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Ching-Shu Hung and Chung-Hsiung Sui

Abstract

The evolution processes for propagating Madden–Julian oscillations with strong magnitude over the Indian Ocean (IO) and Maritime Continent (MC) are investigated through a diagnosis of ECMWF reanalysis data for November–April 1982–2011. A scale-separated lower-tropospheric (1000–700 hPa) moisture budget is analyzed for four stages of composite life cycle: suppressed, cloud developing, convective, and decaying. Overall, the budgets in the IO and MC are dominated by wave-induced boundary layer convergence in the anomalous easterlies (WC) and advection. Starting from the suppressed stage in the central IO, moistening by WC and advection by easterly anomalies contributes to an initiation of the MJO convection in the western IO while surface evaporation and/or shallow convection moistens the central IO. In the following cloud developing and convective stage in the central IO, moistening by WC and advection by the downstream Kelvin–Rossby wave east of central IO lead to eastward propagation of deep convection. In the MC, the suppressed stage coincides with the convective stage in the central IO that promotes anomalous easterlies, subsidence, and enhanced rain rate over islands. Unlike WC and advective moistening in the IO that both occur in the equatorial zone, advective moistening in MC tends to be negative (positive) on windward (leeward) side of the major islands in the equatorial zone and more organized over the Arafura Sea, conducive to a southward detour of the eastward-propagating MJO.

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Kai-Chih Tseng, Chung-Hsiung Sui, and Tim Li

Abstract

Lower-tropospheric (1000–700 hPa) moistening processes of the two Madden–Julian oscillations (MJOs) over the Indian Ocean during Dynamics of the MJO (DYNAMO)/Cooperative Indian Ocean Experiment on Intraseasonal Variability in Year 2011 (CINDY) are investigated by using soundings, operational assimilation, and satellite data. A scale-separated moisture budget is calculated at the sounding site by using time-decomposed wind and moisture fields. Each budget term is projected onto the intraseasonal moisture anomaly and its time tendency change. The projections and the corresponding temporal correlations are analyzed together with the temporal evolution of the budget terms to identify the dominant moistening process responsible for the MJO evolution. Results indicate that broad-scale advection by low-frequency and MJO flow and moisture fields are dominant moisture sources, while the residual of the moisture budget (−Q 2) is a dominant sink contributing to the tendency term (propagation) and intraseasonal moisture anomaly (growth and decay). Dividing their life cycles into four phases (suppressed, cloud developing, convective, and decaying phases), the two MJOs exhibit different budget balances in the premoistening stage from the suppressed phase to the cloud-developing phase when low-frequency vertical motion is downward in MJO1 but upward in MJO2. The corresponding drying and moistening are balanced by negative Q 2 (reevaporation in nonraining cloud) in MJO1 and positive Q 2 in MJO2. The result implies that low-frequency flow (>60 days) can affect the initiation of MJOs. The premoistening in the lower troposphere by boundary layer moisture convergence leading the deep convection is observed but only in the cloud-developing phase to convective phase of the MJOs. Nonlinear moisture advection by synoptic disturbances always acts as a diffusive term. It is the dominant moisture source in the suppress phase of the two MJOs.

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Jau-Ming Chen, Ching-Hsuan Wu, Pei-Hsuan Chung, and Chung-Hsiung Sui

Abstract

Influences of intraseasonal–interannual oscillations on tropical cyclone (TC) genesis are evaluated by productivity of TC genesis (P TCG) from the developing (TCd) and nondeveloping (TCn) precursory tropical disturbances (PTDs). A PTD is identified by a cyclonic tropical disturbance with a strong-enough intensity, a large-enough maximum center, and a long-enough lifespan. The percentage value of PTDs evolving into TCd is defined as P TCG. The analysis is performed over the western North Pacific (WNP) basin during the 1990–2014 warm season (May–September). The climatological P TCG in the WNP basin is 0.35. Counted in a common period, mean numbers of PTDs in the favorable and unfavorable conditions of climate oscillations for TC genesis [such as equatorial Rossby waves (ERWs), the Madden–Julian oscillation (MJO), and El Niño–Southern Oscillation (ENSO)], all exhibit a stable value close to the climatological mean [~31 (100 days)−1]. However, P TCG increases (decreases) during the phases of positive-vorticity (negative-vorticity) ERWs, the active (inactive) MJO, and El Niño (La Niña) years. P TCG varies from 0.17 in the most unfavorable environment (La Niña, inactive MJO, and negative-vorticity ERW) to 0.56 in the most favorable environment (El Niño, active MJO, and positive-vorticity ERW). ERWs are most effective in modulating TC genesis, especially in the negative-vorticity phases. Overall, increased P TCG is facilitated with strong and elongated 850-hPa relative vorticity overlapping a cyclonic shear line pattern, while decreased P TCG is related to weak relative vorticity. Relative vorticity acts as the most important factor to modulate P TCG, when compared with vertical wind shear and 700-hPa relative humidity.

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Jau-Ming Chen, Ching-Hsuan Wu, Jianyun Gao, Pei-Hsuan Chung, and Chung-Hsiung Sui

Abstract

This study focuses on the migratory tropical cyclones (TCs) that form in the western North Pacific (WNP) and move into the South China Sea (SCS). Their movements are found to be modulated differently by intraseasonal oscillations (ISOs) and climatological circulations through the TC-active months. The modulating processes of climatological circulations vary from a westward intensifying western Pacific subtropical high (WPSH) in July and August to a southeastward extending monsoon trough (MT) in September, and a strengthening equatorial trough (ET) in October and November. In July and August, enhanced tropical ISO convections in the SCS are accompanied by a 30–60-day anomalous anticyclone to the northeast of the SCS. The migratory TCs move along the southern peripheries of this anomalous anticyclone and the WPSH into the SCS. In September, enhanced ISO convections in the SCS coincide with a meridional 30–60-day circulation pair with an anomalous anticyclone to the north of 20°N and an anomalous cyclone to the south. TCs move in between this meridional 30–60-day circulation pair and the northern periphery of the MT toward the SCS. In October and November, enhanced ISO convections in the SCS and WNP coexist with an overlying 30–60-day anomalous cyclone and an intensified ET. The migratory TCs move along the northern sections of this 30–60-day anomalous cyclone and the ET toward the SCS. With a different track, TCs recurving northward from the tropical WNP into the region east of Taiwan are modulated by completely different variability features of the 30–60-day ISO and climatological circulations.

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Han-Ching Chen, Zeng-Zhen Hu, Bohua Huang, and Chung-Hsiung Sui

Abstract

This study shows the sudden basinwide reversal of anomalous equatorial zonal transport above the thermocline at the peaking phase of ENSO triggers rapid termination of ENSO events. The anomalous equatorial zonal transport is controlled by the concavity of anomalous thermocline meridional structure across the equator. During the developing phase of ENSO, opposite zonal transport anomalies form in the western-central and central-eastern equatorial Pacific, respectively. Both are driven by the equatorial thermocline anomalies in response to zonal wind anomalies over the western-central equatorial ocean. At this stage, the anomalous zonal transport in the east enhances ENSO growth through zonal SST advection. In the mature phase of ENSO, off-equatorial thermocline depth anomalies become more dominant in the eastern Pacific because of the reflection of equatorial signals at the eastern boundary. As a result, the meridional concavity of the thermocline anomalies is reversed in the east. This change reverses zonal transport rapidly in the central-to-eastern equatorial Pacific, joining with the existing reversed zonal transport anomalies farther to the west, and forms a basinwide transport reversal throughout the equatorial Pacific. This basinwide transport reversal weakens the ENSO SST anomalies by reversed advection. More importantly, the reversed zonal transport reduces the existing zonal tilting of the equatorial thermocline and weakens its feedback to wind anomalies effectively. This basinwide reversal is built in at the peak phase of ENSO as an oceanic control on the evolution of both El Niño and La Niña events. The reversed zonal transport anomaly after the mature phase weakens El Niño in the eastern Pacific more efficiently than it weakens La Niña.

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Joo-Hong Kim, Chang-Hoi Ho, Hyeong-Seog Kim, Chung-Hsiung Sui, and Seon Ki Park

Abstract

The variability of observed tropical cyclone (TC) activity (i.e., genesis, track, and landfall) in the western North Pacific (WNP) is examined in relation to the various categories of the Madden–Julian oscillation (MJO) during summer (June–September) for the period 1979–2004. The MJO categories are defined based on the empirical orthogonal function analysis of outgoing longwave radiation data.

The number of TCs increases when the MJO-related convection center is located in the WNP. The axis of a preferable genesis region systematically shifts like a seesaw in response to changes in the large-scale environments associated with both the eastward and northward propagation of the MJO and the intraseasonal variability of the WNP subtropical high. Furthermore, the authors show that the density of TC tracks in each MJO category depends on the systematic shift in the main genesis regions at first order. Also, the shift is affected by the prevailing large-scale steering flows in each MJO category. When the MJO-related convection center is found in the equatorial Indian Ocean (the tropical WNP), a dense area of tracks migrates eastward (westward). The effects of extreme ENSO events and the variations occurring during ENSO neutral years are also examined.

A statistical analysis of TC landfalls by MJO category is applied in seven selected subareas: the Philippines, Vietnam, South China, Taiwan, East China, Korea, and Japan. While a robust and significant modulation in the number of TC landfalls is observed in south China, Korea, and Japan, the modulation is marginal in the remaining four subareas.

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Chung-Hsiung Sui, Xiaofan Li, Michele M. Rienecker, Ka-Ming Lau, Istvan Laszlo, and Rachel T. Pinker

Abstract

The impacts of high-frequency surface forcing in the upper ocean over the equatorial Pacific are investigated using a nonlinear reduced-gravity isopycnal ocean circulation model forced by daily and monthly mean forcing. The simulated sea surface temperature (SST) in the daily forcing experiment is colder than that in the monthly forcing experiment near the equator. A mixed layer heat budget calculation shows that the net surface heat flux is primarily responsible for the SST difference in the western Pacific, while zonal advection accounts for the SST difference in the eastern Pacific where other budget terms are large but canceling each other. The daily forcing primarily enhances vertical mixing that reduces the vertical shear of the upper ocean. It also changes the net heat into the ocean through two contrasting processes: one is the increased surface latent heat loss induced by transient winds and the other is colder SST due to stronger mixing, which further reduces heat loss at the surface. As a result, the annual mean net surface heat flux into the ocean is reduced and the meridional thermal advection is weaker. The daily forcing also impacts the variation of the thermocline through a changing mixed layer depth so that the temperature in the simulation with the daily forcing is warmer around the thermocline.

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Han-Ching Chen, Chung-Hsiung Sui, Yu-Heng Tseng, and Bohua Huang

Abstract

The Simple Ocean Data Assimilation, version 2.2.4 (SODA 2.2.4), analysis for the period of 1960–2010 is used to study the variability of Pacific subtropical cells (STCs) and its causal relation with tropical climate variability. Results show that the interior STC transport into the equatorial basin through 9°S and 9°N is well connected with equatorial sea surface temperature (SST) (9°S–9°N, 180°–90°W). The highest correlation at interannual time scales is contributed by the western interior STC transport within 160°E and 130°W. It is known that the ENSO recharge–discharge cycle experiences five stages: the recharging stage, recharged stage, warmest SST stage, discharging stage, and discharged stage. A correlation analysis of interior STC transport convergence, equatorial warm water volume (WWV), wind stress curl, and SST identifies the time intervals between the five stages, which are 8, 10, 2, and 8 months, respectively. A composite analysis for El Niño–developing and La Niña–developing events is also performed. The composited ENSO evolutions are in accordance with the recharge–discharge theory and the corresponding time lags between the above denoted five stages are 4–12, 6, 2, and 4 months, respectively. For stronger El Niño events, the discharge due to interior STC transport at 9°N terminates earlier than that at 9°S because of the southward migration of westerly winds following the El Niño peak phase. This study clarifies subsurface transport processes and their time intervals, which are useful for refinement of theoretical models and for evaluating coupled ocean–atmosphere general circulation model results.

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Li-Huan Hsu, Li-Shan Tseng, Shu-Yu Hou, Buo-Fu Chen, and Chung-Hsiung Sui

Abstract

This study evaluates the model simulation of interaction between convectively coupled tropical disturbances in the South China Sea (SCS) and Maritime Continent (MC). The Model for Prediction Across Scales (MPAS) is used to simulate the major interaction events in December 2016 with a fixed 60-km horizontal resolution and a variable 60–15-km resolution. Compared with an observational analysis, the overall spatial and temporal evolution of simulated rainfall and circulation reveals the capability of MPAS for reproducing equatorial Kelvin waves (KWs), and the interactions with equatorial Rossby waves and off-equatorial mixed Rossby–gravity (MRG)/TD-type waves up to a 5–7-day lead in both fixed 60-km and variable 60–15-km resolutions. Two interaction events are further examined. One involves an MRG/TD wave, prevailing northeasterlies, and a Borneo vortex developed in SCS during 6–11 December. The other involves a KW converging with the easterly trade wind that led to an MRG/TD-type wave and the formation of Typhoon Nock-ten during 16–20 December. The MPAS 60–15-km resolution tends to produce stronger precipitation and more coherent vorticity structures in both interaction events. Increasing the resolution to 15 km contributes to better representation of finer spatial vorticity and rainfall structures.

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Joo-Hong Kim, Chang-Hoi Ho, Chung-Hsiung Sui, and Seon Ki Park

Abstract

The present study examines variations in summertime (July–September) tropical cyclone (TC) activity over East Asia during the period 1951–2003. To represent TC activity, a total of 853 TC best tracks for the period were converted to TC passage frequencies (TPFs) within 5° × 5° latitude–longitude grids; TPFs are defined as the percentage values obtained by dividing the number of TC appearances in each grid box by the total number of TCs each year. Empirical orthogonal function analysis of the TPF showed three leading modes: two tropical modes that represent the long-term trend and the relationship with ENSO and one midlatitude mode that oscillates between south of Korea and southeast of Japan with an interannual time scale. The latter proved to be the most remarkable climatic fluctuation of summertime TC activity in the midlatitudes and is referred to as the East Asian dipole pattern (EADP) in this paper.

Anomalous atmospheric flows directly connected to the EADP are an enhanced anticyclonic (cyclonic) circulation centering around Japan when the TPF is high south of Korea (southeast of Japan), thereby showing an equivalent barotropic structure in the entire troposphere. This regional circulation anomaly varies in conjunction with the zonally oriented quasi-stationary Rossby wave train in the upper troposphere. This wave train is meridionally trapped in the vicinity of the summer-mean jet stream; therefore, the mean jet stream alters its internal meandering structure according to the phase of the wave train.

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