CO 2 flux in the North Pacific

1. CO2 flux in the North Pacific Alan Cohn May 10, 2006

2. Introduction • Oceans contain ~50x as much CO2 as atmosphere • Mean annual rate of oceanic CO2 uptake by oceans for past few decades is estimated at about 2 Pg-C yr-1 (Takahashi et al., 2005) • Only a few stations in the ocean CO2 monitoring network  lack of ocean CO2 time-series limits scientists’ ability to estimate interannual changes in oceanic CO2 uptake • Researchers have looked at atmospheric time series of CO2, 13CO2, and O2 to infer interannual changes in oceanic and terrestrial CO2 uptake

3. Distribution of climatological mean annual sea-air CO2flux (moles CO2m-2 yr-1) for reference year 1995 representing non-El Niño conditions. This map yields an annual oceanic uptake flux for CO2of 2.2 ± 0.4 Pg C yr-1. http://www.pmel.noaa.gov/pubs/outstand/feel2331/mean.shtml

4. Why the North Pacific? • I concentrate on North Pacific as it is a region of strong climate variability with implications for variability of atmospheric CO2 • One of most frequently sampled regions of oceans for CO2 variability and nutrient chemistry • Strongly influenced by strength of wintertime Aleutian Low through changes in surface wind stress, Ekman advection, surface ocean mixing, and heat fluxes • In winter, surface water pCO2 values are governed primarily by physical processes because of reduced biological activity (McKinley et al., 2006) • Photosynthesis has significant effects on pCO2 come spring and summer

5. pCO2 sensitivity • pCO2 of seawater is sensitive function of temperature as well as total concentration of CO2  TCO2 depends on net biological community production, rate of upwelling of CO2­rich subsurface waters, and air-sea CO2 flux  Revelle factor measures sensitivity of pCO2 to changes in total CO2

6. SST vs. Biology • Influences of SST on surface ocean pCO2 oppose effects of biological and physical influences on dissolved inorganic carbon (DIC) • Temperatures lead to low pCO2 in winter and high pCO2 in summer, i.e. they’re positively correlated (McKinley et al., 2006) • In mixed layer, lower total CO2 from photosynthesis counteracts effect of seasonal warming on pCO2  often evident during spring blooms

7. Mixed Layer Variability • Upwelling of CO2-rich subsurface waters in winter counteracts effect of cooling on pCO2 (Takahashi et al., 2005) • Interannual variability of CO2 in surface ocean strongly correlated with changes in mixing depth during winter  Deep surface-mixed layers can lead to increased CO2 uptake and higher levels of photosynthesis than during normal years (Quay, 2002). http://www.pmel.noaa.gov/~cronin/encycl/cartoon.jpg

8. Aleutian Low is a wintertime semi-permanent cyclone Strong Low:  strong westerly winds in central N. Pacific  cooler SSTs, deeper mixed layer  enhanced southerly winds in eastern N. Pacific  warmer SSTs, upwelling supressed Strength of Low associated with PDO and ENSO. http://www.ecy.wa.gov/programs/sea/coast/storms/weather.html

9. PDO • Pacific Decadal Oscillation (PDO) is measure of climate variability with possible impacts on CO2 flux; it has a 20 – 30 year periodicity Positive Phase: • SSTs cold, mixing layer deep in central and western North Pacific • Warm SSTs in Alaska Gyre, along coast of North America, and into tropics http://tao.atmos.washington.edu/pdo/

10. PDO Aleutian Low Strong Low Weak Low

11. PDO • In positive phase, upwelling of high CO2 waters suppressed due to anomalously northward wind off of Canada • Patra et al. (2005) finds that sea-air CO2 flux over North Pacific is significantly associated with PDO at 5 months lag • Believe that delayed effect may be result of slow response of marine ecosystems and other environments to changes in climate mode

12. PDO • May also influence pCO2 via changes in ocean circulation • station located near Hawaii is believed to have shifted from a weak CO2 sink to weak source due to increased transport of high salinity waters from the north(Keeling et al., 2004) • shift may be linked to a possible 1997 regime shift in the PDO http://kela.soest.hawaii.edu/ALOHA/images/hawaii.jpg

13. ENSO • El Nino-Southern Oscillation (PDO) has its primary signature in tropics; it has a 3-7 year periodicity • Linked to PDO through the variability of the Aleutian Low • Patra et al. find that CO2 flux over North Pacific is significantly associated with ENSO at three months lag http://tao.atmos.washington.edu/pdo/

14. PDO ENSO La Nina predominates when PDO is in negative phase El Nino predominates when PDO is in positive phase http://tao.atmos.washington.edu/pdo/ http://www.pmel.noaa.gov/~kessler/ENSO/soi-1950-98.gif

15. Physical Mechanisms • Upwelling regions in central and eastern equatorial Pacific are a strong source of CO2 throughout year • Kuroshio Current and extension are strong CO2 sink in winter due primarily to cooling, and a weak source in summer due to warming • Western subarctic areas are strong CO2 source in winter because of convective mixing of waters rich in respired CO2 and nutrients  become strong sink in winter since nutrients help fuel intense photosynthesis

16. Takahashi et al., 2005

17. pCO2 variability • Many studies have shown increased uptake in tropics and subtropics in recent decades, but their temporal structures are inconsistent • A few areas show decreasing pCO2; these are in or near the Bering and Okhotsk Seas due to increased biological activity  may be result of changing nutrient supplies caused by changes in land hydrology or by increases in river or airborne inputs of nutrients http://www.pmel.noaa.gov/np/images/maps/npacific6.gif

18. Summary • Seasonal temperature changes are primary cause for seasonal changes of pCO2 in subtropical gyres • Changes in total CO2 concentration caused by winter upwelling and springtime plankton blooms are primary cause for seasonal changes in sub-polar and polar regions (Takahashi et al., 2006) • Takahashi et al. (2006) find that observed increase in pCO2 is not affected significantly by SST changes, but is primarily due to change in seawater chemistry most likely by uptake of atmospheric CO2

19. Conclusion • Important to study seawater chemistry as well as temperature and circulation changes throughout world’s oceans, as these can affect future uptake or outgassing of CO2 • Vital to understand role of various mechanisms for changes in CO2 flux in order to accurately quantify potentially changing role of the ocean as a sink for future climate scenarios

20. References Keeling, C.D., H. Brix, and N. Gruber (2004), Seasonal and long-term dynamics of the upper ocean carbon cycle at Station ALOHA near Hawaii, Global Biogeochem. Cycles, 18¸ GB4006, doi:10.1029/2004GB002227. McKinley, G.A., T. Takahashi, E. Buitenhuis, F. Chai, J.R. Christian, S.C. Doney, M.-S. Jiang, K. Lindsay, J.K. Moore, C. Le Quéré, I. Lima, R. Murtugudde, L. Shi, and P. Wetzel (2006), North Pacific Carbon Cycle Response to Climate Variability on Seasonal to Decadal Timescales, submitted to J. Geophys. Res. Oceans Patra, P., S. Maksyutov, M. Ishizawa, T. Nakazawa, T. Takahashi, and J. Ukita (2005), Interannual and decadal changes in the sea-air CO2­ flux from atmospheric CO2 inverse modeling, Global Biogeochem. Cycles, 19, GB4013, doi:10.1029/2004GB002257. Quay, P. (2002), Ups and Downs of CO2 Uptake, Science, 298, 2344. Takahashi, T., S.C. Sutherland, R.A. Feely, and R. Wanninkhof (2005), Decadal Change of the Surface Water pCO2 in the North Pacific: A Synthesis of 35 Years of Observations, submitted to J. Geophys. Res.