Source of the 70-Day Mesoscale Eddy Variability in the Coral Sea and the North Fiji Basin

Large-amplitude sea surface height (SSH) variations with a wave period of 70 days and a wavelength of about 700 km are detected in the Coral Sea centered along similar to 16 degrees S and in the North Fiji Basin along similar to 18 degrees S. These mesoscale eddy signals have a well-defined annual cycle with a maximum in December/January and a minimum in May/June. As the southeasterly trades intercept the island mountains of Vanuatu and Fiji, they generate localized wind stress curl dipoles, which in turn induce the eastward-flowing Coral Sea Countercurrent (CSCC) and Fiji Basin Countercurrent (FBCC) in the Coral Sea and the North Fiji Basin, respectively. The high eddy variability band along 16 degrees S is hypothesized in this study as resulting from barotropic instability due to the lateral shear between the eastward-flowing CSCC and its neighboring westward-flowing North Caledonian and North Vanuatu jets (NCJ and NVJ). Based on stability analyses on the background NCJ-CSCC-NVJ system, it is found that the most unstable mode of the system has an equivalent barotropic structure and is insensitive to the specification of the background flow's vertical shear. This barotropic instability hypothesis is also supported by observations; specifically, the Reynolds stresses inferred from the observed SSH data are tilted against the meridional shear of the background NCJ-CSCC-NVJ flow. As the intensity of the NCJ-CSCC-NVJ shear fluctuates with the season, the growth rate of instability changes, resulting in seasonal amplitude modulations in the observed 70-day eddy signals.