10B.3
The role of tropospheric temperature in the simulated remote tropical surface temperature and precipitation response to the 97-98 El Niņo
John C.H. Chiang, University of California, Berkeley, Berkeley, CA; and B. R. Lintner
Motivated by a previous single column model (SCM) study (Chiang and Sobel, 2002) for the potential impact of ENSO-forced tropospheric temperature anomalies on the tropical climate (hereafter referred to as the TT mechanism), we explore the simulated impact of the 97-98 El Niņo on tropical surface temperature and precipitation fields outside the Pacific(hereafter the remote tropics). Using an ensemble of AGCM simulations with imposed 97-98 SSTs in the tropical Pacific and a uniform fixed-depth slab ocean elsewhere, we find that the large-scale surface temperature response over the remote tropical land and ocean regions is generally consistent with the TT mechanism prediction that the surface temperature variability is tied to the overlying tropospheric temperature variability in regions of moist convection. Moreover, the amplitude of the precipitation response over remote ocean regions is found to depend on the ocean mixed layer depth, also in agreement with the TT mechanism. Identical simulations with an intermediate model (the Quasi-equilibrium Tropical Circulation Model or QTCM) for the tropical atmosphere corraborates these findings.
The composition of surface fluxes leading to the remote surface warming is nontrivial as well as region-dependent. In general, wind speed and shortwave variability induced by cloud changes--mechanisms previously identified as being important for the remote surface temperature response to ENSO--are first order effects, but so are the changes in net surface longwave radiation and humidity in the atmospheric boundary layer. The latter two influences have yet to be emphasized in the literature. We argue, however, that the more fundamental aspect of the ENSO-related remote surface temperature warming over convective regions is that it is required in order for boundary layer moist static energy changes to match the El Niņo-induced tropospheric warming.
The precipitation response to the 97-98 El Niņo forcing is more complex than the surface temperature response. Although many regions of the remote tropics experience drying in response to the warm event SSTs, the spatial distribution of precipitation anomalies is quite nonuniform. Analysis of the precipitation anomalies in the QTCM suggests that the response is caused by a number of mechanisms, including changes in convergence and moisture transport. As with the surface temperature anomalies, there are sizeable regional differences in the terms contributing to the precipitation response.
In general, our results imply a significant role for ENSO-induced tropospheric temperature anomalies in impacting the climate of the remote tropics. However, we also find that a substantial fraction of the precipitation response--particularly over the equatorial South America/Atlantic region--cannot be directly explained by the TT mechanism; thus the operation of additional mechanisms is implicated. The nature of these additional mechanisms is currently under investigation.
Session 10B, Interannual variability I (Note new session start time from the original published program)
Wednesday, 5 May 2004, 3:45 PM-5:00 PM, Napoleon I Room
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