An empirical rule for extended range prediction of duration of Indian summer monsoon breaks

Prediction of the duration of the Indian summer monsoon breaks is highly desirable. It will help in planning water resource management, sowing and harvesting. Applicability of the recently discovered regime transition rules for the Lorenz model in predicting the duration of monsoon breaks, is explored in this paper. Using several indices of the observed summer monsoon intraseasonal oscillation (ISO), it is shown that the peak anomaly in an active regime can be used as a predictor for the duration of the subsequent break spell. It is also found that the average growth rate around the threshold to an active condition can be used as a predictor of the peak anomaly in the active spell. Average growth around the threshold to an active condition can give useful prediction of the duration of the following break, on an average, about 23 days (38 days) in advance of its commencement (end)

On Pacific Subtropical Cell Variability over the Second Half of the Twentieth Century

Abstract The evolution of the Pacific subtropical cells (STC) is presented for the period 1948–2007. Using ocean models of different resolutions forced with interannually varying atmospheric forcing datasets, the mechanisms responsible for the observed STC weakening and late recovery during the period of study are analyzed. As a result of the STC weakening (strengthening), warming (cooling) trends are found in the equatorial Pacific sea surface temperatures (SSTs). Model results agree well with observed estimates of STC transport, STC convergence, and equatorial SST anomalies. It is shown that subtropical atmospheric variability is the primary driver of the STC and equatorial SST low-frequency evolution and is responsible for both the slowdown during the second half of the twentieth century and the rebound at the end of the century. Subtropically forced STC variability is identified as a major player in the generation of equatorial Pacific decadal SST anomalies, pacing tropical Pacific natural climate variability on interdecadal time scales, as observed in historical records. The natural mode of variability has implications for the evolution of equatorial SST in the coming decades under the concomitant effects of climate change

Investigating the role of air-sea forcing on the variability of hydrography, circulation, and mixed layer depth in the Arabian Sea and Bay of Bengal

Summary: An effort is made to understand and quantify the influence of near surface zonal and meridional winds, incoming shortwave radiation, and freshwater flux air-sea forcings on the seasonal variability of the hydrography, circulation, and mixed layer depth of the Arabian Sea (AS) and Bay of Bengal (BoB). Sensitivity experiments using an ocean general circulation model are carried out for this purpose in the Indian ocean around 65°–95°E, 5°–22°N during 1998–2014 (17 years). In the absence of near surface wind forcing, the sea surface temperature of the region greatly increases in all the seasons, whereas, in the absence of incoming shortwave radiation forcing, exactly opposite is the case. The sea surface salinity of the AS and BoB decreases in the absence of wind and shortwave radiation forcings, whereas, in the northern BoB it increases in the absence of freshwater flux forcing. The sub-surface changes in the stratification of temperature and salinity are also investigated. The influence of the air-sea forcings on the mixed layer depth of the region is found to be highly seasonally dependent. The effect of air-sea forcings on the seasonal variability of the upper ocean vertical stability is studied using the vertical shear of the horizontal velocity, buoyancy frequency, and energy required for mixing as quantifiers. The near surface wind forcing has highest contribution in changing the surface circulation of the region. Keywords: Arabian Sea and Bay of Bengal, Air-sea forcing, Ocean general circulation model, Hydrography and circulation, Vertical stabilit

Energetically consistent scale-adaptive stochastic and deterministic energy backscatter schemes for an atmospheric model

Current climate models still suffer from many biases which are partly due to excessive subgrid-scale dissipation. Here we systematically develop energetically consistent stochastic energy backscatter (SEB) and deterministic energy backscatter (DEB) parametrization schemes. We implement our schemes in a simplified spectral atmospheric General Circulation Model (GCM). It is shown that the SEB scheme performs better than the DEB scheme at low horizontal resolutions (T21 and T31), whereas the performance of both schemes becomes comparable as the resolution increases to T42 when comparing with our reference simulation at T127 resolution. The energy backscatter parametrization schemes improve eddy variability in low-resolution models and correctly capture the dominant mode of zonal-mean zonal wind variability. The autocorrelation time-scale of low-resolution models is also found to be more consistent with the reference simulation when applying the SEB and DEB parametrizations. Our schemes are scale-adaptive and computationally efficient

A study of the forced Lorenz model of relevance to monsoon predictability

209-216The forced Lorenz model introduced as a paradigmatic model for monsoon prediction is explored in greater detail. In particular, a study is made of the bifurcation structure of the forced Lorenz model as the forcing is varied. Shift in the probability distribution function between the two branches of the Lorenz attractor as a function of the forcing is also studied

Predictive skill of DEMETER models for wind prediction over southern subtropical Indian Ocean

62-69The ensemble mean prediction of winds at 850 hPa from individual models of DEMETER project has been compared from NCEP observation over southern subtropical Indian Ocean during summer monsoon season (JJAS) for the time domain 1980-2001. Predictability of U850 hPa (U850) and V850 hPa (V850) has been tested by different statistical approach like root mean square error (RMSE) for the region between Madagascar and western Australia in view of the importance of this region in anomalous variation of south central African rainfall variability as evidenced by some recent studies. A dichotomous forecast skill measure has been performed by calculating predictive skill measures like accuracy, bias, probability of detection (POD), false alarm ration (FAR), probability of false detection (POFD), threat score (TS), equivalent threat score (ETS) and Heidke skill score (HSS) for model produced U850 and V850 from all the individual models and multi model ensemble (MME). It has been found that the root mean square error has been reduced by applying MME but there is no effect on dichotomous predictive skill measures