in capturing annual TCG meridional distribution over the ENP basin. June–October ENSO, as defined by Niño‐3.4, can be skillfully predicted with ∼1 season lead time, while ITD can be skillfully predicted ∼3 seasons in advance. This predictability allows for the potential for seasonal outlooks of TCG meridional movement over the ENP. The influence of ITD on TCG meridional migration over the ENP has repercussions on climate change timescales as well, due to the potential TCG meridional migration over the ENP in response to global warming. Under the Shared Socioeconomic Pathways 2–4.5 scenario, the ITD shows a robust increase among the 23 models of version 6 of the Coupled Model Intercomparison Project. However, the considerable uncertainty in the model response of ENSO to global warming reduces confidence in projections of TCG meridional migration over the ENP.

What Caused the Unprecedented Absence of Western North Pacific Tropical Cyclones in July 2020?

AbstractWhile the Atlantic basin experienced the busiest hurricane season in 2020, the typhoon activity over the western North Pacific (WNP) was also record‐setting with no tropical cyclone (TC) formation in July 2020, which is the first time in available historical records. The unprecedented absence of TC formation is consistent with the extremely unfavorable large‐scale conditions that are linked to an anomalous anticyclonic circulation over the WNP, which results mainly from sea surface temperature (SST) anomalies across the tropical oceans. Numerical experiments suggest that the tropical Indian Ocean SST anomalies, which were warmest in July 2020 since 1965, play a dominant role in the anomalous WNP anticyclonic circulation, while the increased zonal SST gradient across the tropical Pacific and the Atlantic SST warming also have important contributions. This study suggests that the configuration of SST anomalies across the tropical basins is quite important to the WNP TC activity.

What Modulates the Intensity of Synoptic-Scale Variability over the Western North Pacific during Boreal Summer and Fall?

AbstractThe present study investigates the factors that affect the year-to-year change in the intensity of synoptic-scale variability (SSV) over the tropical western North Pacific (TWNP) during boreal summer and fall. It is found that the intensity of the TWNP SSV in summer is associated with the equatorial central-eastern Pacific sea surface temperature (SST) anomalies that modulate the background fields through a Rossby wave response both in the source region and along the propagation path of the synoptic-scale disturbances. In fall, the intensity of the TWNP SSV is related to an SST anomaly pattern with opposite anomalies in the equatorial central Pacific and TWNP that modulates the background fields from the equatorial central Pacific to TWNP. However, the equatorial central Pacific SST anomalies alone fail to change the intensity of the TWNP SSV as the induced background field changes are limited to the equatorial central Pacific. It is shown that tropical western Pacific SST anomalies may induce notable changes in the intensity of the TWNP SSV. The relation of the TWNP SSV to the equatorial eastern Pacific SST is weak due to opposite SST anomalies in different types of years. Both seasonal mean and intraseasonal flows provide sources of barotropic energy for the change in the intensity of the TWNP synoptic-scale disturbances in summer. Seasonal mean flow has a main contribution to the barotropic energy conversion for the change in the intensity of the TWNP synoptic-scale disturbances in fall.

Different Configurations of Interannual Variability of the Western North Pacific Subtropical High and East Asian Westerly Jet in Summer

This study investigates the circulation and precipitation anomalies associated with different configurations of the western North Pacific subtropical high (WNPSH) and the East Asian westerly jet (EAJ) in summer on interannual timescales. The in-phase configuration of the WNPSH and EAJ is characterized by the westward (eastward) extension of the WNPSH and the southward (northward) shift of the EAJ, which is consistent with the general correspondence between their variations. The out-of-phase configuration includes the residual cases. We find that the in-phase configuration manifests itself as a typical meridional teleconnection. For instance, there is an anticyclonic (cyclonic) anomaly over the tropical western North Pacific and a cyclonic (anticyclonic) anomaly over the mid-latitudes of East Asia in the lower troposphere. These circulation anomalies are more conducive to rainfall anomalies over the Yangtze River basin and south Japan than are the individual WNPSH or EAJ. By contrast, for the out-of-phase configuration, the mid-latitude cyclonic (anticyclonic) anomaly is absent, and the lower-tropospheric circulation anomalies feature an anticyclonic (cyclonic) anomaly with a large meridional extension. Correspondingly, significant rainfall anomalies move northward to North China and the northern Korean Peninsula. Further results indicate that the out-of-phase configuration is associated with the developing phase of ENSO, with strong and significant sea surface temperature (SST) anomalies in the tropical central and eastern Pacific which occur simultaneously during summer and persist into the following winter. This is sharply different from the in-phase configuration, for which the tropical SSTs are not a necessity.

Influence of the QBO on tropical convection and its impact on tropical cyclone activity over the western North Pacific

On the basis of 70 hPa zonal wind (U70)-defined quasi-biennial oscillation (QBO) events, after removing the El Nino–Southern Oscillation (ENSO) signal, the present study investigates the process by which the QBO modulates tropospheric circulation and convection during summer (between July and October), when tropical cyclone (TC) activities enter their peak period. Concurrent with the western phase of the QBO (QBOW), significant tripole pattern modulations over the tropical Indo-Pacific Ocean are regressed onto the residual part of U70 after removing the ENSO signal, with enhanced convection observed over its central branch (0°–10°N, 120°E–180°) and the inactive convection branches located to both sides. A ventilation opening-like effect is exerted on the monsoon trough, which is shifted southward under the QBOW phase. According to the QBO-associated changes in the circulations over the tropical western North Pacific (WNP), equatorial environments (with low-level relative vorticity, high-level divergence, tropospheric vertical wind shear (VWS), and midlevel humidity) tend to be favorable for TC genesis. Consequently, the off-equatorial TC tracks show a significantly decreased occurrence frequency in the northern monsoon trough region. The present study provides a summary sketch showing the QBO-tropospheric circulation modulation process. Considering that the westerly or weak easterly is observed over the upper central Pacific (CP), QBOW phase-associated weak VWS can induce anomalous upward motion at equatorial latitudes, together with the upwelling between the equatorial westerly and off-equatorial easterly, leading to an enlarged convective window over the CP. Affected by the prevailing easterly within the tropics, the actual pumping shifts westward over the tropical WNP.

The cyanobacterium Prochlorococcus has divergent light-harvesting antennae and may have evolved in a low-oxygen ocean

Marine picocyanobacteria of the genus Prochlorococcus are the most abundant photosynthetic organisms in the modern ocean, where they exert a profound influence on elemental cycling and energy flow. The use of transmembrane chlorophyll complexes instead of phycobilisomes as light-harvesting antennae is considered a defining attribute of Prochlorococcus Its ecology and evolution are understood in terms of light, temperature, and nutrients. Here, we report single-cell genomic information on previously uncharacterized phylogenetic lineages of this genus from nutrient-rich anoxic waters of the eastern tropical North and South Pacific Ocean. The most basal lineages exhibit optical and genotypic properties of phycobilisome-containing cyanobacteria, indicating that the characteristic light-harvesting antenna of the group is not an ancestral attribute. Additionally, we found that all the indigenous lineages analyzed encode genes for pigment biosynthesis under oxygen-limited conditions, a trait shared with other freshwater and coastal marine cyanobacteria. Our findings thus suggest that Prochlorococcus diverged from other cyanobacteria under low-oxygen conditions before transitioning from phycobilisomes to transmembrane chlorophyll complexes and may have contributed to the oxidation of the ancient ocean.

The Thermocline Biases in the Tropical North Pacific and Their Attributions

AbstractThe tropical thermocline plays an important role in regulating equatorial sea surface temperature (SST); at present, it is still poorly simulated in the state-of-the-art climate models. In this paper, thermocline biases in the tropical North Pacific are investigated using the newly released CMIP6 historical simulations. It is found that CMIP6 models tend to produce an overly shallow thermocline in the northwestern tropics, accompanied by a deep thermocline in the northeastern tropics. A pronounced thermocline strength bias arises in the tropical northeastern Pacific, demonstrating a dipole structure with a sign change at about 8°N. These thermocline biases are accompanied with biases in the simulations of oceanic circulations, including a too weak North Equatorial Countercurrent (NECC), a reduction in water exchanges between the subtropics and the equatorial regions, and an eastward extension of the equatorward interior water transport. The causes of these thermocline biases are further analyzed. The thermocline bias is primarily caused by the model deficiency in simulating the surface wind stress curl, which can be further attributed to the longstanding double-ITCZ bias in the tropical North Pacific. Besides, thermocline strength bias can be partly attributed to the poor prescription of oceanic background diffusivity. By constraining the diffusivity to match observations, the thermocline strength in the tropical northeastern Pacific is greatly increased.

Trivalent chromium isotopes in the eastern tropical North Pacific oxygen-deficient zone

Changes in chromium (Cr) isotope ratios due to fractionation between trivalent [Cr(III)] and hexavalent [Cr(VI)] are being utilized by geologists to infer oxygen conditions in past environments. However, there is little information available on Cr in the modern ocean to ground-truth these inferences. Transformations between the two chromium species are important processes in oceanic Cr cycling. Here we present profiles of hexavalent and trivalent Cr concentrations and stable isotope ratios from the eastern tropical North Pacific (ETNP) oxygen-deficient zone (ODZ) which support theoretical and experimental studies that predict that lighter Cr is preferentially reduced in low-oxygen environments and that residual dissolved Cr becomes heavier due to removal of particle-reactive Cr(III) on sinking particles. The Cr(III) maximum dominantly occurs in the upper portion of the ODZ, implying that microbial activity (dependent on the sinking flux of organic matter) may be the dominant mechanism for this transformation, rather than a simple inorganic chemical conversion between the species depending on the redox potential.

Vertical distribution of planktic foraminifera through an oxygen minimum zone: how assemblages and test morphology reflect oxygen concentrations

Abstract. Oxygen-depleted regions of the global ocean are rapidly expanding, with important implications for global biogeochemical cycles. However, our ability to make projections about the future of oxygen in the ocean is limited by a lack of empirical data with which to test and constrain the behavior of global climatic and oceanographic models. We use depth-stratified plankton tows to demonstrate that some species of planktic foraminifera are adapted to life in the heart of the pelagic oxygen minimum zone (OMZ). In particular, we identify two species, Globorotaloides hexagonus and Hastigerina parapelagica, living within the eastern tropical North Pacific OMZ. The tests of the former are preserved in marine sediments and could be used to trace the extent and intensity of low-oxygen pelagic habitats in the fossil record. Additional morphometric analyses of G. hexagonus show that tests found in the lowest oxygen environments are larger, more porous, less dense, and have more chambers in the final whorl. The association of this species with the OMZ and the apparent plasticity of its test in response to ambient oxygenation invites the use of G. hexagonus tests in sediment cores as potential proxies for both the presence and intensity of overlying OMZs.

Quantifying Nitrous Oxide Cycling Regimes in the Eastern Tropical North Pacific Ocean With Isotopomer Analysis

AbstractNitrous oxide (N2O), a potent greenhouse gas, is produced disproportionately in marine oxygen deficient zones (ODZs). To quantify spatiotemporal variation in N2O cycling in an ODZ, we analyzed N2O concentration and isotopologues along a transect through the eastern tropical North Pacific (ETNP). At several stations along this transect, N2O concentrations reached a near surface maximum that exceeded prior measurements in this region, of up to 226.1 ± 20.5 nM at the coast. Above the σθ = 25.0 kg/m3 isopycnal, Keeling plot analysis revealed two sources to the near‐surface N2O maximum, with different δ15N2Oα and δ15N2Oβ values, but each with a site preference (SP) of 6‰–8‰. Given expected SPs for nitrification and denitrification, each of these sources could be comprised of 17%–26% nitrification (bacterial or archeal), and 74%–83% denitrification (or nitrifier‐denitrification). Below the σθ = 25.0 kg/m3 isopycnal, box model analysis indicated that the observed 46‰–50‰ SPs in the anoxic core of the ODZ cannot be reproduced in a steady state context without an SP for N2O production by denitrification, and may indicate instead a transient net consumption of N2O. Furthermore, time‐dependent model results indicated that while δ15N2Oα and δ18O‐N2O reflect both N2O production and consumption in the anoxic core of the ODZ, δ15N2Oβ predominantly reflects N2O sources. Finally, we infer that the high (N2O) observed at some stations derive from a set of conditions supporting high rates of N2O production that have not been previously encountered in this region.

Contribution of precipitation events with different consecutive days to rainfall change over Asia during ENSO years

The present study investigates the contributions of four types of precipitation events with different consecutive days (1–3-day, 4–7-day, 8–14-day, and over-14 day) of precipitation to April–October rainfall anomalies over Asia and roles of the three factors (intensity, number, duration) in area-mean rainfall anomalies of four types of precipitation events in El Nino and La Nina years. It is found that precipitation tends to be below normal in the entire tropical and extratropical regions except for northeastern Asia and east and west Indochina Peninsula during El Nino years. During La Nina years, precipitation tends to increase in eastern China, India, the south hills of the Himalayas, west Indochina Peninsula-southwest China, and the Maritime Continent, but decreases in northeast China, east Kazakhstan, Afghanistan, and east Indochina Peninsula. The precipitation change at the low latitudinal regions has a larger contribution from the over-14-day events. The number of events is a main factor in changes of precipitation of primary events during El Nino years. The precipitation changes in El Nino and La Nina years are consistent with changes in the activity of cyclonic wind anomalies corresponding to the precipitation events. The intraseasonal wind anomalies leading to the precipitation events in China are preceded by northwestward move of cyclonic systems from the tropical western North Pacific that promotes the start of precipitation in the concerned regions. The present study points to the impacts of intraseasonal wind systems with various lifespans on precipitation changes over Asia in El Nino and La Nina years.

Emerging Pacific Quasi‐Decadal Oscillation Over the Past 70 Years

AbstractQuasi‐decadal variation in various Pacific regions has been recognized for decades but its spatial‐temporal characteristics and generation mechanisms remain poorly understood. Here we show that the sea surface temperature (SST) in the equatorial central Pacific (ECP) exhibits a prominent 11‐year periodicity since 1951 but not before. An episodic‐like quasi‐decadal warm event is initiated in a northeast‐southwest tilted belt from the United States' west coast to the ECP. The initial development involves positive feedback between atmospheric heating‐induced Rossby waves and underlying SST in the tropical North Pacific. The amplification of ECP warming is primarily due to equatorward heat advection and deepening thermocline, while the zonal advective feedback mainly controls its decay. The quasi‐decadal oscillation (QDO) tends to follow the 11‐years solar cycle with a 1‐to‐2‐year phase delay, meanwhile, it possibly links to the nonlinear behavior of ENSO. The cause of intensification of the QDO over the past 70 years, however, remains elusive.

Western North Pacific Tropical Cyclone Database Created by the China Meteorological Administration

This paper describes the access to, and the content, characteristics, and potential applications of the tropical cyclone (TC) database that is maintained and actively developed by the China Meteorological Administration, with the aim of facilitating its use in scientific research and operational services. This database records data relating to all TCs that have passed through the western North Pacific (WNP) and South China Sea (SCS) since 1949. TC data collection has expanded over recent decades via continuous TC monitoring using remote sensing and specialized field detection techniques, allowing collation of a multi-source TC database for the WNP and SCS that covers a long period, with wide coverage and many observational elements. This database now comprises a wide variety of information related to TCs, such as historical or real-time locations (i.e., best track and landfall), intensity, dynamic and thermal structures, wind strengths, precipitation amounts, and frequency. This database will support ongoing research into the processes and patterns associated with TC climatic activity and TC forecasting.

Nitrification and nitrous oxide dynamics in the Southern California Bight

AbstractThe amount of primary production fueled by upwelled “new” nitrate can be used to estimate the amount of organic carbon available for export to the deep ocean. Nitrate production in the euphotic zone from the microbial process of nitrification affects these estimates, yet the controls on nitrification in the upper ocean are debated. This study examines how seasonal cycles in primary production influence rates of nitrification fueled by both ammonia and urea‐derived nitrogen (N), and how these processes relate to the distribution of the greenhouse gas nitrous oxide (N2O) using monthly rate measurements from the San Pedro Ocean Time‐series (SPOT) station. Nitrification rates were highest at the onset of upwelling and were correlated with depth‐integrated primary production in the lower euphotic zone. Similar ammonia and urea‐derived N oxidation rates suggest urea is a significant N source fueling nitrification, particularly below the euphotic zone. Nitrification supplied a large proportion of phytoplankton N demand in the lower euphotic zone, implying significant regenerated production. The Southern California Bight was always a source of N2O to the atmosphere, which likely was advected into the system from the eastern tropical North Pacific, rather than produced locally from nitrification, and ventilated to the atmosphere during upwelling. Together, the results suggest the coupling of N remineralization and primary production in the upper ocean have important implications for the amount of organic carbon available for export out of the surface ocean, but that transport may dominate over local production in explaining local N2O dynamics.

Future Changes in Western North Pacific Tropical Cyclone Genesis Environment in High-Resolution Large-Ensemble Simulations

This study applied the database for Policy Decision making for Future climate change (d4PDF) and tropical cyclone (TC) genesis (TCG) environment factors to project future changes in the frequency and characteristics of TCs over the western North Pacific. We examined current and future TCG environmental conditions in terms of the contribution of five factors: shear line (SL), confluence region (CR), monsoon gyre, easterly wave (EW), and Rossby wave energy dispersion from a preexisting TC (PTC). Among summer and autumn TCs, the contributions of SL and EW to future TCG increased by about 4% and 1%, respectively, whereas those of CR and PTC decreased by the same amounts. In future climate projections, the average lifetime maximum intensity (LMI) of TCs associated with EW (EW-TCs) was significantly higher than those of TCs associated with other factors except PTC. At higher sea surface temperatures and wetter conditions, higher lower-tropospheric relative vorticity was related to increases in the development rate of EW-TCs. Findings of this study suggest that increases in the average LMI of all future TCs were caused by large contributions from the average LMI of future EW-TCs.