Equatorial Pacific Region Research Articles

Preliminary result of Central Equatorial Pacific Ocean Tomography Experiment

JAMSTEC (Japan Marine Science and Technology Center) and APL (Applied Physics Laboratory, University of Washington) started a joint program to directly measure part of the circulation cell of the Central Equatorial Pacific Ocean using acoustic tomography. The tropical and subtropical Pacific Ocean are suggested to connect via a shallow meridional circulation cell by Gu and Philander (1997). This subtropical cell (STC) has been hypothesized to the El Nino-Southern Oscillation phenomena in the tropics, which then, in turn, can affect the subtropical ocean via rapid atmospheric forcing. But no direct measurements exist and indirect measurements and modeling support the hypothesis. The observational centerpiece is tomography systems located in the Central Equatorial Pacific Region. JAMSTEC began the field work by deploying five acoustic transceivers in the observational region and this work will continue until January 2001 (two-year plan). Measurement data were sent to JAMSTEC in realtime using satellite telemetry system. These data were analyzed and compared with CTD observations. The time series fluctuation of the temperature was solved by this analysis.

Ocean color variability of the tropical Indo‐Pacific basin observed by SeaWiFS during 1997–1998

High‐quality ocean color data (chlorophyll) provided by the Sea‐viewing Wide Field of view Sensor (SeaWiFS) satellite were analyzed for the first complete year of coverage (October 1997 to September 1998) in the tropical Indo‐Pacific basin. This period coincides with the peak of one of the strongest El Niño events during December 1997 and the La Niña of 1998 that appeared dramatically in less than a month as a sea surface temperature (SST) change of over 6°C in the central equatorial Pacific during June 1998. The tropical Indian Ocean also underwent a highly anomalous series of events with negative SST anomalies (SSTA) of over 3°C in the eastern equatorial and coastal regions during October‐December 1997 and warm SSTA in the west that peaked at over 2°C during February 1998. The ocean color variability is interpreted using other satellite data such as sea level from TOPEX/Poseidon and also in terms of the dynamics and thermodynamics of the region from simulations with an ocean general circulation model. The El Niño‐related reductions in equatorial production and the off‐equatorial increase in biological activity, and their basin scale evolution is clearly seen for the first time. Persistent northerly wind anomalies resulted in a northward shift of the equatorial divergence and the upwelling Kelvin wave which signalled the end of the 1997–1998 El Niño. The anomalous surface chlorophyll associated with this Kelvin wave was also clearly shifted north of the equator by nearly 300 km and appeared more than a month before the negative sea level anomalies seen by TOPEX/Poseidon. On the equator near 165°E, the disappearance of the barrier layer appeared to coincide with a localized bloom that occurred in response to the easterly wind bursts over the western Pacific that lasted from December 1997 through the boreal summer. The ecosystem response to the cold La Niña conditions is clearly seen as elevated chlorophyll during the boreal summer of 1998 in the equatorial Pacific cold tongue region. In the Indian Ocean, an anomalous phytoplankton bloom was observed by SeaWiFS during October‐December 1997 coincident with the anomalous upwelling in the eastern equatorial region and off the coast of Sumatra. A stronger than normal northeast monsoon is seen as higher than climatological values of surface chlorophyll. The open ocean Ekman pumping and the shoaling of the thermocline near 60°E and 10°S and the eastward extension of mixed layer entrainment in the same latitude band is seen as a region of higher biological activity during the boreal summer.

Influence of El Niño on the equatorial Pacific contribution to atmospheric CO2 accumulation

The equatorial oceans are the dominant oceanic source of CO2 to the atmosphere, annually amounting to a net flux of 0.7–1.5 Pg (1015 g) of carbon, up to 72% of which emanates from the equatorial Pacific Ocean1,2,3. Limited observations indicate that the size of the equatorial Pacific source is significantly influenced by El Nino events4,5,6,7,8,9,10, but the effect has not been well quantified. Here we report spring and autumn multiannual measurements of the partial pressure of CO2 in the surface ocean and atmosphere in the equatorial Pacific region. During the 1991–94 El Nino period, the derived net annual sea-to-air flux of CO2 was 0.3 Pg C from autumn 1991 to autumn 1992, 0.6 Pg C in 1993, and 0.7 Pg C in 1994. These annual fluxes are 30–80% of that of 1996, a non-El-Nino year. The total reduction of the regional sea-to-air CO2 flux during the 1991–94 El Nino period is estimated to account for up to one-third of the atmospheric anomaly (the difference between the annual and long-term-average increases in global atmospheric CO2 content) observed over the same period.

Aircraft measurements of ozone, NOx, CO, and aerosol concentrations in biomass burning smoke over Indonesia and Australia in October 1997: Depleted ozone layer at low altitude over Indonesia

The 1997 El Niño unfolded as one of the most sever El Niño Southern Oscillation (ENSO) events in this century and it coincided with massive biomass burning in the equatorial western Pacific region. To assess the influence on the atmosphere, aircraft observations of trace gases and aerosol were conducted over Kalimantan in Indonesia and Australia. Over Kalimantan in Indonesia, high concentrations of O3, NOx, CO, and aerosols were observed during the flight. Although the aerosol and NOx decreased with altitude, the O3 had the maximum concentration (80.5 ppbv) in the middle layer of the smoke haze and recorded very low concentrations (∼20 ppbv) in the lower smoke layer. This feature was not observed in the Australian smoke. We proposed several hypotheses for the low O3 concentration at low levels over Kalimantan. The most likely are lack of solar radiation and losses at the surface of aerosol particles.

An iron‐based ecosystem model of the central equatorial Pacific

The central and eastern equatorial Pacific region is characterized by lower than expected phytoplankton biomass and primary production given the relatively high ambient nitrate concentrations. These unusual conditions have spawned several field programs and laboratory experiments to determine why this high nitrate‐low chlorophyll pattern persists in this region. To synthesize the results from these field programs, as well as providing additional evidence in support of the iron hypothesis, we developed a one‐dimensional, nine‐component ecosystem model of 0°N 140°W. The model components include two phytoplankton size fractions, two zooplankton size fractions, two detrital size fractions, dissolved iron, nitrate, and ammonium. The model was run for 5 years (1990–1994) and was forced using an atmospheric radiative transfer model, an ocean general circulation model (GCM), and in situ data. To our knowledge, this is the first ecosystem model at 0°N 140°W to synthesize the Joint Global Ocean Flux Study Equatorial Pacific Process Study (JGOFS EqPac) data set, as well as to use both in situ and modeled physical data to drive the model. Modeled phytoplankton, zooplankton, and iron all varied on interannual timescales due to El Niño events. Total phytoplankton biomass increased by as much as 40% from early 1992 (El Niño warm) to 1993 (normal). The results also indicate that the biomass increase during a cool period is not constant for each phytoplankton component, but instead the increase is most evident in the netphytoplankton (>10 μm). Netphytoplankton increase from a low of 0.1% of the total chlorophyll in 1992 to a high of 30% of the total in 1993. Microzooplankton grazing rates fluctuated in response to changes in nanophytoplankton growth rates, whereas mesozooplankton grazing was unrelated to netphytoplankton growth rates. The magnitude and temporal variability of phytoplankton chlorophyll agreed well with in situ data collected during 1992. Modeled primary production was lower than measured during El Niño but agreed with observations during normal conditions. The low primary productivity was probably a result of downwelling produced by the physical model. New production was calculated from total and recycled iron rather than nitrate‐based production and was more variable in general and almost 3 times the nitrate‐based new production during non‐El Niño conditions.

Equatorial spread F effects on an HF path: Doppier spread, spatial coherence, and frequency coherence

In August 1990 we participated in the Equatorial Ionospheric Studies sounding rocket campaign near Kwajalein Atoll in the equatorial Pacific region. The campaign included measurements of plasma density using rocket probes and coherent and incoherent scatter radar. During the campaign we fielded high‐frequency ionospheric sounders over a bistatic path between Maloelap Atoll and Bikini Atoll in the Marshall Islands. The distance between the transmitters and receivers was 700 km; the ionospheric‐reflection region was at 10.18°N, 168.40°E, near the magnetic equator. We made three types of measurements: Doppler spread and spatial coherence for a single‐frequency CW path; frequency coherence of multiple CW paths; and Doppler spread and time‐delay spread for a 60‐kHz bandwidth path. We obtained such data over a period of 2 weeks for approximately 2 hours each evening; during this period spread F was common. Fifty percent of the evenings showed Doppler spread of greater than 6 Hz at the −10 dB level (relative to the peak signal power) and greater than 15 Hz at the −30 dB level. Forty percent of the evenings showed spatial coherence distance of less than 180 m in the direction normal to the bistatic path; 40% of the evenings showed spatial coherence of less than 75 m in the direction parallel to the path. Seventy‐five percent of the evenings showed coherence bandwidths of less than 1.5 kHz.

Measuring heat storage changes in the equatorial Pacific: A comparison between TOPEX altimetry and Tropical Atmosphere‐Ocean buoys

Heat storage variations in the equatorial Pacific have been studied from December 1992 through February 1997, using sea level data from the TOPEX altimeter and temperatures measured by 42 tethered buoys that are part of the Tropical Ocean‐Atmosphere (TAO) array. The TOPEX measurements are converted to heat storage anomalies using a coefficient determined from mean climatological values. For 30‐day averages the two measurements agree well over most of the region, except for the southwestern quadrant of the warm pool and a small region of the north‐central equatorial Pacific. In the southwestern quadrant the TOPEX measurements indicate a smaller long‐term heating rate than the TAO measurements, differing by as much as 30%. After examining conductivity‐temperature‐depth data in this region, it appears that the difference is due to a change in ocean salinity which is reflected in the TOPEX sea level measurements but not in the TAO heat measurements. The signal in the north‐central region is predominately at an annual period, but there is not enough external measurements to determine what the source of the difference is. In the remainder of the equatorial Pacific, the agreement between the data suggests that the TOPEX measurements can be used to measure heat storage variations in the upper layer with reasonable accuracy. Thus TOPEX altimeter data can provide information about the heat budget of the equatorial Pacific in regions where there are few or no direct measurements.

Forecasting regional sea surface temperatures in the tropical Pacific by neural network models, with wind stress and sea level pressure as predictors

We constructed two types of neural network models for forecasting the sea surface temperature anomaly (SSTA) over several standard equatorial Pacific regions (Niño 3, 3.4, 3.5, 4, P2, P4, and P5). The first type used the sea level pressure (SLP) as predictors, while the second one used the wind stress. By ensemble averaging over 20 forecasts with random initial weights, the resulting forecasts were much less noisy than those in our earlier models. The models performed best in the western‐central equatorial regions and less well in the eastern boundary regions. At longer leads of 9–12 months, the cross‐validated skills (1952–1993) for the models using the tropical Pacific SLP as predictors were statistically higher than those using the wind stress. Overall, the models using the tropical SLP showed usable cross‐validated skills up to 12‐month lead. The true out‐of‐sample forecast performances during the 1982–1993 period for the Niño 3.5 SSTA at lead times of 9, 12, and 15 months attained correlation skills of 0.78, 0.80, and 0.75, respectively.

A Tropical Influence on Global Climate

Abstract A potential influence of tropical sea surface temperature on the global climate response to a doubling of the CO2 concentration is tested using an atmospheric general circulation model coupled to a slab mixed layer ocean. The warming is significantly reduced when sea surface temperatures in the eastern equatorial Pacific cold tongue region between latitudes 2.25°N and 2.25°S are held at the control simulation values. Warming of the global mean temperature outside of the cold tongue region is reduced from 2.4°C in the unconstrained case to 1.9°C when the sea surface temperature constraint is applied. The decrease in the warming results from a positive net heat flux into the ocean cold tongue region and implicit heat storage in the subsurface ocean, induced by horizontal atmospheric heat fluxes. The reduced surface temperature warming outside of the cold tongue region is due to reduction in the downward longwave radiative flux at the surface, caused in turn by reduced atmospheric temperature and mo...

Diurnal Variations in Tropical Oceanic Cumulus Convection during TOGA COARE

Diurnal variations in atmospheric convection, dynamic/thermodynamic fields, and heat/moisture budgets over the equatorial Pacific warm pool region are analyzed based on data collected from different observation platforms during the Intensive Observation Period of the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE). Results reveal that the diurnal variations in rainfall/convection over the TOGA COARE region can be classified into three distinct stages: warm morning cumulus, afternoon convective showers, and nocturnal convective systems. Afternoon rainfall comes mostly from convective cells, but the nocturnal rainfall is derived from deeper convective cells and large areas of stratiform clouds. Results further show that afternoon convective showers are more evident in the large-scale undisturbed periods when the diurnal SST cycle is strong, but the nocturnal convective systems and morning cumulus are more enhanced in the disturbed periods when more moisture is available. The primary cause of the nocturnal rainfall maximum is suggested to be associated with more (less) available precipitable water in the night (day) due to the diurnal radiative cooling/heating cycle and the resultant change in tropospheric relative humidity.

The total organic carbon export rate based on 13C and 12C of DIC budgets in the equatorial Pacific region

Carbon stable isotopic composition (δ 13C) of total dissolved inorganic carbon (DIC) is independent of DIC as a tracer for net organic carbon production rate in the surface ocean due to the Significantly different equilibration times for CO 2 and 13CO 2 during air-sea exchange. The rates of the organic carbon export and net physical supply of DIC, can therefore, be determined from the mixed layer mass balances for DIC and DIC 13 (DIC· 13C/ 12C), given an air-sea C0 2 flux estimated from wind speed and measured pCO 2. This method does not rely on specifying the absolute advetive and diffusive transports of DIC. The mass balance approach was applied to measurements of concentration and δ 13C of DIC made in the equatorial Pacific ocean in the Spring and Fall of 1991–1992 during WOCE P16C and US JGOFS EqPac cruises. The calculated organic carbon export rate out of mixed layer during Spring 1992 El Nin˜o was 3.2±1.4 mmol C m −2 d −1 (between 2°N to 2°S and 170°W to 110°W). No spatial trend for the total organic carbon export rate was determined despite the large variation in C02 degassing rate from ∼0.1 mmol C m −2 d −1 at 170°W to ∼4.0 mmol Cm −2 d −1 at 110°W over this region. Under the cold tongue conditions in the end of August 1992, a higher organic carbon export rate, about 11.5±6.3 mmol C m -2 d −1 was obtained on the equator at 140°W while the rate for the previous September (in 1991) was 4.9 ± 2.4 mmol C m −2 d −1. Most of the uncertainty in our estimated results from the uncertainty in gas exchange coefficient. The comparison between the estimated organic carbon export rates and the particulate organic carbon (POC) export rates derived from thorium budget and sediment drifting traps at the same stations (Buesseler et al., 1995; Murray et al., 1996) suggests that particulate organic carbon probably accounts for about 70% of the organci export. Since physical, rather than biological, processes primarily control the distribution of DIC, NO 3 O 2 and other nutrient-related properties in this region, it is very difficult to accurately obtain the net organic carbon export rate from advection rates and nutrient gradients, as demonstrated by a 3-D calculation.

Biogenic budgets of particle rain, benthic remineralization and sediment accumulation in the equatorial Pacific

Budgets of organic C ( C org), CaC0 3 and opal have been constructed for the Palisades, NY Pacific equatorial region at 140°W between 5°N and 5°S. Measurements of the rain and benthic remineralization rate of biogenic materials have been adjusted and normalized to account for sampling biases. Sea surface temperature serves as a master variable in normalizing sediment trap and benthic remineralization data to average conditions. The rain and remineralization rates for C org are nearly equal: 0.40±0.05 and 0.46±0.06 mmol m −2 d −1 respectively; thus only a minor fraction of this constituent is buried. Rain and dissolution rates for biogenic opal are similarly balanced (0.3±0.06 and 0.36±0.01 mmol m −2 d −1) and consistent with the value for opal burial (0.0±0.004). The CaC0 3 budget appears to have changed during the Holocene. The best estimates of modern CaC0 3 dissolution (0.58±0.03 mmol m −2 d −1) and rain rate (0.61±0.06) are consistent with 230Th-normalized carbonate accumulation rates for the late Holocene (0.1 mmol m −2 d −1). However, the balance between dissolution and rain is not consistent with early Holocene carbonate accumulation (0.3 mmol m −2 d −1 ), and this imbalance suggests: 1) a recent increase in the rate of CaC0 3 dissolution on the sea floor, or 2) a decrease in the rain rate of carbonate particles. Modeling 230Th profiles in sediments from this region define the last 3000 years as the duration of increased dissolution or decreased particle rain. 231Pa/ 230Th ratios in sediments indicate that particle rain rates have remained constant or possibly increased slightly through the Holocene. Two potential causes for increased dissolution were investigated; a change in deep water carbonate saturation or a change in Co,g/CaC0 3 rain ratios. A model describing carbonate dissolution as a function of the degree of undersaturation and the amount of organic carbon oxidation within sediments indicates that the recent increase in dissolution is more likely due to changes in bottom water chemical composition. We propose that Pacific Ocean bottom water carbonate ion concentration has decreased by 10–15 μM over the last 3000 years.

A coupling experiment of an atmosphere and an ocean model with a monthly anomaly exchange scheme

A nine-layer spectral atmospheric general circulation model is coupled to a twenty-layer global oceanic general circulation model with the “prediction-correction” monthly anomaly exchange scheme which has been proposed at the Institute of Atmospheric Physics (IAP). A forty-year integration of the coupled model shows that the CGCM is fairly successful in keeping a reasonable pattern of the modelled SST although most of the Pacific become warmer than those given by the uncoupled ocean model. The model tends to reach a more realistic state than the uncoupled one in terms of downward surface heat flux into ocean particularly in the equatorial Pacific region. Also, the model is capable to simulate interannual variability of sea surface temperature in tropical region.

Reef coral reproduction in the eastern Pacific: Costa Rica, Panamá, and Galápagos Islands (Ecuador). III. Agariciidae (Pavona gigantea and Gardineroseris planulata)

The reproductive ecology of Pavona gigantea Verrill and Gardineroseris planulata (Dana) was investigated in the equatorial eastern Pacific region from 1985 to 1994. These zooxanthellate scleractinian corals were adversely affected in this region during the 1982–1983 El Nino warming event. Both species were hermaphroditic, with individual colonies showing sequential cosexual development, thus resulting in dominantly outbreeding reproduction. Sexuality was mixed, with high percentages of gonochoric and hermaphroditic colonies in both species. Approximately 1:1 male-to-female gonad ratios were found in gonochoric and hermaphroditic colonies combined. Broadcast spawning was observed in P. gigantea in the Galapagos Islands, and the sudden disappearance of mature gametes and the presence of spent gonads suggest that G. planulata is also a broadcast spawner. Colonies of both species with ≲200 cm2 (10 cm diam) live tissue were nonreproductive. Estimated ages of the youngest reproductive colonies were 11 yr for P. gigantea and 20 yr for G. planulata. The percentage of all colonies of P. gigantea with gonads at nonupwelling sites (Cano Istand, Costa Rica and Uva Island, Panama) ranged from 37 to 47%, respectively; colonies with gonads from upwelling environments (Saboga and Taboga Islands, Panama) ranged from 31 to 39%, respectively, and reproductively active colonies from the thermally variable Galapagos islands comprised 40% of the collections. Compared with P. gigantea, the numbers of sexually active G. planulata colonies were roughly onehalf at nonupwelling Cano Island (20%) and Uva Island (25%) sites, or less (10%) at the upwelling Saboga Island site. Peak reproductive activity in P. gigantea occurred during the rainy season at all study sites. In the nonupwelling Costa Rican (Cano Island) and Panamainan (Uva Island) sites, mean monthly sea-surface temperatures (SSTs) were high (28 to 29°C), but slightly lower than in the dry season (29°C). In the upwelling Gulf of Panama (Saboga and Taboga Islands), reproduction occurred after mean monthly SSTs increased from 24 to 28–29°C. In the Galapagos Islands, reproductive activity peaked during sea warming, when mean monthly SSTs reached 25°C. Sexually active colonies of G. planulata, present only at the main collection sites of Cano and Uva Islands, were also observed during the wet season. The presence of mature or spawned gonalds in both species mostly around new and full lunar phases suggests that spawning is at least weakly synchronized with moon phase. Fecundity estimates disclosed the following nonsignificant differences between sites for P. gigantea, expressed as egg production cm-2 colony surface surface yr-1: Galapagos (10 300 to 30 800), Uva Island (4900 to 9800), Cano Island (1800 to 7400), Saboga Island (600 to 1300) Taboga Island (1200 to 2400). Fecundity estimates for G. planulata were considerably lower: Uva Island (700 to 1400), Cano Island (500 to 1000). The sexual recruitment of P. gigantea into El Nino-Southern Oscillation (ENSO) 1982–1983-disturbed, equatorial eastern Pacific coral communities has been low, with only moderate recovery evident since 1983. G. planulata has revealed no sexual recruitment where seed populations are absent or rare (Cano Island, Galapagos Islands), and only low recruitment (Panama) in areas with colonies that survived the ENSO disturbance.

Early diagenesis of organic material in equatorial Pacific sediments: stpichiometry and kinetics

Benthic incubation chambers and sediment pore water profiles were used to study early diagenesis of organic matter in equatorial Pacific sediments. Replicate measurements with a flux chamber covering 720 cm 2 indicated that the spatial variability of oxygen, TCO 2, alkalinity, nitrate and silica fluxes at a single station did not exceed 10–35%. In contrast, diffusive fluxes of oxygen from replicate cores covering 70 cm 2 at a single station often showed greater variation. In January 1992, benthic oxygen consumption was fairly constant along the equator from 103°W to 140°W at 0.6-0.8 mmol m −2 day −1. In November 1992, consumption was roughly symmetrical across the equator along 140°W, with rates of 0.6-0.8 mmol m −2 day −1 between 2°S and 2°N, declining to rates of 0.1-0.2 mmol m −2 day −1 at 12°S and 9°N. Pore water oxygen profiles were fit with a reaction-diffusion model equation to evaluate reaction kinetics. Most profiles were adequately fit with a model that assumed reaction rates declined exponentially with depth, but at low latitudes better fits often were obtained with a model that assumed decomposing organic matter has two labile components and that each decays with first-order kinetics and decreases exponentially with depth. Results of both fits indicate that at least 70% of the organic matter degradation occurs within the upper 1–2 cm of sediment. At the low-latitude stations fit with the two-component model, 70–90% of the flux is attributable to the more labile component which has an average 1/e penetration depth of 0.4 ± 0.1 cm. The more refractory component at these stations has a penetration depth of 4.4 ± 0.4 cm. From estimates of sediment mixing rates, the mean life of all degrading organic matter at the higher latitude stations is 4–55 years, while at the stations fit with the two-component model, the lifetime of the more labile fraction is weeks to months, and the lifetime of the less labile component is 40–300 years. A third carbon fraction exists at all stations that is far more refractory. The O 2:CO 2 stoichiometry of remineralization is −1.45 ± 0.17, and the C:N ratio is 8 ± 1. Both ratios are in good agreement with those observed from sediment trap and hydrographic studies in the water column, and suggest that degrading organic matter has about 70% of its carbon in -CH 2O-groups and 30% in -CH 2-groups. The C:P atom ratios for benthic remineralization differ by a factor of 3 for the two cruises, showing substantial temporal variability and de-coupling from carbon, although the mean for the two cruises (170 ± 85) is not significantly different than remineralization ratios observed in the water column. The aerally-integrated benthic respiration rate for the equatorial Pacific upwelling region is at least 25% of the integrated respiration rate for the continental margin (slope + rise) areas of the Pacific, emphasizing the importance of the equatorial Pacific sediments as a major site of benthic carbon recycling. Benthic carbon remineralization rates determined during the past decade near the equator and 140°W have varied by a factor of 2, which is not surprising given the short lifetime of the majority of the carbon degrading. The temporal patterns of carbon remineralization rates resemble those of sea-surface temperature, suggesting that benthic carbon oxidation at this site may reflect water column productivity over relatively short timescales.

Neutral carbohydrate geochemistry of particulate material in the central equatorial Pacific

Neutral carbohydrate compositions were determined for particulate samples from plankton net tows, shallow floating sediment traps, mid-depth and deep moored sediment traps, and sediment cores collected along a north-south transect in the central equatorial Pacific Ocean during the U.S. JGOFS EqPac program. Total neutral carbohydrate depth profiles and patterns along the transect follow essentially the same trends as bulk and organic carbon (OC) fluxes—attenuating with depth, high near the equator and decreasing poleward. OC-normalized total aldose (TCH 2,O) yields along the transect and with depth do not show any consitent patterns. Relative to a planktonic source, neutral carbohydrate compositions in sediment trap and sediment core samples reflect preferential loss of ribose and storage carbohydrates rich in glucose, and preferential preservation of structural carbohydrates rich in rhamnose, xylose, fucose, and mannose. There is also evidence for an intermediately labile component rich in galactose. It appears that compositional signatures of neutral carbohydrates in sediments are more dependent upon their planktonic source than on any particular diagenetic pathway. Relative to other types of organic matter, neutral carbohydrates are better preserved in calcareous oozes from 12°S to 5°N than in red clays at 9°N based on OC-normalized TCH 2O yields, due to either differing sources or sorption characteristics. Weight per cent glucose generally decreases with increased degradation of organic material in the central equatorial Pacific region. Based on weight per cent glucose, comparisons of samples between Survey I (El Niõn) and Survey II (non-El Niño) indicate that during Survey I, organic material in the epipelagic zone in the northern hemisphere may have undergone more degradation than organic material in the southern hemisphere.

Classification of clouds over the western equatorial Pacific Ocean using combined infrared and microwave satellite data

A new cloud classification scheme is presented that combines infrared and microwave satellite data. Because microwave radiation can penetrate deep into the cloud layer, this scheme is able to determine characteristics for both thin and deep clouds. Additionally, the new scheme can provide information on precipitation, which traditional infrared‐visible cloud classification schemes have been unable to. The proposed cloud classification scheme utilizes the cloud top temperature obtained from infrared measurements and a microwave index that includes both emission and scattering signals. The following eight cloud classes are defined: warm nonprecipitating cloud, warm precipitating cloud, midtop nonprecipitating cloud, midtop precipitating cloud, thin high‐top nonprecipitating cloud, deep high‐top nonprecipitating cloud, anvil with stratiform precipitating cloud, and deep convective precipitating cloud. The classification scheme is validated by aircraft radar data obtained from Tropical Ocean‐Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment. The new cloud classification scheme is used to investigate the clouds in the western equatorial Pacific Ocean warm pool region for the period from November 1992 to February 1993, allowing us to obtain for the first time the spectra of cloud coverage and precipitation distribution as a function of cloud types. The analysis shows that most of the cloudiness in this region is associated with warm nonprecipitating clouds. Precipitating pixels comprise only about 15% of total cloud pixels. Of the precipitating pixels, about 50% have cloud top temperatures warmer than −40°C. Using rainfall rates obtained from microwave satellite data, the relative contribution by each precipitating cloud type is investigated. It is found that the precipitating clouds with cloud top temperature warmer than −40°C contribute at least 23% of the total rainfall amount, while the remaining 77% of total precipitation is divided almost equally by anvil with stratiform precipitating cloud and the deep convective precipitating cloud.

Early diagenesis of biogenic opal: Dissolution rates, kinetics, and paleoceanographic implications

A study was undertaken to measure the rate of biogenic opal dissolution in equatorial Pacific sediments along the equator between 103 and 140°W, and across the equator between 12°S and 9°N. Along the equator, benthic incubation chamber measurements indicate a gradient in the opal dissolution rate, with rates decreasing from ∼0.7 mmol m −2 day −1 at 103°W to 0.4 mol m −2 day −1 at 140°W. Across the equator at 140°W, the pattern of opal dissolution is symmetrical, with dissolution rates of ∼0.4 mmol m −2 day −1 from 2°S to 2°N, decreasing to ∼0.1 mmol m −2 day −1 at the ends of the transect. Benthic fluxes calculated from pore water profiles of silicic acid are in good agreement with incubation chamber measurements. Each pore water profile fits with a function that exponentially approaches a constant value with depth ( C d), and C d co-varies with the dissolution flux. At least three previously published models can explain this relationship: one in which C d is regulated by the solubility of the opal present in the sediments; a second in which C d depends on the availability of easily dissolvable opal; and the sediment mixing rate and a third rate in which C d is controlled by the development of surface coatings. If the first model is correct, the data demonstrate that opal solubility varies spatially and that solubility is positively correlated with the opal rain rate, although the rate at which pore waters become saturated varies little among the stations between 5°N and 5°S. The implication of this model is that the opal burial rate depends on dissolution kinetics and sediment accumulation rate. If the second model is correct, fits to the pore water data and knowing the sediment mixing rate indicate that at least three types of solid phase opal must be present in the equatorial Pacific region, one that is essentially unreactive, one that has a dissolution rate constant between 0.27 ± 0.09 and 0.05 ± 0.02 year −1 , and another that has a dissolution rate constant of 6 ± 4 × 10 −4 year −1. The more reactive phase dominates the dissolution flux between 5°S and 5°N, whereas the less reactive phase dominates the flux at the high latitude extremes of the transect. The implication of this second model is that sedimentary opal in equatorial Pacific sediments provides a record of only the non-reactive opal supply. If the third model is correct, surface coating development and opal preservation may depend upon the kinetics of the opal surface aging process or on the concentration of the coating material within the sediments. Storage experiments suggest that this third model may be the most realistic, but the implications of this model cannot be explored until the factors regulating coating growth are identified.

TOGA-COARE集中観測期間中の上部海洋混合層の発展と維持に関する様々のレヴェルのクロージャーモデルの比較について

Sensitivity of parameterization schemes for upper ocean mixing in a large-scale ocean model has been examined by using observational data of the R/V Hakuho-maru during the TOGA-COARE IOP, which was conducted from November, 1992 until February, 1993 in the equatorial Western Pacific region. The turbulent energy dissipation rate (e) and mixed-layer structure have been intensively observed, associated with surface fluxes at (Equator, 156°E) from 12-26 November, 1992. Time evolution of the mixed layer has been simulated by using 1-dimensional level 2, 2.5, 3 and 4 of the Mellor-Yamada closure model (Mellor and Yamada, 1974; Yamada and Mellor, 1975), and has been compared to the observations conducted by the R/V Hakuho-maru. A special emphasis was placed on the comparison of the turbulent energy dissipation rate, because it is an observed quantity directly related to a turbulent process. In general, the results of the four models are similar and, if compared to the observation, it is noted that the mixing is weak. Especially, when the surface wind is very weak, the discrepancy becomes prominent. It is concluded that there are few differences between these schemes as long as a 1-dimensional model is applied. Sensitivity of the results to vertical resolution has been investigated. General features of the upper mixing layer (∼100 m) are not different between high vertical resolution (Δz = 1 m) and ordinary resolution (Δz = 20 m), although the diurnal variation of SST (0.2°C) cannot be simulated by Δz = 20 m. Time evolution of the mixed layer was simulated by using the level-2 model with eddy diffusion coefficients derived by the observed e. This result explains approximately 70 % of the increase of heat content during this period.

228Ra-derived nutrient budgets in the upper equatorial Pacific and the role of “new” silicate in limiting productivity

228Ra activities in the upper ocean (surface to ∼850 m) of the equatorial Pacific between 9°N and 12°S along ∼140†W were measured at five stations during the JGOFS EqPac 1992 Survey I cruise, when El Niño conditions prevailed in the area. The vertical profile of 228Ra at each station consists of measurements made on 2 to 3 m 3 of water collected using submersible filtration systems in situ from 10–11 depths. 228Ra activities in the surface mixed layer range from ∼5 dpm M −3 at northern stations to ∼1.5 dpm m −3 near the equator. They decrease markedly between about 100 and 300 m, to concentration levels of 0.1–0.4 dpm M −3. The distributions manifest the occurrence of upwelling near the equator and downwelling between ∼3°N and 10°N. On the basis of the 228Ra and nitrate distributions, estimations of upward vertical fluxes of nitrate at various depth horizons at each of the stations have been made. Maximum fluxes of ∼2.0 to 3.5 mmol N m −2 day −1, averaging 2.6 mmol N m −2 day −1, occur near the base of the euphotic zone, about 100 m below sea surface. The average nitrate flux translates to a potential new production of about 17 mmol C m −2 day −1. While close to the result of the 15N tracer experiments, this new production estimate is significantly higher than the reported particulate organic carbon fluxes derived from the Th isotope and floating trap measurements. This implies that a significant fraction of the export production may have occurred in the form of dissolved organic carbon. The 228Ra-derived new production of 0.8 × 10 15 g C year −1 for the equatorial Pacific region east of the dateline is approximately one-half of the value obtained by Chavez and Barber (1987) for a non-El Niño period. The recycled fluxes of silicate, nitrate and phosphate to the euphotic layer bear molar ratios Si:N:P = 0.8:1:0.06. It is proposed that in upwelling regions of the equatorial Pacific, surface productivity is limited by the availability of “new” silicate, the silicate component that is decoupled from nitrate cycling and exported to the deep sea.