Synoptic Wind‐Dependent Sampling Asymmetry Biases Satellite Sea Surface Chlorophyll‐a in the Mid‐to‐High‐Latitude North Pacific and North Atlantic

Abstract

Satellite observations of sea surface chlorophyll‐a concentration (SChl) provide a basin‐scale record for interpreting marine productivity and upper ocean biogeochemical variability. However, clouds and other observational limits create gaps in SChl retrieval, which can bias the SChl record, especially in cloud‐prone mid‐to‐high‐latitude oceans. Here, we use satellite SChl data from 2010 to 2019 to quantify sampling bias associated with synoptic‐scale wind speed conditions. In winter, compared with low wind speed conditions, high wind speed conditions are often cloudier, yielding fewer valid retrievals of SChl. At the same time, observed SChl is generally lower under high wind speed conditions across the mid‐to‐high latitudes of the North Atlantic and the North Pacific. Together, these differences bias mean SChl upward. The largest bias occurs in winter, when the bias of gyre‐mean SChl is about 1.4%, with local values exceeding 10%. The resulting SChl bias also propagates into satellite‐based estimates of ocean productivity.