Subseasonal Forecasts of the Northern Queensland Floods of February 2019: Causes and Forecast Evaluation

Wayne Yuan-Huai Tsai,Mong-Ming Lu,Chung-Hsiung Sui,Yin-Min Cho

doi:10.3390/atmos12060758

Abstract

During the austral summer 2018/19, devastating floods occurred over northeast Australia that killed approximately 625,000 head of cattle and inundated over 3000 homes in Townsville. In this paper, the disastrous event was identified as a record-breaking subseasonal peak rainfall event (SPRE). The SPRE was mainly induced by an anomalously strong monsoon depression that was modulated by the convective phases of an MJO and an equatorial Rossby (ER) wave. The ER wave originated from an active equatorial deep convection associated with the El Niño warm sea surface temperatures near the dateline over the central Pacific. Based on the S2S Project Database, we analyzed the extended-range forecast skill of the SPRE from two different perspectives, the monsoon depression represented by an 850-hPa wind shear index and the 15-day accumulated precipitation characterized by the percentile rank (PR) and the ratio to the three-month seasonal (DJF) totals. The results of four S2S models of this study suggest that the monsoon depression can maintain the same level of skill as the short-range (3 days) forecast up to 8–10 days. For precipitation parameters, the conclusions are similar to the monsoon depression. For the 2019 northern Queensland SPRE, the model forecast was, in general, worse than the expectation derived from the hindcast analysis. The clear modulation of the ER wave that enhanced the SPRE monsoon depression circulation and precipitation is suspected as the main cause for the lower forecast skill. The analysis procedure proposed in this study can be applied to analyze the SPREs and their associated large-scale drivers in other regions.

Highlights

In late January and early February 2019, Queensland was hit by a disastrous rainfall event that caused 18,000 residents to lose power and hundreds of others to evacuate
After comparing the difference between the wind shear and the westerly [6,7,26] monsoon indices, we found that the wind shear index has the merit of showing clear seasonal contrast separated by the onset date and the cyclonic structure of the clockwise monsoon depression over the interior of northern Australia [27]
During the 2018/19 Box-A subseasonal peak rainfall event (SPRE), almost all of the 15 days of Outgoing longwave radiation (OLR) were in the Madden–Julian oscillation (MJO) convective phase, of which the percentage is much higher than the SPRE during other years

Summary

Introduction

In late January and early February 2019, Queensland was hit by a disastrous rainfall event that caused 18,000 residents to lose power and hundreds of others to evacuate. King et al [15] pointed out that the extreme rainfall variability is closely related to the mean rainfall variability during austral summer, especially the TCs and east coast low (monsoon depression) They defined the extreme rainfall as the monthly maximum consecutive 5-day precipitation totals. Cowan et al [1] gave a thorough analysis of the large-scale climate conditions of the 2019 Queensland flood. They found this event was a good example to promote the awareness of the benefit of S2S prediction that showed good skill in forecasting the broadscale atmospheric conditions north of Australia a week ahead.

Data and Methods

Identifying MJO and CCEWs

MJO and Equatorial Rossby Wave

Findings

S2S Prediction Evaluation