Factors Affecting the Trajectory and Intensification of Tropical Cyclone Yasi

Tropical Cyclone (TC) Yasi was a rapidly intensifying storm that began cyclogenesis just northeast of Fiji on January 29th 2011. It tracked southwest, continually gaining intensity until reaching category 5 by the time of landfall at Mission Beach on the Australian Eastern coastline in the early hours of February 3rd 2011. Yasi was measured at 600km wide and produced a storm surge of up to 6m in height, wind speeds of up to 300km/h, and a minimum central pressure of 929hPa recorded at landfall. The storm had a large impact on the coastline and particularly the Great Barrier Reef ecosystem, causing mechanical damage to the reef structures, reducing salinity and temperature, and bringing anomalously high levels of nutrient and sediment loading into the reef lagoons as flood waters receded from land.

This study ran an ensemble of simulations with different physics options with the National Center for Atmospheric Research (NCAR) Weather Research and Forecasting (WRF) Model v.3.4.1 to simulate this TC event. The ensemble revealed the difficulties in capturing both the intensity and track of an event of this size and magnitude and the simulated TC's sensitivity physics parameters, especially the cumulus parameterization. The ensemble was able to present the most appropriate model set up and the boundary and initial condition data required to accurately simulate the extreme event based on a criterion of: track, timing and intensity. Further simulations with altered sea surface temperatures (SSTs) demonstrate that a warming along the coastline, analogous to a warmer East Australian Current acts to maintain and increase TC intensification up to landfall as Yasi did. Increasing SSTs over the whole South Pacific acts to greatly increase TC intensity and push the track of the storm further southwards.

This modeling work is combined with analysis and quantification of changes to SST; nutrient and sediment loading; and chlorophyll a concentrations in surface waters along the coastline over the Great Barrier Reef from before to after TC Yasi with remotely sensed data sets such as from MODIS. The data validates the wide spread interaction that TC Yasi and events of this size and intensity have with the ocean surface and the negative and long term impacts they can have on coastal and marine ecosystems. The modeling work and the remote sensing analysis suggest that increasing SSTs have the potential to increase TC intensity and the extent of the negative impacts to coastal ecosystems.