Impact of Madden–Julian Oscillation upon Winter Extreme Rainfall in Southern China: Observations and Predictability in CFSv2

Abstract

Abstract:The impact of Madden-Julian oscillation (MJO) upon extreme rainfall in southern China was studied using the Real-time Multivariate MJO (RMM) index and daily precipitation data from high-resolution stations in China. The probability-distribution function (PDF) of November-March rainfall in southern China was found to be skewed toward larger (smaller) values in phases 2-3 (6-7) of MJO, during which the probability of extreme rainfall events increased (reduced) by 30-50% (20-40%) relative to all days in the same season. Physical analysis indicated that the favorable conditions for generating extreme rainfall are associated with southwesterly moisture convergence and vertical moisture advection over southern China, while the direct contributions from horizontal moisture advection are insignificant. Based on the above results, the model-based predictability for extreme rainfall in winter was examined using hindcasts from the Climate Forecast System version 2 (CFSv2) of NOAA. It is shown that the modulations of MJO on extreme rainfall are captured and forecasted well by CFSv2, despite the existence of a relatively small bias. This study suggests the feasibility of deriving probabilistic forecasts of extreme rainfall in southern China based on RMM indices.

Combined Effects of Synoptic-Scale Teleconnection Patterns on Summer Precipitation in Southern China

“…By day −2, the western boundary of the anomalous low-latitude anticyclone reaches west of 125° E. During typical EAP/SR-C events, owing to the further intensification and westward extension of the WPSH, the greatly enhanced low-level southwesterlies to the northern flank of the low-latitude anticyclone advect anomalously abundant moisture toward the YRV, resulting in a more significant moisture flux with a magnitude anomaly more than 1.0  above normal. This diagnosis of moisture conditions during EAP/SR-C events indicates that moisture transport is primarily ascribed to the anomalous anticyclone associated with the westward-extended WPSH, rather than the southwesterlies from the Bay of Bengal [43]. Moreover, accompanying development of the anomalous mid-latitude cyclone are northerlies from mid-latitudes (45° N), which prevail in the upstream of the YRV and strengthen significantly from day 0 onward, steering the low-level cold and dry air required by the frontogenesis into the YRV [25].…”

“…Meanwhile, northerly flows from the mid-latitudes, steered by the deep trough, prevail in the middle troposphere, promoting the southward intrusion of cooling/dry air from mid-high latitudes required by the frontogenesis into the YRV [43]. Therefore, the confluence of northerlies and southwesterlies contributes to stronger moisture convergence, enhancing atmospheric instability and local ascent [25,28].…”

“…However, the WPSH weakens rapidly and retrogrades to its normal position, decelerating the southwesterlies. In contrast to the long duration of the significant moisture flux and strong moisture convergence during EAP/SR-C events, due to the weakening of southwesterlies [43], both the moisture flux and convergence within region-2 rapidly weaken to dissipation during EAP-S events (Figure 8e), terminating the precipitation process. Correspondingly, the daily normalized domain-averaged precipitation of 1.0  above normal shows a relatively shorter duration (from day 0 to day 4).…”

“…Owing to the relatively weaker WPSH and the absence of an anomalous low-latitude anticyclone during the SR-S events, the southwesterlies originating from the Bay of Bengal account for moisture transport to the coastal areas of southeastern China (Figure 8c). This implies that the anomalous anticyclone/cyclone pair associated with the EAP teleconnection pattern is an essential factor for moisture transport in the YRV [25,43].…”

The 10–30‐day oscillation of winter rainfall in southern China and its relationship with circulation patterns in different latitudes

“…A Rossby wave propagates from the northwest to the southeast, and at the surface, the intraseasonal cyclone and anti-cyclone also move from high latitude to middle and low latitudes, which then leads to continuous precipitation and a coldwave (e.g., Hu et al, 2016;Yang and Li, 2016). On the other hand, the tropical MJO (Madden-Julian oscillation) also influences the rainfall in China (e.g., Noh et al, 2008;Jia and Liang, 2011;Bai et al, 2013;Ren and Ren, 2017). Strong MJO convection was markedly active over the Indian Ocean, especially with the convection centre maintaining 9 days in MJO phase 3 of and corresponding to two of the strongest ice-snow processes in China in November 2009 (Jia and Liang, 2011).…”