Description: Surface ocean iron (Fe) fertilization can affect the marine primary productivity (MPP), thereby impacting on CO2 exchanges at the atmosphere-ocean interface and eventually on climate. Mineral (aeolian or desert) dust is known to be a major atmospheric source for the surface ocean biogeochemical iron cycle, but the significance of volcanic ash is poorly constrained. We present the results of geochemical experiments aimed at determining the rapid release of Fe upon contact of pristine volcanic ash with seawater, mimicking their dry deposition into the surface ocean. Our data show that volcanic ash from both subduction zone and hot spot volcanoes (n = 44 samples) rapidly mobilized significant amounts of soluble Fe into seawater (35–340 nmol/g ash), with a suggested global mean of 200 ± 50 nmol Fe/g ash. These values are comparable to the range for desert dust in experiments at seawater pH (10–125 nmol Fe/g dust) presented in the literature (Guieu et al., 1996; Spokes et al., 1996). Combining our new Fe release data with the calculated ash flux from a selected major eruption into the ocean as a case study demonstrates that single volcanic eruptions have the potential to significantly increase the surface ocean Fe concentration within an ash fallout area. We also constrain the long-term (millennial-scale) airborne volcanic ash and mineral dust Fe flux into the Pacific Ocean by merging the Fe release data with geological flux estimates. These show that the input of volcanic ash into the Pacific Ocean (128–221 × 1015 g/ka) is within the same order of magnitude as the mineral dust input (39–519 × 1015 g/ka) (Mahowald et al., 2005). From the similarity in both Fe release and particle flux follows that the flux of soluble Fe related to the dry deposition of volcanic ash (3–75 × 109 mol/ka) is comparable to that of mineral dust (1–65 × 109 mol/ka). Our study therefore suggests that airborne volcanic ash is an important but hitherto underestimated atmospheric source for the Pacific surface ocean biogeochemical iron cycle.

Description: Using a global ocean model with regionally focused high resolution (1/10°) in the East China Sea (ECS), we studied the oceanic heat budget in the ECS. The modeled sea surface height variability and eddy kinetic energy are consistent with those derived from satellite altimetry. Significant levels of eddy kinetic energy are found east of the Ryukyu Islands and east of Taiwan, where the short-term variability is spawned by active mesoscale eddies coalescing with the circulation. Furthermore, the simulated vertical cross-stream structure of the Kuroshio (along the Pollution Nagasaki line) and the volume transport through each channel in the ECS are in good agreement with the observational estimates. The time-averaged temperature fluxes across the Taiwan Strait (TWS), Tsushima Strait (TSS), and the 200 m isobath between Taiwan and Japan are 0.20 PW, 0.21 PW, and 0.05 PW, respectively. The residual heat flux of 0.04 PW into the ECS is balanced by the surface heat loss. The eddy temperature flux across the 200 m isobath is 0.005 PW, which accounts for 11.2% of the total temperature flux. The Kuroshio onshore temperature flux has two major sources: the Kuroshio intrusion northeast of Taiwan and southwest of Kyushu. The Ekman temperature flux induced by the wind stress in the ECS shows the same seasonal cycle and amplitude as the onshore temperature flux, with a maximum in autumn and a minimum in summer. We conclude that the Ekman temperature flux dominates the seasonal cycle of Kuroshio onshore flux.

Description: Earth system climate models generally underestimate dissolved oxygen concentrations in the deep eastern equatorial Pacific. This problem is associated with the "nutrient trapping" problem, described by Najjar et al. [1992], and is, at least partially, caused by a deficient representation of the Equatorial Intermediate Current System (EICS). Here we emulate the unresolved EICS in the UVic earth system climate model by locally increasing the zonal isopycnal diffusivity. An anisotropic diffusivity of ∼50,000 m 2 s-1 yields an improved global representation of temperature, salinity and oxygen. In addition, it (1) resolves most of the local "nutrient trapping" and associated oxygen deficit in the eastern equatorial Pacific and (2) reduces spurious zonal temperature gradients on isopycnals without affecting other physical metrics such as meridional overturning or air-sea heat fluxes. Finally, climate projections of low-oxygenated waters and associated denitrification change sign and apparently become more plausible

Description: We analyze an extensive set of global coupled biogeochemical ocean circulation models. The focus is on the equatorial Pacific. In all simulations, which are consistent with observed standing stocks of relevant biogeochemical species at the surface, we find spuriously enhanced (reduced) macronutrient (oxygen) concentrations in the deep eastern equatorial Pacific. This modeling problem, apparently endemic to global coupled biogeochemical ocean circulation models, was coined “nutrient trapping” by Najjar et al. (1992). In contrast to Aumont et al. (1999), we argue that “nutrient trapping” is still a persistent problem, even in eddy-permitting models and, further, that the scale of the problem retards model projections of nitrogen cycling. In line with previous work, our results indicate that a deficient circulation is at the core of the problem rather than an admittedly poor quantitative understanding of biogeochemical cycles. More specifically, we present indications that “nutrient trapping” in models is a result of a spuriously damped Equatorial Intermediate (zonal) Current System and Equatorial Deep Jets—phenomenon which await a comprehensive understanding and have, to date, not been successfully simulated.