Zonal Oscillation of Western Pacific Subtropical High and Subseasonal SST Variations during Yangtze Persistent Heavy Rainfall Events

Abstract

This study examines the relationship between the zonal oscillation of the western Pacific subtropical high (WPSH) and underneath sea surface temperature (SST) variation on a subseasonal time scale, associated with the persistent heavy rainfall (PHR) events over the middle and lower reaches of the Yangtze River valley (MLYRV) in China. A total of 76 PHR events and 45 break events in the summers of 1979-2011 are first identified over the MLYRV and divided into early and late summer groups. During the PHR events over the MLYRV for both groups, the WPSH stretches more westward, accompanied by the positive anomalies of the 500-hPa geopotential height field over East Asia and its coastal region south of 308N and the subseasonal warmer SSTs beneath the WPSH western edge. The time-lagged composites suggest that the WPSH western edge exhibits westward-then-eastward migration on a subseasonal time scale for the PHR events. The zonal changes of the WPSH and anomalous circulation and SST anomaly (SSTA) signals for break events is almost the mirror image of that for the PHR events for the early summer group. Accompanied by the WPSH westward extension, the increased incident solar radiation and decreased latent heat flux over the coastal region of East Asia contribute to the positive SSTAs beneath the western part of the WPSH. The positive SSTAs construct a convective instability that provides an adverse condition for maintaining the anticyclonic anomalies in the mid-lower levels. The persistent SST warming is also favorable to the transition of low-level circulation from anticyclonic to cyclonic anomalies over the coastal region. As a result, the WPSH withdraws eastward after the peak of the rainfall events over the MLYRV.

Mechanisms for concurrent low-latitude circulation anomalies responsible for persistent extreme precipitation in the Yangtze River Valley

“…A SAHI value over 12,520 gpm suggests anomalously eastward shift of the SAH. All the variables and indices firstly go through a five-point binomial filtering procedure to remove very high-frequency signals, which are reckoned as noise for sub-monthly analyses (Ren et al 2013). The sub-monthly scale refers to the period short than 30 days.…”

“…It is necessary to adopt relaxed thresholds of both intensity and persistence for combined regimes to ensure an adequate sample size (Archambault et al 2010). 3-day was used as the typical duration in previous studies (Chen and Zhai 2013;Ren et al 2013). However, combined with (2) and (3), the adoption of 3-day would yield large sample size (about 50), among which the break between two events may be less than 1 week.…”

“…Actually, during PEPEs, the coastal region with strong descent atop experienced increasing incident solar radiation and decreasing upward latent heat flux from surface, both of which contribute to positive sea surface temperature anomalies (SSTAs) (see Ren et al 2013). The warmer SSTs and more incident solar radiation accelerate the evaporation from local sea surface.…”

“…After confirming key circulation systems and their possible configurations, significant precursors with lead time of 1-2 weeks have also been introduced in some recent studies (e.g. Zhai 2014b, 2015;Ren et al 2013). Interestingly, in spite of diverse cold air sources from mid-high latitudes in individual cases, one common feature shared by most cases lies in a distinct zonal approach between the western Pacific subtropical high (WPSH) and the South Asia High (SAH) prior to PEPEs, i.e.…”

“…Simultaneously, descending motion of these two cells increases local solar radiation and decreases upward latent heat flux from surface, facilitating warmer underlying surface and swift accumulation of lower-level moisture. Correspondingly, enhanced heating long-lasting precipitation extremes with duration of 3 days or longer have been defined as persistent extreme precipitation events (PEPEs, Chen and Zhai 2013;Ren et al 2013). Subsequently, particular attention has been paid to PEPEs in the Yangtze River Valley (YRV), where is significantly influenced by Asian monsoon during boreal summer.…”