Abstract
This study presents a synthesis of surface water partial pressure of CO2 (pCO2) and nutrient observations in the tropical Pacific (20°S–20°N) from 1981 to 2015 and characterizes the spatio-temporal variability. We used data from the Surface Ocean CO2 Atlas version 5, which include about 2 million pCO2 measurements in the tropical Pacific. Unlike many previous studies that estimated pCO2 fields from relationships with other variables like sea surface temperature, we developed gridded products of monthly means using a technique to interpolate measured pCO2 values. Large seasonal variation of pCO2 appears in the cold tongue region (EQ–20°S, east of 120°W) corresponding to the seasonal variation of coastal upwelling, and in the off-equatorial region where the thermodynamic effect on pCO2 dominates. Consistent with previous findings, pCO2 along the equator declines during El Niño due to weakening of the easterly winds and therefore reduced upwelling of CO2 rich subsurface water. We also quantified the spatial distribution of the long-term pCO2 trend beyond the area-averaged trend presented previously. The long-term trend of pCO2 is positive in all regions with an area average of 1.8 ± 0.1 μatm yr−1. However, along the equator the trend is > 2 μatm yr−1 linked to the Pacific Decadal Oscillation forcing. Using the same methodology, we also analyzed about 20,000 surface nutrient measurements from the Global Ocean Data Analysis Project version 2, World Ocean Database 2013, and ship-of-opportunity sampling by the National Institute for Environmental Studies, Japan. We present the spatial patterns of reduced surface nutrient concentration in the central to eastern tropics along the equator during El Niño, but there are not enough data to characterize the trends of nutrients in the tropical Pacific.Decorrelation analyses are also applied using covariance between pCO2 anomalies separated by a lag increment for zonal, meridional, and temporal directions. The e-folding scale of pCO2 is estimated to be 6° in latitude, 13° in longitude, and 2 months with a signal-to-noise ratio of 4, which are used as input for the interpolation as well as an assessment of ideal observation density and frequency. Decorrelation analyses such as this are critical for evaluating existing observing system design and informing future sampling strategies.
Highlights
The tropical Pacific is characterized by the warm pool in the west and the cold tongue in the east
We present the spatial patterns of reduced surface nutrient concentration in the central to eastern tropics along the equator during El Nin~o, but there are not enough data to characterize the trends of nutrients in the tropical Pacific
Seasonal variation of pressure of CO2 (pCO2) is in-phase with seasonal variation of dissolved inorganic carbon (DIC) and out-of-phase with seasonal variation of sea surface temperature (SST) in the cold tongue regions; pCO2 is high when and where DIC is high and SST is low (Fig. 3b, 3c and 3g). nDIC shows less north-south contrast in the eastern tropics, while the amplitude and phase of seasonal vari ation are quite similar to those of DIC
Summary
The tropical Pacific is characterized by the warm pool in the west and the cold tongue in the east. In the cold tongue region, the upwelled water is cool and rich in dissolved inorganic carbon (DIC) and nutrients, but iron-limited, which causes moderate productivity and strong CO2 release to the atmosphere (Chavez and Barber, 1987; Feely et al, 1987). The position of the warm pool and the cold tongue fluctuates seasonally and interannually (Horel, 1982; Rasmusson and Carpenter, 1982; McPhaden and Picaut, 1990; Yan et al, 1992). During El Nin~o, upwelling weakens and the cold tongue retreats to the east.
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