Teleconnection Mechanisms for Tropical Pacific Descent Anomalies during El Niño*

Abstract

Teleconnection mechanisms in relative descent zones are examined using the quasi-equilibrium tropical circulation model (QTCM). The regions of anomalous descent neighboring the warm sea surface temperature (SST) anomalies in the eastern and central Pacific Ocean during the 1997/98 El Niño are used as an example, and results are verified for three other El Niño warm events (1982, 1987, and 1991). By analyzing the moisture and moist static energy budgets over these regions, it is found that the physical processes responsible for the descent anomalies are different for each region. Anomalous dry advection is the dominant mechanism for the reduced precipitation to the north of the El Niño warm region. On the other hand, the relative descent to the south of the El Niño warm SSTs is caused by reduced surface wind speed and reduced air–sea contrast and resulting weaker than normal evaporation. Budget analysis based on the National Aeronautics and Space Administration Seasonal-to-Interannual Prediction Project atmospheric general circulation model simulations is consistent with the QTCM results. Radiative cooling associated with the warming of tropospheric temperature is relatively small, on the order of a few watts per square meter. The traditional assumption that compensating subsidence is caused by radiative cooling due to increased tropospheric temperature thus does not apply in this context. An experiment to test this by suppressing the radiative cooling due to temperature anomalies showed little impact on the descent anomalies. Thus radiative cooling is not even an initiator of descent anomalies. Instead, relative descent tends to occur in particular regions where other larger cooling or drying tendencies act. Reductions in convective heating and cloud radiative heating behave like a reduction in effective static stability, enhancing the descent anomalies. The complexity of the mechanisms for the descent anomalies may be responsible for the difficulty of descent zone simulation and forecast in general circulation models.