Atmospheric Sciences V
Presiding: C Jones, Univ of California, Santa Barbara; M Kanamitsu, Univ of California San Diego
A24A-01 15:30h
Charney Lecture: Interpretation of recent climate change
The observed climate change of the past few decades is characterized not only by increases in global mean temperatures, but also by substantial changes in the structure of the atmospheric circulation. In this overview talk, I will review the observed changes in the atmospheric flow, discuss the implications of these changes for regional weather, and outline a series of possible causal mechanisms. In both hemispheres, recent changes in the extratropical circulation are dominated by trends in the extratropical zonal flow that extend from the surface of the Earth to the middle stratosphere. In the Northern Hemisphere, the circulation trends are largest in winter, and account for a substantial fraction of recent changes in such widely varying fields as Arctic sea-ice, precipitation over Europe, and temperatures throughout North America and northern Eurasia. The Northern Hemisphere trends were substantial through the late 1990s, but have relaxed in recent years. In the Southern Hemisphere, the circulation trends peak during the summer months, and have contributed to much of the observed cooling of eastern Antarctica and a portion of the warming of the Antarctic Peninsula during that season. It is argued that a substantial fraction of the Southern Hemisphere trends can be traced to anthropogenic emissions of stratospheric ozone-depleting gases. Northern Hemisphere stratospheric ozone depletion is likely too small to account for the trends there, and it remains unclear how increasing greenhouse gases will impact the extratropical circulation in either hemisphere. The recent trends in the extratropical circulation are juxtaposed against an unanticipated cooling maximum in the tropical stratosphere, where temperatures have decreased by several K over the past few decades. The tropical stratospheric cooling maximum is readily apparent at virtually all radiosonde stations, but is not expected based on numerical simulations of recent climate change.
A24A-02 16:30h
The Role of Sea Surface Temperature in Reanalysis
Aiming at understanding the role of SST in the Reanalysis for the pre-radiosonde, pre-satellite and satellite era, a number of observation system experiments were performed using the NCEP/DOE reanalysis system. Five pairs of experiments were performed using observed and climatological SSTs for cases 1) without any observation, 2) surface pressure observation only with the observation density of 1915, 3) surface pressure observation with the observation density of 1997, 4) surface observation and radiosondes and 5) all observations, including satellite retrievals. The analyses were run for 4 months in 1979 (strong El Nino) and 1993 (near normal SST). The impact of SST and the change in the observation system on the analysis of near surface parameters, upper level field and some diagnostic fields were compared against the control analysis with observed SST and all available observations. The most important finding of this study is that the impact of SST varies with the time scale of the analysis, which is most apparent in surface pressure only observation experiments. The impact of SST is largest for the low frequency (seasonal) analysis and smaller for the high frequency (daily) analyses. This is particularly apparent for near surface temperature and upper level height field analyses. In our extreme case of the strong El Nino year, the simulation with observed SST without any observations (AMIP type run) produced seasonal mean 2-meter temperature and 500 hPa height fields, agreeing better with the control analysis than the analysis with surface pressure observation only with climatological SST. On the contrary, the impact of surface pressure observation shows up more on the higher frequency analyses, and less on the low frequency analyses. Generally speaking, accurate analysis of SST is important when very little atmospheric observation is available. But even for the full atmospheric observation system, climatological SST produces inferior analysis not only over ocean but also over land. The introduction of radiosonde data to the system drastically reduces errors in all the analyses, and thus, radiosonde data are indispensable for accurate estimation of the atmospheric state both in the short and long time scales.
A24A-03 16:45h
Decadal Variations in the Madden-Julian Oscillation
The Madden-Julian Oscillation (MJO) is the main mode of tropical intraseasonal variability. The MJO strongly influences the precipitation patterns associated with the monsoons in Asia, Australia, North America and South America. This influence has been shown to modulate rainfall variability and extreme events in the Americas as well. Some studies have additionally indicated that interaction of the MJO with extra-tropical regions can influence weather forecasts on medium and extended ranges. This study presents an observational analysis to characterize decadal changes in the MJO. It is shown that the MJO exhibits significant decadal chages alternating in regimes of high and low activity. These regimes seem to alternate in cycles of about 6 yrs and 14 yrs.