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Prediction of Indian Summer Monsoon Onset Using Dynamical Subseasonal Forecasts: Effects of Realistic Initialization of the Atmosphere

Andrea Alessandri Agenzia Nazionale per le nuove Tecnologie, l’energia e lo sviluppo economico sostenibile, Rome, Italy, and International Pacific Research Center, Honolulu, Hawaii

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Andrea Borrelli Centro Euro-Mediterraneo sui Cambiamenti Climatici, Bologna, Italy

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Annalisa Cherchi Centro Euro-Mediterraneo sui Cambiamenti Climatici, and Istituto Nazionale di Geofisica e Vulcanologia, Bologna, Italy

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Stefano Materia Centro Euro-Mediterraneo sui Cambiamenti Climatici, Bologna, Italy

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Antonio Navarra Centro Euro-Mediterraneo sui Cambiamenti Climatici, and Istituto Nazionale di Geofisica e Vulcanologia, Bologna, Italy

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June-Yi Lee Research Center for Climate Sciences, Pusan National University, Busan, South Korea

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Bin Wang International Pacific Research Center, Honolulu, Hawaii

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Print Publication:
01 Mar 2015
DOI:
https://doi.org/10.1175/MWR-D-14-00187.1
Page(s):
778–793
Restricted access

Abstract

Ensembles of retrospective 2-month dynamical forecasts initiated on 1 May are used to predict the onset of the Indian summer monsoon (ISM) for the period 1989–2005. The subseasonal predictions (SSPs) are based on a coupled general circulation model and recently they have been upgraded by the realistic initialization of the atmosphere with initial conditions taken from reanalysis. Two objective large-scale methods based on dynamical-circulation and hydrological indices are applied to detect the ISM onset. The SSPs show some skill in forecasting earlier-than-normal ISM onsets, while they have difficulty in predicting late onsets. It is shown that significant contribution to the skill in forecasting early ISM onsets comes from the newly developed initialization of the atmosphere from reanalysis. On one hand, atmospheric initialization produces a better representation of the atmospheric mean state in the initial conditions, leading to a systematically improved monsoon onset sequence. On the other hand, the initialization of the atmosphere allows some skill in forecasting the northward-propagating intraseasonal wind and precipitation anomalies over the tropical Indian Ocean. The northward-propagating intraseasonal modes trigger the monsoon in some early-onset years. The realistic phase initialization of these modes improves the forecasts of the associated earlier-than-normal monsoon onsets. The prediction of late onsets is not noticeably improved by the initialization of the atmosphere. It is suggested that late onsets of the monsoon are too far away from the start date of the forecasts to conserve enough memory of the intraseasonal oscillation (ISO) anomalies and of the improved representation of the mean state in the initial conditions.

Corresponding author address: Andrea Alessandri, Agenzia Nazionale per le nuove Tecnologie, l’energia e lo sviluppo economico sostenibile, Via Anguillarese, 301 Sp. 118 CR Casaccia, 00123 Santa Maria di Galeria, Rome, Italy. E-mail: andrea.alessandri@enea.it

Abstract

Ensembles of retrospective 2-month dynamical forecasts initiated on 1 May are used to predict the onset of the Indian summer monsoon (ISM) for the period 1989–2005. The subseasonal predictions (SSPs) are based on a coupled general circulation model and recently they have been upgraded by the realistic initialization of the atmosphere with initial conditions taken from reanalysis. Two objective large-scale methods based on dynamical-circulation and hydrological indices are applied to detect the ISM onset. The SSPs show some skill in forecasting earlier-than-normal ISM onsets, while they have difficulty in predicting late onsets. It is shown that significant contribution to the skill in forecasting early ISM onsets comes from the newly developed initialization of the atmosphere from reanalysis. On one hand, atmospheric initialization produces a better representation of the atmospheric mean state in the initial conditions, leading to a systematically improved monsoon onset sequence. On the other hand, the initialization of the atmosphere allows some skill in forecasting the northward-propagating intraseasonal wind and precipitation anomalies over the tropical Indian Ocean. The northward-propagating intraseasonal modes trigger the monsoon in some early-onset years. The realistic phase initialization of these modes improves the forecasts of the associated earlier-than-normal monsoon onsets. The prediction of late onsets is not noticeably improved by the initialization of the atmosphere. It is suggested that late onsets of the monsoon are too far away from the start date of the forecasts to conserve enough memory of the intraseasonal oscillation (ISO) anomalies and of the improved representation of the mean state in the initial conditions.

Corresponding author address: Andrea Alessandri, Agenzia Nazionale per le nuove Tecnologie, l’energia e lo sviluppo economico sostenibile, Via Anguillarese, 301 Sp. 118 CR Casaccia, 00123 Santa Maria di Galeria, Rome, Italy. E-mail: andrea.alessandri@enea.it
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