Atmospheric General Circulation Model Integrations Research Articles

Optimal Tropical Sea Surface Temperature Forcing of North American Drought

Abstract The optimal anomalous sea surface temperature (SST) pattern for forcing North American drought is identified through atmospheric general circulation model integrations in which the response of the Palmer drought severity index (PDSI) is determined for each of 43 prescribed localized SST anomaly “patches” in a regular array over the tropical oceans. The robustness and relevance of the optimal pattern are established through the consistency of results obtained using two different models, and also by the good correspondence of the projection time series of historical tropical SST anomaly fields on the optimal pattern with the time series of the simulated PDSI in separate model integrations with prescribed time-varying observed global SST fields for 1920–2005. It is noteworthy that this optimal drought forcing pattern differs markedly in the Pacific Ocean from the dominant SST pattern associated with El Niño–Southern Oscillation (ENSO), and also shows a large sensitivity of North American drought to Indian and Atlantic Ocean SSTs.

Anomalous Rainfall over Southwest Western Australia Forced by Indian Ocean Sea Surface Temperatures

Abstract The potential impact of Indian Ocean sea surface temperature (SST) anomalies in modulating midlatitude precipitation across southern and western regions of Australia is assessed in a series of atmospheric general circulation model (AGCM) simulations. Two sets of AGCM integrations forced with a seasonally evolving characteristic dipole pattern in Indian Ocean SST consistent with observed “dry year” (PDRY) and “wet year” (PWET) signatures are shown to induce precipitation changes across western regions of Australia. Over Western Australia, a significant shift occurs in the winter and annual rainfall frequency with the distribution becoming skewed toward less (more) rainfall for the PDRY (PWET) SST pattern. For southwest Western Australia (SWWA), this shift primarily is due to the large-scale stable precipitation. Convective precipitation actually increases in the PDRY case over SWWA forced by local positive SST anomalies. A mechanism for the large-scale rainfall shifts is proposed, by which the SST anomalies induce a reorganization of the large-scale atmospheric circulation across the Indian Ocean basin. Thickness (1000–500 hPa) anomalies develop in the atmosphere mirroring the sign and position of the underlying SST anomalies. This leads to a weakening (strengthening) of the meridional thickness gradient and the subtropical jet during the austral winter in PDRY (PWET). The subsequent easterly offshore (westerly onshore) anomaly in the thermal wind over southern regions of Australia, along with a decrease (increase) in baroclinicity, results in the lower (higher) levels of large-scale stable precipitation. Variations in the vertical thermal structure of the atmosphere overlying the SST anomalies favor localized increased convective activity in PDRY because of differential temperature lapse rates. In contrast, enhanced widespread ascent of moist air masses associated with frontal movement in PWET accounts for a significant increase in rainfall in that ensemble set.

Storm Track Predictability on Seasonal and Decadal Scales

This paper is concerned with estimating the predictable variation of extratropical daily weather statistics (''storm tracks'') associated with global sea surface temperature (SST) changes on interannual to interdecadal scales, and its magnitude relative to the unpredictable noise. The SST-forced storm track signal in each northern winter in 1950-99 is estimated as the mean storm track anomaly in an ensemble of atmospheric general circulation model (AGCM) integrations for that winter with prescribed observed SSTs. Two sets of ensembles available from two modeling centers, with anomalous SSTs prescribed either globally or only in the Tropics, are used. Since the storm track signals cannot be derived directly from the archived monthly AGCM output, they are diagnosed from the SST-forced winter-mean 200-mb height signals using an empirical linear storm track model (STM). For two particular winters, the El Nino of January-February-March (JFM) 1987 and the La Nina of JFM 1989, the storm track signals and noise are estimated directly, and more accurately, from additional large ensembles of AGCM integrations. The linear STM is remarkably successful at capturing the AGCM's storm track signal in these two winters, and is thus also suitable for estimating the signal in other winters. The principal conclusions from this analysis are as follows. A predictable SST-forced storm track signal exists in many winters, but its strength and pattern can change substantially from winter to winter. The correlation of the SST-forced and observed storm track anomalies is high enough in the Pacific-North America (PNA) sector to be of practical use. Most of the SST-forced signal is associated with tropical Pacific SST forcing; the central Pacific (Nino-4) is somewhat more important than the eastern Pacific (Nino-3) in this regard. Variations of the pattern correlation of the SST-forced and observed storm track anomaly fields from winter to winter, and among five-winter averages, are generally consistent with variations of the signal strength, and to that extent are identifiable a priori. Larger pattern correlations for the five-winter averages found in the second half of the 50- yr record are consistent with the stronger El Nino SST forcing in the second half.None of these conclusions, however, apply in the Euro-Atlantic sector,where the correlations of the SST-forced and observed storm track anomalies are found to be much smaller. Given also that they are inconsistent with the estimated signal-to-noise ratios in this region, substantial AGCM error in representing the regional response to tropical SST forcing, rather than intrinsically lower Euro-Atlantic storm track predictability, is argued to be behind these lower correlations.

Modelling low frequency variability in Southern Hemisphere extra‐tropical cyclone characteristics and its sensitivity to sea‐surface temperature

The outflow regions of the western boundary currents of the Southern Hemisphere and their extensions into the South Indian Ocean, South Pacific and South Atlantic Currents are prominent areas of low frequency variability in sea-surface temperature (SST). One mechanism of decadal/multidecadal variability involves modulation of surface heat flux exchange and the track and intensity of extra-tropical cyclones and associated frontal systems in response to a zonal mid-latitude SST anomaly. Here, the influence of this type of SST pattern on cyclone properties is investigated by applying a vortex tracking scheme to atmospheric general circulation model integrations. In order to assess the subdecadal to multidecadal scale response, these integrations were conducted for a 21-year period with forcing consisting of a warm SST anomaly stretching across the Southern Hemisphere mid-latitudes. In response to this forcing, a large-scale trough at mid-latitudes and ridging at higher latitudes are generated in the model. Associated with this change in Southern Hemisphere baroclinicity are increases in the density and flux of cyclonic systems in the 40°–55°S zone but decreases poleward of this latitude belt. It is found that the model response weakens throughout the Southern Hemisphere mid- to high-latitudes in the second decade compared to the first but that there is less evidence of a hemispheric weakening in the second quinquennium relative to the first. Implications of these results for mechanisms of decadal/multidecadal climate variability are discussed. Copyright © 2001 Royal Meteorological Society.

Modelling low frequency variability in Southern Hemisphere extra‐tropical cyclone characteristics and its sensitivity to sea‐surface temperature

AbstractThe outflow regions of the western boundary currents of the Southern Hemisphere and their extensions into the South Indian Ocean, South Pacific and South Atlantic Currents are prominent areas of low frequency variability in sea‐surface temperature (SST). One mechanism of decadal/multidecadal variability involves modulation of surface heat flux exchange and the track and intensity of extra‐tropical cyclones and associated frontal systems in response to a zonal mid‐latitude SST anomaly. Here, the influence of this type of SST pattern on cyclone properties is investigated by applying a vortex tracking scheme to atmospheric general circulation model integrations. In order to assess the subdecadal to multidecadal scale response, these integrations were conducted for a 21‐year period with forcing consisting of a warm SST anomaly stretching across the Southern Hemisphere mid‐latitudes. In response to this forcing, a large‐scale trough at mid‐latitudes and ridging at higher latitudes are generated in the model. Associated with this change in Southern Hemisphere baroclinicity are increases in the density and flux of cyclonic systems in the 40°–55°S zone but decreases poleward of this latitude belt. It is found that the model response weakens throughout the Southern Hemisphere mid‐ to high‐latitudes in the second decade compared to the first but that there is less evidence of a hemispheric weakening in the second quinquennium relative to the first. Implications of these results for mechanisms of decadal/multidecadal climate variability are discussed. Copyright © 2001 Royal Meteorological Society.

Sensitivity of Atlantic Tropical Storm Frequency to ENSO and Interdecadal Variability of SSTs in an Ensemble of AGCM Integrations

A significant reduction (increase) of tropical storm activity over the Atlantic basin is observed during El Niño (La Niña) events. Furthermore, the number of Atlantic tropical storms displays an interdecadal variability with more storms in the 1950s and 1960s than in the 1970s and 1980s. Ensembles of simulations with an atmospheric general circulation model (AGCM) are used to explore the mechanisms responsible for this observed variability. The interannual variability is investigated using a 10-member ensemble of AGCM simulations forced by climatological SSTs of the 1980s everywhere except over the tropical Pacific and Indian Oceans. Significantly fewer tropical storms are simulated with El Niño SSTs imposed over the tropical Pacific and Indian Oceans than with La Niña conditions. Increased simulated vertical wind shear over the Atlantic is the most likely explanation for the reduction of simulated tropical storms during El Niño years. SST forcing from different El Niño events has distinct impacts on Atlantic tropical storms in the simulation: simulated tropical storms are significantly less numerous with 1982 SSTs imposed over the tropical Pacific and Indian Oceans than with 1986 SSTs. The interdecadal variability of tropical storm activity seems to coincide with an interdecadal variability of the North Atlantic SSTs with colder SSTs in the 1970s than in the 1950s. Ensembles of AGCM simulations produce significantly more tropical storms when forced by observed SSTs of the 1950s than when forced by SSTs of the 1970s. This supports the theory that the interdecadal variability of SSTs has a significant impact on the expected number of Atlantic tropical storms and suggests that Atlantic tropical storms may be more numerous in coming years if North Atlantic SSTs are getting warmer. A significant increase of vertical wind shear and a significant decrease in the convective available potential energy over the tropical Atlantic in the 1970s may explain the simulated interdecadal variability of Atlantic tropical storms.

Interaction between Tropical Atlantic Variability and El Niño–Southern Oscillation

The interaction between tropical Atlantic variability and El Nino-Southern Oscillation (ENSO) is investigated using three ensembles of atmospheric general circulation model integrations. The integrations are forced by specifying observed sea surface temperature (SST) variability over a forcing domain. The forcing domain is the global ocean for the first ensemble, limited to the tropical ocean for the second ensemble, and further limited to the tropical Atlantic region for the third ensemble. The ensemble integrations show that extratropical SST anomalies have little impact on tropical variability, but the effect of ENSO is pervasive in the Tropics. Consistent with previous studies, the most significant influence of ENSO is found during the boreal spring season and is associated with an anomalous Walker circulation. Two important aspects of ENSO's influence on tropical Atlantic variability are noted. First, the ENSO signal contributes significantly to the ''dipole'' correlation structure between tropical Atlantic SST and rainfall in the Nordeste Brazil region. In the absence of the ENSO signal, the correlations are dominated by SST variability in the southern tropical Atlantic, resulting in less of a dipole structure. Second, the remote influence of ENSO also contributes to positive correlations between SST anomalies and downward surface heat flux in the tropical Atlantic during the boreal spring season. However, even when ENSO forcing is absent, the model integrations provide evidence for a positive surface heat flux feedback in the deep Tropics, which is analyzed in a companion study by Chang et al. The analysis of model simulations shows that interannual atmospheric variability in the tropical Pacific-Atlantic system is dominated by the interaction between two distinct sources of tropical heating: (i) an equatorial heat source in the eastern Pacific associated with ENSO and (ii) an off-equatorial heat source associated with SST anomalies near the Caribbean. Modeling this Caribbean heat source accurately could be very important for seasonal forecasting in the Central American-Caribbean region.

Seasonal Prediction over North America with a Regional Model Nested in a Global Model

Ensembles of winter and summer seasonal hindcasts have been carried out with an 80-km resolution version of the National Centers for Environmental Prediction, Environmental Modeling Center Eta model over the North American region. The lateral boundary conditions for the Eta model are prescribed from Center for Ocean–Land–Atmosphere R40 atmospheric general circulation model integrations, which used observed atmospheric initial conditions and observed global sea surface temperature (SST). An examination of 15 seasonal winter hindcasts and 15 seasonal summer hindcasts shows that the nested model reduces the systematic errors in seasonal precipitation compared to the global model alone. The physical parameterizations, enhanced resolution, and better representation of orography in the Eta model produces better simulations of precipitation, and in some cases, its interannual variability. In particular, the precipitation difference between the 1988 drought and 1993 flood over the United States was much better simulated by the nested model. The predictions of circulation features were generally as good or better than those from the global model alone. Estimates of external (SST forced “signal”) and internal (dynamics generated “noise”) variability were made for both the global model and the nested model predictions. Contrary to the expectation that a higher-resolution model would have higher internal-dynamics-generated variability, the signal, noise, and signal-to-noise ratios of the near-surface temperature and precipitation fields were generally quite similar between the nested model and the global model predictions. In the winter season the nested model had larger signal-to-noise ratios in both temperature and precipitation than did the global model alone.

Stochastic variability at the air‐sea interface on decadal timescales

Multiple integrations of an atmospheric general circulation model (AGCM) exhibit differences in the decadal mean of the surface heat flux, Q. The differences (typically 2–3 Wm−2) can be as large as 10 Wm−2 in places, which are shown to produce sea‐surface temperature (SST) changes up to approximately 0.5°C in a hybrid coupled atmosphere/ocean/sea‐ice model or HCM. This underscores a significant mechanism by which long‐lived SST anomalies can arise, independent of any internal ocean variability, for which there is very little predictive capability. Consequently, AGCM integrations using prescribed SSTs provide upper bounds on the predictability of atmospheric variability. The HCM is used to estimate the magnitude of the drift in the climatology of coupled models if flux adjustments are based on AGCM integrations of decadal duration.The random interannual fluctuations in Q averaged over the ocean are most closely associated with fluctuations in surface latent heating and long wave radiation to space. The same fluctuations are not associated with land surface variability.The impact of variability in other fluxes on SST in the HCM was also analysed. Changes due to wind‐stress were approximately half those due to heating, while changes due to freshwater forcing were relatively unimportant except at polar latitudes.