Abstract
Global temperature anomalies associated with ENSO are investigated, making use of a 13-year record of gridded temperature and precipitation data from the microwave sounding unit (MSU). The warm phase of the ENSO cycle during this period was characterized by an overall warming of the tropical troposphere, superimposed upon a distinctive equatorially symmetric dumbbell-shaped pattern straddling the equator near 140°W, accompanied by negative anomalies along the equator over the western Pacific. By means of singular value decomposition (SVD) analysis it is shown that this pattern fluctuated in phase with the displacements of convective activity over the equatorial Pacific, as reflected in the anomalies in outgoing longwave radiation (OLR) and MSU precipitation fields. Fluctuations in mean tropical tropospheric temperature lagged the OLR anomalies and the related temperature pattern by about 3 months. The same dumbbell-shaped pattern was evident, with reversed polarity, in the lower stratosphere, together with the zonally symmetric signature of the quasi-biennial oscillation.
The dumbbell-shaped temperature pattern is related to the off-equatorial upper-tropospheric gyres that have been identified in previous studies. It can be interpreted as the dynamical response to shifts in the distribution of diabatic heating in the equatorial belt. It resembles the linear response to an equatorial heat source, but its major centers of action are shifted slightly eastward. It is detectable in SVD analysis for each season, but appears to be best organized around March, the season in which the equatorial cold tongue is weakest and precipitation anomalies associated with the ENSO cycle impact the equatorial dry zone most strongly.
The fluctuations in mean tropical tropospheric temperature that occur in association with the ENSO cycle are highly coherent with the fluctuations in surface air temperature over the tropical landmasses and sea surface temperatures over the tropical Indian and North Atlantic Oceans. It is argued that these fluctuations represent a thermodynamic response to the perturbations in the surface heat balance induced by the ENSO cycle in the eastern equatorial Pacific. They can be simulated by means of a simple thermodynamic model with a linear damping, an empirically determined heat capacity, and forcing proportional to the observed sea surface temperature anomalies in the cold tongue region of the equatorial eastern Pacific.
The warming of the tropical troposphere is accompanied by a strengthening of the zonally averaged jet stream in both hemispheres induced by an intensified Hadley circulation.
