Miller, A. J., W. B. White and D. R. Cayan, 1997:
North Pacific thermocline variations on ENSO time scales
Journal of Physical Oceanography, 27, 2023-2039.
Abstract.
The North Pacific 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(0.1C) amplitude at 400m,
are found to
exhibit westward propagation
throughout the extra-tropical
North Pacific 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 fluctuations which
have spatial scale and phase speeds similar to the observations
are also found
in a coarse resolution model of the Pacific Ocean
forced by observed wind and heat flux anomalies over the 1970-88 period.
In the midlatitudes, north of 30N, large-scale Ekman pumping
by interannual wind stress curl variations provides a significant 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 fluctuations.
A tropical, remotely forced
component is prominant near the eastern
boundary, but this only contributes weakly in the model open ocean.
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