Richard H. Gammon

Observations of the Mindanao Current during the western equatorial Pacific Ocean circulation study

The Western Equatorial Pacific Ocean Circulation Study (WEPOCS) III expedition was conducted from June 18 through July 31, 1988, in the far western equatorial Pacific Ocean to observe the low-latitude western boundary circulation there, with emphasis on the Mindanao Current. This survey provides the first quasi-synoptic set of current measurements which resolve all of the important upper-ocean currents in the western tropical Pacific. Observations were made of the temperature, salinity, dissolved oxygen, and current profiles with depth; of water mass properties including transient tracers; and of evolving surface flows with a dense array of Lagrangian drifters. This paper provides a summary of the measurements and a preliminary description of the results. The Mindanao Current was found to be a narrow, southward-flowing current along the eastward side of the southern Philippine Islands, extending from 14°N to the south end of Mindanao near 6°N, where it then separates from the coast and penetrates into the Celebes Sea. The current strengthens to the south and is narrowest at 10°N. Direct current measurements reveal transports in the upper 300 m increasing from 13 Sv to 33 Sv (1 Sverdrup = 1 × 106 m3 s−1) between 10°N and 5.5°N. A portion of the Mindanao Current appears to recurve cyclonically in the Celebes Sea to feed the North Equatorial Countercurrent, merging with waters from the South Equatorial Current and the New Guinea Coastal Undercurrent. Another portion of the Mindanao Current appears to flow directly into the NECC without entering the Celebes Sea. The turning of the currents into the NECC is associated with the Mindanao and Halmahera eddies.

Basin-wide distributions of chlorofluorocarbons CFC-11 and CFC-12 in the North Pacific: 1985–1989

All of the dissolved chlorofluorocarbon measurements made between 1985 and 1989 along several long zonal and meridional hydrographic sections in the North Pacific are presented in this manuscript. Chlorofluorocarbon (CFC) concentrations are displayed as functions of depth and density along the sections. Over much of the region studied, dissolved CFCs are observed to have penetrated to densities greater than those that outcrop at the surface in the North Pacific (σθ > 26.8). Maxima in CFC concentration are associated with remnant winter mixed layers and with mode waters. When the observed CFC concentrations from these sections are normalized to a common date and mapped onto five density surfaces in the North Pacific, it becomes apparent that the Sea of Okhotsk is an important location for the ventilation of the intermediate waters of the North Pacific. The CFC observations are used together with hydrographic data to study the pathways and timescales of circulation and ventilation processes in the upper and intermediate waters of the North Pacific. Using models of the increases of these compounds as a function of time, CFC “apparent ages” are calculated on these isopycnal surfaces. The CFC apparent ages are used together with observed apparent oxygen utilization to estimate oxygen utilization rates along these sections.

A reevaluation of the open ocean source of methane to the atmosphere

Seawater and atmospheric methane (CH4) mixing ratios were measured on five cruises throughout the Pacific Ocean from 1987 to 1994 to assess the magnitude of the ocean-atmosphere flux. The results showed consistent regional and seasonal variations with surface seawater concentrations ranging from 1.6 to 3.6 nM and saturation ratios ranging from 0.95 to 1.17. The equatorial Pacific Ocean was supersaturated with respect to atmospheric CH4 partial pressures, while areas outside the tropics often were undersaturated during fall and winter. Although atmospheric CH4 mixing ratios over the North Pacific during April increased 3.4% from 1988 to 1993, the saturation ratios remained constant. Based on the concentration fields, the data were divided into two seasons and 10 latitude zones from 75°S to 75°N. Using monthly Comprehensive Ocean-Atmosphere Data Set (COADS) wind and surface seawater temperature data and the Wanninkhof [1992] wind speed/transfer velocity relationship, the calculated zonal average fluxes ranged from −0.1 to 0.4 μmol m−2 d−1. The combined seasonal and zonal fluxes result in a total global ocean-to-atmosphere flux of 25 Gmol yr−1 (0.4 Tg CH4 yr−1), which is an order of magnitude less than previous estimates [Intergovernmental Panel on Climate Change (IPCC), 1994]. The estimated uncertainty in this number is approximately a factor of 2.

Regional and seasonal variations in the flux of oceanic carbon monoxide to the atmosphere

Carbon monoxide (CO) is produced photochemically in the surface ocean and emitted to the atmosphere. To assess the magnitude of this ocean-atmosphere flux, seawater and atmospheric CO mole fractions were measured on six cruises throughout the Pacific Ocean from 1987 to 1994. The results showed consistent regional and seasonal variations in surface seawater CO concentrations with daily averaged concentrations ranging from 0.1 to 4.7 nM. Based on the concentration fields, the data were divided into four seasons and 10 latitude zones from 75°S to 75°N. Using monthly Comprehensive Ocean-Atmosphere Data Set wind and surface seawater temperature data and the Wanninkhof [1992] wind speed/transfer velocity relationship, the calculated zonal average fluxes ranged from 0.25 to 13 μmol/m2/d. The combined seasonal and zonal fluxes result in a total global flux of 0.46 Tmol CO/y with 2/3 of this flux in the southern hemisphere. The estimated uncertainty in this number is approximately a factor of 2.

Assessment of the air‐sea exchange of CO2 in the south Pacific during austral autumn

Measurements of CO2 concentrations in the atmosphere and in the surface waters of the South Pacific Ocean were made by NOAA scientists between 1984 and 1989. These basin-wide measurements were all taken during austral autumn and provide data for evaluation of the seasonal flux of CO2 from this region. The sensitivity of this flux to the uncertainty in the CO2 gas exchange coefficient was evaluated using four different wind data sets and two formulations for the wind dependence of gas transfer velocity. The integrated net flux of CO2 to the atmosphere during austral autumn (February to May) ranges from −0.03 (ocean influx) to +0.09 (ocean efflux) GT of carbon depending on the combination of wind field and wind-dependent exchange coefficient used.

Reevaluation of the open ocean source of carbonyl sulfide to the atmosphere

Carbonyl sulfide (COS) concentrations were measured in surface seawater samples and the overlying marine boundary layer of the Pacific Ocean by using gas chromatography (GC) and electron capture sulfur detection (ECD-S). A wide latitudinal range was covered (55°N–70°S) on two cruises 9 months apart. COS saturation ratios (SRs) in seawater were found to be less than 1 (undersaturated) across wide regions of the open ocean, especially in the subtropical gyres and wintertime subpolar waters. SRs were highest in coastal/shelf regions and in spring/summertime temperate waters. Extensive undersaturation is attributed to a low COS photoproduction potential of the water, limited sunlight, and/or a rapid hydrolysis rate constant. Decreasing COS concentrations during diurnal cycles in tropical waters were fitted to first-order exponentials, with resulting decay times agreeing with calculated hydrolysis lifetimes to within 15%. Air-sea fluxes of COS from the open ocean were calculated by using two different expressions for the transfer velocity and averaged into six latitude bands and three seasons. On the basis of these data we report a global open ocean sea-air flux of −0.032 (−0.010 to −0.054) Tg COS/yr, which is much lower than and of different sign from the current global estimate (0.14–0.58 Tg COS/yr). Atmospheric COS mixing ratios averaged 470 pptv on the first cruise and 442 pptv on the second cruise, with much of the difference possibly a result of a seasonal decrease in the northern hemisphere COS mixing ratio of up to 10%.

Autumn air-sea disequilibrium of CO2 in the South Pacific ocean

Accurate assessment of the role of the South Pacific in the global carbon cycle has been hindered by a paucity of data. In particular, the South Pacific has been suggested as a major sink for CO2 on the basis of measurements made principally in the western basin. The PMEL/NOAA Marine CO2 Program has made basin-wide CO2 measurements over 6 years which now permit estimation of surface CO2 fluxes during the austral autumn. These results suggest that the South Pacific is not a significant sink for atmospheric CO2 during the austral autumn.

Examining a coupled climate model using CFC-11 as an ocean tracer

Anthropogenic CFC-11 dissolved in seawater is used to analyze ocean ventilation simulated in a global coupled air-sea model. Modeled CFC-11 distributions are compared to observations gathered on three Southern Hemisphere research cruises. The total amount of CFC-11 absorbed by the models Southern Ocean is realistic, though some notable differences in the vertical structure exist. Observed and simulated CFC-11 distributions are qualitatively consistent with the coupled models predictions that the ocean may delay greenhouse gas-induced warming of surface air temperatures at high southern latitudes. The sensitivity of model-predicted CFC-11 levels in the deep Southern Ocean to the choice of gas exchange parameterization suggests that quantitative assessments of model performance based upon simulated CFC-11 distributions can be limited by air-sea gas flux uncertainties in areas of rapid ocean ventilation. Such sensitivities can complicate the quantitative aspects of CFC-11 comparisons between models and observations, and between different models.

North Pacific Ocean CO2 disequilibrium for spring through summer, 1985–1989

Extensive measurements of CO2 fugacity in the North Pacific surface ocean and overlying atmosphere during the years 1985–1989 are synthesized and interpreted to yield a basin-wide estimate of ΔfCO2. The observations, taken from February through early September, suggest that the subtropical and subarctic North Pacific is a small sink for atmospheric CO2 (0.07 to 0.2 Gton C (half year)−1 for the region north of 15°N). Objective analysis techniques are used to estimate uncertainty fields resulting from constructing basin-wide contours of oceanic fCO2 on the basis of individual cruise transects. The uncertainties are significant and imply that future sampling programs need to recognize that estimating oceanic uptake of anthropogenic CO2 from ship-transect observations of oceanic fCO2 alone will require very extensive sampling.

Atmospheric 14CO: A tracer of OH concentration and mixing rates

Time series measurements of the ground level 14CO concentration were made at Olympic Peninsula, Washington (48°N), and Point Barrow, Alaska (71°N), between 1991 and 1997. Measurements of the meridional gradients of the 14CO concentration at sea level were made during five oceanographic cruises in the Pacific Ocean between 55°N and 65°S during 1991–1995. These measurements were combined with earlier time series measurements of atmospheric 14CO at 41°S and 77°S [Brenninkmeijer, 1993] and at 13°N [Mak and Southon, 1998] and meridional transects of 14CO at 6–8 km [Mak et al., 1994]. These 14CO data sets were analyzed using a two-dimensional atmospheric circulation and chemistry model in order to determine the tropospheric OH concentration that could explain the temporal and spatial trends in 14CO. Additionally, the interannual trend in tropospheric methyl chloroform concentration and the stratospheric time history of bomb 14CO2 were simulated by the model. The results of this analysis indicate that an average tropospheric OH concentration of ∼l0×l05 radicals cm−3 explains both the 14CO and methyl chloroform trends. The model-predicted 14CO concentrations, however, are sensitive to the rate of stratosphere-troposphere exchange and horizontal mixing in the troposphere. Model predictions of tropospheric 14CO at high latitudes improved when the stratosphere-troposphere exchange rate was slowed, based on the results of the stratospheric bomb 14CO2 model simulation. Substantial improvement in the model 14CO simulations occurred with increased horizontal diffusion rates in the troposphere and lower cosmogenic 14CO production rates. Significantly lower 14CO concentrations (∼50%) are observed in the Southern versus Northern Hemisphere. Model simulations indicate that either higher tropospheric horizontal mixing or higher OH concentrations in the Southern Hemisphere can explain the hemispheric asymmetry in 14CO.