by Gavin Schmidt and Michael Mann
In a sure-to-be widely publicized paper in the Dec. 1 Nature, Bryden et al. present results from oceanographic cruises at 25°N across the Atlantic showing a ~30% decline in the ocean overturning circulation. These cruises have been repeated every few years since 1957, and the last two cruises (in 1998 and 2004) show notable changes in the structure of the deep return circulation. In particular, the very deepest part of the return flow (at around 3000 to 5000 m) has reduced and moved up in the water column compared to previous decades. How solid is this result and what might it imply for climate?
The first question that is asked is usually how these calculations are done. Due to the predominantly "geostrophic" nature of the ocean circulation (i.e. velocity is generally horizontally perpendicular to pressure gradients because of the Coriolis effect), you can calculate changes in North-South velocities by only considering the East-West changes in temperature and salinity. So given a section across the ocean (say 25°N), oceanographers can estimate the transport across that section. The error in these numbers is a little hard to know, but Bryden et al estimate around +/- 6 Sv (1 Sv is 10^6 m^3/s, the Amazon output is around 0.1 Sv for perspective).
What did Bryden et al find? Their calculations indicate that the Gulf Stream itself has been remarkably stable over the almost 40 years, and this accords with other measurements of the Gulf Stream flow itself. Since what goes north must eventually go south (after taking into account the very small amounts of atmospheric transport and the amount of flow through the Bering Strait), all of the other changes will balance. They show that the amount of deep return flow seems to have gone down about 8 Sv (out of 25 Sv), and the amount of mid-ocean to surface transport has gone up by about the same amount. This corresponds to a roughly 30% apparent weakening in the so-called "Thermohaline Circulation" (see our previous discussion here). Since the surface flow is warmer than the deep flow, there is a consequent decrease in the northward heat flux of about 0.2 PW (or about 15%).
http://www.realclimate.org/index.php?p=225
I haven't had a chance to read this paper yet, but I thought I'd post this link and discussion here, since if this result stands up it does impact the issue of influences on the activity of the Atlantic hurricane season.
There are definitely some odd aspects to this, since the prevailing theory for our current active storm phase is predicated in part on and increased rate of the THC, whereas this data indicates a decreased rate. Furthermore, if this has been going on for any extended time, we would expect to see some cooling of the north Atlantic surface waters, and we're seeing the opposite.
Jan