11.9
Linear atmospheric response to ENSO associated SST meridional differential
Wilbur Y. Chen, NOAA/NWS/NCEP/CPC, Camp Springs, MD
A new balance between the wind field and the underlying sea surface temperature (SST) meridional differential is examined for El Nino versus La Nina boreal winters. Particular attention is placed on the exit region of the Western Pacific jet stream, where sensitive response to a change in temperature differential is found. A near linear atmospheric response to tropical Pacific forcing can be accounted for by a new balance between these two parameters. El Nino events expand eastward the warm SST and deep convection into eastern equatorial Pacific, causing a wide separation in region of diabatic heating between warm and cool phases of the El Nino/Southern Oscillation (ENSO). Atmospheric responses to widely separated diabatic heating are speculated to be also widely separated. Supporting observations and model simulations have been presented for this nonlinear response scenario. Unfortunately, there are also results showing disagreement: a near co-location instead of far apart in the response. Some insights into this unsettled relationship are presented. Most noteworthy is the fact that large ENSO events also result in near linear difference in meridional SST differential at Nino3.4 longitudes, in addition to result in a widely separated regions of deep convection. On long time scales such as the seasonal means, while there is no direct link between tropical diabatic heating and the atmospheric response, a direct link is present between the SST meridional differential and the atmospheric response. Since the meridional temperature differential is near linear in difference between the El Nino and La Nona winters, the atmospheric response is accordingly near linear. The near linear characteristics can be observed in various variables in upper-level tropics, subtropics and extratropics. Simulation studies by a comprehensive atmospheric general circulation model (AGCM) are also conducted. The model results also show strong support to this linear physical process instead of the nonlinear diabatic heating scenario.
Session 11, Observed Seasonal/Interannual Variability (Room 608)
Thursday, 15 January 2004, 11:00 AM-3:00 PM, Room 608
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