Interannual variability in Pacific Ocean circulation and transport during
D. Roemmich, J. Gilson, J. Andueza, R. Bailey, B. Cornuelle, M. Feng,
K. Hanawa, M. McCarthy, M. Morris, K. Ridgway, S. Rintoul, J. Sprintall,
P. Sutton, L. Talley, R. Weller, S. Wijffels
Interannual variability in Pacific Ocean circulation and heat transport
has been observed throughout the 1990s using ongoing High Resolution XBT/XCTD
(HRX) transects and TOPEX/Poseidon altimetric data. The HRX network was
created as part of WOCE to observe variability of the upper ocean, including
boundary current and interior circulation as well as choke point transports
in the Southern Ocean and the Indonesian Throughflow (ITF).
The North Pacific:
The similarity of geostrophic transport in the North Pacific to the North
Pacific Index of SLP demonstrates the dominant role of atmospheric forcing
and a rapid basin-wide oceanic response. Poleward Ekman transport and
thermocline transport across 22°N peaked simultaneously in early 1994
and early 1997. These maxima of the shallow overturning circulation resulted
in corresponding interannual maxima in northward heat transport - about
1 pW compared to a mean of 0.8 pW. The interannual time-series of meridional
heat transport was similar to that of air-sea heat loss north of the 22°N
XBT transect, suggesting a secondary role for heat storage in that region.
The South Pacific and ITF:
In the subtropical South Pacific (31°S), net meridional transport (13.0
Sv northward mean) balances the ITF transport (11.9 Sv westward). As in
the North Pacific, the South Pacific/ITF combination shows maxima in the
shallow overturning cell in 1994 and 1997 (export of the warmest layers
from the tropical Pacific, import of cooler waters). A 15-year time-series
of zonal geostrophic transport in the southern tropics shows strong
and decadal signals.
The tropical Pacific box:
Large exports of heat from the tropical Pacific - about 1.6 pW in early
1994 and 1997 - accompanied maxima in the shallow overturning circulation
(in phase in the two hemispheres). This occurred during the development
of El Nino episodes. The minimum heat export, about 0.8 pW, was in early
1998 during the developing La Nina. While heat storage and air-sea flux
in the tropical Pacific are primary elements of the heat budget, ocean
also plays a significant role in interannual variability.
HRX sampling continues, with enhancements to include improved meteorological
sensors, research-quality 2000-m XBTs and salinity from the Argo float
McCarthy, M., L. Talley and D. Roemmich, 2000. Seasonal to interannual
from expendable bathythermograph and TOPEX/Poseidon altimetric data in the
South Pacific subtropical gyre. Journal of Geophysical Research, 105,
Roemmich, D. and J. Gilson, 2001. Eddy transport of heat and thermocline
waters in the North Pacific: A key to interannual/decadal climate variability.
Journal of Physical Oceanography, 31, 675-687.
Roemmich, D., J. Gilson, B. Cornuelle and R. Weller, 2001. The mean and
time-varying meridional heat transport at the tropical/subtropical boundary
of the North Pacific Ocean. Journal of Geophysical Research, 106, 8957-8970.