Residual Flows in Cook Strait, a Large Tidally Dominated Strait

Flows at subtidal frequencies are resolved from 20 months of current observations at multiple sites across the narrows of Cook Strait, New Zealand. The substantial tidal flows (spring tides >3 m s−1) are challenging to measure with moored ADCPs. Along-strait residual flows are on the order of 0.05–0.1 m s−1, but there were differences across the strait and also through the annual cycle. The calculated net residual transport is −0.25 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) (i.e., negative sign implies to the south, from Tasman Sea to Pacific Ocean). This is smaller (by a factor of 2–3) than that estimated from previous sparse current measurements. Crucially, however, these new data also suggest that, over the austral summer–autumn period, there is persistent residual flow northward on the western side of the strait so that the summer–autumn net −0.15-Sv flow can be separated into +0.065-Sv westside and −0. 22-Sv eastside flow. This western northward flow had a seasonal bias, appearing only in the summer–autumn period. The observations also show persistent temperature stratification on the order of 2°C over the full depth of the water column in this late summer–autumn period. An implication is that there is vertical shear influencing the residual flow estimates. A range of exchange flow scenarios are considered with the most extreme indicating that the flow on the western side of the strait in the January–June period might be as much as +0.08 Sv northward. This has substantial implications for regional circulation as well as for the nutrient supply for the nearby Marlborough Sounds.