SIO HIGH RESOLUTION XBT/XCTD TRANSECTS
Link to NOAA Strategic Plan: NOAA's Mission Goal 2: Understand Climate Variability and Change to Enhance Society's Ability to Plan and Respond;
Goal 3: Serve Society's Needs for Weather and Water Information
RESEARCH OBJECTIVES AND SPECIFIC PLANS TO ACHIEVE THEM
The SIO High Resolution XBT Network is a set of basin-spanning shipping routes (Fig. 1) along which eddy-resolving temperature transects are collected four times per year. The HRX Network was initiated in 1986 along a commercial shipping route between New Zealand, Fiji, and Hawaii. It was subsequently expanded during the 1990’s to include basin-spanning temperature transects in all of the oceans. Major partners in the HRX network include Scripps (Pacific and Indian Ocean), NOAA/AOML (Atlantic), and CSIRO (SW Pacific, Indian). Typically, each transect is repeated on a quarterly basis to resolve variability in temperature, geostrophic circulation and transport on annual and longer periods. Scientific objectives of the HRX Network are:
• Measure the seasonal and interannual fluctuations in the transport of mass, heat, and freshwater across transects which define large enclosed ocean areas.
• Determine the long-term mean, annual cycle and interannual fluctuations of temperature, geostrophic velocity and large-scale ocean circulation in the top 800 m of the ocean.
• Obtain long time-series of temperature profiles at precisely repeating locations in order to unambiguously separate temporal from spatial variability.
• Determine the space-time statistics of variability of the temperature and geostrophic shear fields.
• Provide appropriate in situ data (together with Argo profiling floats, tropical moorings, air-sea flux measurements, sea level etc.) for testing ocean and ocean-atmosphere models.
• Determine the synergy between HRX transects, satellite altimetry, Argo, and models of the general circulation. What are the minimal sampling requirements for in situ data?
• Identify permanent boundary currents and fronts, describe their persistence and recurrence and their relation to large-scale transports.
• Estimate the significance of baroclinic eddy heat fluxes.
RESEARCH ACCOMPLISHMENTS
Quarterly HRX transects have been maintained along basin-spanning routes that include:
• PX37/10/44 (North Pacific – San Francisco to Hawaii to Guam to Hong Kong)
• PX38 (North Pacific – Hawaii to Alaska)
• PX08 (South Pacific – New Zealand to Panama)
• PX06/31 (Central Pacific – New Zealand to Fiji to Los Angeles)
• PX30 (South Pacific – Brisbane to Fiji, joint with CSIRO Australia)
• IX15/21 (South Indian – Fremantle to Mauritius to Durban)
Logistical assistance or XBT probes are provided collaboratively for:
• PX34 (South Pacific – Wellington to Sydney, CSIRO Australia)
• IX28 (Southern Ocean – Hobart to Antarctica, CSIRO Australia)
• AX22 (Southern Ocean – Drake Passage)
During each cruise, XBT temperature profiles (0-800 m) were collected at spatial intervals from 10 km near ocean boundaries and the equator, to 50 km in interior regions—resolving boundary currents and interior eddies for calculation of ocean-wide integrals of geostrophic transport. A web site for the project, with downloadable data, is maintained at http://www-hrx.ucsd.edu. HRX data are transmitted on the GTS immediately after collection for real-time applications. Due to the rapidly evolving nature of the present-day commercial shipping industry, a strong focus of the work has been to maintain sampling along routes as ships and shipping companies change.
Scientific analysis of the HRX dataset is progressing both on a stand-alone basis and in the context of ocean data assimilation (ODA) modeling. The first HRX line, New Zealand to Fiji, was begun in 1986, and the resulting 21-year time series is a rich source of information on seasonal to decadal variability of the eastward limb of the subtropical South Pacific gyre (Fig. 2). In the North Pacific, two different studies have (i) improved estimates of the mean transport of heat and freshwater from the tropics to the subtropics and (ii) provided a closed heat budget on interannual timescales, including heat transport, air-sea flux and heat storage, using the ECCO ODA model. Large interannual variability was observed in the heat budget, including a maximum in ocean heat transport in 1998 coinciding with minimal heat loss to the atmosphere and maximum in heat storage. In a separate study the ECCO model was used to understand the limitations of ocean data assimilation in the tropics and to learn whether the satellite and in situ datasets can be combined in a consistent fashion with the model dynamics.
Fig. 1
The High Resolution XBT network, indicating lines sampled by SIO (blue) and by partners (black), and indicating first year of sampling (red).
Fig. 2
20-year mean (oC, top panel) and variance (oC2, middle panel) of temperature, and 20-year mean of zonal geostrophic velocity (cm/s, bottom panel), HRX Line PX06, New Zealand to Fiji. This dataset demonstrated that the eastward limb of the shallow subtropical gyre is broken into distinct thin filaments that are persistent over decades of sampling.
