Interactions between Boreal Summer Intraseasonal Oscillations and Synoptic-Scale Disturbances over the Western North Pacific. Part I: Energetics Diagnosis*

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.

Barotropic Interactions Between Summertime Tropical Cyclones/Sub-Monthly Wave Patterns and Intraseasonal Oscillations over the Western North Pacific

“…Dissipation is represented by the last term. Equation (1) describes the energetic budget terms between the mean state and perturbations and is different from that in Hsu et al (2011) because they included terms that involve vertical fluxes and eddy-eddy interaction terms that are from the perturbation advection term. According to Mu and Zhang (2008), these terms are small (though they are non-zeroes) and the present study focuses on the interaction between the mean state and perturbations.…”

“…The definitions of other terms render them unsuitable for examining the interaction between the TSM and ISO components (Hsu et al 2011), and were thus ignored. This study focused on the kp si term to delineate the interaction between the TSM and ISO.…”

“…Studies have focused mostly on the intraseasonal oscillation (ISO) modulation effect on tropical cyclones (hereafter TCs) along with the submonthly or synoptic perturbations. A recent study by Hsu et al (2011) found that the low frequency background flow always supplied energy to the synoptic eddies regardless of the ISO phases. Intraseasonal perturbations lost energy to synoptic eddies in the ISO active phase; whereas they gained energy from the synoptic eddies in the ISO suppressed phase.…”

A Paper on the Tropical Intraseasonal Oscillation Published in 1963 in a Chinese Journal

“…The oscillation was also shown to be of a more broadband pattern (20-90 days) than the original 40-50-day period (e.g., Krishnamurti and Subrahmanyam 1982;Wang and Rui 1990;Hendon and Salby 1994;Annamalai and Slingo 2001;Zhang 2005). Recent theoretical and diagnostic studies indicated that equatorial waves interacting with moisture, convection, and boundary layer dynamics build the fundamental mechanism for the eastward propagation of the MJO (Hsu and Li 2012;Sobel and Maloney 2013;Jiang et al 2015;Wang et al 2016;Wang et al 2017 mostly takes place over the western equatorial Indian Ocean (Zhang and Ling 2017) and is caused by anomalous low-level moisture advection (Zhao et al 2013), warm advection-induced anomalous ascending motion (Li et al 2015), intraseasonal SST anomalies (Li et al 2008), and/or extratropical forcing (Hsu et al 1990;Zhao et al 2013). Li (2014) provides a comprehensive review on the dynamics of MJO initiation and propagation.…”

Precursor synoptic‐scale disturbances associated with tropical cyclogenesis in the South China Sea during 2000–2011

“…The TD-type disturbances may be enhanced under an active ISO phase due to barotropic energy conversion that involves both the rotational and divergent components of ISO wind (e.g. Hsu et al, 2011). The atmospheric ISO consists of two dominant modes of quasi-biweekly oscillation (QBW) and Madden-Julian oscillation (MJO) (e.g.…”