Climate models suggest that the meridional overturning circulation (MOC) in the Atlantic, and the accompanying oceanic heat flux, vary considerably on interannual time scales. In addition to abrupt climate change scenarios in which the MOC can virtually shut off, the "normal" interdecadal variation may range from 20% to 30% of its long-term mean value, according to some models. However, until recently no direct measurement system had been put in place that could provide regular estimates of the meridional overturning circulation to determine its natural variability or to assess these model predictions. Such a system is now deployed along 26.5-N in the Atlantic as part of the joint U.K./U.S. RAPID-MOCHA program, which has been continuously observing the MOC since March 2004.
Intellectual merit: The goal of this project is to maintain this measurement system for an additional 6 years, from 2008 through 2014, so that a decade-long time series of the Atlantic MOC and associated meridional ocean heat flux can be obtained. This goal is different from, but complementary to, the more established method of occupying high-resolution zonal trans-basin hydrographic sections to estimate the MOC and heat flux at irregular intervals. Regular estimates are needed to determine the magnitude and dominant time scales of variability, the climatological seasonal cycle, and to help filter out the higher frequency "noise" that may alias the determination of longer term heat flux variations from trans-basin sections that are widely spaced in time. The proposed observations will provide the data necessary to document the spectrum of the observed MOC variability and to assess the MOC variability in ocean reanalysis models. The broader impacts of the proposal are to provide the basis for long-term monitoring of the MOC, so that its relationship to observed climate fluctuations can be understood, and our ability to assess climate model predictions can be improved. The proposed program continues a successful collaborative effort with investigators from the U.K. National Oceanography Centre and the Max Planck Institute in Germany. This component of the project will maintain the moored component in the eastern Atlantic for a six-year period extending from March 2008 to March 2014, and continue the collaboration with NOAA scientsits in the maintenance of the semiannual hydrographic/lowered-ADCP sections east of the Bahamas. The U.K. will support trans-basin hydrographic cruises and an extended array of moorings along 26.5-N that will complete the continuous time series observing system.
Broader Impacts: The MOC observing system along 26.5-N in the Atlantic will provide the definitive data set necessary to document the spectrum of the observed MOC variability, and to assess the realism of MOC variability in ocean reanalysis models. The anticipated legacy of this program will be in laying the groundwork for long-term observation of the MOC, so that its relationship to observed climate fluctuations can be understood, and our ability to assess climate model predictions can be further improved. The international collaboration with UK scientists initiated under the first phase of the project will be continued. Training in state of the art ocean observational techniques will be provided to two graduate students.
This project is a contribution to the U.S. CLIVAR (CLImate VARiability and predictability) program
The large-scale circulation of the ocean plays an important role in the meridional transport of water properties such as heat, freshwater, carbon, and nutrients. In the northern hemisphere subtropics, where the Atlantic Ocean heat transport is close to its maximum, the circulation carries about 1.3 PW (1 PW = 1015 W) of heat northward. This is approximately 70% of the net poleward heat flux carried by the global oceans and 25% of the poleward heat transport by the ocean and atmosphere together. This poleward heat transport is dominated by the meridional overturning circulation (MOC), in which upper ocean waters moving northward in the basin are transformed into deep waters and transported southward below about 1000 m. Previous direct estimates of the ocean heat transport in the Atlantic, as well as in the other ocean basins, have been derived primarily from a few individual research cruises that were widely spaced in time. Variability of ocean heat transport on different time scales has remained largely unknown, but is critical to understanding long-term climate variability. Beginning in 2004, the RAPID-MOCHA observational array was deployed in the Atlantic Ocean along 26.5ºN with the purpose of continuously monitoring the MOC at this latitude. Funded jointly by the U.S. (NSF, precursor to this grant) and the U.K., this array successfully monitored the MOC from 2004-2008. The present grant continued the funding of the U.S. portion of this effort for an additional 6 years, from 2008-2014, and provided support to develop estimates of the ocean heat transport across 26.5 ºN. We were able to show from these observations that both the MOC and heat transport vary dramatically on relatively short time scales (weeks to months), casting doubt on the representativeness of snapshot estimates made from single cruises. The continuous measurements provided a substantially reduced uncertainty in the heat transport estimate with respect to one-time estimates. A seasonal cycle for the MOC and heat transport was also resolved, and significant interannual variability was observed. Over the 8.5 year time series analyzed in this grant, the MOC showed a significant downward trend. The associated decline in heat transport was shown to account for a significant cooling of upper ocean waters in the mid-latitudes of the North Atlantic after 2009, which may have played a role in the harsh European winters of 2010 and 2011. The resulting time series of the MOC and ocean heat transport has provided an important benchmark for indirect estimates derived from surface climatologies and satellite methods, and for comparison with numerical models.
