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Alice M. Grimm
,
A. K. Sahai
, and
Chester F. Ropelewski

Abstract

Global climate models forced by sea surface temperature are standard tools in seasonal climate prediction and in projection of future climate change caused by anthropogenic emissions of greenhouse gases. Assessing the ability of these models to reproduce observed atmospheric circulation given the lower boundary conditions, and thus its ability to predict climate, has been a recurrent concern. Several assessments have shown that the performance of models is seasonally dependent, but there has always been the assumption that, for a given season, the model skill is constant throughout the period being analyzed. Here, it is demonstrated that there are periods when these models perform well and periods when they do not capture observed climate variability. The variations of the model performance have temporal scales and spatial patterns consistent with decadal/interdecadal climate variability. These results suggest that there are unmodeled climate processes that affect seasonal climate prediction as well as scenarios of climate change, particularly regional climate change projections. The reliability of these scenarios may depend on the time slice of the model output being analyzed. Therefore, more comprehensive model assessment should include a verification of the long-term stability of their performance.

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R. Chattopadhyay
,
A. K. Sahai
, and
B. N. Goswami

Abstract

The nonlinear convectively coupled character of the summer monsoon intraseasonal oscillation (ISO) that manifests in its event-to-event variations is a major hurdle for skillful extended-range prediction of the active/break episodes. The convectively coupled character of the monsoon ISO implies that a particular nonlinear phase of the precipitation ISO is linked to a unique pattern of the large-scale variables. A methodology has been presented to capture different nonlinear phases of the precipitation ISO using a combination of a sufficiently large number of dynamical variables. This is achieved through a nonlinear pattern recognition technique known as self-organizing map (SOM) involving six daily large-scale circulation indices. It is demonstrated that the nonlinearly classified states of the large-scale circulation isolated at the SOM nodes without involving any information on rainfall are strongly linked to different phases of evolution of the rainfall ISO, including the active and break phases. While a lower SOM classification involving 9 different states identify the composite phases of the rainfall ISO, a higher SOM classification involving 81 states can identify different shades of composite phase of the rainfall ISO. The concept of isolating the nonlinear states, as well as the technique of doing so, is robust as almost identical phases of precipitation ISO are identified by the large-scale circulation indices derived from two different reanalysis datasets, namely, the 40-yr ECMWF Re-Analysis (ERA-40) and the NCEP–NCAR reanalysis.

The ability of the SOM technique to isolate spatial structure and evolutionary history of nonlinear convectively coupled states of the summer monsoon ISO opens up a new possibility of extended-range prediction of summer monsoon ISO. This knowledge is used to develop an analog technique for predicting different phases of monsoon ISO. Skillful four-pentad lead prediction of rainfall over central India is demonstrated with the model using only large-scale circulation fields. A major strength of the model is that it can easily be used for real-time extended-range prediction of monsoons.

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Susmitha Joseph
,
A. K. Sahai
,
R. Phani
,
R. Mandal
,
A. Dey
,
R. Chattopadhyay
, and
S. Abhilash

Abstract

Under the National Monsoon Mission Project initiated by the government of India’s Ministry of Earth Sciences, an indigenous dynamical ensemble prediction system (EPS) has been developed at the Indian Institute of Tropical Meteorology based on the state-of-the-art Climate Forecast System Model version 2 (CFSv2) coupled model, for extended-range (~15–20 days in advance) prediction. The forecasts are generated for the entire year covering the southwest monsoon, the northeast monsoon, and the summer and winter seasons. As the forecast of rainfall is important during the southwest and northeast monsoon seasons, along with that of the temperature during the summer and winter seasons, the present study documents the deterministic as well as probabilistic skill of the EPS in predicting the results in the respective seasons, over various meteorological subdivisions throughout India, on a pentad-lead time scale. The EPS is found to be skillful in predicting rainfall during the southwest and northeast monsoon seasons, as well as temperature during the summer and winter seasons, across different subdivisions of India. In addition, the EPS is noted to be skillful in predicting selected extremes in rainfall and temperature. This affirms the reliability and usefulness of the present EPS from an operational perspective.

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Susmitha Joseph
,
A. K. Sahai
,
S. Abhilash
,
R. Chattopadhyay
,
N. Borah
,
B. E. Mapes
,
M. Rajeevan
, and
A. Kumar

Abstract

This study reports an objective criterion for the real-time extended-range prediction of monsoon onset over Kerala (MOK), using circulation as well as rainfall information from the 16 May initial conditions of the Grand Ensemble Prediction System based on the coupled model CFSv2. Three indices are defined, one from rainfall measured over Kerala and the others based on the strength and depth of the low-level westerly jet over the Arabian Sea. While formulating the criterion, the persistence of both rainfall and low-level wind after the MOK date has been considered to avoid the occurrence of “bogus onsets” that are unrelated to the large-scale monsoon system. It is found that the predicted MOK date matches well with the MOK date declared by the India Meteorological Department, the authorized principal weather forecasting agency under the government of India, for the period 2001–14. The proposed criterion successfully avoids predicting bogus onsets, which is a major challenge in the prediction of MOK. Furthermore, the evolution of various model-predicted large-scale and local meteorological parameters corresponding to the predicted MOK date is in good agreement with that of the observation, suggesting the robustness of the devised criterion and the suitability of CFSv2 model for MOK prediction. However, it should be noted that the criterion proposed in the present study can be used only in the dynamical prediction framework, as it necessitates input data on the future evolution of rainfall and low-level wind.

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S. Abhilash
,
A. K. Sahai
,
N. Borah
,
S. Joseph
,
R. Chattopadhyay
,
S. Sharmila
,
M. Rajeevan
,
B. E. Mapes
, and
A. Kumar

Abstract

This study describes an attempt to overcome the underdispersive nature of single-model ensembles (SMEs). As an Indo–U.S. collaboration designed to improve the prediction capabilities of models over the Indian monsoon region, the Climate Forecast System (CFS) model framework, developed at the National Centers for Environmental Prediction (NCEP-CFSv2), is selected. This article describes a multimodel ensemble prediction system, using a suite of different variants of the CFSv2 model to increase the spread without relying on very different codes or potentially inferior models. The SMEs are generated not only by perturbing the initial condition, but also by using different resolutions, parameters, and coupling configurations of the same model (CFS and its atmosphere component, the Global Forecast System). Each of these configurations was created to address the role of different physical mechanisms known to influence error growth on the 10–20-day time scale. Last, the multimodel consensus forecast is developed, which includes ensemble-based uncertainty estimates. Statistical skill of this CFS-based Grand Ensemble Prediction System (CGEPS) is better than the best participating SME configuration, because increased ensemble spread reduces overconfidence errors.

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Suryachandra A. Rao
,
B. N. Goswami
,
A. K. Sahai
,
E. N. Rajagopal
,
P. Mukhopadhyay
,
M. Rajeevan
,
S. Nayak
,
L. S. Rathore
,
S. S. C. Shenoi
,
K. J. Ramesh
,
R. S. Nanjundiah
,
M. Ravichandran
,
A. K. Mitra
,
D. S. Pai
,
S. K. R. Bhowmik
,
A. Hazra
,
S. Mahapatra
,
S. K. Saha
,
H. S. Chaudhari
,
S. Joseph
,
P. Sreenivas
,
S. Pokhrel
,
P. A. Pillai
,
R. Chattopadhyay
,
M. Deshpande
,
R. P. M. Krishna
,
Renu S. Das
,
V. S. Prasad
,
S. Abhilash
,
S. Panickal
,
R. Krishnan
,
S. Kumar
,
D. A. Ramu
,
S. S. Reddy
,
A. Arora
,
T. Goswami
,
A. Rai
,
A. Srivastava
,
M. Pradhan
,
S. Tirkey
,
M. Ganai
,
R. Mandal
,
A. Dey
,
S. Sarkar
,
S. Malviya
,
A. Dhakate
,
K. Salunke
, and
Parvinder Maini

Abstract

In spite of the summer monsoon’s importance in determining the life and economy of an agriculture-dependent country like India, committed efforts toward improving its prediction and simulation have been limited. Hence, a focused mission mode program Monsoon Mission (MM) was founded in 2012 to spur progress in this direction. This article explains the efforts made by the Earth System Science Organization (ESSO), Ministry of Earth Sciences (MoES), Government of India, in implementing MM to develop a dynamical prediction framework to improve monsoon prediction. Climate Forecast System, version 2 (CFSv2), and the Met Office Unified Model (UM) were chosen as the base models. The efforts in this program have resulted in 1) unparalleled skill of 0.63 for seasonal prediction of the Indian monsoon (for the period 1981–2010) in a high-resolution (∼38 km) seasonal prediction system, relative to present-generation seasonal prediction models; 2) extended-range predictions by a CFS-based grand multimodel ensemble (MME) prediction system; and 3) a gain of 2-day lead time from very high-resolution (12.5 km) Global Forecast System (GFS)-based short-range predictions up to 10 days. These prediction skills are on par with other global leading weather and climate centers, and are better in some areas. Several developmental activities like coupled data assimilation, changes in convective parameterization, cloud microphysics schemes, and parameterization of land surface processes (including snow and sea ice) led to the improvements such as reducing the strong model biases in the Indian summer monsoon simulation and elsewhere in the tropics.

Open access
Emily Shroyer
,
Amit Tandon
,
Debasis Sengupta
,
Harindra J. S. Fernando
,
Andrew J. Lucas
,
J. Thomas Farrar
,
Rajib Chattopadhyay
,
Simon de Szoeke
,
Maria Flatau
,
Adam Rydbeck
,
Hemantha Wijesekera
,
Michael McPhaden
,
Hyodae Seo
,
Aneesh Subramanian
,
R Venkatesan
,
Jossia Joseph
,
S. Ramsundaram
,
Arnold L. Gordon
,
Shannon M. Bohman
,
Jaynise Pérez
,
Iury T. Simoes-Sousa
,
Steven R. Jayne
,
Robert E. Todd
,
G. S. Bhat
,
Matthias Lankhorst
,
Tamara Schlosser
,
Katherine Adams
,
S. U. P Jinadasa
,
Manikandan Mathur
,
M. Mohapatra
,
E. Pattabhi Rama Rao
,
A. K. Sahai
,
Rashmi Sharma
,
Craig Lee
,
Luc Rainville
,
Deepak Cherian
,
Kerstin Cullen
,
Luca R. Centurioni
,
Verena Hormann
,
Jennifer MacKinnon
,
Uwe Send
,
Arachaporn Anutaliya
,
Amy Waterhouse
,
Garrett S. Black
,
Jeremy A. Dehart
,
Kaitlyn M. Woods
,
Edward Creegan
,
Gad Levy
,
Lakshmi H. Kantha
, and
Bulusu Subrahmanyam

Abstract

In the Bay of Bengal, the warm, dry boreal spring concludes with the onset of the summer monsoon and accompanying southwesterly winds, heavy rains, and variable air–sea fluxes. Here, we summarize the 2018 monsoon onset using observations collected through the multinational Monsoon Intraseasonal Oscillations in the Bay of Bengal (MISO-BoB) program between the United States, India, and Sri Lanka. MISO-BoB aims to improve understanding of monsoon intraseasonal variability, and the 2018 field effort captured the coupled air–sea response during a transition from active-to-break conditions in the central BoB. The active phase of the ∼20-day research cruise was characterized by warm sea surface temperature (SST > 30°C), cold atmospheric outflows with intermittent heavy rainfall, and increasing winds (from 2 to 15 m s−1). Accumulated rainfall exceeded 200 mm with 90% of precipitation occurring during the first week. The following break period was both dry and clear, with persistent 10–12 m s−1 wind and evaporation of 0.2 mm h−1. The evolving environmental state included a deepening ocean mixed layer (from ∼20 to 50 m), cooling SST (by ∼1°C), and warming/drying of the lower to midtroposphere. Local atmospheric development was consistent with phasing of the large-scale intraseasonal oscillation. The upper ocean stores significant heat in the BoB, enough to maintain SST above 29°C despite cooling by surface fluxes and ocean mixing. Comparison with reanalysis indicates biases in air–sea fluxes, which may be related to overly cool prescribed SST. Resolution of such biases offers a path toward improved forecasting of transition periods in the monsoon.

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