Interannual variability in Pacific Ocean circulation and transport during the 1990s.

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

Introduction

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 equatorward 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 quasi-biennial 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 transport 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 project.

Related Work:

McCarthy, M., L. Talley and D. Roemmich, 2000. Seasonal to interannual variability from expendable bathythermograph and TOPEX/Poseidon altimetric data in the South Pacific subtropical gyre. Journal of Geophysical Research, 105, 19535-19550.

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.