A REVIEW OF GLOBAL OCEAN TEMPERATURE
OBSERVATIONS: IMPLICATIONS FOR OCEAN
HEAT CONTENT ESTIMATES AND
CLIMATE CHANGE
J. P. Abraham,
1
M. Baringer,
2
N. L. Bindoff,
3,4,5
T. Boyer,
6
L. J. Cheng,
7
J. A. Church,
4
J. L. Conroy,
8
C. M. Domingues,
5
J. T. Fasullo,
9
J. Gilson,
10
G. Goni,
2
S. A. Good,
11
J. M. Gorman,
1
V. Gouretski,
12
M. Ishii,
13
G. C. Johnson,
14
S. Kizu,
15
J. M. Lyman,
14,16
A. M. Macdonald,
17
W. J. Minkowycz,
18
S. E. Moffitt,
19,20
M. D. Palmer,
11
A. R. Piola,
21
F. Reseghetti,
22
K. Schuckmann,
23
K. E. Trenberth,
9
I. Velicogna,
24,25
and J. K. Willis
25
Received 24 April 2013; revised 9 August 2013; accepted 13 August 2013; published 23 September 2013.
[1] The evolution of ocean tempera ture measurement systems
is pre sented with a focus on the development and accuracy of
two critical devices in use toda y (expenda ble bathythe rmo-
graphs and conductivity-temperature-depth instruments used
on Argo floats). A detailed discussion of the accu racy of these
devices and a projection of the future of ocean temperature
measurements are provided. The accuracy of ocean tempera-
ture measurements is discussed in detail in the context of ocean
heat content, Earth’s energy imbala nce, and thermosteric se a
level rise. Up-to-date estimates are provided for these three
important quantities. The total energy imbalance at the to p of
atmosphere is best assessed by taking an inventory of changes
in energy storage. The main storage is in the ocean, the late st
values of whic h are presented. Furthe rmore, despite differences
in measurement methods and analysis techniques, multiple
studies sh ow that there has been a multidecadal increase in
the heat content of both the upper and deep ocean regions,
which reflects the impact of anthropogenic warming. With
respect to sea level rise, mutually reinforcing information from
tide gauges and radar altimetry sh ows that presently, sea level
is rising at approximately 3 mm yr
1
with contributions from
both thermal expansion and mass accumulation from ice melt.
The latest data for thermal expansio n sea level rise are inc luded
here and analyzed.
1
School of Engineering, University of St. Thomas, St. Paul, Minneapolis,
USA.
2
Atlantic Oceanographic and Meteorological Laboratory, National
Oceanic and Atmospheric Administration, Miami, Florida, USA.
3
IMAS, University of Tasmania, Hobart, Tasmania, Australia.
4
Centre for Australian Weather and Climate Research, CSIRO Marine
and Atmospheric Research, Hobart, Tasmania, Australia.
5
Antarctic Climate and Ecosystems Cooperative Research Centre,
University of Tasmania, Hobart, Tasmania, Australia.
6
National Oceanographic Data Center, NOAA, Silver Spring, Maryland,
USA.
7
Institute of Atmospheric Physics, Chinese Academy of Science, Bejing,
China.
8
School of Earth and Atmospheric Sciences, Georgia Institute of
Technology, Atlanta, Georgia, USA.
9
National Center for Atmospheric Research, Boulder, Colorado, USA.
10
Scripps Institution of Oceanography, La Jolla, California, USA.
11
Met Office Hadley Centre, Exeter, UK.
12
Klima Campus, Hamburg University, Hamburg, Germany.
13
Climate Research Department, Meteorological Research Institute,
Tsukuba, Japan.
14
Pacific Marine Environmental Laboratory, NOAA, Seattle, Washington,
USA.
15
Department of Geophysics, Tohoku University, Sendai, Japan.
16
Joint Institute for Marine and Atmospheric Research, University of
Hawai’i at Manoa, Honolulu, Hawaii, USA.
17
Woods Hole Oceanographic Institution, Woods Hole, Massachusettes,
USA.
18
Department of Mechanical and Industrial Engineering, University of
Illinois at Chicago, Chicago, Illinois, USA.
19
Bodega Marine Laboratory, Bodega, California, USA.
20
Graduate Group in Ecology, University of California, Davis, California,
USA.
21
Departamento Oceanografia, Servicio de Hidrografia Naval and
Departamento de Ciencias de la Atmosfera y los Oceanos/UMI IFAECI,
Universidad de Buenos Aires, Buenos Aires, Argentina.
22
ENEA–Italian National Agency for New Technologies, Energy
Sustainable Economic DevelopmentUTMAR-OSS, La Spezia, Italy.
23
Ifremer, Toulon, France.
24
Department of Earth System Science, University of California, Irvine,
California, USA.
25
Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, California, USA.
Corresponding author: J. P. Abraham, School of Engineering, University
of St. Thomas, 2115 Summit Ave., St. Paul, MN 55105-1079, USA.
(jpabraham@stthomas.edu)
©2013. American Geophysical Union. All Rights Reserved. Reviews of Geophysics, 51 / 2013
450
8755-1209/13/10.1002/rog.20022 Paper number 2013RG000432
Citation: Abraham, J. P., et al. (2013), A review of global ocean temperature observations: Implications for
ocean heat content estimates and climate change, Rev. Geophys., 51, 450–483, doi:10.1002/rog.20022.
1. INTRODUCTION
[
2] The broad topic of climate science includes a multitude
of subspecialties that are associated with various components
of the climate system and climate processes. Among these
components are Earth’s oceans, atmosphere, cryosphere, and
terrestrial regions. Processes include all forms of heat transfer
and fluid mechanics within the climate system, changes to
thermal energy of various reservoirs, and the radiative balance
of the Earth. The incredible diversity of climate science makes
it nearly impossible to cover all aspects in a single manuscript,
except perhaps for within massive assessment reports [e.g.,
Intergovernmental Panel on Climate Change (IPCC), 2007].
Nevertheless, it is important to periodically provide detailed
surveys of the aforementioned topical areas to establish the
current state of the art and future directions of research.
[
3]Itisfirmly established that changes to the Earth’s atmo-
spheric concentrations of greenhouse gases can and have
caused a global change to the stored thermal energy in the
Earth’s climate system [Hansen et al., 2005; Levitus et al.,
2001]. To assess the impact of human emissions on climate
change and to evaluate the overall change to Earth’sthermal
energy (whether from natural or human causes), it is essential
to comprehensively monitor the major thermal reservoirs.
The largest thermal reservoirs are the Earth’s oceans; their
extensive total volume and large thermal capacity require
a larger injection of energy for a change in temperature
compared to other reservoirs.
[
4] Despite the importance of accurately measuring the ther-
mal energy of the ocean, it remains a challenging problem for
climate scientists. Measurements covering extensive spatial
and temporal scales are required for a determination of the
energy changes over time. While there have been significant
advancements in the quantity and quality of ocean temperature
measurements, coverage is not yet truly global. Furthermore,
past eras of ocean monitoring have provided extensive data
but variable spatial coverage. Finally, chang es in measurement
techniques and instrumenta tion have resulted in biases, many
of which have been discovered with some account made .
[
5] This review focuses on subsurface ocean temperature
measurements that are required for climate assessment, with
an emphasis on the status of oceanographic temperature
measurements as obtained from two of the key historical and
modern measurement instruments. 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. A historical discussion
of other families of probes will also be provided along with
discussions of the accuracy of those families.
[
6] While most of the analyses reviewed here are done by
individuals or small groups of investigators, they would not
have been possible without strong international coordination
and cooperation. International, observational programs and
projects are vital to the data used in these analyses. Early
examples are the International Geophysical Year in 1957–
1958, with its extensive Nansen bottle sections, and the
1971–1980 International Decade of Ocean Exploration, which
endorsed the North Pacific Experiment (greatly increasing
North Pacific shallow XBT use in the 1970s) and
Geochemical Ocean Sections Study (a global high-quality
and full-depth, if sparse, baseline oceanographic survey).
[
7] Since its inception, the World Climate Research
Program (WCRP) has taken international leadership with the
Tropical Ocean-Global Atmosphere project which focused
on observation in the equatorial region in the 1980s, including
initiating the Tropical Atmosphere Ocean/Triangle Trans-
Ocean Buoy Network (TAO/TRITON) moored array and the
World Ocean Circulation Experiment (WOCE) which took a
truly global set of oceanographic coast-to-coast full-depth
sections and expanded the XBT network in the 1990s. The
WOCE provides a global-scale benchmark against which
change can be assessed. More recently, the WCRP formulated
the Climate Variability and Predictability Project (CLIVAR),
further fostering the Argo float array and reoccupation of some
of the full-depth WOCE hydrographic sections under the aus-
pices of the Global Oceanographic Ship-Based Hydrographic
Investigations Program. The Global Climate Observing
System, in partnership with WCRP, has formulated a global
ocean observing system and encourage d contri bution to it, par-
ticularly through the OceanObs workshops in 1999 and 2009.
[
8] Oceanographic data centers, both national and interna-
tional, are also vital to the studies reviewed here. These centers
accept, collect, and actively seek out data (from large programs
and small); then archive and quality control them; and make
the results readily and publically available. The collection,
assembly, and quality control of a comprehensive data set are
invaluable for all sorts of global analyses, including those of
ocean temperature, heat content, and thermal expansion.
2. THE EVOLVING SUBSURFACE TEMPERATURE
OBSERVING SYSTEM: A HISTORICAL PERSPECTIVE
[
9] 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 subsur-
face 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)
[
10] On Captain James Cook’s second voyage (177 2–1775),
water samples were obtained from the subsurface Southern
Ocean and it was found that surface waters were colder
than waters at 100 fathoms (~183 m) [Cook, 1777]. These mea-
surements, although not very accurate, are among the first in-
stances of oceanographic profile data recorded and preserved.
ABRAHAM ET AL.: REVIEW OF OCEAN OBSERVATIONS
451
Slightly more than 100 years later, the Challe nger 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 measure-
ment. The Challenger was equipped with a pressure-shielded
thermometer [Anonymous, 1870; Wolla ston, 1782; Roemmich
et al., 2012] to partially counteract the effe cts of pressu re on
temperature at great depths.
2.2. The Nansen Bottle Observation System
(From 1900)
[
11] Around the time of the Challenger expedition, the
reversing thermometer [Negretti and Zambra, 1873] was
introduced and remained the standard instrument for subsur-
face temperature measurements until 1939. It is still in limited
use today. A protected reversing thermometer was typically
accurate to 0.01°C or better when properly calibrated. Pairs
of protected and unprotected reversing thermometers were
used to determine temperature and pressure, with pressure de-
termined to an accuracy of ±5 m depth in the upper 1000 m.
The development of the Nansen bottle [Mill,1900;Helland-
Hansen and Nansen, 1909] which attached the thermometers
to a sealed water sample bottle completed the instrumentation
package which constituted the subsurface upper ocean temper-
ature observing system for the 1900–1939 time period. The
problems during this time period with regard to a global ocean
observing system were that Nansen bottle/reversing thermom-
eter 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. It
was also difficult to get properly equipped ships to most areas
of the ocean. Many of the open ocean temperature profiles
were measured during a small number of major research
cruises [Wust, 1964]. 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)
[
12] 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 hid-
ing places. As related in Couper and LaFond [1970], sonar
operators were aware of an “afternoon effect” where sonar
ranges were shorter in the afternoon than in the morning,
but did not understand that the effect was due to diurnal
warming. The wide vertical spacings of Nansen bottle casts
did not capture the gradients at the bottom of the mixed layer
or indeed the vertical extent of the mixed layer.
[
13] Early in the 1930s, Carl-Gustaf Rossby had ex-
perimented with an “oceanograph” which could draw a
continuous pressure/temperature trace on a smoked brass foil
[Rossby and Montgomery, 1935]. Rossby enlisted Athelstan
Spilhaus to develop this idea into a cheap, reliable, reusable
instrument. Spilhaus created the first version of the instrument
that we now call the mechanical bathythermograph (MBT)
[Spilhaus, 1938]. Oceanographers now had the means with
which to acquire detailed sets of measurements to map the
mixed layer and shallow thermocline [Spilhaus, 1940].
[
14] The U.S. Navy funded research to improve the design
and operation of the MBT, as Drs. Vine, Ewing, and Worzel
modified Spilhaus’s design to allow operational use of the
instrument by the Navy and oceanographers [Spilhaus,
1987]. The U.S. Navy, in conjunction with Scripps Institute
of Oceanography and the Woods Hole Oceanographic
Institution, facilitated the first coordinated worldwide subsur-
face temperature measurement system, which grew up during
World War II and continued afterward. The MBT itself is a
cylinder approximately 31.5 inches (~0.8 m) long and 2 inches
(~0.51 m) in diameter with a nose weight, towing attachment,
Figure 1. Geographic distribution of subsurface tempera-
ture profiles for (a) 1934, (b) 1960, (c) 1985, and (d) 2009.
Red = Nansen bottle or conductivity-temperature-depth
(CTD), light blue = mechanical bathythermograph (MBT),
dark blue = expendable bathythermograph (XBT), orange =
tropical moored buoy, green = profiling float.
ABRAHAM ET AL.: REVIEW OF OCEAN OBSERVATIONS
452
and tail. Inside the cylinder is a Bourdon tube enclosing a
capillary tube with xylene (a hydrocarbon obtained from wood
or coal tar) inside. As temperature increases, the pressure on
the xylene increases, causing the Bourdon tube to unwind. A
stylus attached to the Bourdon tube captures the movement
as temperature change horizontally scratched on a plate of
smoked glass. A spring and piston measuring pressure simul-
taneously pulls the stylus vertically down the glass, complet-
ing the depth/temperature profile. The instrument free-falls
from a winch that is used to recover the instrument; it can be
used at speeds up to 15 kt. Initially, MBTs were built to reach
depths of 400 feet (~122 m). By 1946, MBTs could reach to
900 feet (~275 m), although the shallower version was
deployed more often every year except 1964 (49% shallower
version). The 900 foot MBTs had significant depth calibration
issues if they were lowered the full 900 feet, and for this rea-
son, most MBTs were not lowered deeper than 400–450 feet.
The accuracy of the MBT instrument was ±5 dbar in pressure
and ±0.3°C in temperature.
[
15] The Navy’s interest in MBTs was for temperature
gradient information, but a system of careful calibration
was put in place to accurately preserve the full temperature
information for future study. Later, more than 1.5 million
MBT temperature traces from1939 to 1967 were digitized
at 5 m intervals and stored on index cards. These cards were,
in turn, electronically digitized and archived at the U.S.
National Oceanographic Data Center [Levitus, 2012]. It was
reported that 73% of all 1939–1967 MBTs were U.S.
devices, but other countries, notably Japan and the Soviet
Union, also dropped MBTs. However, these traces were not
distributed under the U.S. Navy system. MBTs continued
to be used after 1967, with ~800,000 traces gathered in
1968–1990. Geographic coverage of MBTs was limited by
areas of interest to navies, merchant ship routes, and research
cruises. So, while a sketch of the upper ocean waters was
being recorded by the MBT network, geographic distribution
was uneven and temperature measurements from depths
deeper than 250 m were still reliant on sparse Nansen
bottle observations.
2.4. Ship-Based Conductivity-Temperature-Depth
Instruments (From 1955)
[
16] 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 our current observing
system and for the backbone large-scale measurement cruises
of the World Ocean Circulation Experiment (WOCE) among
others. But, since it was an instrument that was mainly
deployed from research ships, the CTD could not replace the
MBT observing network. The development of the CTD was
precipitated by advances in temperature measurement
before and during World War II. 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].
However, it was not until 1946 that technological advances
made commercial production of these thermal resistors
(coined “thermistors”)possible[Becker, 1946]. Similarl y,
platinum resistance thermometers, which had been under-
stood for some time [Callendar, 1887], became practical
for oceanographic applications owing to more recent
technological advances [Barber, 1950].
[
17] An early attempt to measure a continuous temperature
profile [Jacobsen, 1948 ] inspired Hamon and Brown [Hamon,
1955; Hamon and Brown, 1958] to engineer a similar instru-
ment. Hamon and Brown deployed their first STD in 1955
[Baker, 1981]. Their instrument, which was lowered by a
winch, used a thermistor, as well as a conductivity sensor
and pressure sensor connected by a sealed cable to an analog
strip chart on deck. The pressure sensor was a Bourdon tube
connected to a potentiometer. Commercial production of
CTDs began in 1964. 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]. Most modern
CTDs now use thermistors, often in pairs, and strain gauge
pressure sensors. While Hamon’s original STD experiments
had an accuracy of 0.1°C and 20 m in depth, the modern
CTD is accurate to 0.001°C and 0.15% of full scale for
pressure (1.5 m at 1000 m depth) and fully digital. 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. When combined with the lowering speed
(~1 m s
1
), a vertical resolution of 0.06 m is obtained,
although in practice, data are often reported in 1 or 2 m
averages, since ship-roll-induced motions alias the tempera-
ture data on finer vertical scales.
2.5. The Expendable Bathythermograph Observing
System (From 1967)
[
18]AsSnodgrass [1968] relates, by the early 1960s, the
search was on for a replacement for the MBT. The replace-
ment needed to be cheaper and easier to deploy , calibrate,
and retrieve data, and had to be able to profile deeply from
ships moving faster than 15 knots. 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.
Advances in thermistor manufacture made it practical to
deploy these temperature sensors cheaply, with no need to
retrieve instruments after deployment. More than 12 compa-
nies attempted to create the expendable bathythermograph
(XBT). Three succeeded, but only one, Sippican (Lockheed
Martin Sippican (LMS)), went on to dominate the XBT
market due to their winning of a contract with the U.S.
Navy [Kizu et al., 2011]. Their design was a torpedo-shaped
probe smaller than the MBT, containing a thermistor in the
central hole through the zinc nose. A wire connected the
probe to the ship deck. Part of the wire is wrapped around
the XBT itself and part in a canister shipboard.
ABRAHAM ET AL.: REVIEW OF OCEAN OBSERVATIONS
453
[19] 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]. About
60% of all publicly available XBT data in 1967–1989 were
U.S. drops. In 1990, a global system of distributing XBT data
was implemented (see below discussion of the Global
Temperature and Salinity Profile Program (GTSPP)).
[
20] The new probe almost immediately revolutionized
subsurface ocean temperature observations with their low
cost and easy deployment from Navy, merchant, and research
ships. Estimates of upper ocean global mean yearly heat
content anomaly exhibit reduced sampling uncertainty
starting from around year 1967, the first year of widespread
use of the XBT [Lyman and Johnson, 2008; Boyer et al.,
1998]. The success of the XBT and the concurrent Fleet
Numerical Weather Center (FNWC) Ship-of-Opportunity
Program (SOOP) led to more systematic designs of XBT
observing networks for the Pacific[White and Bernstein,
1979] and the Atlantic [Bretherton et al., 1984; Festa and
Molinari, 1992] which were implemented and continue still.
The switch to digital recorders in the 1980s made the use and
dissemination of XBT data even easier.
[
21] With the advent of the ARGOS positioning and data
transmission system, set up by the French and U.S. Space agen-
cies in 19 78, XBT profiles began to be transmitted from ships
in real time and distributed on the World Meteorological
Organization’s Global Telecommunications System (GTS).
The Global Temperature and Salinity Profile Program
(GTSPP) began in 1990 to systematically capture subsurface
temperature data off the GTS, perform quality check and
control, and distribute XBT temperature profiles (and other
subsurface data) to the scientific and operational communities
in near-real time. The XBT response time, at 0.15 s, is slower
than modern shipboard CTDs, its accuracy likewise, at 0.15°C
and 2% or 5 m in depth, whichever is greater. LMS is still the
main manufacturer of XBTs. TSK, a Japanese company
(Tsurumi-Seiki Co.), started manufacturing T6s in 1972 and
T7s in 1978 [Kizu et al., 2011]. These designators follow a
model-naming scheme that uses letter/number combinations
to identify probe types. A Canadian company, Sparton, also
briefly manufactured XBTs of their own design.
[
22] Despite their widespread use, XBTs are not free of
problems. Section 3 of this review will discuss these
problems in detail. 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.
However, it was still limited to major shipping routes and
Navy and research cruise paths, leaving large parts of the
ocean undersampled for many years. The XBT is also depth
limited. While there are deep falling XBTs such as the T-5
that reach to nearly 2000 m, they are of limited use due to cost
and the lower ship speed necessary for the drops.
[
23] There is another expendable probe that contempora-
neously measures conductivity and temperature (XCTD). It
is available from TSK; however, it has appeared in far fewer
numbers than the XBT devices described here.
2.6. Tropical Moored Arrays (From 1984)
[
24] 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]. After initial
experiments in 1979, an array of moored buoys, spaced at
2°–3° latitude and 10°–15° longitude, was set up across the
equatorial Pacific. Work began on the array in 1984, and it
was completed in 1994. The temperature sensor is often just
a thermistor but is sometimes paired with a conductivity or
pressure sensor depending on geographic location and depth.
Each buoyed sensor is attached to a mooring line and hung
at depths from the surface to 500 m. The measurements are
relayed to a satellite and then the GTS at 12 min intervals.
The TAO/TRITON array requires regular maintenance and
calibration cruises.
[
25] The PIRATA array (Pilot Research moored Array
in the Tropical Atlantic) [Bourles et al., 2008] was set up
in the Atlantic starting in the mid-1990s. The RAMA
array (Research Moored Array for African-Asian-Australian
Monsoon Analysis and Prediction) [McPhaden et al., 2009],
begun in the Indian Ocean in the early 2000s, is still not
complete. Both follow similar setup and data transmission
patterns as TAO/TRITON. 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.
2.7. Argo Profiling Float Observing System
(From 2001)
[
26] By the 1990s, all the pieces were in place for a global
ocean observing system: a scientifically based blueprint for
systematic observations, a satellite network for real-time data
delivery, technology for easy and accurate temperature and
pressure (depth) measurements, and a reliable data distribution
network. But the observing system was still limited by the need
to take most measurements from ships, geographically limited,
seasonally biased, and often costly to outfit and deploy. As
with previous obstacles to the observing system, the answer
to these limitations lay in a combination of older ideas and
new technological applications. The Swallow float was a neu-
trally buoyant float developed in the 1950s [Swallow, 1955].
These floats sank to a neutrally buoyant level and were tracked
by a nearby surface ship. 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. In the 1980s, the
RAFOS floatreversedthisideabyhavingthefloat listen for
stationary underwater sound sources [Rossby et al., 1986].
[
27] The Autonomous Lagrangian Circulation Explorer re-
moved entirely the need for a system of underwater sound
sources by having the float surface periodically and its position
determined by ARGOS satellites [Davis et al., 1992]. The
floats surface by increasing their buoyancy relative to the
surrounding water by transferring mass and volume between
the float’s pressure case and an external bladder. The process
ABRAHAM ET AL.: REVIEW OF OCEAN OBSERVATIONS
454
![Figure 8. (a) Zonal and time-averaged temperature differences between XBT and quasi-collocated reference data, (b) zonal and time-averaged temperature gradient. and (c) zonal and time-averaged temperature. Reproduced from Gouretski and Reseghetti [2010]. Note that negative numbers are used for latitudes in the south and positive numbers for latitudes in the north.](/figures/figure-8-a-zonal-and-time-averaged-temperature-differences-1qerhr5h.webp)
![Figure 7. Global average temperature differences between XBT and quasi-collocated reference data versus depth and year of the measurements. Reproduced from Gouretski and Reseghetti [2010].](/figures/figure-7-global-average-temperature-differences-between-xbt-7kdpfxb3.webp)
![Figure 14. (a1–a3) Yearly time series of global upper OHC and (b–f) linear trends observed for the upper 400m [Gouretski et al., 2012], upper 900m [Boening et al., 2012], and 700m (others), with details in Table 1. Results from CMIP3 model simulations (Figure 14a1) with the most complete set of natural (e.g., solar and volcanic) and anthropogenic (e.g., greenhouse gases and aerosols) forcings are shown for individual runs (thin gray lines) and as a multimodel ensemble mean (black bold dashed line), with the latter repeated in Figures 14a2 and 14a3 for visual reference. Updated observational time series (colored bold lines) and their error bars when available (colored thin lines) are provided by the originators. Time series stopping before 2011 were originally included in Palmer et al. [2010] or Lyman et al. [2010]. All observational time series are relative to 2005–2007 (except the “robust average” time series which was referenced to the 1993–2001 period from Johnson et al. [2013]), while the model time series are relative to 1960–1999 plus an offset for plotting purposes. In Figures 14b–14f, the simple least squares (SLS) linear trends are shown as crosses and the weighted least square (WLS) linear trends are shown as circles. The median rates of the plotted trends for the upper 700m are indicated by horizontal lines, solid for WLS (accompanied by the gray shading uncertainties) and dashed for SLS. In Figure 14e, the horizontal green line is the rate of the robust average, solid for WLS (accompanied by shading uncertainties) and dashed for SLS. All rates are relative to the entire Earth’s surface (5.1 × 1014 m2) to indicate the upper ocean’s contribution to changes in the planetary heat storage. The OHC time series from Domingues et al. [2008] has been smoothed by a 3 year running mean to reduce noise.](/figures/figure-14-a1-a3-yearly-time-series-of-global-upper-ohc-and-b-1x4u53ib.webp)