Impact of effective ocean optical properties on Pacific subtropical cell and its mechanism for interdecadal variability

The Pacific subtropical cell (STC) is a shallow overturning circulation confined to the upper 500 m. The STC connects subduction regions of the subtropical gyre with upwelling regions in the tropics. It has been pointed out that variations in the STC transport are closely related to changes in equatorial SSTs on interdecadal timescales. Thus, the STC variations could be a possible mechanism for low-frequency variability in the tropics. This study examines the mechanism for the interdecadal variability of the STC by an ocean general circulation model (OGCM), especially focusing on the treatment of solar radiation. The OGCM we employed is based on Meteorological Research Institute Community Ocean Model (MRI.COM). It covers a global domain with the tripolar grid. The horizontal resolution is 1 deg. in longitude and 0.5 deg. in latitude. The model has 51 levels in vertical. It is driven by surface fluxes of momentum, heat, and freshwater derived from CORE ver.2 (Large and Yeager, 2008) for the period 1948-2006 after a spin-up integration of 3,000 years. First, we examine an impact of ocean optical properties on the STC transport using two types of ocean radiant heating schemes. One is a conventional ocean radiant heating scheme (Paulson and Simpson, 1977), and another a new ocean radiant heating scheme introducing the effect of the solar angle (Ishizaki and Yamanaka, 2010, Ocean Modeling). Theoretical consideration suggests that the effective attenuation depth is reduced in the new scheme, where the amount of radiation absorption increases in the upper layer and decreases in the lower layer. It is found that the new scheme has a significant impact on ocean conditions in the equatorial Pacific. As a result of a dynamical response to a change in the effective optical properties of water, a mixed layer depth is reduced and an equatorial upwelling is enhanced. This leads to the increased STC transport by about 5 % in the long-term mean field and by about 10 % on interannual to decadal timescales. Hence, care about the treatment of solar radiation in OGCMs is needed for the STC modeling. Next, we evaluate the interdecadal variability of the model STC using the run with the new scheme. The model was able to successfully simulate the observed interdecadal variability: a slowdown from 1960s to mid-1990s and a rebound after mid-1990s. It is shown that variations in the STC pycnocline transport are controlled mainly by changes in the interior ocean transport, which are out of phase with those in the western boundary transport. It is also found that the interdecadal variability of the STC pycnocline transport corresponds well with that of the Ekman transport. These results suggest that the mechanism for the interannual variability of the STC, the combined effect of variability in off-equatorial wind stress curl in the western Pacific and near-equatorial zonal wind stress (Lee and Fukumori, 2003), is still at work even on interdecadal timescales.