GREENHOUSE WARMING, DECADAL VARIABILITY, OR EL-NINO - AN ATTEMPT TO UNDERSTAND THE ANOMALOUS 1990S

The dominant variability modes in the Tropics are investigated and con trasted with the anomalous situation observed during the last few year s. The prime quantity analyzed is anomalous sea surface temperature (S ST) in the region 30 degrees S-60 degrees N. Additionally, observed tr opical surface wind stress fields were investigated. Further tropical atmospheric information was derived from a multidecadal run with an at mospheric general circulation model that was forced by the same SSTs. The tropical SST variability can be characterized by three modes: an i nterannual mode [the El Nino-Southern Oscillation (ENSO)], a decadal m ode, and a trend or unresolved ultralow-frequency variability. The dom inant mode of SST variability is the ENSO mode. It is strongest in the eastern equatorial Pacific, but influences also the SSTs in other reg ions through atmospheric teleconnections, such as the Indian and North Pacific Oceans. The ENSO mode was strong during the 1980s, but it exi sted with very weak amplitude and short period after 1991. The second most energetic mode is characterized by considerable decadal variabili ty. This decadal mode is connected with SST anomalies of the same sign in all three tropical oceans. The tropical Pacific signature of the d ecadal mode resembles closely that observed during the last few years and can be characterized by a horseshoe pattern, with strongest SST an omalies in the western equatorial Pacific, extending to the northeast and southeast into the subtropics. It is distinct from the ENSO mode, since it is not connected with any significant SST anomalies in the ea stern equatorial Pacific, which is the ENSO key region. However, the i mpact of the decadal mode on the tropical climate resembles in many re spects that of ENSO. In particular, the decadal mode is strongly linke d to decadal rainfall fluctuations over northeastern Australia in the observations. It is shown that the anomalous 1990s were dominated by t he decadal mode. Considerable SST variability can be attributed also t o a linear trend or unresolved ultra-low-frequency variability. This t rend that might be related to greenhouse warming is rather strong and positive in the Indian Ocean and western equatorial Pacific where it a ccounts for up to 30% of the total SST variability. Consistent with th e increase of SST in the warm pool region, the trends over the tropica l Pacific derived from both the observations and the model indicate a strengthening of the trade winds. This is inconsistent with the condit ions observed during the 1990s. If the wind trends reflect greenhouse warming, it must be concluded that the anomalous 1990s are not caused by greenhouse warming. Finally, hybrid coupled ocean-atmosphere model experiments were conducted in order to investigate the sensistivity of ENSO to the low-frequency changes induced by the decadal mode and the trend. The results indicate that ENSO is rather sensitive to these ch anges in the background conditions.