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Bermuda-Labrador Basin Transport Index
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Bermuda-Labrador Basin Transport Index

The transport index (bottom panel) estimates the strength of the baroclinic gyre circulation in the North Atlantic, or the strength of the Gulf Stream and North Atlantic Current system. The units (Mtons s^-1) are within a few percent of the volumetric unit of transport (Sverdrups = 10⁶ m³s^-1). It is calculated from the difference of the potential energy anomalies (PEA) near Bermuda and in the Labrador Basin (top two panels).

The index was defined by Curry and McCartney (J. Phys. Oceanogr., 2001). The evolution of the index from 1950 through 2000 shows a circulation reducing through the low NAO period in the 1960s, then strengthening during the period of persistently high NAO over the next 25 years, reaching a peak in the mid-1990s. They found that the timing and mechanisms associated with PEA changes in each gyre varied, and were dependent on both locally and remotely-forced changes in the ocean (see paper for details).

Ocean transports of heat, salt, and other climate-relevant properties can be modified by changes in the gyre circulation, feeding back on decadal to interdecadal time scales to the atmosphere.

Data source

The index is calculated using the Coriolis Centre subsurface ocean objective analysis of temperature and salinity, which uses data from the Argo profiling float programme (now providing the majority of profiles of both temperature and salinity), as well as other subsurface data coordinated by JCOMM SOT and research vessels. The index is based on the Coriolis Atlantic Ocean analysis for the years 2000 to end 2004 (marked by the vertical line in the plots), and on the global analysis from the beginning of 2005 to the present (last updated 24-JUN-2009). The analysis is calculated every Wednesday based on the previous four weeks of data, and the index is updated every Thursday night.   get transport index (netcdf 8kB) »   get Bermuda PEA (netcdf 8kB) »   get Labrador Basin PEA (netcdf 8kB) »

Calculation

The index is calculated as the difference in ocean potential energy anomaly near the centers of the subpolar and subtropical gyres, in an analog to the NAO index calculation as the difference in sea level pressure between Iceland and Portugal. The potential energy anomaly (PEA) is defined as

,

the vertical integral in pressure p from 2000 db to the surface of the pressure-weighted specific volume anomaly δ, divided by the acceleration due to gravity g. The PEA approximates the stream function for the baroclinic mass transport. For Bermuda, the PEA is averaged over the box defined by 63.5-65°W and 31.5-33°N. For the Labrador Basin, the PEA is averaged over the box defined by 54-48°W and 55-59°N. These represent zones near the centers of the subtropical and subpolar wind-driven oceanic gyres. The transport index is then calculated as the difference in PEA between Bermuda and the Labrador Basin,

,

where the appropriate Coriolis parameter ƒ is taken for 40°N, the average latitude of the concentrated PEA gradient in the Gulf Stream-North Atlantic Current system.

Uncertainty

see a plot showing an indication of the uncertainty »

The uncertainty of the calculation is shown in an indicative manner, as an index varying from 0 to 1, with 1 being the a priori error (no data constraining the analysis, high uncertainty) and zero being a perfectly constrained analysis (no uncertainty). These values are obtained by taking the mean of the reported estimates of the variance divided by the a priori variance, a product of the Coriolis analysis, over the geographic zones involved in the calculations. The uncertainty is shown separately for temperature (red) and salinity (tan) over the Bermuda and Labrador Basin volumes, and the mean of these four uncertainty indicators is shown in the final plot as an uncertainty indicator for the transport estimate, the final plot above. The drop in uncertainty in the broad Labrador zone in early 2002, as the Argo network was being deployed, is notable.

get uncertainty index for the transport (netcdf 8kB) »   get uncertainty index for Bermuda temperature (netcdf 8kB) »   get uncertainty index for Bermuda salinity (netcdf 8kB) »   get uncertainty index for Labrador temperature (netcdf 8kB) »   get uncertainty index for Labrador salinity (netcdf 8kB) »