Abstract:
The onset of cumulus convection in a grid column is a catastrophe, a.k.a. a subcritical instability. Accordingly, in designing a cumulus parameterization scheme the onset of cumulus convection requires that a parameter crosses a critical value and the termination requires that the same or a different parameter crosses a different critical value. Once started, cumulus convection stays on, regardless if the onset criterion is still met, until the termination criterion is met. Also, the intensity of cumulus precipitation is related to how far the state is from the termination, not the onset, criterion. In contrast, the cumulus parameterization schemes currently in use treat the onset of cumulus convection as a supercritical instability. Namely, convection is on when a parameter exceeds a critical value and is off when the same parameter falls below the same critical value. Also, the intensity of cumulus precipitation is related to how much this critical value has been exceeded. Among the adverse consequences of the supercritical-instability-concept-based cumulus parameterization schemes are that over relatively flat land the precipitation peak occurs around noon-4~6 hours too soon-and that the amplitude of the precipitation diurnal cycle is too weak. Based on the above concept, a new cumulus parameterization scheme has been designed by taking advantage of the existing infrastructure in the relaxed Arakawa-Schubert scheme (RAS), but replacing RAS's guiding principle with the catastrophe concept. Test results using NASA's GEOS-5 GCM show dramatic improvement in the phase and amplitude of the precipitation diurnal cycle over relatively-flat land.

Abstract:
El Nino events, the warm phase of the El Nino/Southern Oscillation (ENSO), are known to affect other tropical ocean basins through teleconnections. Conversely, mounting evidence suggests that temperature variability in the Atlantic Ocean may also influence ENSO variability. Here we use reanalysis data and general circulation models to show that sea surface temperature anomalies in the north tropical Atlantic during the boreal spring can serve as a trigger for ENSO events. We identify a subtropical teleconnection in which spring warming in the north tropical Atlantic can induce a low-level cyclonic atmospheric flow over the eastern Pacific Ocean that in turn produces a low-level anticyclonic flow over the western Pacific during the following months. This flow generates easterly winds over the western equatorial Pacific that cool the equatorial Pacific and may trigger a La Nina event the following winter. In addition, El Nino events led by cold anomalies in the north tropical Atlantic tend to be warm-pool El Nino events, with a centre of action located in the central Pacific, rather than canonical El Nino events. We suggest that the identification of temperature anomalies in the north tropical Atlantic could help to forecast the development of different types of El Nino event.

Abstract: