Scales of spatial variability for surface ocean pCO2 in the Gulf of Alaska and Bering Sea: Toward a sampling strategy

Abstract

Twenty‐four near‐exact repeat tracks (over a 48‐month period) of ocean surface pCO2 data from the North Pacific offer an uprecedented opportunity to examine the space scales of pCO2 in this region. Because previous basin scale studies of air‐sea carbon flux have shown that the field Of ΔpCO2 (air‐sea disequilibrium) largely controls the field of flux (and atmospheric pCO2 is relatively constant), knowing how to sample pCO2 in seawater is a crucial element of the design of a basin scale carbon flux observing system. Unbiased (within 3 µatm) along‐track means for ΔpCO2 can be obtained from measurements made every 40 km (∼ hourly for a ship traveling 20 knots). We find distinctly different characteristics of spatial variability in two open ocean regions, the Bering Sea and the Gulf of Alaska. The first zero crossing of the mean spatial autocorrelation function is 1.5° longitude in the Bering Sea and 3.5° in the Gulf of Alaska, when the cruise data are linearly detrended to remove the larger scale field. There is a strong seasonal variation in the zero crossing, with shorter scales in the summer. In near‐shore waters there is extreme variability, often with very small space scales and very large cruise‐to‐cruise differences. Neither the mean nor variability statistics from a single ship cruise data set appear likely to produce reliable information for designing an observing system.