Hobart

Seminar Abstract

Friday 6 June 2008, 11.30am (Tas time)
CSIRO Auditorium, Hobart

Jaclyn N Brown
OCE Postdoctoral Fellow/CAWCR
CSIRO Marine and Atmospheric Research, Hobart

Understanding Subsurface ENSO Dynamics for Improving Model Analysis

Jaclyn N Brown
Centre for Australian Weather and Climate Research (CAWCR),
CSIRO Marine and Atmospheric Research, Hobart.

Alexey Fedorov
Department of Geology and Geophysics, Yale University.

We present a physical energy-based framework that is a useful tool for understanding ENSO dynamics. We focus on three key characteristics of the tropical Pacific Ocean: the wind power, the buoyancy power, and the available potential energy. Wind energy is supplied to the ocean when the wind blows in the same direction as the surface currents. It is removed from the system when the wind blows in the opposite direction. The wind power generates a buoyancy power that raises or lowers the isopycnals, leading to thermocline depth anomalies. The available potential energy is largely a measure of the thermocline slope. A La Niña year has a strongly sloped thermocline which has a high available potential energy, and El Niño has a flatter thermocline, and less available potential energy. The wind power and available potential energy are approximately 90 degrees out of phase, with wind power leading available potential energy by four to eight months. The system described is similar to the recharge-discharge oscillator but is based on rigorous conservation laws. It also helps us understand how energy is transferred from the surface to the subsurface

From analyzing these energy variables we derive two energy-based metrics, which allow us to compare how models simulate ENSO events. The first of our metrics (the efficiency of energy transfer from the surface to the thermocline) measures how efficient the wind power is in creating available potential energy, hence thermocline slope anomalies and ENSO events. The second metric measures the decay rate of the available potential energy.

These metrics provide a useful tool for understanding models differences in the way they represent ENSO. Our energy-based metrics provide a way to give a basin-wide measure of how energy is transferred differently between models and how they dissipate this energy, regardless of smaller model differences such as grid size and friction schemes. In this study we compare ocean-only models, data assimilations, and coupled IPCC AR4 models. For example, some models have exceptionally strong windstress, but compensate by dissipating most of the energy in the mixed layer. Typically, coupled models are less efficient in transferring energy to the thermocline and more dissipative than ocean-only models and data assimilations. Our metric provides a simple, yet effective method of understanding model differences so that further progress can be made.

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Location:
CSIRO = Marine Laboratories Auditorium, Castray Esplanade, Hobart

For further information, or to schedule a seminar, contact:
To schedule a seminar, contact:
Bernadette Sloyan, (Oceanographic seminars) CSIRO Marine and Atmospheric Research (03) 6232 5152
Thomas Kunz, (Biological seminars) CSIRO Marine and Atmospheric Research
(03) 6232 5076
Natalie Dowling, (Fisheries Modelling) CSIRO Marine and Atmospheric Research
(03) 6232 5148
Jillian Enraght-Moony, (seminar administrator) CSIRO Marine and Atmospheric Research (03) 6232 5320
Communications Manager, Antarctic Climate and Ecosystems CRC (03) 6226 2265
Margaret Hazelwood, Institute of Antarctic and Southern Ocean Studies (IASOS) University of Tasmania (03) 6226 2971

Last updated 21/07/09