The Deep Equatorial Ocean Circulation in Wind-Forced Numerical Solutions*
Journal of Physical Oceanography
The deep equatorial ocean circulation in wind-forced numerical solutions. J. Phys. Oceanogr. Abstract We perform eddy-resolving and high-vertical-resolution numerical simulations of the circulation in an idealized equatorial Atlantic Ocean in order to explore the formation of the deep equatorial circulation (DEC) in this basin. Unlike in previous studies, the deep equatorial intraseasonal variability (DEIV) that is believed to be the source of the DEC is generated internally by instabilities of
... by instabilities of the upper ocean currents. Two main simulations are discussed: Solution 1, configured with a rectangular basin and with wind forcing that is zonally and temporally uniform; and Solution 2, with realistic coastlines and with an annual cycle of wind forcing varying zonally. Somewhat surprisingly, Solution 1 produces the more realistic DEC: The large-vertical-scale currents (Equatorial Intermediate Currents or EICs) are found over a large zonal portion of the basin, and the small-vertical-scale equatorial currents (Equatorial Deep Jets or EDJs) form low-frequency, quasi-resonant, baroclinic equatorial basin modes with phase propagating mostly downward, consistent with observations. We demonstrate that both types of currents arise from the rectification of DEIV, consistent with previous theories. We also find that the EDJs contribute to maintaining the EICs, suggesting that the nonlinear energy transfer is more complex than previously thought. In Solution 2, the DEC is unrealistically weak and less spatially coherent than in the first simulation probably because of its weaker DEIV. Using intermediate solutions, we find that the main reason for this weaker DEIV is the use of realistic coastlines in Solution 2. It remains to be determined, what needs to be modified or included to obtain a realistic DEC in the more realistic configuration. 2 of 68 25 30 35 40 1 Latitudinally alternating large-vertical-scale zonal jets are also found at more poleward latitudes (see Ollitrault et al. 2006, Cravatte et al. 2012 and Qiu et al. 2013 for a recent account). We do not include them in the present paper but note that the dynamical similarity between these jets and the EICs is still unclear. 3 of 68 45 50 55 60 fields in global, coupled, biogeochemical ocean models have been attributed to inaccuracies of the simulated DEC (Dietze and Loeptien 2013; Getzlaff and Dietze 2013). Finally, Brandt et al. (2011) provide evidence that the upward-propagating energy and interannual variability of the EDJs in the Atlantic Ocean might be indirectly responsible for a portion of the interannual atmospheric variability via their modulation of sea surface temperature (SST). On the modeling side, a realistic DEC is absent in most ocean general circulation model (OGCM) solutions. At the same time, recent theory and idealized numerical simulations have shown that the DEC may arise from the rectification of the deep equatorial intraseasonal variability (DEIV) (d'Orgeville et al. 2007; Hua et al. 2008; Ménesguen et al. 2009; Ascani et al. 2010). The reasons for this inconsistency are not clear. One possible cause is that DEIV, which arises internally from instabilities of the mean circulation in OGCMs, may be poorly reproduced in these models. Present research In this study, we continue the effort to understand DEC dynamics and improve its modeling; for this purpose, we focus on the Atlantic Ocean. Specifically, we seek to reduce the gap between idealized and OGCM simulations by obtaining a series of numerical solutions that generate DEIV internally. We have tested many configurations with varying parameters and degrees of realism. Here, we focus on two: Solution 1, with a rectangular basin and with winds varying only with latitude; and Solution 2, with realistic coastlines and with an annual cycle of zonally and meridionally varying winds based on Atlantic climatology. We also briefly mention the results of two intermediate solutions (Solutions 1.5 4 of 68 65 70 75 80 8 of 68 155 160 165 240 245 250 255 Schott, F. A., L. Stramma, and J. Fischer, 1995: The warm water inflow into the western tropical Atla nt ic bounda ry regim e, spring 1994.