The relationship between the North Atlantic jet and tropical convection over the Indian and western Pacific Oceans

Boreal winter jet variability over the North Atlantic is investigated using ERA-40 reanalysis data, where the variability is defined as the first EOF of the zonal wind on seven vertical levels. The principal component time series of this EOF, termed as the North Atlantic Jet (NAJ) index, is correlated with the North Atlantic Oscillation (NAO) index at a value of 0.48. A pattern correlation analysis indicates that the NAJ pattern more accurately describes the actual zonal wind field than does the zonal wind pattern associated with the NAO.

A series of composite analyses with daily data shows that polarity of the NAJ is tied to Indo-western Pacific precipitation, and reveals the following statistically significant intraseasonal time-scale relationship: (1) the negative NAJ leads enhanced Indian Ocean precipitation; (2) enhanced Indian Ocean precipitation is followed by the positive NAJ; (3) the positive NAJ leads enhanced western Pacific Ocean precipitation; (4) enhanced western Pacific Ocean precipitation is followed by the negative NAJ. These intraseasonal relationships are found to be linked through the circumglobal teleconnection pattern (CTP). The CTP index is negative during positive NAJ events and vice versa for negative NAJ events. This sign paring suggests that relationships (1) and (3) may arise from cold air surges associated with the CTP over these oceans.

Relationships (2) and (4), on the other hand, provide insight into interdecadal NAO variability. The upward trend in the NAO index from the early 1970s until the late 1990s coincided with increased precipitation over both the Indian and western Pacific Oceans. In contrast, the downward trend in the NAO index during the following decade was accompanied by enhanced precipitation over the western Pacific Ocean and depressed precipitation over the Indian Ocean. This result suggests that the interdecadal NAO trend can be interpreted as arising from competition between convective heating over the tropical Indian and western Pacific Oceans and their impact on the frequency of occurrence of positive and negative NAO events.