Xuejuan Ren

Seasonal Variations of the Synoptic-Scale Transient Eddy Activity and Polar Front Jet over East Asia

Abstract Seasonal variations of the synoptic-scale transient eddy activity (STEA) and the jet streams over East Asia are examined through analysis of the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data. Extracted from the 6-hourly upper-level wind fields, the distribution of the jet core numbers exhibits a distinct geographical border for the East Asian subtropical jet (EASJ) and the East Asian polar front jet (EAPJ) at the latitudes of the northern Tibetan Plateau (TP). In the cool seasons, two branches of the STEA and low-level baroclinicity exist over the East Asian landmass, accompanied by the two-jet state of the EASJ and EAPJ. In the warm seasons, a single jet pattern of the EASJ along the north flank of the TP is accompanied by the weakened STEA over the mid- to high latitudes of East Asia. Further analysis shows two distinct features of the seasonal variations of the STEA over East Asia, compared with that over the North Pacific. First, during the transit...

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

AbstractThis 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 30°N 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) signal...

Characteristics and Mechanisms of the Subseasonal Eastward Extension of the South Asian High

AbstractThis study investigates the features of eastward extension of the South Asian high (SAH) and its connection with diabatic heating and rainfall over eastern Asia on subseasonal time scales. The causes of SAH’s eastward extension are examined by potential vorticity (PV) diagnosis with emphasis on the joint role of diabatic heating feedback and midlatitude wave train. The SAH’s eastward extension features eastward propagation of a wave train across Eurasia. Among the wave train, the migration of weak high from the western flank of the Tibetan Plateau (TP) to the east of TP contributes to the SAH’s eastward extension at the early stage. When the SAH approaches its easternmost position, a strong negative PV (positive geopotential height) center prevails to the east of the TP at 200 hPa. The associated anomalies in diabatic heating and rainfall include the anomalous heating and above-normal rainfall over the South China Sea (SCS) and subtropical western Pacific occurring 12 days before the SAH’s eastern...

Persistent heavy rainfall over South China during May–August: Subseasonal anomalies of circulation and sea surface temperature

This study investigates the relationship between subseasonal variations of the circulation and sea surface temperature (SST) over the South China-East Asian coastal region (EACR) in association with the persistent heavy rainfall (PHR) events over South China during May-August through statistical analysis. Based on the intensity threshold and duration criterion of the daily rainfall, a total of 63 May-June (MJ) and 59 July-August (JA) PHR events are selected over South China from 1979 to 2011. The lower-level circulation anomalies on subseasonal timescale exhibit an anomalous cyclone over South China and an anomalous anticyclone shaped like a tongue over the South China Sea (SCS) during the PHR events for MJ group. The anomalous cyclone over South China in MJ originates from low-value systems in the mid-high latitudes before the rainfall. The anomalous anticyclone over the SCS is due to the westward extension of the western Pacific subtropical high (WPSH) and the southeastward propagation of the anomalous anticyclone from South China before the rainfall. For JA group, the lower-level anomalous circulation pattern is similar to that for MJ over the South China-EACR, but with different features of propagation. The subseasonal anomalous anticyclone is also related to the westward stretch of the WPSH, while the anomalous cyclone is traced back to the weak anomalous cyclone over the Philippine Sea several days before the rainfall events.Positive SST anomaly (SSTA) is observed over the SCS and the Philippine Sea during the MJ PHR events on the subseasonal timescale. It is closely linked with the variation of local anomalous anticyclone. In contrast, negative SSTA occupies the South China coastal region for the JA PHR events, and it is driven by the anomalous cyclone which propagates northwestward from the Philippine Sea. The subseasonal positive (negative) SSTAs are generated via the local processes of above (below)-normal incident solar radiation and below (above)-normal latent heat fluxes. The possible role of the subseasonal SSTA in the local convective instability is also analyzed in this study.

Role of intraseasonal oscillation in the persistent extreme precipitation over the Yangtze River Basin during June 1998

The persistent extreme precipitation event (PEPE) occurred over the Yangtze River Basin (YRB) during the period of 12–27 June 1998 is the most severe one in recent 60 years, and it is mainly caused by two significant components of intraseasonal oscillation (ISO) (10-30-day and 30-60-day) identified in this study. The two ISOs play different roles in the distributions of YRB rain belt in the PEPE, i.e., the 30-60-day ISO generally maintains the shape and intensity of YRB rain belt with its peak covering the whole PEPE period, however, the 10-30-day ISO mainly determines the south–north swing of the YRB rain belt that features 3 PEPE stages. North Indian Ocean is the major forcing region of 30-60-day ISO, where anomalous warm SST induced local strong convections stimulates a meridional teleconnection wave train over the East Asia, generating the 30-60-day intraseasonal YRB rainfalls. The 10-30-day ISO primarily originates from the Northwest Pacific and the South China Sea (SCS), and along with its northwestward and northeastward propagations due to the air-sea coupling and prevailing winds, suppressed and enhanced convections appear alternatively over the Philippine Sea and the central SCS in the 3 PEPE stages, thus their stimulated downstream wave trains along the coast of East Asia vary accordingly in terms of phase and position, causing 3 stages of 10-30-day intraseasonal YRB rainfalls with different intensities and locations. These results suggest that proper combination of different intraseasonal oscillations is one of the essential and effective ways to produce the PEPEs.

Probabilistic versus deterministic skill in predicting the western North Pacific‐East Asian summer monsoon variability with multimodel ensembles

Based on historical forecasts of three quasi-operational multimodel ensemble (MME) systems, this study assesses the superiority of coupled MME over contributing single-model ensembles (SMEs) and over uncoupled atmospheric MME in predicting the Western North Pacific-East Asian summer monsoon variability. The probabilistic and deterministic forecast skills are measured by Brier skill score (BSS) and anomaly correlation (AC), respectively. A forecast-format-dependent MME superiority over SMEs is found. The probabilistic forecast skill of the MME is always significantly better than that of each SME, while the deterministic forecast skill of the MME can be lower than that of some SMEs. The MME superiority arises from both the model diversity and the ensemble size increase in the tropics, and primarily from the ensemble size increase in the subtropics. The BSS is composed of reliability and resolution, two attributes characterizing probabilistic forecast skill. The probabilistic skill increase of the MME is dominated by the dramatic improvement in reliability, while resolution is not always improved, similar to AC. A monotonic resolution-AC relationship is further found and qualitatively explained, whereas little relationship can be identified between reliability and AC. It is argued that the MMEs success in improving the reliability arises from an effective reduction of the overconfidence in forecast distributions. Moreover, it is examined that the seasonal predictions with coupled MME are more skillful than those with the uncoupled atmospheric MME forced by persisting sea surface temperature (SST) anomalies, since the coupled MME has better predicted the SST anomaly evolution in three key regions.