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 <5kts variable wind, long period S swell easing, flood tide 1 hr before slack. 1km SSW FR Near field – Drifters W of FR travel due east, those to the E of FR travel N with flood tide. General – Circular motion around FR for first 10-12hrs of deployment, when tide changesto flood drifters areadvected away fromFR in a NE direction. Flood tide velocities greater than ebb. Net displacement of drifters to the NE of FR. Two drifters on the western side of FR get caught in wake, lower velocity and travel due eastin the wake region. No eddy structure evident in tracks. Deployment Conditions Deployment location GPS tracksummary Eddies/wake interaction Notes N1 10-12kt S wind, 1m NW swell easing, 3hrs into ebb tide 1km N of FR Near field – Difficult to determine dueto messy data. General – Oscillatory motion along a N-S axis, similar ebb/flood tide velocities, tide turned to the westfor both theebb and floodchanges. Net displacement of drifters to the NNW of release point. Difficult to determine due to messy data longer droguelines dragging GPS unit under water. N2 15-20kts N wind, 1.5m steep NW chop, slack tide starting to ebb 1km N of FR Near field – Drifters passed to westof FR, eastern most drifter caughtin wake. General – Ebb tide velocities much greater than flood, drifters travel SSW of FR. Tide turns through the east and the majority of the drifters end up on eastern side of strait. Highest velocities observed on the ebb tide in the Terawhiti region, once through the narrows drifters are advected to the SE along the south coast of the NI. 1 driftercaught in wake region on western side of FR, significantly slower velocity and unusual current directions but no clear eddy structure in track. N3 <5kts variable wind, ebb tide 1hr before slack water 500m NNE of FR Near field – Drifters part around FR, tide turns to the east advecting drifters away from wake region before flooding to the N. General – Oscillatory motion, similar velocities and distances travelled by the drifters on the flood and ebb tide phases. Driftervelocities had a large westward component leading up to and during the turn of the tide resulting in a net displacement to the WNWof release point. No interaction between drifters and wakeregion. N4 <5kts variable wind, 1.5m S swell easing, 3hrs into flood tide phase 1km SSW of FR Near field – Drifters part around rock,one caught in wake, currentnear rock flowing NE. General – Ebb tide velocities much greater than flood phase,tide turned toward the east, flood velocities have a large easterly component resulting in drifters grouped on the eastern side of the strait near Cape Terawhiti. Thetwo drifters to the westof the maingroup are long drogues. Drifters displaced to the S/SE of release point. 1 Driftercaught in wake, significantly slower velocities and abnormal current direction, eddy structure forms to N of FR. Whenthe tide changes it stays in wake and forms eddy structure to S of FR. No field work previous days due tostrong southerly winds. Longer drogues stay in centre of strait whereas others are pushed east. N5 15-20 kts S wind, 1.5m southerly swell and white caps, 1hr into flood tide 1km SSW of FR Near field – 8/9 drifters are caught in wake of FR, significantly slower velocities than 1 drifter outside of wake region. General– Similar ebb and floodtide velocities, drifters turn west at the tide change and both ebb and flood flows have a large westward component. Net displacement is to the W of release point. 8/9 drifters caught in wake, significantly lower velocities than outside wake region. Helix shape crossover pattern in tracks. No complete eddy loops.