Transient Tropical Diabatic Heating and the Seasonal-Mean Response to ENSO

Abstract

Abstract Boreal winter simulations of the Community Atmospheric Model, version 4.0, were carried out using observed sea surface temperature (SST) fields from the three El Niño events of 1982/83, 1991/92, and 1997/98 [control (CTL) runs] and from observed climatology (CLIM run). In each case, 50 ensemble members were run (1 November–31 March). The diabatic heating Q at every grid point, level, and day of the CTL runs in the Indo-Pacific region was stored and used in four additional suites of experiments, each of which parallels the appropriate CTL suite. In each suite, Q generated by the model is replaced by a specified subset of Q at every time step, grid point, and level spanning the Indo-Pacific. The Q subsets consist of the seasonal ensemble-CTL-mean Q for each ensemble member (suite FIX), the seasonal-mean Q from the appropriate ensemble member of the CTL (suite EFIX), the seasonal mean plus low-frequency component of Q (suite ESUBFIX), and the daily means of Q (suite DAYFIX). The midlatitude ENSO anomalies of the seasonal-mean upper-level height field and time-filtered meridional wind variance are enhanced in the FIX, EFIX, and ESUBFIX suites, with little change in patterns, compared to CTL anomalies. The enhancements have a smaller magnitude in ESUBFIX and especially in DAYFIX; qualitative differences are seen in DAYFIX. These differences are due to (i) the required setup time for midlatitude response, (ii) the altered relationship between vertical structure and vertically integrated heating, and (iii) the lack of midlatitude interactive influence on tropical heating in the experiments.