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Recent Changes in the Pacific Subtropical Cells Inferred from an Eddy-Resolving Ocean Circulation Model

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  • 1 Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington
  • | 2 Pacific Marine Environmental Laboratory, Seattle, Washington
  • | 3 Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, and Pacific Marine Environmental Laboratory, Seattle, Washington
  • | 4 Naval Research Laboratory, Stennis Space Center, Mississippi
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Abstract

In this study the subtropical cells (STC) in the Pacific Ocean are analyzed using an eddy-resolving ocean general circulation model driven by atmospheric forcing for the years 1992–2003. In particular, the authors seek to identify decadal changes in the STCs in the model and to compare them with observations in order to understand the consequences of such changes for the equatorial ocean heat and mass budgets. The simulation shows a trend toward increasing pycnocline volume transport at 9°N and 9°S across the basin from 1992 to 2003. This increase [4.9 ± 1.0 Sv (Sv ≡ 106 m3 s−1)] is in qualitative agreement with observations and is attributed primarily to changes in the interior ocean transport, which are partially compensated by opposing western boundary transports. The subtropical meridional volume transport convergence anomalies in the model pycnocline are found to be consistent with anomalous volume transports in both the observed and modeled Equatorial Undercurrent, as well as with the magnitude of simulated anomalous upwelling transport at the base of the mixed layer in the eastern Pacific. As a result of the increased circulation intensity, heat transport divergence through the lateral boundaries of the tropical control volume (defined as the region between 9°N and 9°S, and from the surface to σθ = 25.3 isopycnal) increases, leading to a cooling of the tropical upper ocean despite the fact that net surface heat flux into the control volume has increased in the same time. As such, these results suggest that wind-driven changes in ocean transports associated with the subtropical cells play a central role in regulating tropical Pacific climate variability on decadal time scales.

* Joint Institute for the Study of Atmosphere and Ocean Contribution Number 1155

Corresponding author address: Dr. Wei Cheng, Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Campus Box 357941, Seattle, WA 98115. Email: wcheng@ocean.washington.edu

Abstract

In this study the subtropical cells (STC) in the Pacific Ocean are analyzed using an eddy-resolving ocean general circulation model driven by atmospheric forcing for the years 1992–2003. In particular, the authors seek to identify decadal changes in the STCs in the model and to compare them with observations in order to understand the consequences of such changes for the equatorial ocean heat and mass budgets. The simulation shows a trend toward increasing pycnocline volume transport at 9°N and 9°S across the basin from 1992 to 2003. This increase [4.9 ± 1.0 Sv (Sv ≡ 106 m3 s−1)] is in qualitative agreement with observations and is attributed primarily to changes in the interior ocean transport, which are partially compensated by opposing western boundary transports. The subtropical meridional volume transport convergence anomalies in the model pycnocline are found to be consistent with anomalous volume transports in both the observed and modeled Equatorial Undercurrent, as well as with the magnitude of simulated anomalous upwelling transport at the base of the mixed layer in the eastern Pacific. As a result of the increased circulation intensity, heat transport divergence through the lateral boundaries of the tropical control volume (defined as the region between 9°N and 9°S, and from the surface to σθ = 25.3 isopycnal) increases, leading to a cooling of the tropical upper ocean despite the fact that net surface heat flux into the control volume has increased in the same time. As such, these results suggest that wind-driven changes in ocean transports associated with the subtropical cells play a central role in regulating tropical Pacific climate variability on decadal time scales.

* Joint Institute for the Study of Atmosphere and Ocean Contribution Number 1155

Corresponding author address: Dr. Wei Cheng, Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Campus Box 357941, Seattle, WA 98115. Email: wcheng@ocean.washington.edu

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