Monitoring the Heat Content of the World Ocean

Because of the enormous mass of the Global Ocean and the high heat capacity of the sea water, changes of the heat energy accumulated in the world ocean is the dominant component dominate the Earth’s heat balance. The uptake of heat by the ocean acts as a buffer, slowing down the rate of the Earth’s surface warming due to increasing green house gases.

The ICDC started a project to monitor the temperature and the heat content of the upper ocean. The assessment of the ocean thermal state is based on the collection of the hydrographic data obtained by means of several types of instrumentation. The majority of the subsurface temperature data comes from the following instrument types:

1. hydrographic bottle casts,
2. Conductivity-Temperature-Depth (CTD) profilers,
3. Mechanical BathyThermographs (MBT),
4. eXpendable BathyThermographs (XBT),
5. Argo profiling floats, and
6. Autonomous Pinniped Bathythermographs (APB) (bathythermographs attached to marine mammals).

Three instrument groups - the bottle cast, CTD and Argo float data – are characterized by a higher precision and together they constitute a reference dataset against which inhomogeneities associated with gradual changes in the mix of instrumentation can be assessed.

NODC World Ocean Data Base 2009 collection of temperature profiles provided the majority of the subsurface hydrographic data used in the analysis. Additional profiles not included in WOD09 were downloaded (December 2011) from:

• the International Council for the Exploration of the Sea (ICES) database,
• the Japanese oceanographic data centre,
• the Mediterranean Oceanic Database (MODB),
• the German Oceanographic Datacenter (DOD), and
• taken from cruises conducted by the Institute for Marine Research of Hamburg University.

WOD09 contributed 97.3% of the 7,615,223 temperature profiles with measurements at least 20 meters deep.

The temperature profiles were interpolated onto 1-meter levels and integrated vertically to obtain point estimates of the mean temperature within the upper 20 meter and 400 meter layers. The data from the upper 20 meters were chosen because they mostly lie within the upper mixed and are therefore most suitable for comparison with SST data. The 0-400 meter layer was chosen because the maximum sample depth is 460 meters for the numerous XBT probes of T4 and T6 types.

The 2001-2010 period was selected as a base period for anomaly calculations, because the profiling float data available in this decade provide a more even and complete spatial coverage than was available in previous decades. The monthly climatologic temperature fields for the base period were calculated on a 0.25° geographical grid and the point temperature anomalies were then calculated simply as a difference between the layer-averaged temperature and the respective monthly climatologic value. The subsurface point anomalies were finally averaged onto a 5° grid using the median as the estimate for the grid-box average. Finally, monthly global temperature anomaly time series were calculated by taking an area-weighted average of the available 5-degree boxes in each month.

The time series of the temperature anomalies within the upper 20-meter layer (Fig.1,2) and 400-meter layer (Fig.3) were extended to the beginning of the twentieth century, although there are gaps around the two world wars for the 0-400 m layer. Previous estimates started around 1950. Both the calculation details and the comparison with the independent sea surface temperature time series may be found in Gouretski et al. (2012).

Time-series of temperature anomalies for selected regions of the World Ocean

References

• Gouretski, V., J. Kennedy, T. Boyer and A. Köhl (2012) Consistent near-surface ocean warming since 1900 in two largerly independent observing networks. Geophysical Research Letters, doi:10.1029/2012GL052975