Air sampling for the shipboard NOAA/CMDL program has been carried out since December 1986 by the merchant vessel Southland Star, operated by Blue Star Line, Ltd. Beginning in May 1990, air sampling has also been carried out by the merchant vessel Wellington Star (also operated by Blue Star Line, Ltd.). Each ship usually sails southward from Los Angeles, passing near Christmas Island, American Samoa, and Fiji en route to its destination in Auckland, New Zealand. Air samples are collected at a latitude of 30° N, and then at every 5° latitude southward to 35° S. The northward return voyage follows a different track, passing Hawaii en route to Seattle. Air samples are collected at 35° S and then at every 5° latitude northward to 30° N (samples are sometimes also collected at 35° N, 40° N, and 45° N, but data at these latitudes were too sparse to calculate averages). Thus, samples are collected at a minimum of two longitudes for a given latitude. Because the longitudinal variations in CO2 mixing ratios are small compared with the latitudinal variations, all samples collected within a given latitudinal band, defined as the center latitude (e.g., 30° N, 0°, 35° S) ± 2.5°, are considered representative of a single latitude "site." The frequency of sampling on average is ~3 weeks per latitude per ship. Since May 1990, when the Pacific Ocean air sampling program was augmented by the contribution of the Wellington Star, the overall frequency of sampling has increased because of the somewhat staggered cruise schedules of the two ships.
The CO2 mixing ratios at Pacific Ocean shipboard sites exhibit both a temporal trend and a latitudinal gradient. The annual mean mixing ratios for 1987 ranged from 347.4 parts per million (ppm) at 30° S to 349.6 ppm at 30° N. For 1992, the annual mean values ranged from 354.4 ppm at 30° S to 357.1 ppm at 30° N. At all latitudes, the mixing ratios increased with time during the period of record (January 1987-December 1992). For any given year, the annual mean mixing ratios generally decreased along a gradient from north to south. Accordingly, CO2 levels were consistently highest at the northernmost latitude, 30° N, and they decreased to their minimum values at 30° S. Overall, the shipboard CO2 mixing ratios were consistent with those obtained at land-based sites at similar latitudes. The shipboard data also exhibited seasonal cycles that were most evident at higher latitudes, particularly in the Northern Hemisphere. This observed seasonality was also in general agreement with results from land-based monitoring sites.
Pacific Ocean Table (continued)
Access to Digital Data
Date revised 4/10/95 (gg)
Previous Section Next Section Contents CDIAC Home
