ENSO Time Scales

The North Paci c thermocline (250m to 400m) is studied using XBT observations acquired during the 1970s and 1980s. Interannual variations (3-5 year time scales) in thermocline temperature, with O(0.1 C) amplitude at 400m, are found to exhibit westward propagation throughout the extra-tropical North Paci c up to 45N. Southwards of 30N, the features propagate intact across the basin from the eastern boundary to the western boundary. Northwards of 30N, the features can be observed to propagate only as far as the dateline. The observed midlatitude thermocline anomalies are often related to tropical ENSO events in that they occur most strongly after the development of tropical El Nino or La Nina conditions and propagate westward from near the eastern boundary in the midlatitudes. But it is found that the observed midlatitude thermocline anomalies have larger phase speeds than theoretically predicted free baroclinic Rossby waves. Also, the observed anomalies have larger wavelength and faster propagation speeds than baroclinic Rossby waves that radiate from coastal Kelvin-like waves near the eastern boundary in well-known high-resolution models. Large-scale thermocline uctuations which have spatial scale and phase speeds similar to the observations are also found in a coarse resolution model of the Paci c Ocean forced by observed wind and heat ux anomalies over the 1970-88 period. In the midlatitudes, north of 30N, large-scale Ekman pumping by interannual wind stress curl variations provides a signi cant driving mechanism for the modeled large-scale thermocline anomalies. The modeled ocean response is a combination of the static thermocline response to large-scale Ekman pumping plus a train of westward traveling Rossby waves which accounts for part of the propagating temperature uctuations. A tropical, remotely forced component is prominant near the eastern boundary, but this only contributes weakly in the model open ocean. 2 North Paci c Thermocline Variations on ENSO Time Scales