THE CONSORTIUM ON THE OCEAN'S ROLE IN CLIMATE (CORC III):
Integrated Boundary Current Observations In the Global Climate System
Link to NOAA Strategic Plan: NOAA's Mission Goal 2: Understand Climate Variability and Change to Enhance Society's Ability to Plan and Respond
RESEARCH OBJECTIVES AND SPECIFIC PLANS TO ACHIEVE THEM
The project plans to develop, demonstrate, and implement a system that can fully monitor the intensity (mass and heat transports) of most boundary currents in a sustained and routine mode, delivering indicators about the state of those regimes in near-real-time. To this end we will merge several technologies and techniques that have been used by the PIs in the past, and that were partly developed in prior CORC phases. These include:
• end-point moorings (with CTD sensors throughout the water column and bottom pressure sensors) at the ends of a section to determine the dynamic height difference, and thus geostropic transports, as a time series.
• underwater gliders to estimate the heat transport through a section, by providing the horizontal (and vertical) distribution of heat content and its correlation with the flow.
• inverted echosounders plus bottom pressure (PIES) distributed along the section to be monitored. These will yield 2 vertical integrals (e.g. dynamic height and heat content) at each location, providing the depth (and time) coverage along the section that the gliders cannot.
• data telemetry for the PIES and (subsurface) moorings using acoustic modems between these and the gliders. In very high (surface intensified) current regions, the gliders may need to remain submerged on one of the round-trip crossings each time. In this case, a navigation capability will be needed in the gliders to pass within close enough proximity of the PIES and moorings.
• data assimilation for determining heat and flow distributions, and thus the full mass and heat transports, that are consistent with all the data types collected, with satellite altimetry, with the forcing fields (wind) and with up/downstream and offshore information.
The pilot and testbed application will be carried out in the California Current which has large climate and socio-economic relevance and does not have a routine monitoring system. Operation along CalCOFI line 90 in southern California assures synergy with other programs, and coincides approximately with the high-resolution XBT line PX31 which will contribute comparison data and connect sampling to the basin interior. In addition automated surface drifter releases will quantify the eddy variability and the Ekman flow in the boundary current region. Later in the project, implementation of the system in the climatically highly relevant western boundary current of the low-latitude western Pacific is planned (which feeds the Equatorial Undercurrent through the Solomon Sea).
RESEARCH ACCOMPLISHMENTS
Task A: Glider Operations in Integrated Boundary Current Observations
In the longer running contribution of gliders to boundary current monitoring, we are using Spray gliders to observe, first, the structure and magnitude of the flows from the tropical South Pacific to the equatorial zone and, eventually, temporal variability of this transport. In the current reporting period, we have, in cooperation with CORC II (R. Davis), (1) completed another crossing of the South Pacific’s Equatorial Current (SEC) between Guadalcanal and New Caledonia and (2) prepared and shipped a second Spray for exploration of the SEC as it enters to Solomon Sea on its way to the equator.
CORC contributions to monitoring climate variability in the California Current were development of a 750 kHz acoustic Doppler current profiler (ADCP) for the Spray glider and the implementation of continuous sampling along CalCOFI Line 90 since 19 October 2006. This is the section where the pilot boundary current array will be deployed.
For the new integrated system using gliders together with PIES and moorings, we have finished the design study, chosen and purchased one acoustic modem and transducer that will be used on Spray to interrogate submerged instruments, devised a way to install a transponder in Spray by lengthening it by only about 5 cm, and construction of the larger hull has begun. A way to physically incorporate the electronics for the modem into Spray has also been designed. Work has further begun for structuring the software expansion that will be needed.
Task B: Moorings and PIES in the Integrated Boundary Current Observations
For the PIES and mooring development work, we have selected an acoustic modem system for use on the moorings and on the inverted echosounders/pressure sensors (PIES), to match the ones described above for the Spray gliders in their role of interrogating subsurface instruments in the pilot boundary current array. Two such modem systems have been purchased for this task (plus one in the glider task). Engineering bench tests with the modems have been completed to check their functionality, power consumption, and performance. Designs for the modem integration (hardware and software) with the PIES and the moorings are completed. Modifications for the PIES have been agreed on and implemented with the manufacturer. Two such PIES ready for external modem interfacing have been purchased and received. The mechanical and interface design for adding the modem to the PIES on their bottom tripod is completed and will be tested soon. For the mooring work, a new controller had been designed and construction is nearing completion which will allow collection of data from moored instruments (inductively from microcats, and via serial line from ADCPs), and which will pre-process the data, and pass them to the acoustic modem.
Some mooring sensors have been acquired already, and more mooring hardware is now being purchased to prepare the first test moorings off San Diego.
Task C: XBT data in support of the California Current observing system
We are organizing CalCOFI and XBT datasets to address questions about interannual variability in the transport of the California Current. First, we have updated, to the present, estimates of geostrophic velocity and transport for CalCOFI lines 80 and 90. Similarly, we have calculated geostrophic velocity for the XBT lines out of Los Angeles and San Francisco. Scientific questions are: (i) do we see the same interannual variability in CalCOFI Line 90 and the Fiji-Los Angeles XBT line, (ii) where is the offshore edge of the California Current in the Fiji-Los Angeles XBT line, (iii) is the interannual transport variability in the California Current at the two XBT lines correlated?
For the research-quality deep XBT probes (LMP-5 T1) development work at Sippican is proceeding slowly. Sippican carried out an unsuccessful test at Bermuda a few months ago. We're hoping for a new batch of probes for evaluation in the next few months.
Task D: Synthesis of transports through the southern CCS
The combined data sets on circulation of the CCS are being analyzed for the purpose of computing both the time mean and the deviations from that mean. In the period of February-April 2007 we have organized the historical drifter and altimeter data sets from 1985-2006 and computed from these the 15m geostrophic velocity distribution in the 1982-2007 CALCOFI survey area. In the present funding period, we will combine these observations with the ADCP data sets acquired from the CALCOFI surveys since 1993 by Dr. T. Chereskin at SIO, as well as the CALCOFI hydrographic data to compute the mass transports of the upper layers through the CORC observation regions. Of interest is both the time mean and time variability of the geostrophic (that contains no divergence) and the ageostrophic (that contains the divergence and upwelling) horizontal circulation patterns.
Task E: Modeling and assimilation of future CORC data in the California Current
For the modeling work, an MITgcm model grid has been made for the CCS region from north of San Francisco to south of San Diego, at 1/6 degree resolution. A few months of test assimilation runs have been performed using altimeter SSH and satellite SST only. The assimilation seems to work as expected so far, but a lot of work is needed to evaluate the choices of parameters (viscosity, diffusivity, Kpp, etc.) and to assemble the datasets.
We are continuing to work on the California Current assimilation, although we have not yet incorporated the in situ observations. We are presently upgrading to the newer version of the ECCO code (which is the assimilation package including the MITgcm), so we are in a debugging process, since many features have changed. We are planning to do hindcast experiments from CalCOFI observations to evaluate the model skill. To that end, we have begun to look at assembling forcing fields and observations.
Fig. 1
SPRAY glider with CORC modifications: acoustic modem transducer for 10km range communication incorporated into tail section (left). Inverted echosounder (white sphere) with the acoustic modem (black cylinder) modification and assembly (right).
Fig. 2
Unbiased 1992-2002 geostrophic 15m velocity in the CCS off southern California from surface drifters and altimetry. Along CalCOFI line 90, the section to be sampled by CORC, the CCS is concentrated in two branches.
