Generation and Decay Mechanisms of Ningaloo Niño/Niña

Abstract

Using an ocean model, generation and decay mechanisms of warm/cool sea surface temperature anomalies (SSTAs) off Western Australia, or Ningaloo Niño/Niña, are investigated through the calculation of a mixed‐layer temperature (MLT) balance taking the mixed‐layer depth (MLD) variation into account. Since Ningaloo Niño/Niña develops owing to local air‐sea interaction and/or remote forcing, events are classified into two cases based on alongshore wind anomalies and analyzed separately. It is revealed that the anomalous meridional advection associated with the stronger Leeuwin Current and the enhanced warming by the climatological shortwave radiation because of the shallower MLD generate warm SSTAs in the coastal region for both cases of Ningaloo Niño. On the other hand, the latent heat flux damps SSTAs only in a case without northerly alongshore wind anomalies. In the decay, larger sensible heat loss is important. Because of the reduced meridional temperature gradient, the meridional advection eventually damps SSTAs. The sensitivity change to the climatological shortwave radiation owing to MLD anomalies explains offshore MLT tendency anomalies for both cases throughout the events. The mechanisms for Ningaloo Niña are close to a mirror image of Ningaloo Niño but differ in that the latent heat flux damps offshore SSTAs. The seasonal phase‐locking nature of Ningaloo Niño/Niña is related to the seasonal variations of MLD and surface heat fluxes, which regulate the amplitude and sign of the sensitivity change to surface heat fluxes. It is also related to the seasonal variations of the Leeuwin Current and meridional temperature gradient through advection anomalies.

Can Ningaloo Niño/Niña Develop Without El Niño–Southern Oscillation?

“…The absence of ENSO in austral spring could offer favorable conditions for local SST forcing because La Niña tends to suppress the atmospheric convection off Western Australia (supporting information Figure S7) and thus, may explain the difference in the amplitude of Ningaloo Niño between the CTRL and noENSO experiment. We note that while latent heat loss is reduced over the offshore region (supporting information Figure S6), which also favors shallower MLD, its direct warming effect is offset by the enhanced cooling by the climatological latent heat flux according to Kataoka et al ().…”

“…We performed two CGCM experiments and demonstrated that in the noENSO sensitivity experiment, Ningaloo Niño/Niña develops even without ENSO with a similar amplitude, duration, and seasonality. The driving mechanism seems to be the intrinsic air‐sea interaction off the west coast of Australia shown in earlier studies (e.g., Kataoka et al, , ; Tozuka et al, ), which can amplify initial atmospheric or oceanic perturbations. In this positive air‐sea feedback, a local warm SST anomaly would strengthen atmospheric deep convection to the northwest of Australia, which is accompanied by low SLP anomalies off Western Australia, northwesterlies extending westward of the convection anomaly, and alongshore‐northerly wind anomalies (Matsuno‐Gill response).…”

“…Positive SSH anomalies, that is, coastal downwelling waves, are seen (Figure d), partly originating from the western Pacific (Clarke, ; Clarke & Liu, ; Meyers, ), where observed SSH signals are more modest than in the CTRL run (Figure a), and partly forced locally by alongshore wind as suggested by the poleward strengthening of SSH anomalies. These coastal waves intensify the Leeuwin Current (Figures a and d), which causes warm advection anomalies (Benthuysen et al, ; Kataoka et al, ; Marshall et al, ). Negative SLP and alongshore‐northerly wind anomalies off the western coast of Australia are also found (Figures b and e).…”

“…The anomalous low is accompanied by alongshore‐northerly wind anomalies, which cause coastal downwelling anomalies as discussed above (Figure h). These coastal downwelling anomalies enhance the initial SST anomalies via a stronger poleward‐flowing current (Kataoka et al, ; Marshall et al, ). In addition, the cyclonic wind anomalies are against the climatological anticyclonic wind, which results in reduced wind speeds and thus shallower MLDs (supporting information Figure S6), consistent with the CTRL run and a previous oceanic modeling study (Kataoka et al, ).…”

“…Interannual oceanic variability off the western coast of Australia has long been considered to be mainly a response to tropical Pacific variability, especially the El Niño–Southern Oscillation (ENSO; M. Feng et al, ; M. Feng, McPhaden, et al, ; Pariwano et al, ; Wijffels & Meyers, ). However, Kataoka et al () showed that some Ningaloo Niño events develop through an intrinsic air‐sea interaction off the west coast of Australia: an anomalous low generated by positive SST anomalies (Tozuka et al, ) forces northerly alongshore wind anomalies due to the land‐sea pressure contrast, which in turn cause coastal downwelling anomalies and further enhance the initial warm SST anomalies through a strengthened poleward‐flowing Leeuwin Current (Benthuysen et al, ; Kataoka et al, ; Marshall et al, ). A subsequent study by Marshall et al () also concluded that Ningaloo Niño is the result of a combination of a local ocean‐atmosphere coupled process and remote forcing.…”