Abstract
Abstract. The southwestern Pacific Ocean sits at a bifurcation where southern subtropical waters are redistributed equatorward and poleward by different ocean currents. The processes governing the interannual variability of these currents are not completely understood. This issue is investigated using a probabilistic modeling strategy that allows disentangling the atmospherically forced deterministic ocean variability and the chaotic intrinsic ocean variability. A large ensemble of 50 simulations performed with the same ocean general circulation model (OGCM) driven by the same realistic atmospheric forcing and only differing by a small initial perturbation is analyzed over 1980–2015. Our results show that, in the southwestern Pacific, the interannual variability of the transports is strongly dominated by chaotic ocean variability south of 20∘ S. In the tropics, while the interannual variability of transports and eddy kinetic energy modulation are largely deterministic and explained by the El Niño–Southern Oscillation (ENSO), ocean nonlinear processes still explain 10 % to 20 % of their interannual variance at large scale. Regions of strong chaotic variance generally coincide with regions of high mesoscale activity, suggesting that a spontaneous inverse cascade is at work from the mesoscale toward lower frequencies and larger scales. The spatiotemporal features of the low-frequency oceanic chaotic variability are complex but spatially coherent within certain regions. In the Subtropical Countercurrent area, they appear as interannually varying, zonally elongated alternating current structures, while in the EAC (East Australian Current) region, they are eddy-shaped. Given this strong imprint of large-scale chaotic oceanic fluctuations, our results question the attribution of interannual variability to the atmospheric forcing in the region from pointwise observations and one-member simulations.
Highlights
The southwestern Pacific Ocean is a region comprising many islands, seamounts and reefs
We focus our analyses on the 1980–2015 period; before 1980 the buoyancy fluxes derived from the DFS5.2 forcing are devoid of interannual variability
The spread due to oceanic chaotic variability in the 36-year average transports, revealing how intrinsic oceanic variability affects the 36-year average, is symbolized by black circles overlaid when such spread is greater than 15 % of the mean. It appears to be negligible in most areas (Fig. 2a). It represents 10 % to 20 % of the mean current south of New Caledonia: 20 % of the mean South Caledonian Jet (SCJ) may be attributed to chaotic oceanic variability, while the East Australian Current (EAC) and Tasman Front exact latitude and meanders vary from one member to the other
Summary
The southwestern Pacific Ocean is a region comprising many islands, seamounts and reefs. Recent studies have highlighted the importance of nonlinear internal ocean dynamics in spontaneously generating interannual variability, which is substantial in eddy-active regions (e.g., Penduff et al, 2011; Sérazin et al, 2015) This so-called intrinsic variability, which can be strong and more importantly has a random phase, can imprint on large-scale and integrated quantities such as the ocean heat content (Sérazin et al, 2017; Penduff et al, 2018) and the volume transport of the overturning circulation (Grégorio et al, 2015; Leroux et al, 2018; Jamet et al, 2020). We show that there are large parts of the region where interannual transport variability is firstly driven by internal chaotic ocean processes, which probably arise from a rectification of the lowerfrequency signal by mesoscale activity This important role played by ocean-only dynamics may hamper our capability to identify the atmospheric drivers of the oceanic interannual variability in the southwestern Pacific and to predict their behavior at interannual timescales.
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