When my kids were younger, they asked me why the ocean was blue. I would answer that the ocean mirrors the blue sky. However, I would not think much more about it, even though it is well-known that the oceans represent the most important source for atmospheric moisture. They also play an important role for many types of internal variations, such as the El Nino Southern Oscillation. Now a new study by Durack et al. (2012) has been published in Science that presents the relationship between the oceans and the atmosphere.
When it rains over the ocean, the salty ocean water gets diluted by fresh water from the clouds, whereas in regions with high evaporation and little rainfall, the evaporation takes away the water and leaves the salt behind in the ocean. If there is a systematic increase in rainfall over some regions and enhanced evaporation in others, then this ought to leave a fingerprint in form of reduced and increased salinity in the respective regions. This is exactly what Durack et al. (2012) have found.
This finding is consistent with earlier model-based suggestions that dry regions will become drier and wet regions will become wetter in response to warming (Giorgi et al., 2011). The Durack et al. (2012) study was based on surface salinity measurements from the ARGO floats over a period of 50 years (Durack et al., 2010). They studied the changes in the spatial structure in terms of linear regression against time over the 50-year period. The change in salinity was consistent with the notion of a strengthening of the hydrological cycle by ~8%/°C.
They also compared their results with global climate model simulations (which involve coupled atmosphere-ocean models) as well as ocean model simulations (forced by atmospheric data). Ocean models use the laws of physics to describe how the ocean currents flow, the water masses mix, and how these processes affect temperature and salinity. The most important inputs to the ocean models are wind forcing, evaporation and rainfall.
The model results give a similar picture as the observations, albeit with an under-estimation of the hydrological cycle speed-up. By looking at projections for the future, the Durack et al. (2012) analysis points to a “substantial (16 to 24%) intensification of the global water cycle [that] will occur in a future 2°C to 3°C warmer world”.
There is a sinister twist to these inferences. A warmer world is predicted to result in both increased rainfall intensity (mean precipitation estimated for the wet days only) as well as increased length of dry spells (Giorgi et al., 2011). Moreover, in a recent paper, I have together with two colleagues shown that the more extreme rainfall amounts closely follow the rainfall intensity (Benestad et al. (2012). From these results, it seems that we can expect more extremes in both floods and droughts.
Finally, the analysis by Durack et al. (2012) provides another fingerprint of a global warming. However, this finding aspect is not entirely new: the last IPCC report already concluded there is an enhancement of the hydrological cycle, based on observed ocean salinity changes. The method and the conclusion are therefore not new, but the new Argo data confirm earlier findings and strengthen previous observations: the global warming and changes to the hydrological cycle are closely entangled.