The NECC is centered near 5°N in the western Pacific and shifts poleward as it flows eastward to about 7°-8°N in the central and eastern Pacific (Donguy and Meyers, 1996; their Figure 2). Its northern boundary is well defined by the adjacent westward flowing North Equatorial Current, but the location of its southern boundary is not always obvious. In the east and central Pacific, although it is clearly bounded near the surface by the westward South Equatorial Current (SEC), its deeper portions merge with the North Subsurface Countercurrent (NSCC; Wyrtki and Kilonsky, 1984; their Figure 2). In the western ocean, its southern edge is located as far south as 2.5°N, and may merge with the Equatorial Undercurrent (EUC) at depth. Likewise, its bottom boundary is not clear everywhere because of the NSCC. Generally, the Pacific NECC extends only slightly below the depth of the 20°C isotherm (Z20), that is, not much deeper than 200 m (Wyrtki and Kilonsky, 1984; their Figure 2; G. Johnson, private communication). These difficulties in defining the boundaries of the NECC are not just definitional, but indicate a blend of dynamical influences that make modeling of this current and its heat and mass transports challenging.

Since Sverdrup's (1947) seminal paper, the NECC is believed to be a direct response to atmospheric forcing by wind-stress curl associated with the ITCZ. According to Sverdrup theory, the depth-integrated, zonal geostrophic current (i.e., the transport per unit latitude) is given by the full Sverdrup transport minus the Ekman transport. Given the success of Sverdrup theory, one might expect that it would be easy to simulate the NECC in numerical models. In fact, simulated NECCs tend to be weak (e.g., Philander et al., 1987; Grima et al., 1999) and/or poorly formed (Fig. 1).

PROJECT GOALS
Understanding why numerical models have this difficulty is the motivation for this research. We seek to answer the following questions: What processes determine the spatial structure and total transport of the Pacific NECC in ocean models? How sensitive are NECC simulations to forcing by different wind products? Do the deficiencies in simulated NECCs result from wind or model error? If it is wind error, can the nature of the error be characterized?

METHODOLOGY
Our approach is to force an ocean model with different wind products, and to compare solutions with new estimates of observed NECC structures and transports determined from XBT and Pacific Ocean Analysis (POA) data, the latter being a data-assimilation product of model and observed thermal fields from the NOAA/National Center for Environmental Prediction. The ocean model used here is a 4.5-layer system with active thermodynamics and mixed-layer physics, essentially a general circulation model of intermediate complexity. Solutions are forced by climatological and interannual versions of FSU and ECMWF winds.

RESULTS AND ACCOMPLISHMENTS
Our results support the idea that wind inaccuracies cause poor NECC simulation, and identify the problematic aspects of the wind fields and the ocean response. One of our main results is that when solutions fail to develop a realistic NECC structure, they do so in a distinct manner by developing a discontinuity in the central ocean. This failure is traceable to two specific aspects of wind accuracy: the meridional derivative of the wind stress curl in the ITCZ region and zonal wind stress in the near-equatorial region. Only when the forcing in these two regions is properly prescribed do solutions develop a NECC with both realistic spatial structure and transport. The model NECC transport is determined mainly by the strength of d(curl J)/dy (the Sverdrup transport term), but its structure depends on the near-equatorial J^x; thus NECC physics involve equatorial as well as Sverdrup dynamics. More details are presented in our publications listed below.

FUTURE WORK|
We hope to improve NECC simulations using satellite-based wind measurements. The new QSCAT winds will soon be released to the community with rain flag information. We are looking forward to using this new data set.

Yu, Z., J.P. McCreary, W.S. Kessler, and K.A. Kelly, 2000: Influence of equatorial dynamics on the Pacific North Equatorial Countercurrent. J. Phys. Oceanogr., (Accepted.)

Yu, Z. and D.W. Moore, 2000: Validating the NSCAT winds in the vicinity of the Pacific Intertropical Convergence Zone. Geophys. Res. Lett., (Accepted.)

Principal Investigator:
Dr. William S. Kessler
kessler@pmel.noaa.gov

Co-Investigator:
Dr. Zuojun Yu
zuojun@pmel.noaa.gov

Institutions:
NOAA / PMEL / OCRD
7600 Sand Point Way NE
Seattle WA 98115 USA

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