Modelling the Ocean Response to a Tropical Cyclone on Australia's Northwest Shelf Madeleine Cahill, CMAR Peter Craig, CMAR Mike Herzfeld - CMAR Lou Mason - UTas

Can we model the ocean response to tropical cyclones ? Tropical cyclones: infrequent, small, energetic Dynamic atmospheric models have little skill in generating TCs Observations of cyclones in Australia are generally land based In situ observations of ocean response are rare. Effects are devastating (oil rigs, coastal regions…)

Ocean observations NWS – 1995 current meter deployment ADCP x current meter mooring

Path of TC Bobby

SHOC - CSIRO’s Coastal Ocean Model  3-D primitive equation hydrodynamic model  Arakawa C-grid  free surface  vertical z coordinates  a choice of turbulence closure schemes Our application  4km & 20km horizontal grid resolution  46 z-levels  k-  turbulence closure scheme  Horizontally uniform vertical temperature profile with a 20m mixed layer

Tropical Cyclone Model Double vortex Holland model - track prescribed - radius to maximum winds determined by fitting to available wind and pressure observations

Available marine wind observing sites

Cyclone model fit to wind observations: Site 1

Model fit to wind observations: site 2

Tropical Cyclone Model Parameters Radius to maximum winds Central pressure deficit Two Holland wind fields: Bobby1 (blue), Bobby2 (pink)

Holland wind fields - wind stress magnitude ocean current vectors (pink) Bobby1 Bobby2

Filtered current vector timeseries ADCP mooring in 125m (central site)

Modelled ocean response at ADCP mooring Wind forcing: Bobby1 Bobby2

Comparing response to two wind fields: Bobby1 (m) and Bobby2 (black) at the ADCP (125m) Alongshore component Cross-Shelf component

Comparing response to Bobby1(m) and Bobby2(black) at offshore mooring (M2 in 300m) Alongshore component Cross-shelf component

Filtered current vector timeseries Offshore mooring in 300m

Modeled response at offshore mooring: windfield1 windfield2

Velocities at ADCP location Observations (blue) v SHOC (mag)

Conclusions Model-data fit is starting to look pretty good Model response is very sensitive to TC extent and location Further work compare SHOC results with ROMS explore the effect of different vertical mixing schemes (using temperature as well velocity for verification) There are a further 11 TCs for which we have some current and temperature observations from the NWS.

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