3.1Oceandynamics
Separating thermal expansion from mass increases in studies of global sea level rise
Global sea level rise is one of the major challenges of climate change (Church et al., 2001).
Whether the sea level will be some centimetres or a few decimetres higher than now at the end of this century is of immediate concern for about half of the population of the Earth and has enor-mous economic consequences. The scientific community is asked for realistic prediction of sea level rise. Predictions vary widely, according to data sets used and principles applied (Church et al., 2001). Clearly it is not sufficient to measure sea level change at tide gauges and by altimetry but we must aim at understanding the underlying processes and quantify them. This holds espe-cially for processes that will become more important in the future such as the response of the glo-bal ocean to local enhanced freshwater forcing from Greenland melting: Hypotheses suggest that Greenland might loose much of its ice volume over the next few decades. This situation could result in a substantial regional and global sea level rise that needs to be understood.
An example of the response of a quasi-global ocean circulation model with 1 degree spatial reso-lution to enhanced Greenland ice melting (on the order of 0.1 Sv freshwater input) is depicted in the Figure 3.1.4. It shows the differences of a control run and one that was perturbed by enhanced Greenland melting with otherwise similar model parameters as it occurs after 5 years (Stam-mer and Ueyoshi, 2004). The results demonstrate that the response is very dynamical leading to strong boundary wave activities that originate from the sub polar North Atlantic and that spread across the Atlantic in terms of boundary, Kelvin and Rossby waves with respective time scales.
Clearly sea level increase is not uniform but leads instead to a dynamically caused depression in the central sub-polar North Atlantic by several centimetres. In contrast, coastal regions “feel” the enhanced freshwater much faster in terms of sea level rise. The figure clearly illustrates the threat of predominantly coastal sea level rise due to climate change.
With respect to sea level, we are primarily interested in monthly scales and periods up to a cen-tury. Local sea level change by surface waves and tides is fairly well understood and does not lead to substantial secular changes. The seasonal cycle of sea level rise and fall is dominated by redis-tribution of mass due to changes in ocean circulation and also by local warming and cooling.
Figure 3.1.4: The figure shows the response of a global ocean circulation model as it results to about 0.1 Sv increased runoff due to Greenland ice melting. The field shows the differences (in cm) of sea surface height as it results after 5 years of increased freshwater forcing due to ice melting. Colour range is +/3cm. See
Stammer and Ueyoshi (2004) for details.
An additional global trend remains that results from an imbalance of the hydrological cycle which is thought to be mainly net inflow of melt water into the sea from glaciers, ice caps and polar ice sheets (Chapter 3.2). Superimposed on a regional basis are strong changes in the thermohaline (temperature and salinity) structure of the ocean. In principle, these can be measured. However, the vastness of the global ocean and the remoteness of large domains make such a task difficult.
Even with novel techniques like thousands of autonomous drifting buoys from the ARGO project performing vertical probing the ocean remains undersampled in regions such as the arctic.
The magnitude of sea level rise due to ocean heating varies considerably. The specific volume of sea water (volume per unit mass) increases when the water is heated. This increase is known as thermosteric expansion. Its magnitude depends strongly on temperature and pressure: heating of warm water has a much bigger effect than that of water around the freezing point. Also water at high pressure in the deep ocean reacts much stronger than surface waters. The specific volume of sea water decreases when salt is added, denoted as halosteric contraction. Its magnitude is fairly constant over the whole ocean.
The stratification of the ocean interior is mostly stable. Heating from above has only local influ-ence and does not reach deep into the water. Only when cooling or salinity increase result in an unstable water column the deep ocean is ventilated and its properties change. Both processes oc-cur in high latitude where the primary ventilation regions of the world ocean reside. Areas with vertical overturning called convection are strongly linked to cooling, fresh water forcing and sea ice processes which will be discussed below.
Locally, sea level change varies strongly and may even change sign. It is difficult to derive stable estimates from tide gauges and even from altimetry (Church et al., 2001). In order to get closed budgets and to separate different contributions to the observed sea level changes it is possible to assimilate the observations in a global ice-ocean circulation model that conserves mass, heat, salt and momentum. Forcing is by the atmosphere only and by inflow of fresh water from land.
Volume change computed from the model results in dynamic topography change which must coincide with altimeter and tide gauge observations (Wenzel et al., 2001). When only surface data are assimilated there is an infinity of possible solutions. Only by using additional measurements from the deeper ocean the problem has a unique solution and the explanation found depends on the measurements in an unambiguous way. In Figures 3.1.5 to 3.1.7 results of such an assimilation experiment which exploits altimetry, geodesy and oceanographic data are depicted. Sea surface variability over the period 1993 to 2001 is modelled successfully.
The analysis of linear trends reveals the dominance of local warming (cf. Figure 3.1.5) while changes in salinity have a much smaller effect as is shown in Figure 3.1.6. An interesting feature of ocean circulation is also found for the trend analysis performed here. Frequently, temperature and salinity variations are correlated in a way that leaves density unchanged. It is evident from the figures that strong temperature variations are not enough to change sea level.
Associated variations in salinity must be considered simultaneously. The third mechanism to change the sea surface involves a change in mass. On the time scales of a decade considered here almost no variability remains (cf. Figure 3.1.7). Net inflow due to an imbalance of the hydrological cycle spreads approximately evenly over the whole globe. What is diagnosed in the assimilation experiment is a decrease in mass near Antarctica which is associated with an increase of transport of the ACC. The other remarkable change is a mass increase over the Arctic Ocean. For this no independent evidence is available at present.
The situation will change once the GRACE measurements can be fully exploited. Temporal anom-alies of gravity field observations provided at periods from two months to the mission life time will yield information about the deep, time varying ocean mass distribution and circulation which otherwise is unobservable. This information, which is independent of steric contributions, can be used to distinguish steric from nonsteric contributions to altimeter measurements of sea surface
3.1Oceandynamics
Figure 3.1.5: Trends of sea level heights (SSH) due to thermal expansion. The effect is integrated over the full water column. Depicted here and in the following two figures are the results of assimilation of satellite altimetry and traditional oceanographic data into an ocean model. The solution is dynamically fully con
sistent and close to observations. Local trends exhibit a large variance due to strong interannual variabil
ity: For slightly different periods the patterns change significantly.
0.0
height. Measured changes in mass distribution are useful for ocean model verification or falsifica-tion and can, in principle, also be assimilated.
The interpretation of the measurements is important. Knowing the steric contributions would make it feasible to approximate the changes in the vertically integrated heat and freshwater
stor-Figure 3.1.6: Local sea level rise due to changes in salinity. In some areas the effect is as strong as that of warming, showing the importance of salinity. Note that for many strong signals an anticorrelation is found:
although local warming is observed it has little impact on sea level as associated changes in salinity largely compensate thermal expansion keeping density fairly constant.
0.5 0.0
age over scales of a few hundred kilometres and larger and would thus contribute significantly to our understanding of global climate change in terms of buoyancy and mass variations. Regional fluctuations in ocean mass are directly linked to the wind field. Changes in mass on global scale are a measure of changes in the Earth freshwater cycle. Knowledge of both is urgently required and will lead to a better understanding of the relation between local and remote forcing in setting the mean and time-varying circulation