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F. Kucharski, A. Bracco, J. H. Yoo, and F. Molteni

Abstract

The Indian monsoon–El Niño–Southern Oscillation (ENSO) relationship, according to which a drier than normal monsoon season precedes peak El Niño conditions, weakened significantly during the last two decades of the twentieth century. In this work an ensemble of integrations of an atmospheric general circulation model (AGCM) coupled to an ocean model in the Indian Basin and forced with observed sea surface temperatures (SSTs) elsewhere is used to investigate the causes of such a weakening.

The observed interdecadal variability of the ENSO–monsoon relationship during the period 1950–99 is realistically simulated by the model and a dominant portion of the variability is associated with changes in the tropical Atlantic SSTs in boreal summer.

In correspondence to ENSO, the tropical Atlantic SSTs display negative anomalies south of the equator in the last quarter of the twentieth century and weakly positive anomalies in the previous period. Those anomalies in turn produce heating anomalies, which excite a Rossby wave response in the Indian Ocean in both the model and the reanalysis data, impacting the time-mean monsoon circulation.

The proposed mechanism of remote response of the Indian rainfall to tropical Atlantic sea surface temperatures is further tested forcing the AGCM coupled to the ocean model in the Indian Basin with climatological SSTs in the Atlantic Ocean and observed anomalies elsewhere. In this second ensemble the ENSO–monsoon relationship is characterized by a stable and strong anticorrelation through the whole second half of the twentieth century.

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C. D. Roberts, F. Vitart, M. A. Balmaseda, and F. Molteni

Abstract

This study uses initialized forecasts and climate integrations to evaluate the wintertime North Atlantic response to an increase of ocean model resolution from ~100 km [low-resolution ocean (LRO)] to ~25 km [high-resolution ocean (HRO)] in the European Centre for Medium-Range Weather Forecasts Integrated Forecasting System (ECMWF-IFS). Importantly, the simulated impacts are time-scale dependent such that impacts in subseasonal and seasonal forecasts cannot be extrapolated to climate time scales. In general, mean biases are reduced in HRO relative to LRO configurations and the impact is increased at longer lead times. At subseasonal to seasonal lead times, surface heating anomalies over the Gulf Stream are associated with local increases to the poleward heat flux associated with transient atmospheric eddies. In contrast, surface heating anomalies in climate experiments are balanced by changes to the time-mean surface winds that resemble the steady response under linear dynamics. Some aspects of air–sea interaction exhibit a clear improvement with increased resolution at all lead times. However, it is difficult to identify the impact of increased ocean eddy activity in the variability of the overlying atmosphere. In particular, atmospheric blocking and the intensity of the storm track respond more strongly to mean biases and thus have a larger response at longer lead times. Finally, increased ocean resolution drives improvements to subseasonal predictability over Europe. This increase in skill seems to be a result of improvements to the Madden–Julian oscillation and its associated teleconnections rather than changes to air–sea interaction in the North Atlantic region.

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L. Ferranti, F. Molteni, Č Branković, and T. N. Palmer

Abstract

Properties of the general circulation simulated by the ECMWF model are discussed using a set of seasonal integrations at T63 resolution. For each season, over the period of 5 years, 1986–1990, three integrations initiated on consecutive days were run with prescribed observed sea surface temperature (SST).

This paper presents a series of diagnostics of extratropical variability in the model, with particular emphasis on the northern winter. Time-filtered maps of variability indicate that in this season there is insufficient storm track activity penetrating into the Eurasian continent. Related to this the maximum of lower-frequency variance in the Euro-Atlantic region is erroneously shifted eastward in the model. By contrast the simulated fields of both high- and low-frequency variability for northern spring are more realistic.

Blocking is defined objectively in terms of the geostrophic wind at 500 mb. Consistent with the low-frequency transience, in the Euro-Atlantic sector the position of maximum blocking in the model is displaced eastward. The composite structure of blocks over the Pacific is realistic, though their frequency is severely underestimated at all times of year.

Shortcomings in the simulated wintertime general circulation were also revealed by studying the projection of 5-day mean fields onto empirical orthogonal functions (E0Fs) of the observed flow. The largest differences were apparent for statistics of EOFs of the zonal mean flow. Analysis of weather regime activity, defined from the EOFS, suggested that regimes with positive PNA index were overpopulated, while the negative PNA regimes were underpopulated. A further comparison between observed and modeled low-frequency variance revealed that underestimation of low-frequency variability occurs along the same axes that explain most of the spatial structure of the error in the mean field, suggesting a common dynamical origin for these two aspects of the systematic error.

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L. Ferranti, T. N. Palmer, F. Molteni, and E. Klinker

Abstract

An observational and modeling study is made of tropical-extratropical interactions on time scales relevant to medium and extended range forecasting. First, an empirical orthogonal function (EOF) analysis is made of outgoing longwave radiation (OLR) in the tropics over seven winters. Having removed the seasonal cycle and interannual variability, the two leading EOFs describe the 30–60 day oscillation. A composite of extratropical 500 mb geopotential height correlated simultaneously with this mode of tropical variability is constructed. In its two phase-quadrature components, this composite has significant projection onto the Pacific/North American teleconnection pattern and onto the North Atlantic oscillation pattern, respectively.

The 500 mb height composite is compared with the Simmons, Wallace and Branstator (SWB) mode of barotropic instability, which has similar periodicity and similar spatial structure in both its phase-quadrature components. A simple theoretical analysis shows that the SWB mode can be strongly excited by a periodic forcing in the tropics whose spatial structure resembles the oscillation in convective activity described by the first two EOFs of OLR. This is confirmed in a barotropic model integration, which is forced using the observed EOFs of OLR. The model response in the extratropics compares well with the observed composite oscillation in 500 mb height.

In the final phase of this study, the ECMWF model has been integrated over four wintertime 20-day periods. For each period, five integrations have been performed; a control forecast, an integration in which the tropics are relaxed towards the verifying analysis, an integration in which the tropics are relaxed towards the initial analysis, an integration in which the extratropics are relaxed towards the verifying analysis and finally an integration in which the extratropics are relaxed towards the initial analysis. The four initial dates were chosen on the basis that in the succeeding 20 days, observed OLR and extratropical height provided a reasonable realization of each separate quarter of the composite oscillation.

It was found that in the extratropics, skill scores in the range of 11–20 days were noticeably improved, particularly over the Pacific/North American region (consistent with expectations from the data analysis). The mean geopotential height error in the extratropics; i.e., the error averaged over the four experiments, was also reduced (mainly in the Pacific area) when the model tropical fields were relaxed towards the verifying analysis. Indeed, maps showing the time evolution of geopotential height from the first 5 days of the forecast were generally correlated with the differences between the integrations with tropics relaxed to the verifying analysis and to the initial analysis indicating a link between tropical and extratropical low-frequency variability.

The impact of the extratropics on the tropics was also studied where it was shown that the largest response was on the nondivergent component of the wind over the tropical east Pacific. Tropical skill scores and model systematic error in upper tropospheric streamfunction were significantly improved with the extratropics relaxed to the verifying analysis. By contrast, extratropical relaxation had a much smaller impact on the divergent component of the tropical wind.

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T.N. Palmer, C. Brankovic, F. Molteni, S. Tibaldi, L. Ferranti, A. Hollingsworth, U. Cubasch, and E. Klinker

Results from a 3 1/2-yr experimental program of extended-range integrations of the European Centre for Medium-Range Weather Forecasts (ECMWF) numerical weather prediction model are summarized. The topics discussed include

Our results are broadly consistent with those from other major centers evaluating the feasibility of dynamical extended-range prediction. We believe that operational extended-range forecasting using the ECMWF model may be viable to day 20—and possibly beyond—following further research on techniques for Monte Carlo forecasting, and when model systematic error in the tropics has been reduced significantly.

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