The global meridional overturning ocean circulation (MOC) has proved difficult to represent in climate models, which are essential to projections of 21st C climate. Recent observational studies give clues, showing that the observed MOC involves new and unexpected elements and not just a calibration of well established flow pathways. Mesoscale eddies are active in driving the actual circulation of mass, heat, fresh water and tracers. The scope of eddy-flux observations has increased greatly in the past several years, due largely to the combination of high-resolution in situ observations with altimetry. In this proposal we focus on mesoscale eddies and their MOC relationship.
Logistically the proposed work supports the US AMOC as well as several NASA OSTST objectives, including future SWOT swath altimetry.
The proposed work involves
- Evaluating the mesoscale eddy contribution to western and eastern warm-branch meridional circulation between subtropics and subpolar Atlantic using altimetry with in situ ARGO float and surface drifters, Seagliders and tracers.
The Gulf Stream warm-water pathway north to the subpolar gyre and the southward cool water transport near the eastern boundary involve complex transitions influenced by seafloor topography. There is important subsurface structure to these transports. Altimetric circulation observations have shed light on this pathway with floats and drifters. Combining ARGO drift velocity with altimetry, Willis (GRL 2010) has developed estimates of northward warm-branch AMOC at 41N. We will study the 3D, eddy-rich circulation of the warm AMOC branch using ARGO and altimetry, with drifters and chemical tracers.
- EOF structure of atmospheric forcing of the AMOC and extension of this work to study the century timescale Atlantic Multidecadal Variability (AMV).
Atlantic sea-surface temperature has strong covariance with blocking patterns in the atmosphere above (Hakkinen et al Science, 2011). Our plan is to diagnose the upper-ocean response of the warm AMOC branch to storm track/blocking dynamics of the atmosphere, and to diagnose from numerical models the response of transient forcing. This connects with objective (1), the altimetry/AMOC connection: A preliminary indication that warm Atlantic SST forces the persistent atmospheric blocking episodes has recently been found (Croci-Maspoli MWR 2009).
And, recently the AMOC RAPID project showed a significant MOC drop of several Sverdrups in 2009-2010. These winters have high atmospheric blocking activity with southward storm track shift.
- Diagnosis of coupled climate models in the eddy-rich regions analyzed in (1) and (2), using an unusual algorithm for the meridional overturning.
This initiative involves the contribution these observations can make to global climate models. We will develop a water-mass transformation (WMT) image of the AMOC and its connection with altimetric/ARGO data. The usual overturning streamfunction metric for the MOC needs to be complemented by the transport across latitude circles resolved in potential temperature/salinity classes. Such a product encompasses the AMOC transports of mass, salt and heat. Initial work (Langehaug et al., 2011, 2012) shows very large differences in the AMOC dynamics of four IPCC-class models. The differences arise from errors in low salinity waters (often ice-melt derived) in the subtropical-subpolar transition zone. It is an important operational product of our research, given that coupled climate models are center-stage in the IPCC scenarios of 21st C. climate change.
We have made progress in several of these areas. Subtropical to subpolar exchange (Hakkinen & Rhines 2011, 2009), atmospheric interaction with the AMV Hakkinen, Rhines and Worthen (2011) and the subtropical to subpolar exchange in climate models Langehaug, Rhines & Eldevik (2012).