Abstract
To what extent is tropical variability forced from the North Pacific through
ocean pathways relative to locally generated variability and variability
forced through the atmosphere? To address this question, in this study we
use an anomaly-coupled model, consisting of a global, atmospheric general
circulation model and a 4.5-layer, reduced-gravity, Pacific-Ocean model.
Three solutions are obtained; with coupling over the entire basin (CNT),
with coupling confined to the Tropics and wind stress and heat fluxes in
the North and South Pacific specified by climatology (TP), and with coupling
confined to the Tropics and wind stress and heat fluxes in the North Pacific
specified by output from CNT (NPF).
It is found that there are two distinct signals forced in the North Pacific
that can impact the Tropics through ocean pathways. These two signals are
forced by wind stress and surface heat flux anomalies in the subtropical
North Pacific. The first signal is relatively fast, impacts tropical
variability less than a year after forcing, is triggered from November
to March, and propagates as a first-mode baroclinic Rossby wave. The
second signal is only triggered during springtime when buoyancy forcing
can effectively generate higher-order baroclinic modes through subduction
anomalies into the permanent thermocline, and it reaches the equator 4-5
years after forcing. The slow signal is found to initiate tropical variability
more efficiently than the fast signal with one standard deviation in
subtropical zonal wind stress forcing tropical SST anomalies centered on
the equator at 135W of approximately 0.5C. Allowing extratropically
forced tropical variability is found to shift primarily 2-year ENSO
variability in a Tropics-alone simulation to a more realistic range of
2-6 years.