Internal variability in the tropical oceans: Noise or stochastic forcing?

Raghu Murtugudde



The concept of internal variability is quite well-established in the atmosphere and is known to be important for weather and climate prediction. The oceanic internal variability has been studied extensively in the midlatitudes with respect to the meanders of the western boundary currents such as the Gulf Stream and the Kuroshio. The tropics have typically been thought of as fairly linear even with respect to ENSO dynamics. In a recent series of studies we demonstrate that the internal variability in the deep tropical oceans is quite significant and generates SST anomalies that are comparable to forced interannual variability with decorrelation scales that are easily capable of affecting the atmospheric response. The role of the internal variability in the tropical Atlantic can potentially affect the ITCZ migrations especially in the boreal spring months leading to oceanic generation of interannual climate variability. In the Pacific, the impact of the internal variability as manifested in the mesoscale activity associated with the tropical instability waves can result in SST anomalies and east-west SST gradient anomalies that are as large as 50% of those associated with most ENSO events. The Indian Ocean has high mean SSTs and the SST variability due to mesoscale internal variability is seen to be as large as any observed interannual variability. The role of the internal variability in the oceans and their potential impact on the coupled climate variability will be discussed with forced OGCM and coupled models.