Surface drifter tracks from central Cook Strait/Te Moana-o-Raukawa

This archive contains ocean drifter results from two experiments in Cook Strait/Te Moana o Ruakawa, Aotearoa New Zealand in the region of Fishermans Rock. Field data were collected during two periods, 15th – 26th of May 2022 (four deployments for a total of 26 tracks) and the 5th – 15th of November 2022 (five deployments for a total of 45 tracks) on NIWA’s research vessel RV Ikatere. The deployments used Pacific Gyre Microstar drifters to track upper-ocean currents in the vicinity of Fishermans Rock. Each drifter comprises a surface float, a tether and a drogue. The surface float is small, smooth, and designed to be close to neutrally buoyant to minimise the surface area exposed to wind forcing. Within the float housing is a GPS receiver (~1 m relative accuracy), battery and temperature/submergence sensors. The GPS transmitter was set to a 5-minute sample frequency, and the tethers were 0.25 m long to ensure the centre of the drogue was 1 m below the surface (referred to as ‘short’ drogue configuration). They were deployed along a short transect perpendicular to the primary current direction, one km upstream of Fishermans Rock. There was 200 m spacing between each drifter deployment location, this ensured the transect extended past each side of Fishermans Rock in order to establish the region of influence of FR. The time and unit number were recorded as each drifter was deployed. The drifters were then left to drift overnight and collected the morning after deployment. Drift periods ranged from 17 – 26 hrs depending on the time of deployment and distance travelled/weather conditions for retrieval. DEPLOYMENT IDENTIFIER START TIME DURATION # OF DRIFTERS MD1 16/05/22 01:00 UTC 21 hrs 6 MD2 17/05/22 01:55UTC 18.8 hrs 7 MD3 22/05/22 22:10 UTC 25 hrs 8 MD4 24/05/22 03:30UTC 17.7 hrs 6 ND1 04/11/22 23:25 UTC 21.5 hrs 7 ND2 06/11/22 22:50UTC 21.6 hrs 9 ND3 08/11/22 02:25 UTC 17.2 hrs 9 ND4 11/11/22 21:15UTC 23.3 hrs 9 ND5 13/11/22 20:05 UTC 29.4 hrs 9 Table of all Lagrangian drifter experiments conducted, the deployment identifiers as they are referred to in figures and text, start time and duration. Deployment Conditions Deployment location GPS track summary Eddies/wake interaction Notes M1 5-8 kt NW wind, southerly swell easing, 3hrs intoebb tide 1km N of FR Near field – drifter tracks part around FR then converges DS. General - Oscillatory motion along a NE-SW axis,at the turn of the tide currents rotate to the East, resulting in the net displacement of the drifters to the ESEof release point. MS-12 caught in wake of rock significantly lower velocity than other drifters, forms smalleddy structure. 12 unableto be found and left for another 24- hr period. M2 15-18 kt WNW wind, 2.5m short steep westerly swell, Ebb tide 2.5hrs before slack 1km N of FR Near field – Drifters part around FR, drifters to the east of FR greater velocity. General - Oscillatory motion along a NE-SW axis, currents at the turnof the tiderotated via eastin the region NE of FR and rotated via the west once in the centre of the Strait. Ebb tide (SW) greater velocity than Flood tide (NE), net displacement of drifters SW of releasepoint. MS-11 caught in wake on SW of FR,significantly lower velocity than other drifters but no clear eddy structure in track. 12 was recovered west of Cape Terawhiti M3 5 ktsS wind, 4m long period southerly swell, deployed at slack tide. 1km S of FR Near field – all drifters passed FR to the East missed wake region. General– Full extentof flood tide (6.5 hrs) travelling NE of FR. Tide turned via the East, very little southward velocity on the ebb tide, only ~2hrs of ebb tide flow before turning back to N direction (flood). Net displacement of drifters to the N of release point. All drifters passed FR to the East missed wake region. No field work previous days due to southerly storm (gale winds and large swell) M4