Distinct North American teleconnection of the strong El Niños as modulated by the ENSO-annual cycle combination mode

The ENSO-annual cycle combination mode (C-mode) derived from the nonlinear interaction between the strong El Niño and tropical annual cycle has been well documented as critical for extending El Niño’s impacts on East Asia. The present study reveals a modulation of this mode on the teleconnection pattern of strong El Niños over the Pacific–North American (PNA) sector. Results show that the teleconnection response to strong El Niños features a distinct meridional pattern over North America, but the response to moderate El Niños tends to be zonally oriented like the classic PNA pattern. The zonally-oriented pattern mainly results from the mid-latitude eastward wave activity fluxes, as usually seen in all El Niños. In contrast, for strong El Niños, additional northeastward wave fluxes emanate from the central-eastern subtropical North Pacific and contribute to the meridional teleconnection pattern in North America. This meridional wave activity can be attributed to the C-mode that peaks in the late boreal winter, which generates an equator-asymmetric circulation response over the central-eastern Pacific and thus the subtropical upper-level Rossby wave source exciting the distinct teleconnection pattern over North America. The effect of annual cycle is further validated with an atmospheric general circulation model, as the meridional teleconnection pattern of strong El Niños will switch to zonal if excluding the annual cycle in the tropical sea surface temperature boundary. The analyses herein on observations and numerical experiments highlight the key role of the annual cycle in shaping the strong El Niño’s teleconnection pattern.