On the Ningaloo Niño/Niña

Abstract

Using both observational and reanalysis data, evolution processes of a regional climate phenomenon off Western Australia named recently “Ningaloo Niño (Niña)” are studied in detail. It is also shown that the Ningaloo Niño (Niña) has significant impacts on the precipitation over Australia. The Ningaloo Niño (Niña), which is associated with positive (negative) sea surface temperature (SST) anomalies and atmospheric anomalies off the western coast of Australia, peaks during austral summer and is classified into two types based on the difference in the evolution process. The first type called a locally amplified mode develops through an intrinsic unstable air–sea interaction off the western coast of Australia; an anomalous cyclone (anticyclone) generated by positive (negative) SST anomalies forces northerly (southerly) alongshore wind anomalies, which induce coastal downwelling (upwelling) anomalies, and enhance the positive (negative) SST anomalies further. The second type called a non-locally amplified mode is associated with coastally trapped waves originating in either the western tropical Pacific, mostly related to El Niño/Southern Oscillation, or the northern coast of Australia. Positive (negative) SST anomalies in both modes are associated with an anomalous low (high) off the western coast of Australia. The sea level pressure (SLP) anomalies in the locally amplified mode are regionally confined with a cell-like pattern and produce a sharp offshore pressure gradient along the western coast of Australia, whereas those in the non-locally amplified mode tend to show a zonally elongated pattern. The difference is found to be related to conditions of the continental SLP modulated by the Australian summer monsoon and/or the Southern Annular Mode.

Appendix: Necessary condition for instability

The method we used to assess whether or not each event satisfy the necessary condition for instability is almost the same as the one derived by Yamagata (1985). We summarize the method here.

The equations of motion for ocean, linearized about a state of no motion, are

$$u_{t} - fv + gh_{x} = \gamma U$$

(1)

$$v_{t} + fu + gh_{y} = \gamma V$$

(2)

$$h_{t} + d\left( {u_{x} + v_{y} } \right) = 0$$

(3)

where (u, v) are the zonal and meridional oceanic velocity component, h is the surface elevation, g is the acceleration due to gravity, and d is the equivalent depth. The wind stress $\left( {\gamma U,\;\gamma V} \right)$ is assumed to enter the ocean as a body force. If we take an energy integral of these equations, we obtain an energy equation:

$$\frac{1}{2}\left\langle {d\left( {u^{2} + v^{2} } \right) + gh^{2} } \right\rangle_{t} = \gamma \left\langle {d\left( {uU + vV} \right)} \right\rangle .$$

(4)

Here, 〈 〉 denotes the integration with respect to x from −∞ to the coast and y over a wavelength of the disturbance. Equation (4) suggests that a positive correlation between atmospheric winds and oceanic currents is necessary for instability. To reduce influences from noise, we take an average over the region near the center of anomalies in our analysis: The same domain as used to derive the CWI (108–114°E, 28–22°S).