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Pang-chi Hsu
and
Tim Li

Abstract

The moisture budget associated with the eastward-propagating Madden–Julian oscillation (MJO) was diagnosed using 1979–2001 40-yr ECMWF Re-Analysis (ERA-40) data. A marked zonal asymmetry of the moisture relative to the MJO convection appears in the planetary boundary layer (PBL, below 700 hPa), creating a potentially more unstable stratification to the east of the MJO convection and favoring the eastward propagation of MJO. The PBL-integrated moisture budget diagnosis indicates that the vertical advection of moisture dominates the low-level moistening ahead of the convection. A further diagnosis indicates that the leading term in the vertical moisture advection is the advection of the background moisture by the MJO ascending flow associated with PBL convergence. The cause of the zonally asymmetric PBL convergence is further examined. It is found that heating-induced free-atmospheric wave dynamics account for 75%–90% of the total PBL convergence, while the warm SST anomaly induced by air–sea interaction contributes 10%–25% of the total PBL convergence.

The horizontal moisture advection also plays a role in contributing to the PBL moistening ahead of the MJO convection. The leading term in the moisture advection is the advection across the background moisture gradient by the MJO flow. In the western Indian Ocean, Maritime Continent, and western Pacific, the meridional moisture advection by the MJO northerly flow dominates, while in the eastern Indian Ocean the zonal moisture advection is greater. The contribution of the moisture advection by synoptic eddies is in general small; it has a negative effect over the tropical Indian Ocean and western Pacific and becomes positive in the Maritime Continent region.

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Pang-Chi Hsu
and
Tim Li

Abstract

The interactions between the boreal summer intraseasonal oscillation (ISO) and synoptic-scale variability (SSV) are investigated by diagnosing the atmospheric apparent heat source (Q 1), apparent moisture sink (Q 2), and eddy momentum transport. It is found that the synoptic Q 1 and Q 2 heating (cooling) anomalies are in phase with cyclonic (anticyclonic) vorticity disturbances, aligned in a southeast–northwest-oriented wave train pattern over the western North Pacific (WNP). The wave train is well organized and strengthened (loosely organized and weakened) during the ISO active (suppressed) phase. The nonlinearly rectified Q 1 and Q 2 fields due to the eddy–mean flow interaction account for 10%–30% of the total intraseasonal Q 1 and Q 2 variabilities over the WNP. During the ISO active (suppressed) phase, the nonlinearly rectified intraseasonal Q 1 and Q 2 heating (cooling) appear to the northwest of the ISO enhanced (suppressed) convection center, favoring the northwestward propagation of the ISO. A diagnosis of the zonal momentum budget shows that the eddy momentum flux convergence forces an intraseasonal westerly (easterly) tendency to the north of the ISO westerly (easterly) center during the ISO active (suppressed) phase. As a result, the eddy momentum transport may contribute to the northward propagation of the boreal summer ISO over the WNP.

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Pang-Chi Hsu
and
Ting Xiao

Abstract

The influences of different types of Pacific warming, often classified as the eastern Pacific (EP) and central Pacific (CP) El Niño events, on Madden–Julian oscillation (MJO) activity over the Indian Ocean were investigated. Accompanied by relatively unstable (stable) atmospheric stratification induced by enhanced (reduced) moisture and moist static energy (MSE) in the lower troposphere, strengthened (weakened) MJO convection was observed in the initiation and eastward-propagation stages during CP (EP) El Niño events. To examine the key processes resulting in the differences in low-level moistening and column MSE anomalies over the Indian Ocean associated with the two types of El Niño, the moisture and column MSE budget equations were diagnosed using the reanalysis dataset ERA-Interim. The results indicate that the enhanced horizontal advection in the CP El Niño years plays an important role in causing a larger moisture and MSE growth rate over the MJO initiation area during CP El Niño events than during EP El Niño events. The increases in horizontal moisture and MSE advection primarily result from advection by mean flow across the enhanced intraseasonal moisture and MSE gradient, as well as by intraseasonal circulation across the mean moisture and MSE gradient associated with the CP El Niño. In the eastward development stage, the enhanced preconditioning comes from positive moisture and MSE advection anomalies in the CP El Niño events. Meanwhile, the strengthened MJO-related convection over the central-eastern Indian Ocean is maintained by increased atmospheric radiative heating and surface latent heat flux during the CP El Niño compared to the EP El Niño events.

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Haoyu Zhou
and
Pang-Chi Hsu

Abstract

The boreal summer intraseasonal oscillation (BSISO), which consists of a 10–30-day component and a 30–90-day component, is vigorous over the East Asian and the western North Pacific (WNP) monsoon regions where synoptic-scale eddies are also active. In this study, we systematically compare and diagnose the modulating effects of the 10–30- and 30–90-day BSISO modes on the development and maintenance of the WNP synoptic-scale eddies in terms of eddy energetics. During the developing phase of the synoptic eddies, the eddies tend to grow faster under the background conditions associated with the 10–30-day BSISO mode, compared with that associated with the 30–90-day mode, indicating the importance of the 10–30-day mode in eddy development. In contrast, the 30–90-day BSISO mode shows a larger contribution to the maintenance of the synoptic eddy intensity after the eddies reach their maximum amplitude. On the basis of the diagnoses of the eddy kinetic energy (EKE) budget equation, we find that the positive EKE advection induced by the 10–30-day flow is the leading process resulting in the increased EKE for eddy development. However, the 30–90-day circulation anomalies provide long-lasting favorable conditions (relative to those of the 10–30-day anomalies) to maintain the eddy intensity by generating a positive barotropic energy conversion from 30- to 90-day kinetic energy to EKE. These results not only advance our understanding of multiscale interactions over the WNP but also help toward better monitoring and prediction of synoptic-scale eddies, including tropical cyclones, using the background BSISO information.

Significance Statement

The western North Pacific is a region with active but complex multiscale interactions among synoptic-scale disturbances, intraseasonal oscillation, and low-frequency background monsoonal activity. The relative effects of 10–30- and 30–90-day intraseasonal variability on the synoptic-scale disturbances were quantitatively examined based on the eddy kinetic energy budget analysis. The results show that the 10–30-day (30–90-day) mode of intraseasonal oscillations plays a key role in the growth (maintenance) of synoptic-scale disturbances through the advection (barotropic energy conversion) process. The results suggest the implication of monitoring and predicting the activity of synoptic-scale disturbances using the information of two different intraseasonal modes.

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Pang-Chi Hsu
,
Tim Li
, and
Hiroyuki Murakami

Abstract

The role of zonal moisture asymmetry in the eastward propagation of the Madden–Julian oscillation (MJO) is investigated through a set of aquaplanet atmospheric general circulation model (AGCM) experiments with a zonally symmetric sea surface temperature distribution. In the control experiment, the model produces eastward-propagating MJO-like perturbations with a dominant period of 30–90 days. The model MJO exhibits a clear zonal asymmetry in the lower-tropospheric specific humidity field, with a positive (negative) anomaly appearing to the east (west) of the MJO convection. A diagnosis of the lower-tropospheric moisture budget indicates that the asymmetry primarily arises from vertical moisture advection associated with boundary layer convergence, while horizontal moisture advection has the opposite effect.

In a sensitivity experiment, the lower-tropospheric specific humidity field is relaxed toward a zonal-mean basic state derived from the control simulation. In this case, the model’s mean state remains the same, but its intraseasonal mode becomes quasi-stationary. The numerical model experiments clearly demonstrate the importance of the zonal moisture asymmetry in MJO eastward propagation.

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Hiroyuki Murakami
,
Tim Li
, and
Pang-Chi Hsu

Abstract

In recent decades, tropical cyclone (TC) activity in the North Atlantic has shown a marked positive anomaly in genesis number, mean lifespan, number of intense hurricanes, and mean maximum intensity. The accumulated cyclone energy (ACE), which is defined as the sum of the square of the maximum surface wind velocity throughout the lifetime of a TC, is one of the measures that can be used to synthesize these factors. Similar to the ACE, the power dissipation index (PDI), which is defined as the integrated third power of maximum surface wind velocity, has also been used to describe TC activity. The basin-total ACE and PDI for the North Atlantic have also followed a large positive anomaly during the period 1995–2012; however, the relative importance of factors such as TC genesis number, TC track property (e.g., duration and lifespan), and TC intensity remains unclear in terms of their contribution to the positive anomalies in ACE and PDI. This study uses a new empirical statistical approach to analyze the TC data and finds that the increase in the TC genesis number is primarily responsible for the positive anomalies in ACE and PDI. Other factors, such as TC track property and TC intensity, appear to be minor influences.

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Zhen Fu
,
Pang-Chi Hsu
, and
Fei Liu

Abstract

This study examined multidecadal changes in the amplitude of the boreal-winter Madden–Julian oscillation (MJO) over the twentieth century using two century-long reanalysis datasets (20CR and ERA-20C). Both revealed reasonable MJO variability compared to other state-of-the-art reanalysis datasets. We detected pronounced multidecadal variations along with an increasing trend in MJO amplitude during the period 1900–2009 in both datasets, although this linear trend was less significant in the reconstructed MJO index proposed by Oliver and Thompson. The two twentieth-century reanalysis datasets and the Oliver–Thompson MJO index consistently showed the intensified amplitude of MJO precipitation and circulation in the later decades (1970–99) compared to the earlier decades (1920–49). The most significant enhancement of MJO precipitation in the later decades appeared over the western Pacific warm pool. To understand the mechanisms controlling the changes in western Pacific MJO precipitation amplitude over the twentieth century, we diagnosed the moisture budget equation. The enhanced MJO precipitation variability in the later decades mainly came from increased moisture associated with a strengthened low-level convergence anomaly working on background mean moisture [ ( q ¯ V ) ]. Further diagnosis showed that the effect of anomalous circulation (∇ ⋅ V′) change on the MJO precipitation amplitude change over the twentieth century was about an order larger than that of mean moisture ( q ¯ ) change, different from the mechanisms (i.e., increased gradient of q ¯ ) responsible for the intensified MJO precipitation amplitude under future warmer climate. The enhanced MJO circulation anomalies during 1970–99 may be caused by an enhanced diabatic heating anomaly, offset partly by the increased mean static stability.

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Sizhuo Wei
,
Pang-Chi Hsu
, and
Jinhui Xie

Abstract

The time of rainy season onset is crucial information for policymakers, especially in densely populated regions such as the Yangtze River Basin (YRB) in China. In this study, we proposed a new grid-based index to objectively detect Meiyu onset timing using reanalysis data and model predictions, and then we identified the key processes via which intraseasonal oscillation (ISO) affects the YRB Meiyu onset and its subseasonal predictability based on scale-decomposed moisture analysis. Climatologically, propagation of an ISO anticyclonic anomaly toward East China supports the moisture convergence required for rainy season onset over the YRB via interaction with the seasonal-mean moisture component. In the years of early Meiyu onset, the ISO was enhanced earlier in May and favored the moisture convergence anomaly in late May–early June, when the Meiyu started. In contrast, the enhanced ISO and associated moistening processes were observed later in June–early July in the years with delayed onset. The European Centre for Medium-Range Weather Forecasts and National Centers for Environmental Prediction models show skillful prediction of Meiyu onset at forecast lead times of 5–6 pentads, whereas the China Meteorological Administration model has limited skill of 3 pentads. The differences in model prediction skill are related to the accuracy of predicted moisture convergence anomalies induced by the ISO. The prediction bias in Meiyu onset timing (early or delayed) is also connected to bias in the occurrence timing of enhanced intraseasonal perturbations, suggesting the vital role of ISO in YRB Meiyu onset on the subseasonal timescale.

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Chih-Hua Tsou
,
Huang-Hsiung Hsu
, and
Pang-Chi Hsu

Abstract

This study formulates a synoptic-scale eddy (SSE) kinetic energy equation by partitioning the original field into seasonal mean circulation, intraseasonal oscillation (ISO), and SSEs to examine the multiscale interactions over the western North Pacific (WNP) in autumn. In addition, the relative contribution of synoptic-mean and synoptic-ISO interactions to SSE kinetic energy was quantitatively estimated by further separating barotropic energy conversion (CK) into synoptic-mean barotropic energy conversion (CK SM ) and synoptic-ISO barotropic energy conversion (CK S−ISO) components.

The development of tropical SSE in the lower troposphere is mainly attributed to CK associated with multiscale interactions. Mean cyclonic circulation in the lower troposphere consistently provides kinetic energy to SSEs (CK SM > 0) during the ISO westerly and easterly phases. However, CK S−ISO during the ISO westerly and easterly phases differs considerably. During the ISO westerly phase, the enhanced ISO cyclonic flow converts energy to SSEs (CK S−ISO > 0). The magnitude of the downscale energy conversion from mean and ISO to SSEs is related to the strength of the SSEs. During the ISO westerly phase, a stronger SSE extracts more kinetic energy from mean and ISO circulation. This positive feedback between SSE-mean and SSE–ISO interactions causes further strengthening of SSEs during the ISO westerly phase.

By contrast, upscale energy conversion from SSEs to ISO anticyclonic flow (CK S−ISO < 0) was observed during the ISO easterly phase. The weaker SSE activity during the ISO easterly phase occurred because the mean circulation provides less energy to SSEs and, at the same time, SSEs lose energy to ISO during the ISO easterly phase. The two-way interaction between the ISO and SSEs has considerable effects on the development of tropical SSEs over the WNP in autumn.

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Pang-chi Hsu
,
Tim Li
, and
Chih-Hua Tsou

Abstract

The role of scale interactions in the maintenance of eddy kinetic energy (EKE) during the extreme phases of the intraseasonal oscillation (ISO) is examined through the construction of a new eddy energetics diagnostic tool that separates the effects of ISO and a low-frequency background state (LFBS; with periods longer than 90 days). The LFBS always contributes positively toward the EKE in the boreal summer, regardless of the ISO phases. The synoptic eddies extract energy from the ISO during the ISO active phase. This positive barotropic energy conversion occurs when the synoptic eddies interact with low-level cyclonic and convergent–confluent ISO flows. This contrasts with the ISO suppressed phase during which the synoptic eddies lose kinetic energy to the ISO flow. The anticyclonic and divergent–diffluent ISO flows during the suppressed phase are responsible for the negative barotropic energy conversion.

A positive (negative) EKE tendency occurs during the ISO suppressed-to-active (active-to-suppressed) transitional phase. The cause of this asymmetric EKE tendency is attributed to the spatial phase relation among the ISO vorticity, eddy structure, and EKE. The southwest–northeast-tilted synoptic disturbances interacting with cyclonic (anticyclonic) vorticity of ISO lead to a positive (negative) EKE tendency in the northwest region of the maximum EKE center.

The genesis number and location and intensification rate of tropical cyclones in the western North Pacific are closely related to the barotropic energy conversion. The enhanced barotropic energy conversion favors the generation and development of synoptic seed disturbances, some of which eventually grow into tropical cyclones.

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