The response of an ENSO model to climate noise, weather noise and intraseasonal forcing

The response of an intermediate coupled model of the: tropical Pacific to different forms of stochastic wind forcing is studied. An estimate of observed Pacific wind variance that is unrelated to Pacific sea surface temperature (SST) has a red spectrum, inconsistent with standard definitions of "weather noise". The reddening is likely due to SST outside the basin; we propose a definition of "climate noise" for such reddened variance. Effects are compared for (i) red climate noise; (ii) the corresponding white weather noise estimate; (iii) intraseasonal and interannual components of the white noise (to test frequency response); and (iv) a noise product with Extra power in the 30-60 day range. Power is not effectively channeled from subannual frequencies to the frequencies associated with ENSO in this model. This suggests that ENSO impacts of the Madden-Julian oscillation are largely restricted to the low-frequency tail rather than the 30-60 day spectral peak. Interannual climate noise originating outside the tropical Pacific appears important.