A study of the tropical response in an idealised global circulation model.

The atmospheric response to idealised Sea Surface Temperature (SST) distributions is investigated using an atmosphere only version of the UKMO Unified Model (UM), which is modified to have an underlying ocean covered surface. The study is motivated by the observational analysis Ward (1992), who found a Tropic Wide Mode of climate variability linked to variations in tropical SST. The model climate exhibits strong sensitivity to meridional curvature in SST. For sufficiently weak near-equator SST gradients there is no Hadley circulation and tropical precipitation is no longer organised by large-scale dynamics. Localised tropical SST anomalies give precipitation increases centred to the west of them. The pattern of precipitation suppression seen is consistent with a tropically confined wave response. Tropical precipitation suppression remote from the SST anomaly is accompanied by a lowering of the cloud top height and a weakening of the Hadley circulation. Intense equatorial westerlies remote from an SST anomaly result due to stationary eddy momentum flux convergence in the horizontal and vertical. A wavenumber-frequency analysis of model tropical precipitation reveals that the intra-seasonal oscillation is absent from this configuration of the UM. The introduction of zonal non-uniformity due to SST anomalies reduces the magnitude of significant spectral peaks. Decoupling of SST from the near-surface temperature in the UM restricts a simple boundary layer model when used to reproduce the strength of the UM flow. An intermediate GCM based upon quasi-equilibrium shows a sensitivity to SST significantly different from the UM. The simple dynamical formulation of this model effectively removes the constraint of local angular momentum conservation seen in the UM. Idealised GCM experimentation is useful in clearly identifying model differences, testing model parameterisations and their interaction with the dynamics, and may provide a bridge between single column models and full GCMs