Outstanding Issues in Seasonal to Interannual Climate Prediction I
Presiding: G P Compo, NOAA-CIRES Climate Diagnostics Center; A W Robertson, International Research Institute for Climate Prediction, Columbia University
A33B-01 13:30h
Circumglobal Teleconnection in the Northern Hemisphere Summer
Analysis of 56-year NCEP/NCAR reanalysis data reveals a recurrent circumglobal teleconnection (CGT) pattern in the summertime midlatitude circulation of the Northern Hemisphere. This pattern represents the second leading empirical orthogonal function of interannual variability of the upper tropospheric circulation. The CGT, having a zonal wavenumber five structure, is primarily positioned within a waveguide associated with the westerly jetstream. The spatial phases of CGT tend to lock to preferred longitudes. The geographically phase-locked patterns bear close similarity during June, August and September, but the pattern in July shows shorter wavelengths in the North Pacific to North America sector. The CGT is accompanied by significant rainfall and surface air temperature anomalies in the continental regions of West Europe, European Russia, India, East Asia and North America. This implies that the CGT may be a source of climate variability and predictability in the above midlatitude regions. The CGT has significant correlations with the Indian summer monsoon (ISM) and El Nino-Southern Oscillation (ENSO). However, in normal ISM years the CGT-ENSO correlation disappears; on the other hand, in the absence of El Nino or La Nina, the CGT-ISM correlation remains significant. It is suggested that the ISM acts as a conductor ± connecting the CGT and ENSO. When the interaction between the ISM and ENSO is active, ENSO may influence northern China via the ISM and the CGT. Additionally, the variability of the CGT has no significant association with the Arctic Oscillation and the variability of the Western North Pacific summer monsoon. The circulation of the wavetrain shows a barotropic structure everywhere except the cell located to the northwest of India where baroclinic circulation structure dominates. Two possible scenarios are proposed. The abnormal ISM may excite anomalous west-central Asian High and downstream Rossby wavetrain extending to North Pacific and North America. On the other hand, a wavetrain excited in the jet exit region of the North Atlantic may affect west-central Asian High and thus the intensity of the ISM. It is hypothesized that the interaction between the global wavetrain and the ISM heat source may be instrumental in maintaining the boreal summer CGT.
A33B-02 13:45h
Prediction of Tropical Atlantic SST Using a Markov Model Trained Upon NCEP's Global Ocean Data Assimilation System
Both statistical and numerical models have been used to predict the Tropical Atlantic SST. However, the forecast skill of the Tropical Atlantic SST is quite poor, partially due to weak signal and complexity of the underling physical processes. Diagnostic studies on Tropical Atlantic Variability largely relate atmospheric circulation patterns and precipitation anomalies with SST anomalies in the Tropical Atlantic. The multivariate EOF analysis by Ruiz-Barradas et al. (2000) suggests that the Tropical Atlantic SST is not only associated to precipitation and surface wind stress but also to subsurface ocean temperature. We will use the ocean reanalysis for 1997-2004 produced with the NCEP's global ocean data assimilation system (GODAS) to search for coupled modes of atmosphere-ocean interaction in the tropical Atlantic and to study impacts from remote forcings such as ENSO and NAO. A prediction system will be developed with the coupled modes using the Markov model approach (Xue et al. 2000). The hindcast skill will be cross-validated and compared with that of the NCEP's new Climate Forecast System (CFS). The goal is to extract oceanic predictors within GODAS and to use them to improve the prediction skill of SST and precipitation in the tropical Atlantic where the forecast skill of numerical models is still poor.
A33B-03 14:00h
Interannual Climate Variability in the South Atlantic: Linking Tropics and Subtropics.
Climate variability in the tropical Atlantic is of capital importance for climate in the adjacent semi-arid regions of North Eastern Brazil, Sahel and Southern Africa. Standard multivariate analyses have isolated the dominant patterns of variability in the region, related or not to ENSO, but most of the studies have considered the tropics and the sub-extratropics separately. Moreover, most of the studies focused on the interannual and decadal time scales, the latter explaining a large part of the total variability in the Atlantic. However, despite the differences between the regions, climate variability in the South Atlantic is linked to the variability of the intensity and location of the subtropical High. This work focuses on the dominant modes of variability common to the tropical and subtropical parts of the South Atlantic and their specific time scales. Special focus is put on interannual time scales and variability not related to ENSO. The dominant modes are reproduced by the UCLA AGCM coupled to a simple, constant depth, oceanic mixed layer in the Atlantic. In both, the observations and the model, the interannual variability in the South Atlantic is dominated by two separate times scales. The longer, ca. 5 years, seems to arise from interhemispheric interactions in the Atlantic while the shorter, quasi-biennial, is characteristic of the South Atlantic itself. Its main feature is an apparent anticlockwise propagation of the anomalies within the South Atlantic basin, from the subtropics in boreal spring to the tropical and equatorial regions in boreal summer. It provides a useful link between the modes of variability identified separately in the tropics and the subtropics and in different seasons. The agreement between simulated and observed spatio-temporal characteristics provides a potential for short term seasonal predictability of the South Atlantic SST. Such predictability, based on air-sea thermodynamical interactions, links between tropics and subtropics and recognition of the different time scales is further explored in this work.
A33B-04 14:15h
Land and SST Feedback on Seasonal Climate Variability in the COLA AGCM
From a number of seasonal integrations of the COLA AGCM using observed SST, the signal and noise components will be diagnosed. A similar computation will be made from seasonal integrations conducted with the same model but forced with climatological SST. The difference between these two integrations, in effect will provide us the role of varying SST on seasonal climate variability exhibited by the COLA AGCM. Furthermore, the variability in the seasonal integrations forced by climatogical SST is forced from land-atmosphere integrations and or the internal dyanmics of the model. Active regions of atmospheric variability forced by land, ocean and both the components will be identified from these experiments. In effect, this study will show areas of potential predictability of the COLA AGCM that is forced from land and ocean components of the climate system.
A33B-05 14:30h
An Upgraded FSU Global/Regional Climate Model System and its use in Crop Model Forecasting
The current Florida State University (FSU) global and regional climate models are upgraded by coupling the National Center for Atmospheric Research community land model (NCAR CLM2) as its land component in order to make a better simulation of land surface variables on the seasonal time scale which is important for crop model application. Climatological and seasonal simulations with the FSU climate model coupled to the CLM2 (hereafter, FSUCLM) are compared to those of the control (the FSU model with he original simple land surface treatment). The current version of the FSU model is known to have a cold bias in the temperature field and a wet bias in precipitation. The implementation of FSUCLM has reduced or eliminated this bias due to reduced latent heat flux and increased sensible heat flux. The role of the land model in seasonal simulations is shown to be more important during summer time than winter time. An additional experiment that assimilates atmospheric forcings produces improved land model initial conditions, which in turn reduces the biases further. The regional model is placed over the Southeast United States and run at 20 km resolution, roughly resolving the county level. Outputs from the models (max/min surface temperature, precipitation, and shortwave radiation at the surface) are used as inputs into the crop model to determine the crop yields. The inclusion of the CLM2 helps produce better crop yield forecasts.
A33B-06 14:45h
Real Time Seasonal Forecasts at FSU and its Application to Crop Models
An experimental real time seasonal forecast has been issued by FSU for the past two Southeast U.S. growing seasons (March-August 2004 and October-March 2004-05). These forecasts are conducted using both the FSU global spectral model (T63L17) and the FSU regional spectral model (~20km resolution). Eight to ten member ensembles are generated for each growing season. The ensembles are developed using the time lagged initial condition approach. Results, along with comparisons to climatology, will be presented from the October-March 2004-2005 forecast period for 13 selected locations in Florida, Georgia and Alabama. Application to the modeling of the crop yields will be discussed.