Surface ocean‐lower atmosphere interactions in the Northeast Pacific Ocean Gyre: Aerosols, iron, and the ecosystem response

TLDR

Measurements of iron and aluminum were taken in surface and subsurface waters along transects between central California and Hawaii in late March and May 2001, and soil aerosol concentrations were estimated using the Navy Aerosol Analysis and Prediction System. A large Asian dust event in April 2001 caused a measurable increase in surface water iron and aluminum along the southern transect, with concentrations rising by 0.5–2 nM and matching crustal metal ratios, indicating soil aerosol deposition, but ecosystem responses were weak.

Abstract

Here we report measurements of iron and aluminum in surface and subsurface waters during late March and late May of 2001 on transects between central California and Hawaii. A large cloud of Asian dust was detected during April 2001, and there was a clear signal in surface water iron due to aerosol deposition on the May transect. Iron and aluminum concentrations increased synchronously by 0.5 and 2 nM along the southern portion of the transect, which includes the Hawaii Ocean Time series (HOT) station, from background values in March (0.1 to 0.2 nM Fe). These changes occurred in a ratio that is close to the crustal abundance ratio of the metals, which indicates a soil aerosol source. A vertical profile of dissolved iron was also measured at the HOT station in late April and this profile also shows a large increase near the surface. Direct observations of aerosol iron concentration at Mauna Loa Observatory on Hawaii indicate that aerosol concentrations were significantly lower than climatological values during this period. Soil aerosol concentrations along the transect were estimated using the real‐time Navy Aerosol Analysis and Prediction System (NAAPS). The NAAPS results show a large meridional gradient with maximum concentrations in the boundary layer north of 30°N. However, the deposition of iron and aluminum to surface waters was highest south of 25°N, near Hawaii. There were only weak signals in the ecosystem response to the aerosol deposition.