Alan Cohn

AOSC 658a

May 10, 2006

 

CO2 flux in the North Pacific

 

            The oceans contain about fifty times as much CO2 as the atmosphere, and are continually absorbing large amounts of excess CO2 from anthropogenic emissions.  The mean annual rate of oceanic CO2 uptake by the oceans for the past few decades is estimated at about 2 Pg-C yr-1 (Takahashi et al., 2005).  There are only a few stations in the ocean CO2 monitoring network, so that the lack of ocean CO2 time-series limits scientists’ ability to estimate interannual changes in oceanic CO2 uptake.  Instead of direct CO2 measurements, researchers have looked at atmospheric time series of CO2, 13CO2, and O2 to infer interannual changes in oceanic and terrestrial CO2 uptake (Quay, 2002).  While many studies have shown increased uptake in the tropics and subtropics in recent decades, their temporal structures are inconsistent (McKinley et al., 2006).

            I chose to concentrate on the North Pacific as it is a region of strong climate variability with implications for the variability of atmospheric CO2.  In addition, it is one of the most frequently sampled regions of the oceans for CO2 variability and nutrient chemistry.  The North Pacific is strongly influenced by the strength of the 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, however, come spring and summer.

The pCO2 of seawater is a sensitive function of temperature and total concentration of CO2, the latter depending on the net biological community production, rate of upwelling of CO2­-rich subsurface waters, and air-sea CO2 flux.  The Revelle factor measures the sensitivity of pCO2 to changes in total CO2.  The influences of SST on surface ocean pCO2 oppose the effects of biological and physical influences on dissolved inorganic carbon (DIC).  Temperatures lead to low pCO2 in winter and high pCO2 in summer; the two are positively correlated (McKinley et al., 2006).  In the mixed layer, lower total CO2 from photosynthesis counteracts the effect of seasonal warming on pCO2; this is often evident during spring blooms.  Also, upwelling of CO2-rich subsurface waters in winter counteracts the effect of cooling on pCO2 (Takahashi et al., 2005).  Interannual variability of CO2 in the surface ocean is strongly correlated with changes in mixing depth during the winter.  Deep surface-mixed layers can lead to increased CO2 uptake and higher levels of photosynthesis than during normal years (Quay, 2002).

            A measure of climate variability with possible impacts on CO2 flux is the Pacific Decadal Oscillation (PDO).  When the PDO is in its positive phase, SSTs are cold and the mixed layer is deep in the central and western North Pacific, with warm SSTs in the Alaska Gyre, along the coast of North America, and into the tropics. Also in the positive phase, upwelling of high CO2 waters is suppressed due to anomalously northward wind off of Canada.  One study by Patra et al. (2005) finds that the sea-air CO2 flux over the North Pacific is significantly associated with the PDO at five months lag (Figure 1a).  They believe that the delayed effect may be the result of a slow response of marine ecosystems and other environments to changes in the climate mode.  The PDO may also influence pCO2 via changes in ocean circulation.  For example, a 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.  This shift may be linked to a possible 1997 regime shift in the PDO (Keeling et al., 2004).  PDO is also linked to El Niño-Southern Oscillation (ENSO) through the variability of the Aleutian Low.  Patra et al. find that CO2 flux over the North Pacific is significantly associated with ENSO at three months lag (Figure 1b).

            Upwelling regions in the central and eastern equatorial Pacific are a strong source of CO2 throughout the year.  The Kuroshio Current and extension are a strong CO2 sink in winter due primarily to cooling, and a weak source in summer due to warming.  Western subarctic areas are a strong CO2 source in the winter because of convective mixing of waters rich in respired CO2 and nutrients; they become a strong sink in the winter since the nutrients help fuel intense photosynthesis.  The only regions that have shown a decrease of pCO2 are the Bering and Okhotsk Seas, due to increased biological activity.  This may be the result of changing nutrient supplies caused by changes in land hydrology or by increases in river or airborne inputs of nutrients.  Overall, seasonal temperature changes are the primary cause for seasonal changes of pCO2­ in subtropical gyres, while changes in total CO2 concentration caused by winter upwelling and springtime plankton blooms are the primary cause for seasonal changes in sub-polar and polar regions (Takahashi et al., 2006).

            Generally, increases in pCO2 in the North Pacific have been following the atmospheric CO2 trend.  The rate of increase varies, however, due to differences in local oceanographic processes: upwelling of subsurface waters, lateral mixing, and biological activity.  Additionally, Takahashi et al. (2006) find that the observed increase in pCO2 is not affected significantly by SST changes, but is primarily due to change in seawater chemistry most likely by the uptake of atmospheric CO2.  It is important to study seawater chemistry as well as temperature and circulation changes throughout the world’s oceans, as these can affect future uptake or outgassing of CO2.  In addition, it is vital to understand the role of various mechanisms for changes in CO2 flux in order to accurately quantify the potentially changing role of the oceanic sink for climate projections.

 

 

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.

 

(a)

(b)

 

Figure 1.  Patra et al. (2005) found that the sea-air CO2 flux over the North Pacific is (a) significantly associated with the PDO at five months lag and (b) with ENSO at three months lag.  In (a), the green line represents the south part of the North Pacific, the blue line represents the north part, and the red line is aggregated to TransCom-3 region size (see Patra et al. for reference).  In (b), NP(N) and NP(S) represent the northern and southern regions of the Pacific, respectively.  The broken line represents the 95% significance level and the solid line represents the 99% significance level.