Evidence of Growing Bred Vector Associated with Tropical Intraseasonal Oscillation

Stability property of the tropical intraseasonal oscillation (ISO) is examined using tropical bred vectors obtained from the operational numerical weather forecast system of the Japan Meteorological Agency (JMA). The JMA one-month weather forecast system is conducted by a global spectral model (T106L40) with a prognostic Arakawa-Schubert scheme. The tropical bred vectors are produced by revised operational breeding cycle in which the perturbation is damped over the extratropics and rescaled by 3.3% of climatological variance of 200 hPa velocity potential. The obtained tropical bred vector has a spatial structure similar to the eastward propagating dry Kelvin wave with phase speed of 30 m/s. The time-mean growth rate (Lyapunov exponent) of the fast growing tropical bred vector has a positive value of 0.1/day. Although its growing rate is smaller than extratropical baroclinic instability mode (the extratropical Lyapunov exponent is 0.4/day), this result suggests that the tropical ISO is unstable to infinitesimal perturbations.