Plausible causes of the interdecadal change of the North Pacific teleconnection pattern in boreal spring around the late 1990s

An interdecadal change of the North Pacific teleconnection pattern in the upper troposphere in boreal spring was noted around the late 1990s. During the period of 1979–1998, a meridional dipole pattern with alternative cyclone and anticyclone to the south and the north of 45° N over North Pacific, was identified as the leading mode by applying the empirical orthogonal function (EOF) technique onto the 300-hPa geopotential height anomaly after removing its zonal average. This pattern is similar to the upper-tropospheric structure of North Pacific Oscillation (NPO)/western Pacific (WP) pattern. During the period of 1999–2018, the meridional dipole is replaced by a northeast–southwest orientated teleconnection with centers of activity over the subtropical central Pacific and northeastern North Pacific, resembling a Pacific-North America (PNA) teleconnection. That is, the upper-level atmospheric teleconnection shifted from a NPO/WP-like pattern to a PNA-like pattern in 1998/1999. For the plausible causes of this interdecadal change, the effect of synoptic high-frequency (HF, 2–8 day) eddy and tropical convection over the central Pacific are emphasized. Before 1998/1999, the NPO/WP-like teleconnection could be ascribed to the vorticity forcing of the HF transient eddy activity over North Pacific between 40° and 60° N, with the role of tropical heating ignorable. After 1998/1999, the spatial distribution of HF transient eddy activity shifted significantly from a north–south pattern to a northeast–southwest pattern, sustaining a PNA-like teleconnection through the transportation of the eddy vorticity fluxes, while the vorticity forcing of the subseasonal low-frequency (LF, 10–90 day) transient eddy activity plays a role in the maintenance of the teleconnection lobe over the subtropical central Pacific. In addition, the tropical convection anomaly centers shifted towards the equatorial central Pacific and Maritime Continent, which can induce a poleward-propagating Rossby wave train, and amplify the PNA-like teleconnection pattern.