Relations between interannual and intraseasonal monsoon variability as diagnosed from AMIP integrations

Abstract

Monsoon variability on intraseasonal and interannual time-scales is analysed using data from five 10-year European Centre for Medium-Range Weather Forecasts Atmospheric Model Intercomparison Project integrations, which differ only in their initial conditions. The results show that monsoon fluctuations within a season and within different years have a common dominant mode of variability. The spatial pattern of the common dominant mode in precipitation has a pronounced zonal structure, with one band of anomalous rainfall extending from 20"N to 5"N, covering most of the land areas, with the other band, of opposite sign, lying between 5"N and lo's, mostly over the Indian Ocean. This mode therefore describes both the activebreak monsoon spells associated with fluctuations of the Tropical Convergence Zone (TCZ) between the continental and the oceanic regime and the principal pattern of interannual variability of monsoon rainfall. In the observations the oscillations between active and break monsoon spells have similar behaviour, although the model is deficient in representing the rainfall variability over India.On the intraseasonal time-scale the transition between the two regimes seems to have a chaotic nature. In addition the probability density function of the principal mode is bimodal for the years in which this mode is particularly dominant. These two results indicate a possible similarity with the Lorenz 3-component chaotic model. Northward-propagating convective regions, simulated by the model, are not clearly associated with the phase transitions of the TCZ regime. The timing of the monsoon onset appears to be modulated by the phase of the El Nifio/Southern Oscillation during the preceding season, consistent with observational studies.The results suggest that the dominant mode may also represent some components of the observed monsoon variability. The interannual fluctuations of the dominant mode exhibit only a weak level of reproducibility compared with the relatively large predictability of a broad-scale monsoon wind-shear index.

Antecedents and self‐induction of active‐break south Asian monsoon unraveled by satellites

“…A strong easterly vertical shear favors developing westward propagating Rossby waves and deep convections in the Northern Hemisphere [Wang and Xie, 1996]. This observation has allowed a new interpretation about the westward and poleward tilted rainband, which were found in previous statistical analyses of other datasets [Ferranti et al, 1997]. This titled rain band may form as a result of the Rossby wave emanation from the equatorial convection Figure 1.…”

Indo‐Pacific Ocean response to atmospheric intraseasonal variability: 2. Boreal summer and the Intraseasonal Oscillation

“…Band-pass filtering was performed using a 35-95-day Lanczos filter [Duchon, 1979]. In order to isolate the canonical form of the ISO, that which is most prevalent in boreal summer [Wang and Rui, 1990;Ferranti et al, 1997;Annamalai et al, 1999;Sperber et al, 2000;Jones et al, 2003], only Northern Hemisphere ''summer'' (hereafter, defined as May-October) data were retained in the analysis. From these band-passed summer data, EEOFs are computed for ±7 pentad time lags on the region between 30°E and 180°E.…”

Diagnosis of systematic forecast errors dependent on flow pattern

“…The computation of SVD is described by Ferranti et al (1997) and is based on PCs (see Barnett and Preisendorfer (1987)). The data used are ECMWF operational forecast errors and verifying analyses of geopotential height at 500 hPa for the seven winters (DJF) 1992 to 1999.…”