U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications

The upper ocean heat balance in the western equatorial Pacific warm pool during September-December 1992

Meghan F. Cronin and Michael J. McPhaden

Pacific Marine Environmental Laboratory, NOAA, Seattle, Washington

Journal of Geophysical Research, 102(C4), 8533-8553 (1997)
This paper is not subject to U.S. copyright. Published in 1997 by the American Geophysical Union.

6. Comparison to a One-dimensional Mixed Layer Model Simulation

SST in the warm pool is relatively homogenous in comparison to other regions of the equatorial Pacific, and therefore traditionally it has been assumed that one-dimensional models should be able to simulate temperature fairly well in this region. Our analysis of the thermal response at 0°, 156°E, however, shows that heat advection can be at times a dominant heating mechanism, as, for example, prior to and during the early stage of the October 1992 wind burst and possibly in early December. Furthermore, since the heat storage and entrainment cooling rates depend upon the layer depth, the upper layer heat balance is affected by three-dimensional processes such as Ekman convergence and divergence that result in variability in the pycnocline depth. To further investigate the effects of these three-dimensional processes, the observations will be compared to a one-dimensional model simulation.

The Price-Weller-Pinkel ("PWP") [Price et al., 1986] mixed layer model will be used for the one-dimensional simulation. The PWP model is initialized with CTD and ADCP data at the mooring site from September 15, 1995. The temperature, salinity, and momentum within the top vertical level (z = 20 cm) are then integrated forward one time step (1 hour) by absorbing the observed shortwave radiation using the Siegel double exponential profile (3) and by applying the observed longwave radiation and turbulent heat flux, evaporation minus precipitation, and wind stress as surface boundary conditions for the one-dimensional heat, salinity, and momentum equations. Additionally, a linear drag function (-Cu) with an e-folding timescale of 7 days is included in the momentum balance to account for unresolved dynamics. As will be discussed later, without this linear drag the surface equatorial currents become unrealistically large. The temperature and salinity profiles are then combined to form a density profile, assuming constant thermal expansion and haline contraction coefficients. Temperature, salinity, and velocity profiles are mixed down each vertical step of 20 cm until the three stability criteria are satisfied: (1) convective stability (-/z 0); (2) mixed layer stability (bulk Richardson number = -gh-1 |v | 0.65); and (3) shear flow stability (gradient Richardson number = -g /z |v/z| 0.25).

As shown in Figure 12a, by the end of the 3-month record the one-dimensional simulated SST is nearly 0.8°C cooler than the observed SST. Much of this drift occurs during the second week of October (Figures 12a and 12b), precisely when zonal advection dominated the observed heat balance (Figure 9). The deepening of the thermocline during the October wind burst is not captured (Figure 13), consistent with our interpretation that the deepening is due to dynamical processes rather than turbulent erosion of the pycnocline. Thus the differences between the observed and one-dimensional simulated low-frequency tendency rates can be interpreted as due to "missing" three-dimensional processes in the upper ocean heat, mass, and momentum balances. On the other hand, the simulated SST variability on timescales less than 5 days is correlated quite well (0.9) with the observed 5-day high-passed SST (Figure 12c). The diurnal cycle is determined primarily by daytime warming due to shortwave radiation, and nighttime turbulent mixing similar to that found in the eastern equatorial Pacific [Bond and McPhaden, 1995]. Thus diurnal variations are more reasonably described by a one-dimensional balance in the warm pool.

 

 

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Figure 12. Price-Weller-Pinkel one-dimensional mixed layer simulation at 0°, 156°E from September 17 to December 20, 1992. (a) Observed SST (thin curve) and simulated SST (thick curve) at 1 m depth. (b) The 5-day triangular filtered observed SST tendency rate (thin curve) and simulated SST tendency rate (thick curve). (c) The 5-day high-passed SST (thin curve) and simulated SST (thick curve).

 

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Figure 13. Daily averaged PWP simulated temperature stratification. The CIs are the same as in Figure 4a.

 

We note that when the model was run without the linear drag in the momentum equation, the winds generated surface currents with speeds of up to 180 cm s. Consequently, the surface temperatures cooled by an additional 1°C over the 3-month period due to the increased vertical entrainment. This "no drag" run is not realistic because of the intense current velocities generated. However, it gives an indication of the sensitivity of the one-dimensional model simulations to parameter variations.


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