Environmental Research Division

The prior schematic showed that a large region of low SLP near Indonesia is coupled to two areas of high SLP in the extratropics. On interannual time scales, SLP anomalies at the NPH and SPH are negatively correlated with those over the western tropical Pacific and southeast Asia, as shown in the figure below.

The figure shows correlation of annual mean SLP in the NPH with SLP globally. Areas significantly correlated with the NPH are shaded. The two regions of positive correlation (yellow-red) in the northeast and southeast Pacific, and the large region of negative correlation (green-blue) in the western tropical Pacific show that interannual changes in the NPH are strongly coupled with variations in the Hadley-Walker circulation. Figure based on image provided by the NOAA-CIRES Climate Diagnostics Center, Boulder Colorado from their Web site at http://www.cdc.noaa.gov/.

The tropical-based SOI emphasizes variations in the zonal Walker circulation. However the SOI tells little about the meridional circulation (the Hadley cells) and its variability, which is an important link between the tropical and mid-latitude atmosphere. The Hadley circulation is specifically associated with the trade winds, which emanate from the extratropical high pressure systems, including the NPH and SPH. The upper level Hadley circulation feeds into the extratropical highs.

Likewise, a number of extratropical teleconnection indices relate information about zonal or higher-latitude atmospheric connections, but do not clearly describe tropical-extratropical or cross-equatorial teleconnections. Extratropical indices do not reflect variability in the trade winds, important vectors for the transport of momentum, energy, and mass between the tropics and extratropics. The difference in SLP between the extratropical NPH (SPH) and Darwin, the NOI (SOI*), is an index of the Pacific trades, and is a clear index of meridional as well as zonal atmospheric patterns.