Draft of OKMC Cruise Plan (R/V Revelle June 1-14, 2012 RR1205)

Transcription

1 Draft of OKMC Cruise Plan (R/V Revelle June 1-14, 2012 RR1205) Ren-Chieh Lien and Thomas B Sanford 1. Objectives In the RR1205 cruise, we will deploy five subsurface moorings, one surface mooring (TBD), and 3-5 HPIES and conduct shipboard survey NE of Luzon (Fig. 1). The primary scientific objective is to quantify the Kuroshio velocity, front, and transport, and its seasonal and spatial variations before it enters the Luzon Strait. 2. Scientific Party ~21 APL (6: Lien, Sanford, Dunlap, Carlson, Ma, Snyder), NTU (2 engineers: Her, Chang), Philippine observers (11), Taiwan observer (1), NTOU students (3) 3. Shipboard Equipment Requirement Some major equipment is listed as follows. TSE winch Trawl winch Multibeam and Knudsen Internet Connection Standard equipment for mooring deployment an, e.g., tugger, capstan, Shipboard ADCP Revelle HDSS Met sensors Intake sea surface temperature, conductivity CTD Marine radar GPS position and heading 4. Mooring and HPIES Positions (tentative): Five subsurface moorings, three-five HPIES (depending on the availability of HPIES), and one surface mooring (TBD) will be deployed during RR1205 (Fig. 1). Locations are shown in Fig. 1 and in Table 1. Other surface moorings and additional HPIES will be deployed in the later Revelle cruise, Jan These mooring positions are chosen because previous observations from driftesr, historical shipboard ADCP, recent ADCP, and glider consistently indicate the Kuroshio flows through this narrow path (red horizontal lines) before enters the Luzon Strait (Fig. 2). However, these are still considered as tentative positions. After thorough bathymetry survey, we will determine exact positions. At the current plan, two subsurface moorings are

2 within Philippine territorial water, < 12 nm. We are waiting for the decision from Philippine. If the deployment in the territorial water is not granted, we will move the mooring line slightly off shore to be outside of the territorial water. Positions of HPIES will follow those of subsurface moorings. Figure 1: Bathymetry North of Luzon. Black contour curves are isobaths at 1000-m interval. The thick black contour is the coast line. Filled circles represent subsurface mooring. Filled squares represent surface mooring. Filled stars represent HPIES. Red filled circles, square, and stars are those to be deployed in RR1205. Filled yellows will be deployed in the later cruise. Tentative positions of subsurface and surface moorings are shown. Table 1 Mooring positions and water depths. Bold Italic fonts represent those moorings to be deployed in RR1205. Others will be deployed in later cruise. Mooring Lat Lon Water Depth Cruise Anchor Weight (lb) SOK RR SOK RR SOK RR SOK RR SOK RR OK RR13?? 7500

3 OK RR OK RR13?? 7500 Figure 2:Surface drifter measurements of current speed (left panel) and historical shipboard ADCP measurements at 100-m depth. Both long-term observations indicate strong Kuroshio current flows through a narrow path (red horizontal lines) before enters the Luzon Strait. Figure 3: Shipboard ADCP survey in Luzon Strait in Panel shows the contour plot of meridional velocity and bathymetry. The vertical scale of shipboard ADCP is exaggerated by a factor of 5 for illustration of the Kurosion. Panels (b) and (c) show vertical profiles of zonal and meridional velocity in Balingtang Channel and in Luzon

4 Strait. The horizontal dashed blue lines in panels (b) and (c) represent the depth range of proposed OKMC subsurface ADCP velocity measurements. 5. Surface and Subsurface Mooring 5.1 Configuration Each of subsurface moorings will be equipped with one upward looking 75-kHz ADCP at about 450-m depth, providing vertical profile of 3-D velocity between ~430 m and 40 m depth range (Fig. 4 right panel). Previous measurements show that Kuroshio is not deeper than 400 m and the upper 40 m is strongly coherent. Therefore, our measurements will provide adequate observations of entire vertical profile of Kuroshio. Surface mooring will be equipped with a string of CTD and temperature sensors in order to measure the Kuroshio TS properties (Fig. 4 left panel). However, considering the risk of vandalism and aggressive finishing activity in the area, we might decide not to deploy the surface mooring. 5.2 Deployment Procedure Bathymetry survey and determine exact mooring target position Ship drift estimate Deploying subsurface/surface buoy at setup point Paying out mooring line while steaming toward anchor drop point at 2-3 kt relative to the water. Deploying glass floats and acoustic releases Deploying anchor at anchor drop point (2000lb for subsurface moorings and 7500lb for surface moorings) Triangulation of mooring position CTD cast

5 Figure 4: Configuration of surface mooring (left panel) and subsurface mooring (right panel). The surface mooring will be equipped with a GPS beacon on the buoy, a series of CTD and temperature sensors on the mooring line, backup glass floats, double acoustic releases and three stacks of train wheels, a total weight of 7500 lb. The subsurface mooring is equipped with a 45 syntactic float at 450m, a 75-kHz LongRanger, Iridium GPS, Kevlar line, backup glass floats, dual acoustic releases and a stack of train wheel of 2000 lb weight. 6. HPIES 6.1 Description HPIES consists of two instrument systems. PIES is a COTS instrument purchased from URI and installed with the HEF built at APL-UW. PIES consists of highly accurate absolute pressure gauge and acoustic travel time for 12 khz pulses. From these data we compute geostrophic shear from t (using dynamic height inferred from a lookup table) and deep geostrophic flow from bottom pressure. The HEF system provides the reference velocity. Technical information about PIES is available at PIES specifications are contained in the supplemental section of this document. The HEF consists of Ag/AgCl electrodes attached to low-noise preamplifiers that measure the ocean electric field between the ends of seawater-filled PVC pipes. The ocean signal is generally much less than 20 μv/m. However, the

6 electrodes can exhibit an offset potential of order 1 mv. Thus, it is necessary to remove the electrode offset in order to obtain ocean electric field observations. Each horizontal axis of the HEF instrument contains a water switch to distinguish the electrode offset and low-frequency preamp noise from the component of the ocean electric field parallel to the pipe axis. Consider the schematic depicted in Fig. 5. The B silicon tubing is pinched, making the resistance very large (~1 MΩ) through the tubing between the B(+) and B(-) electrodes. In this case, electrode pair B observes the potential between the ends of the PVC pipes (i.e., E(+) E(-)) plus the B pair s offset voltage. On the other hand, with the large resistance across the B tubing, there is little electric current passing between the B(+) and B(-) electrodes. So, the voltage sensed by electrode pair A is just that pair s offset. Thus, as the tubing segments are alternately pinched or unpinched, the A amplifier alternately measures the electrode pair offset followed by the sum of the offset and the ocean potential. The same occurs for the B amplifier with a delay of half a rotation of the pinching mechanism. The A and B observations are individually denoted as a half cycle. In this way, the measurement of the ocean potential is redundantly and nearly continuously measured based on both A and B amplifiers. Two water switches and PVC electrode arms are used to determine the horizontal components of the ocean electric field. This arrangement is diagrammed in Fig. 6. The water switches and arms are mounted on a bottom lander built around platforms with components and systems (Fig. 7). The landers weighed over 300 lbs in air. It is important to prevent the instrument from being tipped during an event of strong bottom flow. 6.2 Deployment Procedure Bathymetry survey and determine exact HPIES target position Ship drift estimate Deploying HPIES at the targeted positions using A frame and quick release (~300lb) Triangulation of mooring position CTD cast

7 Figure 5: HEF Water Switch. A motor alternately pinches silicon-tubing sections A and B. In doing so, the electrode offsets can be determined for electrode pairs A and B. The electrodes are connected to the ocean through PVC pipes. The extra circuits above the saltwater filled tubes are ancillary circuits to determine resistance of the pinched tubes. Figure 6: Left panel top view of HPIES configured for AESOP deployment near R/P FLIP. Right panel side view of HPIES ready for deployment. The PIES unit is on top of the HEF.

8 Figure 7: Full-up HPIES. PIES unit is on top. Water switches and arms on the bottom lander for HEF 1. The #8 water switch is mounted upward on the right hand side, while the #7 switch is mounted upside down on the left side. The measuring electrodes are inside the grey PVC blocks and fittings. The D electrodes are outer-most on the arms, while the C electrodes are between the D fittings and the water switch. The A and B electrodes and amplifiers are in the central PVC housing with the motor. The two black rubber bulbs connected to the housings are partially filled with Fluorinert that provide pressure equalization for the motor and electronics. 7. Ship Schedule The following schedule assumes that RR1205 is to be postponed by 4 days. The mob date for RR1205 is May 30-May 31. Revelle will depart Kaohsiung on June 1 and make the port call to Kaohsiung on June 14. The demob date is June 15. The transit between Kaohsiung and the experiment site will take about one day. We will make extensive shipboard CTD, ADCP, and bathymetry survey for up to 4 days to determine the Kuroshio location and water depths. The operation of mooring and HPIES deployment will take about 4 days. After the deployment, if we have extra time, we will make further shipboard observations in the area.