Tropical-Subtropical Interactions and Decadal Variability in the Pacific and Atlantic Oceans

Antonio J. Busalacchi, Jurgen Kroger, Rong-Hua Zhang, Yiyong Luo, Joaquim Ballabrera-Poy, Paola Malanotte-Rizzoli, and Lewis Rothstein

(The Earth System Science Interdisciplinary Center)


Pacific Ocean model simulations indicate that decadal variability of temperature along the equator originates from subsurface thermal anomalies in the tropical/subtropical South Pacific. Through western boundary and interior pathways in the thermocline, the subsurface anomalies that are subducted in the eastern tropical/subtropical South Pacific are first transferred westward and then northward, eventually appearing along the equator. The anomalies then propagate eastward along the equator in the Equatorial Undercurrent, and eventually upwell to the surface in the eastern equatorial ocean. These temperature anomalies are accompanied by salinity anomalies so that compensation of temperature and salinity exists on decadal time scales along subduction pathways from the South Pacific to the equator. Furthermore, an active tracer experiment reveals how tropical/subtropical SST anomalies move along this corridor to reach low latitudes in the western Pacific.

The role of such subsurface temperature variability in modulating El Niħo-Southern Oscillation (ENSO) properties is examined using an intermediate coupled model (ICM). An empirical procedure is used to parameterize the temperature of subsurface water entrained into the mixed layer (Te) from sea level (SL) anomalies via a singular value decomposition (SVD) analysis for use in simulating sea surface temperature anomalies (SSTAs). The ocean model is coupled to a statistical atmospheric model that estimates wind stress anomalies also from a SVD analysis. Using the empirical Te models constructed from two sub-periods 1963-79 (Te 63-79) and 1980-96(Te 80-96), the coupled system exhibits strikingly different properties of interannual variability (the oscillation period, spatial structure, and temporal evolution). For the Te 63-79 model, the system features a 2-year oscillation and westward propagation of SSTAs on the equator, while for the Te 80- 96 model, it is characterized by a 5-year oscillation and eastward propagation. These changes in ENSO properties are consistent with the behavior shift of El Niħo observed in the late 1970s. Heat budget analyses further demonstrate a dictating role played by the vertical advection of subsurface temperature anomalies in determining the ENSO properties.

For the Atlantic Ocean, the relative role of extra-equatorial mechanisms modulating decadal sea surface temperature anomalies (SSTA) in the equatorial Atlantic is investigated using a suite of sensitivity experiments based on an ocean general circulation model. The model is forced by observed wind stress and/or computed heat flux from an associated advective atmospheric mixed layer model. In addition, the surface forcing is optionally applied on the equator or in off-equatorial regions. Although the long term response of equatorial SST is dominated by local forcing, a weak but significant part of it is caused by remotely induced variability. Subtropical Cells (STCs) provide the oceanic bridging of the climate signals. The dynamical forcing leads to a spin-up and -down of the shallow cells, which, in the case of local forcing included, coincides with cold and warm SSTA. The local heat flux forcing reveals an overall damping tendency on the dynamical SST response. When excluding the local forcing, the isolation of the effect of the northern remote forcing from the one in the south appears to be essential in understanding the respective mechanisms at work. In the northern hemisphere the spin-up and -down of the STC is highly correlated with the (lagging) SSTA, the effect of off- equatorial heat flux forcing on SSTA is negligible. In the southern hemisphere, on the other hand, both long term heat and momentum fluxes that were induced in the subtropics lead to a significant SST response on the equator.