A review of global ocean temperature observations: Implications for ocean heat content estimates and climate change
Summary (8 min read)
1. INTRODUCTION
- The broad topic of climate science includes a multitude of subspecialties that are associated with various components of the climate system and climate processes.
- It remains a challenging problem for climate scientists.
- Finally, changes in measurement techniques and instrumentation have resulted in biases, many of which have been discovered with some account made. [5].
- International, observational programs and projects are vital to the data used in these analyses.
- The Global Climate Observing System, in partnership with WCRP, has formulated a global ocean observing system and encouraged contribution to it, particularly through the OceanObs workshops in 1999 and 2009. [8].
2. THE EVOLVING SUBSURFACE TEMPERATURE OBSERVING SYSTEM: A HISTORICAL PERSPECTIVE
- An understanding of ocean heat content changes is only as good as the subsurface ocean temperature observations upon which these calculated changes are based.
- The subsurface temperature observing system is still relatively young when compared to atmospheric observing systems.
- What follows is a look at the developments and ideas that enabled implementation and precipitated changes in the observing system.
- As a guide, Figure 1 shows geographical coverage during the height of each iteration of the observing system.
2.1. Early Measurements (From 1772)
- On Captain James Cook’s second voyage (1772–1775), water samples were obtained from the subsurface Southern Ocean and it was found that surface waters were colder than waters at 100 fathoms (~183m) [Cook, 1777].
- Slightly more than 100 years later, the Challenger expedition (1873–1876) circumnavigated the globe, taking temperature profiles from the surface to the ocean bottom along the way, ushering in an increased interest in subsurface oceanography and new technology developments which facilitated measurement.
- Pairs of protected and unprotected reversing thermometers were used to determine temperature and pressure, with pressure determined to an accuracy of ±5m depth in the upper 1000m.
- The development of the Nansen bottle [Mill, 1900; HellandHansen and Nansen, 1909] which attached the thermometers to a sealed water sample bottle completed the instrumentation package which constituted the subsurface upper ocean temperature observing system for the 1900–1939 time period.
- Hence, the long-term mean seasonal variations, the year-to-year variance, and vertical structure of the ocean were not well described.
2.3. Mechanical Bathythermograph Observation System (From 1939)
- Quickly and accurately mapping the temperature variation of the upper ocean became a military priority in the lead-up to World War II for the accurate interpretation of sonar readings to locate submarines and their potential hiding places.
- Oceanographers now had the means with which to acquire detailed sets of measurements to map the mixed layer and shallow thermocline [Spilhaus, 1940]. [14].
- Inside the cylinder is a Bourdon tube enclosing a capillary tube with xylene (a hydrocarbon obtained fromwood or coal tar) inside.
- A stylus attached to the Bourdon tube captures the movement as temperature change horizontally scratched on a plate of smoked glass.
2.4. Ship-Based Conductivity-Temperature-Depth Instruments (From 1955)
- The development of the salinity-temperature-depth (STD) and later the conductivity-temperature-depth (CTD) instruments augmented existing observations by eventually replacing the discrete reversing thermometer observations with continuous profiles of temperature.
- The development of the CTD also laid the groundwork for their current observing system and for the backbone large-scale measurement cruises of the World Ocean Circulation Experiment (WOCE) among others.
- The basic physical concept of a thermal resistor was known as early as 1833 when Faraday noted that the conductivity of certain elements was affected by changes in temperature [Faraday, 1833].
- Brown later modified the CTD design to use both a fast-response thermistor and a platinum resistance thermometer as well as a wire strain gauge bridge transducer to measure pressure in order to correct transients in the conductivity signal [Brown, 1974].
2.5. The Expendable Bathythermograph Observing System (From 1967)
- Technological advances in wire and wire insulation made it possible to create an instrument electrically connected to the ship and able to transmit information through a thin conducting wire.
- U.S. Navy traces were sent to the Fleet Numerical Weather Center (FNWC) where they were digitized, used for weather prediction and other projects, and then passed to the U.S. National Oceanographic Data Center (NODC) for archive and public release [Magruder, 1970].
- In 1990, a global system of distributing XBT data was implemented (see below discussion of the Global Temperature and Salinity Profile Program (GTSPP)). [20].
- A Canadian company, Sparton, also briefly manufactured XBTs of their own design. [22].
2.6. Tropical Moored Arrays (From 1984)
- The tropical moored arrays were set up to continuously monitor the tropical ocean.
- The first tropical moored array, the Tropical Atmosphere Ocean (TAO) array (later TAO/ TRITON), was set up to help monitor and understand the El Niño phenomenon [McPhaden et al., 1998].
- Later, the SOFAR (Sound Fixing and Ranging) float [Webb and Tucker, 1970; Rossby and Webb, 1970] improved on this system by enabling tracking of the float by underwater listening devices.
- Each cycle, they dive to a nominal 2000 dbar target and typically measure pressure, temperature, and salinity from there to the surface where the information is transmitted to a satellite.
- So, the Argo Program governs the floats from deployment planning through quality control and dissemination, a true end-to-end observation system.
2.8. Summary of Ocean Temperature Measurements
- Interspersed within the main observing system data are high-quality bottle and CTD temperature measurements from projects such as WOCE (1990–1998).
- Historic studies of ocean heat content and other related variables need to take into consideration the changes in the observing system and the limitations of the system during each time period to fully interpret their results.
- Gliders, undulating CTDs, and sensor-outfitted animals are already starting to extend and expand the observing system, and full-depth Argo floats are under development with a goal of allowing an ever-improving understanding of ocean heat content variability and its place in the Earth’s climate system.
3.1.1. The XBT Instrument
- As discussed earlier, an XBT is a probe that measures temperature as it free-falls through the water column.
- There is a hint of temperature dependence in this bias (Figure 5), as also indicated by probe calibrations in the laboratory [Gouretski and Reseghetti, 2010]. [48].
- Wijffels et al. [2008] contributed two sets of corrections, both of which attempt to remove bias by applying time-varying multiplicative factors to the measurement depths (effectively assuming that there are no pure temperature or depth offset adjustments).
- Cowley et al. [2013] derive corrections for the common shallow and deep types of XBT; those with missing data are assigned to a type by their maximum depth and country of origin.
- Many correction schemes have been proposed to adjust for these biases. [63].
3.2. Dynamic Models for XBT Devices
- While traditionally, the descent of XBT probes into the ocean water is handled through the use of standardized FREs, it is also possible to use dynamic models, allowing independent predictions of probe depths.
- For the dynamic modeling technique, it is possible to incorporate changes to the drop conditions that are not reflected in the experiments during which the FRE was obtained.
- And with a dynamic model, it is possible for users to calculate the depth of XBT devices independent of FRE models. [69].
- Probe mass and drop height may have a significant impact on probe depths; however, a recent set of experiments suggest that the effect of drop height might be overpredicted [Abraham et al., 2012a].
- Surface effects (such as sudden impact forces at entry, angle of impact with the ocean surface, ship motion, entrainment of air, etc.) may negate the larger impact velocities. [71].
3.3. The Global XBT Measurement Network
- [72] XBT deployments are designated by their spatial and temporal sampling goals or modes of deployment (low density, frequently repeated, and high density) and sample along repeated, well-observed transects, on either large or small spatial scales, or at special locations such as boundary currents and chokepoints (Figure 11).
- Frequently repeated transects typically target 12–18 realizations per year, with XBTs deployed at 100–150 km spacing, and are designed to obtain high spatial resolution observations in consecutive realizations in regions where temporal variability is strong and resolvable with an order of 20 day sampling.
- Given the advances in global observing system, the global XBT network is currently focused on the monitoring of boundary currents and heat transport and not exclusively on the upper ocean thermal field.
- The scientific objectives of HD sampling and examples of research targeting these objectives are as follows [Goni et al., 2010]: [75].
- Measure the seasonal and interannual fluctuations in the transport of mass and heat across transects which define large enclosed ocean areas and investigate their links to climate indices. [76].
3.4. Future of the XBT Network
- The XBT network reflects the recommendations of OceanObs99 and OceanObs09 [Goni et al., 2010] and includes several transects that the scientific community has added during the last 12 years (Figure 11).
- Ship recruitment is an ongoing issue in implementing the XBT network, resulting in gaps or shifts in sections.
- Thirteen years after OceanObs99, the XBT HD transects continue to increase in value, not only through the growing length of decadal time series, but also due to integrative relationships with other elements of the ocean observing system, including the following: [85].
- HD transects together with Argo float data provide views of the large-scale ocean interior and small-scale features near the boundary, as well as of the relationship of the interior circulation to the boundary-to-boundary transport integrals. [86].
- Almost 20 years of continuous global satellite altimetric sea surface heights are matched by contemporaneous HD sampling on many transects.
4. ACCURACY/BIASES OF ARGO FLOATS
- The Argo Program, an array of over 3000 autonomous floats designed to return materially important oceanic climate data, is a vitally important component of the present oceanic Earth observing system and a strong complement to satellite observations.
- This program, designed to complement the Jason altimeter missions, provides observed climate signals that, when globally averaged, are sensitive to the presence of data bias.
- The Argo Program has advanced the breadth, quality, and distribution of oceanographic data as compared to the broad-scale XBT network while continuing to supplement ship-based CTD programs (section 2).
- The immediate distribution of data results in a two-tiered quality control system, each with distinct expectations of bias within the data.
- A more careful analysis termed “delayedmode quality control” (DMQC) is performed by the float provider 6–18months after data acquisition [Wong et al., 2012].
4.1. Argo Float CTD Sensors
- The majority of floats within the “Core Argo” array measure temperature, salinity, and pressure with SeaBird Electronics, Inc. (SBE) conductivity-temperature-depth (CTD) packages.
- In recent years, the Argo array has become nearly homogenous in the use of the SBE CTDs due to their high accuracy, modest conductivity sensor drift (both in numbers of floats with drift and, if present, the rate of drift), and the lack of a suitable alternative sensor provider.
- The last FSI-equipped Argo float was deployed in December 2006.
- The SBE41CP can also be used in spot sampling mode.
- The SBE-41 has historically been installed in more Argo floats.
4.2. Sensor Drift: The Causes, Identification, and Correction
- Sensor drift is a continuous concern for instruments that are designed to obtain extended duration measurements in the climate system.
- The manufacturing process of the Druck pressure sensor was modified, and Argo floats with rigorously tested Druck pressure sensors were again being deployed by late 2009.
- For pressure drifts larger than 10 dbar, a temperature component to the nonlinear correction is necessary, with cold water at depth requiring greater correction (N. Larson, Sea-Bird Electronics, personal communication, 2012). [99].
- Regardless of the method, the float is transmitting profile and trajectory data that use corrected pressures.
4.3. Temperature Sensor Bias
- No example of significant temperature drift has been identified within the Argo array.
- Small numbers of instruments recovered and recalibrated after 4–9month missions have shown no appreciable drift within manufacturer’s stated temperature accuracy [Oka and Ando, 2004].
- Argo float models report the Core Argo profile parameters—temperature, salinity, and pressure—as either a point measurement or vertical pressure average (bin averaged).
- Both sampling methods are equally valid and each provides advantageous properties depending on the application, but the sampling mode should be known for most accurate use.
- The globally averaged temperature gradient from 2000 up to approximately 200 dbar (Figure 12b, black lines) results in a warm bias for floats recording bin-averaged data.
4.4. Biases Introduced by Float Firmware
- Two recent, unrelated issues affected a single (but different) model of Argo float and introduced pressure bias into the Argo data set.
- Data users may make their own determination of TNDP status by referring to the SP variables included in a float technical parameter netCDF file available at the GDAC.
- A subset of Argo floats (SOLO), manufactured prior to 2007 by the Woods Hole Oceanographic Institution (WHOI), had assigned temperature and salinity values to incorrect pressure levels [Willis et al., 2009].
- The float profile data that were in error due to incorrect pressure level assignment either have been corrected to the proper pressure (all SOLO WHOI SBE models and a subset of SOLOWHOI FSI) or have been assigned bad quality control flags for those models that are uncorrectable (subset of SOLO WHOI FSI).
4.5. Discussion
- The Argo Program float array is an important component of the present oceanic Earth observing system, extending broad-scale monitoring of ocean temperature, among other variables, from what was achieved by previous research programs.
- The most apparent spatial bias in Argo float density in the pelagic ocean is found within seasonal ice zones.
- Development of a Sea-Bird CTD for use in Deep Argo floats is ongoing.
- Advantages include the recording of profiles at higher resolution (2 dbar and higher), reduced float mortality due to shorter surface periods, and the ability to modify the float sampling midmission driven by scientific objective.
5. GLOBAL OCEAN HEAT CONTENT, EARTH ENERGY BUDGET, AND THERMOSTERIC SEA LEVEL RISE
- The amount of heat accumulating in the global ocean is vital for diagnosing Earth’s energy imbalance and sea level rise.
- The uptake of heat by the ocean acts as a buffer to climate change, slowing the rate of surface warming [Raper et al., 2002], and so is an important element in the evolution of the climate over land and between the Northern and Southern Hemispheres. [117].
- This section will present an abbreviated update of ocean heat content estimates, the present Earth energy balance, and thermosteric sea level rise with a particular focus on the accuracy of the temperature measurements and the impact of accuracy on the certainty of these measurements.
5.1.1. Background
- Changes to ocean heat content (OHC) can be calculated from measurements of the temperature evolution of the ocean.
- It is now widely accepted that the large decadal variability in the 1970s–1980s in the earlier estimates was mostly an artifact caused by XBT biases.
- These time-dependent biases, if left uncorrected and when integrated in depth and over the global ocean, lead to substantial errors in OHC estimates, in terms of both temporal variability and trends [e.g., Domingues et al., 2008; Wijffels et al., 2008; Levitus et al., 2009]. [122].
- Domingues et al. [2008] and Church et al. [2011] use a reduced-space optimal interpolation in which a reduced set of near-global spatial functions (derived from satellite altimeter sea level measurements) is combined with thermal expansion observations to produce spatially complete fields from 1950 onward.
5.1.2. Current Observational Estimates
- Updated and recent observational analyses of global upper OHC (Table 1) all show significant multidecadal warming, with a steady increase in OHC since the 1970s (Figures 14a1–14a3).
- More generally, confidence in interannual variability in the global upper OHC has improved after 2005, following the dramatic improvement in open ocean coverage by the Argo floats, at least for the upper ~2000m.
- Further systematic comparisons between OHC analyses are needed to understand the spread in multidecadal rates, to isolate the impact of individual structural uncertainties, and to develop best practices for analyses.
5.2. Deep Ocean Heating
- Though variations in deep ocean temperatures are small compared to the upper ocean, the large volume of the deep ocean makes its contribution to the global energy balance significant [Purkey and Johnson, 2010].
- Variability of the heat content of the deep ocean modulates both the energy budget of the climate system and global sea level [IPCC, 2007].
- At present, only hydrographic observations provide data of the required accuracy and at a decadal frequency (the minimum needed to observe climate change).
5.3. Impact of Ocean Measurements on Earth Energy Balances
- The key issues for the Earth from an overall energy standpoint are the energy imbalances at the top and bottom of the atmosphere and their changes over time.
- Other observing systems in place can nominally measure the major storage and flux terms, but owing to errors and uncertainty, it remains a challenge to track anomalies with confidence.
- Estimates of OHC trends above 700m from 2005 to 2012 (Figure 14f) range from 0.2 to 0.4Wm 2, with large, overlapping uncertainties, highlighting the remaining issues of adequately dealing with missing data in space and time and how OHC is mapped, in addition to remediating instrumental biases, quality control, and other sensitivities.
5.4. Ocean Temperature Measurements and Thermosteric Sea Level Rise
- Both the volume and mass of the global ocean, and thus sea level (global mean sea level), change across a variety of timescales, due to expansion and contraction of water as ocean temperatures and heat content change, and the growth of ice sheets and glaciers.
- Because the ocean has the largest heat capacity of the climate system, and the ocean thermosteric rise is one of the largest contributors to the late 20th (and projected 21st) century sea level rise [Church et al., 2011; Meehl et al., 2007], the Earth’s energy and sea level budgets must be consistent.
- To address the implicit assumption of zero anomaly where there were no data and XBT biases, Domingues et al. [2008] used a reduced-space optimal interpolation scheme in combination with the XBT corrections of Wijffels et al. [2008].
- For waters deeper than 700m, ocean heat content estimates remained dependent on deep ocean bottle and CTD casts until the implementation of the Argo Program [Gould et al., 2004], which dramatically improved global sampling to a depth of 2000m, particularly in the Southern Ocean.
6. CONCLUDING REMARKS AND FUTURE DIRECTIONS
- This paper brings together a broad set of perspectives and information on oceanographic temperature measurements and their implications for climate change.
- Included are discussions of the history of temperature measurements, the primary instrumentations which have been used to complete the measurements, and their associated accuracy.
- With respect to (ii), even with the advances to the observing system culminating in the Argo array, more than 50% of the ocean is without routine observations.
- Important areas such as boundary currents, which are responsible for large poleward heat transport, need higher-frequency observations than are currently provided by Argo.
6.1. Some Future Directions of Instrumentation
- Fortunately, technological advances are being made on all fronts.
- Arvor3500, a profiling float capable of diving to 3500m, has been constructed in prototype, and Deep NINJA Argo (4000m capable float) test floats have already been deployed (Argonautics newsletter 13, August 2012, http://www.argo.ucsd.edu/newsletter.html).
- Another method for gathering under-ice temperature profiles is with the use of instrumented animals, most commonly pinnipeds [Fedak, 2012].
- Gliders are under the control of the pilot, so they can be deployed to carry out a set geographic and depth sampling plan, with updated instructions when needed.
6.2. Improved Observational OHC Estimates and Analysis Methodologies
- A high-quality subsurface ocean temperature database along with accurate and comprehensive metadata is an important prerequisite for advancing knowledge on instrumental biases (e.g., XBT/MBT) and devising more accurate corrections to help further reduce uncertainties in OHC estimates.
- Improvements to the quality of the historical ocean temperature database and metadata information for climate research purposes are currently being planned through a global coordinated effort (http://www.clivar.org/ organization/gsop/activities/clivar-gsop-coordinated-qualitycontrol-global-subsurface-ocean-climate). [170].
- To better understand and quantify the structural uncertainties arising from methods used in publications, a comprehensive project is underway [Boyer et al., 2013].
- A series of systematic intercomparisons is being carried out for a number of sensitivity tests based on different parameter choices but using agreed temperature databases (e.g., same input data).
- It is hoped that this project will provide helpful guidance on best practices to be developed for how best, for instance, to infill observational gaps. [171].
6.3. Conclusion
- Two recent detection and attribution analyses [Gleckler et al., 2012; Pierce et al., 2012] have significantly increased confidence since the last IPCC AR4 report that the warming (thermal expansion) observed during the late twentieth century, in the upper 700m of the ocean, is largely due to anthropogenic factors.
- T.B., J.M.L., and G.C.J. were supported by the NOAA Climate Program Office and NOAA Research.
- The Editor on this paper was Eelco Rohling.
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Frequently Asked Questions (17)
Q2. What are the future works in "A review of global ocean temperature observations: implications for ocean heat content estimates and climate change" ?
Despite these potential future improvements to ocean monitoring, past and present measurements show that the Earth is experiencing a net gain in heat, largely from anthropogenic factors [ Hansen et al., 2005 ; Levitus et al., 2001 ], although the magnitude differs among individual studies.
Q3. What made it possible to create an instrument electrically connected to the ship?
Technological advances in wire and wire insulation made it possible to create an instrument electrically connected to the ship and able to transmit information through a thin conducting wire.
Q4. How long does it take to maintain the 3000 float goal?
The expected lifetime of an Argo float is 3–5 years, so the fleet must be continually renewed to maintain the 3000 float goal. [28]
Q5. How fast can a shipboard temperature sensor be deployed?
Modern shipboard CTD temperature sensors have a time response of 0.065 s (compared to 0.2–0.4 s for the MBT stylus), which allow the acquisition of accurate pressure/temperature profiles at a fairly rapid deployment rate from the surface to the deep ocean.
Q6. What was the purpose of the reversing thermometer?
The Challenger was equipped with a pressure-shielded thermometer [Anonymous, 1870; Wollaston, 1782; Roemmich et al., 2012] to partially counteract the effects of pressure on temperature at great depths.
Q7. What was the main reason for the CTD being used?
since it was an instrument that was mainly deployed from research ships, the CTD could not replace the MBT observing network.
Q8. What is the recent data on sea level rise?
With respect to sea level rise, mutually reinforcing information from tide gauges and radar altimetry shows that presently, sea level is rising at approximately 3mmyr 1 with contributions from both thermal expansion and mass accumulation from ice melt.
Q9. What was the accurate temperature and pressure measured?
Pairs of protected and unprotected reversing thermometers were used to determine temperature and pressure, with pressure determined to an accuracy of ±5m depth in the upper 1000m.
Q10. What was the first evidence of the subsurface temperature?
On Captain James Cook’s second voyage (1772–1775), water samples were obtained from the subsurface Southern Ocean and it was found that surface waters were colder than waters at 100 fathoms (~183m) [Cook, 1777].
Q11. What was the role of the XBT in the subsurface temperature observing system?
From 1967 to 2001, the XBT was a major contributor to the subsurface temperature observing system and was responsible for the growth of this system.
Q12. What is the importance of the array for local heat content calculations?
The array is important for local heat content calculations [e.g., Xue et al., 2012], and even the exclusion of one meridional set of buoys from the heat content calculation during the 1997–1998 El Niño led to a significant underestimate of heat content anomaly.
Q13. What are the important instruments for assessing ocean temperatures?
Those instruments (the expendable bathythermograph (XBT) and the Argo floats) are among the most important instruments for assessing ocean temperatures globally, and they provide up-to-date ocean subsurface temperature measurements.
Q14. How many times did the MBT reach 900 feet?
By 1946, MBTs could reach to 900 feet (~275m), although the shallower version was deployed more often every year except 1964 (49% shallower version).
Q15. How long does the XBT response time take?
The XBT response time, at 0.15 s, is slower than modern shipboard CTDs, its accuracy likewise, at 0.15°C and 2% or 5m in depth, whichever is greater.
Q16. What was the problem with the Nansen bottle/reversing thermometer system?
The problems during this time period with regard to a global ocean observing system were that Nansen bottle/reversing thermometer systems could only measure at a few discrete levels at each oceanographic station and that it was time consuming to deploy the instrumentation and make the measurements.
Q17. What is the purpose of a smoked glass stylus?
A stylus attached to the Bourdon tube captures the movement as temperature change horizontally scratched on a plate of smoked glass.