With support from NASA, the McKinley group has developed the MITgcm.NA, a North Atlantic regional version of the MITgcm and applied it to questions of carbon cycle variability. Below are highlights from the three papers resulting from this study:

I. Trends in the North Atlantic carbon sink: 1992 - 2006

A biogeochemical general circulation model is used to assess the impact of climate variability from 1992 to 2006 on air-sea CO2 fluxes and ocean surface pCO2 in the North Atlantic and to understand trends in the North Atlantic carbon sink over this time period. The model indicates that the North Atlantic carbon sink increased from the mid-1990s to the mid-2000s. Consistent with observations, the model output indicates large changes in the physical and chemical systems of the basin. An analysis of the changes in dissolved inorganic carbon (DIC), alkalinity (ALK), and sea-surface temperature (SST), combined with model-derived DIC tendency terms, allow for an investigation of the mechanisms that dominate the spatial variability and magnitude of the trends in the air-sea fluxes and pCO2. Modeled parameters compare favorably with available data from the Bermuda Atlantic Time Series in the subtropical gyre and the SURATLANT volunteer observation ship data in the subpolar gyre. Subtropical changes are controlled primarily by changes in sea-surface temperature. Subpolar changes in pCO2 are instead driven dynamically, primarily through changing vertical supply of DIC. The amplitude of the ocean pCO2 and air-sea flux trends are largely related to the increase in atmospheric CO2, but changes to the forcing and circulation of the North Atlantic during this period set the spatial patterns. Model changes are consistent with variation in the North Atlantic Oscillation over the period of study.

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Figure 6. Trend in the (a) pCO2, (b) pCO2-ALK, (c) pCO2-DIC, and (d) pCO2-SST, calculated by taking the difference between the 2003-2006 averages and the 1992-1995 averages. Areas with values between -3 and 3 uatm have been whited out to accentuate smaller differences around the zero line (heavy contour).

Ullman, D. J., G. A. McKinley, V. Bennington, and S. Dutkiewicz (2009), Trends in the North Atlantic carbon sink: 1992 - 2006, Global Biogeochem. Cycles, 23, GB4011, doi:10.1029/2008GB003383.Download PDF Supplementary Material

II. What does chlorophyll variability tell us about export and air-sea CO2 flux variability in the North Atlantic?

The importance of biology to the ocean carbon sink is often quantified in terms of export, the removal of carbon from the ocean surface layer. Satellite images of sea surface chlorophyll indicate variability in biological production, but how these variations affect export and air-sea carbon fluxes is poorly understood. We investigate this in the North Atlantic using an ocean general circulation model coupled to a medium-complexity ecosystem model. We find that biological CO2 drawdown is significant on the mean and dominates the seasonal cycle of pCO2, but variations in the annual air-sea CO2 flux and export are not significantly correlated. Large year-to-year variability in summertime pCO2 occurs, because of changing bloom timing, but integrated bloom strength and associated carbon uptake and export do not vary substantially. The model indicates that small biological variability, quantitatively consistent with SeaWiFS (1998-2006), is not sufficient to be a first-order control on annual subpolar air-sea CO2 flux variability.

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Figure 2. Climatology of bloom peak day in SeaWiFS satellite data (1998-2006)
and in the model (1982 - 2006).

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Figure 7. (a, b) The percent the standard deviation is of the mean daily chlorophyll between fall 1998 and 2006 in (Figure 7a) SeaWiFS and (Figure 7b) the model. Eight-day weeks 10 through 37 of the year are used in this analysis, because SeaWiFS data in the subpolar region is too sparse during the omitted weeks. To compare the model to SeaWiFS satellite data, a daily average chlorophyll value for each year was calculated in both the data and model. Because of cloud cover and other satellite issues, not all grid points have observational data every 8-day period, so area-weighted averages for 5° x 5° regions were used with observational data. Within a 5° x 5° area, the available data for each 8-day record is assumed representative of the entire area and an area-weighted average is created, ignoring missing data points. The model is able to capture the percent variability observed and much of the pattern of variability magnitude. (c) Twenty-five years of modeled chlorophyll at region near Iceland boxed in Figure 7b. Thick black line is modeled climatology. Annually integrated chlorophyll varies by only 10.9% of the mean, and annual export varies by only 4.8% of the mean between 1982 and 2006. SeaWiFS daily average chlorophyll (1998-2006) varies by 18% of its mean in this region.

Bennington, V, G.A. McKinley, S. Dutkiewicz, D. Ullman (2009) What does chlorophyll variability tell us about export and air-sea CO2flux variability in the North Atlantic? Global Biogeochem. Cycles., 23, GB3002, doi:10.1029/2008GB00341. Download PDF Supplementary Material

III. Do hurricanes cause significant interannual variability in the air-sea CO2 flux of the subtropical North Atlantic?

Observations at Bermuda and in the Caribbean Sea indicate that hurricanes influence surface ocean pCO2 (pCO2ocean) and air-sea CO2 fluxes at short time scales. We use a regional version of the MIT ocean general circulation model to study impacts on interannual variability in air-sea CO2 fluxes in the North Atlantic subtropical gyre (25 x 40N). Consistent with observations, enhanced wind speeds dominate the hurricane's effect on the flux, driving CO2 out of the ocean due to the negative air-sea gradient in pCO2 (pCO2atm < pCO2ocean) that occurs in response to warm sea surface temperatures (SSTs) during hurricane season. With a storm, vertical mixing causes negative SST anomalies that depress pCOo2cean, but not enough to reverse the gradient. Though hurricanes drive a substantial local CO2 efflux, we find no evidence for a relationship between year-to-year variability in hurricane frequency and variability in basin- integrated air-sea CO2 fluxes across the subtropical North Atlantic.

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Figure 1. Modeled and observed variables at Bermuda (31°N, 64°W) from 15 May 1995 through 15 December 1995. SST and pCO2 data from B07 and MLDs calculated from CTD profiles (downloaded from http://bats.bios.edu/) indicated with asterisks. Hurricanes Felix, Luis, and Marilyn are denoted by the gray bars, beginning one day before closest approach to Bermuda and ending one day after: (a) NCEP reanalysis 10-meter wind speeds, (b) MLD, (c) SST, (d) total pCO2 (thin solid bold black), pCO2-T (thin red), pCO2-nontT (dashed red), and pCO2atm (thin black), and (e) air-sea CO2 flux, positive to the atmosphere.

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Figure 3. Scatterplots of year, hurricane, and tropical storm (TS) frequency and modeled CO2 fluxes, as well as best-fit lines when a linear trend is able to reasonably define the relationship. (a) Hurricane and tropical storm frequency (NHC data) versus year, (b) annual flux (25° x 40°N, including the Gulf of Mexico and Caribbean) per year, (c) sum of the daily fluxes along hurricane and tropical storm tracks (NHC data) versus number of hurricanes and tropical storms, and (d) basin-integrated annual flux versus hurricane and tropical storm frequency.

Koch, J., G. A. McKinley, V. Bennington, and D. Ullman (2009), Do hurricanes cause significant interannual variability in the air-sea CO2 flux of the subtropical North Atlantic?, Geophys. Res. Lett., 36, L07606, doi:10.1029/2009GL037553. Download PDF, AGU webversion Download PDF , highlighted by Nature Reports Climate Change Download PDF Supplementary Material