Ocean Surface Layer Research Articles

Formation of molecular chlorine from the photolysis of ozone and aqueous sea-salt particles

Halogen atoms from the reactions of sea-salt particles may play a significant role in the marine boundary layer. Reactions of sodium chloride, the major component of sea-salt particles, with nitrogen oxides generate chlorine atom precursors. However, recent studies suggest there is an additional source of chlorine in the marine troposphere. This study shows that molecular chlorine is generated from the photolysis of ozone in the presence of sea-salt particles above their deliquescence point; this process may also occur in the ocean surface layer. Given the global distribution of ozone, this process may provide a global source of chlorine.

Seasonality in Exchange of Organochlorines between Arctic Air and Seawater

Decreases in atmospheric concentrations of organochlorine (OC) pesticides in the Northern Hemisphere during the last decade have resulted in Arctic Ocean surface layer water that is now supersaturated for some compounds. Seasonal measurements of several OCs in air and seawater in the Canadian arctic archipelago during 1993 were made to study changes in air−sea exchange over an annual cycle. Fugacity ratios and Henry's law constants, with cor rection for seasonal changes in ice cover, were used to calculate net air−sea fluxes on a monthly basis. The lack of concentration gradients between 1 and 50 m indicated that advection of polar mixed layer water flowing southward from under the polar ice cap was an important factor controlling seawater concentrations. Out-gassing of HCHs, HCB, and dieldrin during the ice-free period could have lowered surface layer inventories by 4−20%. In contrast, net deposition of CHBs, chlordanes, and endosulfan-I during the open water period was equivalent to 50−>100% of the surface layer inventory. Air−sea fluxes of OCs were influenced by the combined effects of changes in atmospheric vapor pressure and water mass advection. Potential removal of OCs on sedimenting particles was calculated to be less important in controlling surface layer concentrations.

Variability of surface layer hydrography in the tropical Pacific Ocean

The purpose of this study is to examine the variation of ocean surface layer hydrography on interannual timescales in the tropical Pacific Ocean using conductivity‐temperature‐depth measurements from 1976 to 1994. We demonstrate that associated with interannual variations in atmospheric forcing, there were distinct changes in mixed layer temperature, salinity, depth, and barrier layer thickness between normal, El Niño, and La Niña time periods. During El Niño a warm, fresh mixed layer accompanied by an underlying barrier layer develops in the central and eastern Pacific in association with increased precipitation and reduced trade wind forcing. Conversely, during La Niña, when unusually cold conditions prevail in the central and eastern equatorial Pacific and the warm pool is confined to the far western Pacific, a thick barrier layer is found west of 160°E. Statistically, we demonstrate that barrier layers occur with increasing frequency as mixed layer temperatures increase from 20°C to 30°C, suggesting that barrier layer thickness may be one of the factors affecting surface temperatures. The physical mechanism underlying this relationship is likely to be related to the reduced efficiency of vertical turbulent mixing in cooling the surface via entrainment of thermocline water when the barrier layer is thick. Changes in mixed layer temperature, on the other hand, can affect precipitation and therefore mixed‐layer salinity, leading to the possibility of feedbacks on interannual timescales involving the mixed‐layer temperature balance and the hydrologic cycle over the ocean. The extent to which such feedbacks, if operative, may influence the detailed evolution of large‐scale, lower‐frequency variability in the tropical Pacific needs to be critically assessed in the context of coupled ocean‐atmosphere models.

Turbulence Decay and Restratification in the Equatorial Ocean Surface Layer following Nighttime Convection

Abstract Although the process of restratification of the ocean surface layer at the equator following nighttime convection is similar in many ways to the process at midlatitudes, there are important differences. A composite day calculated from 15 days of consistent conditions at 140°W on the equator was compared with midlatitude observations by Brainerd and Gregg. In the depth range of 20–40 m, 1) minimum nighttime stratification was similar [N2 of 1.2–3.2 (× 10−6 s−2) vs 0.4–1.7 (× 10−6 s−2)], 2) maximum daytime stratification was significantly larger, as might be expected from the greater surface heat input [N2 of 8–21 (× 10−6 s−2 vs 3–7 (× 10−6 s−2)], and 3) minimum nighttime shear was similar [shear-squared was 1.4–4.6 (× 10−6 s−2) vs 0.8–1.9 (× 10−6 s−2)], but the maximum daytime shear was much larger [shear-squared of 24–41 (× 10−6 s−2) vs 3–7 (× 10−6 s−2)]. For much of the surface layer, the dominant identifiable cause of restratification in both cases was the divergence of the penetrating shortwav...

A novel delta-subdivision proteobacterial lineage from the lower ocean surface layer.

A small-subunit ribosomal RNA (16S rRNA) gene lineage (SAR324) affiliated with the delta-subdivision of the class Proteobacteria (DP) was discovered in a 16S rRNA gene clone library prepared from a water sample collected from 250 m in the western Sargasso Sea. This clone library of nearly full-length amplicons of bacterial 16S rRNA genes has been the subject of previous studies aimed at identifying bacteria that inhibit the lower ocean surface layer. The novel lineage was identified by randomly sequencing clones that did not hybridize to oligonucleotide probes specific for several abundant bacterioplankton groups identified in previous studies. Phylogenetic analysis indicated that SAR324 was most closely affiliated with the DP, although it showed no specific relationship to any DP 16S rRNA genes in databases. Eight of the clones in the library of 148 clones were identified as members of the SAR324 lineage by hybridization to an oligonucleotide probe specific for SAR324. Subsequent hybridizations showed that the SAR324 group is stratified in the lower surface layer of both the Atlantic and Pacific Oceans, with maxima between 160 and 500 m. The repeated discovery of sequences belonging to different gene clusters with similar distributions in this region of the water column suggests that microbial communities in the lower surface layer may be functionally specialized.

Multidecadal climate variability in the Greenland Sea and surrounding regions: A coupled model simulation

Pronounced oscillations of ocean temperature and salinity occur in the Greenland Sea in a 2000 year integration of a coupled ocean‐atmosphere model. The oscillations, involving both the surface and subsurface ocean layers, have a timescale of approximately 40–80 years, and are associated with fluctuations in the intensity of the East Greenland Current. The Greenland Sea temperature and salinity variations are preceded by large‐scale changes in near‐surface salinity in the Arctic, which appear to propagate out of the Arctic through the East Greenland Current. These anomalies then propagate around the subpolar gyre into the Labrador Sea and the central North Atlantic. These oscillations are coherent with previously identified multi‐decadal fluctuations in the intensity of the North Atlantic thermohaline circulation. The oscillations in the Greenland Sea are related to atmospheric variability. Negative (cold) anomalies of surface air temperature are associated with negative (cold) sea surface temperature (SST) anomalies in the Greenland Sea, with amplitudes up to 2°C near Greenland declining to several tenths of a degree C over northwestern Europe. The cold SST anomalies and intensified East Greenland Current are also associated with enhanced northerly winds over the Greenland Sea.

Modes of North Atlantic Decadal Variability in the ECHAM1/LSG Coupled Ocean–Atmosphere General Circulation Model

Abstract The climate variability in the North Atlantic sector is investigated in a 325-yr integration of the ECHAM1/ LSG coupled ocean–atmosphere general circulation model. At the interannual timescale, the coupled model behaves realistically and sea surface temperature (SST) anomalies arise as a response of the oceanic surface layer to the stochastic forcing by the atmosphere, with the heat exchanges both generating and damping the SST anomalies. In the ocean interior, the temperature spectra are red up to a period of about 20 years, and substantial decadal fluctuations are found in the upper kilometer or so of the water column. Using extended empirical orthogonal function analysis, two distinct quasi-oscillatory modes of ocean–atmosphere variability are identified, with dominant periods of about 20 and 10 years, respectively. The oceanic changes in both modes reflect the direct forcing by the atmosphere through anomalous air–sea fluxes and Ekman pumping, which after some delay affects the intensity of t...

The warm oceanic surface layer: Implications for CO2 fluxes and surface gas measurements

The ocean and air‐sea interface are important for the exchange of heat, momentum, water vapour and carbon‐dioxide. The details of the exchange mechanisms, which are often coupled and complex, have to be understood in order to assess the future role of the global oceans in climate change. Recently, much attention has focused on the ‘thermal skin effect’, cooling or warming of the uppermost millimeters of the sea surface, and its global implications for enhancing CO2 uptake [Robertson and Watson, 1992; Van Scoy et al., 1995]. Routinely, air‐sea flux estimates derived from oceanic pCO2 measurements are corrected for the usually cooler thermal skin by applying a correction factor [Sarmiento and Sundquist, 1992; Wong et al., 1995]. Here we describe how near surface warming of the upper few meters of the ocean by solar radiation, routinely observed but not fully appreciated in this context, can significantly affect the net daily exchange of CO2 over and above the skin effect, and can even lead to a reversal of the direction of the air‐sea flux calculated from pCO2 measurements and wind speed alone. The warming and cooling cycle produces a net asymmetry between CO2 invasion and evasion, having the effect of decreasing CO2 invasion and increasing CO2 evasion.

Local ocean response to a multiphase westerly wind burst: 2. Thermal and freshwater responses

A westerly wind burst observed in the warm pool of the western equatorial Pacific Ocean cooled the ocean's surface layer by about 0.8°C. Turbulent entrainment at the base of this layer caused cooling but also heating due to the reversal of the vertical temperature gradient during rain events. Consequently, the cumulative effect of turbulent entrainment was minimized. Following the wind burst, a sustained eastward surface current contributed to high current shear and turbulent dissipation rates at the top of the thermocline. As a result, most of the heat transfer into the thermocline occurred after the wind burst had ended. The cruise‐averaged turbulent flux into the thermocline was 17 ± 10 W m−2, which suggests that the annual mean is only a few watts per square meter. The restratification of the upper ocean in the aftermath of the wind burst is accounted for partly (but not wholly) by local turbulent entrainment. Despite heavy precipitation, upper ocean salinity generally increased during the cruise. Advection appears to have been the dominant factor governing local salinity changes.

Measuring bubbles, boundary layers, and related processes in the ocean surface layer

Bubbles entrained by breaking waves are important in the active and passive acoustics of the ocean surface layer, and also play an important role as tracers for acoustic Doppler techniques. Bubbles are not just passive tracers but may also be dynamically significant, thereby affecting the structure of the wave-zone boundary layer. The processes affecting the distribution of entrained air and the distribution of bubble sizes are reviewed in the context of the available measurement techniques, models, and observations of the surface boundary layer. In many respects, the knowledge of the fluid dynamic structure of the surface layer is no better than the knowledge of the acoustics and bubble phenomena, and the talk will emphasize those processes which are important for both fields. These include wave breaking, convection, Langmuir circulations, and turbulent fluxes and dissipation. Indeed, acoustic instrumentation plays an important role in probing the surface layer, and a better understanding of bubble-related phenomena is necessary to better understand the performance of these instruments.

Remote sensing of sea surface Sun-induced chlorophyll fluorescence: consequences of natural variations in the optical characteristics of phytoplankton and the quantum yield of chlorophyll a fluorescence

Abstract The rate of Sun-induced chlorophyll a fluorescence (SICF) observed at sea surface is determined by chlorophyll a concentration, incident irradiance, and optical and fluorescence properties of the phytoplanktonic population. In this study, the impact of natural variations in the two latter on the use of remotely sensed SICF to determine ocean surface chlorophyll a concentration, is assessed using a simple parameterization of phytoplankton optical properties and a model describing the variations in the quantum yield of chlorophyll a fluorescence as a function of environmental factors, such as excess irradiance and nutrient limitations. It is shown that (1) variations in the optical properties of phytoplankton are the main cause of non-linearity in the relationship between SICF and chlorophyll a concentration, (2) the extent of spatial variations in the rate of fluorescence per unit chlorophyll a concentration and irradiance, at the level of a typical sensor scene, prevents the use of a linear relationship between SICF and chlorophyll a concentration even at local scales and (3) the optical properties of the ocean surface layer play an important role in modifying the SICF signal. Nevertheless, the parameterizations presented in this paper may represent a reasonably sound approach for a meaningful use of SICF in view of detecting the chlorophyll a concentration within the upper layer of the ocean. Interestingly, it is also shown that the detection threshold of SICF could be significantly lower than the one currently expected.

16S rRNA genes reveal stratified open ocean bacterioplankton populations related to the Green Non-Sulfur bacteria.

Microorganisms play an important role in the biogeochemistry of the ocean surface layer, but spatial and temporal structures in the distributions of specific bacterioplankton species are largely unexplored, with the exceptions of those organisms that can be detected by either autofluorescence or culture methods. The use of rRNA genes as genetic markers provides a tool by which patterns in the growth, distribution, and activity of abundant bacterioplankton species can be studied regardless of the ease with which they can be cultured. Here we report an unusual cluster of related 16S rRNA genes (SAR202, SAR263, SAR279, SAR287, SAR293, SAR307) cloned from seawater collected at 250 m in the Sargasso Sea in August 1991, when the water column was highly stratified and the deep chlorophyll maximum was located at a depth of 120 m. Phylogenetic analysis and an unusual 15-bp deletion confirmed that the genes were related to the Green Non-Sulfur phylum of the domain Bacteria. This is the first evidence that representatives of this phylum occur in the open ocean. Oligonucleotide probes were used to examine the distribution of the SAR202 gene cluster in vertical profiles (0-250 m) from the Atlantic and Pacific Oceans, and in discrete (monthly) time series (O and 200 m) (over 30 consecutive months in the Western Sargasso Sea. The data provide robust statistical support for the conclusion that the SAR202 gene cluster is proportionately most abundant at the lower boundary of the deep chlorophyll maximum (P = 2.33 x 10(-5)). These results suggest that previously unsuspected stratification of microbial populations may be a significant factor in the ecology of the ocean surface layer.

A model of the effects of horizontal density gradients on mixing processes in the oceanic surface layer

A diurnal Eulerian model of the upper mixed layer incorporating the effects of horizontal density gradients is developed based on that of Janowitz and Kamykowski [1991] and is utilized to study the effects of wind‐driven cross‐isopycnal transport on mixing processes in the layer. Four runs of the model are conducted for a constant horizontal density gradient of 2.5 × 10−5 kg m−4 with wind directions at 0°, 90°, 180°, and 270° to this gradient. Two runs are conducted with half this density gradient and wind directions 90° to the left and right of the gradient and one run with no density gradient. In all cases the model is first run with a constant heat loss rate of 150 W m−2 through the sea surface for 11 hours to simulate nighttime cooling and to provide initial conditions for daytime runs. The model is then run for 24 hours with a constant wind speed of 7 m s−1 and a maximum solar radiation of 900 W m−2 during the first 13 hours of the 24‐hour simulation. The results show that the shallowest mixed layer occurs when the wind direction is 90° to the left of the density gradient and the deepest layer occurs with the wind direction 90° to the right of the gradient. After 24 hours, the ratio of the deepest to the shallowest mixed layer depth is 2.35 for the stronger gradient and 1.83 for the weaker gradient. These results are explained by the effect of vertical shear of the wind‐driven advection of horizontal density variations, which changes the vertical density gradient and can destabilize or stabilize the mixed layer. The results indicate that wind‐driven, cross‐isopycnal transport can have significant effects on mixing processes in the layer. Dimensionless parameters are developed to estimate the relative importance of cross‐isopycnal, wind‐driven transport on mixed layer properties.

Modeling the diurnal cycle of carbon monoxide: Sensitivity to physics, chemistry, biology, and optics

As carbon monoxide within the oceanic surface layer is produced by solar radiation, diluted by mixing, consumed by biota, and outgassed to the atmosphere, it exhibits a diurnal cycle. The effect of dilution and mixing on this cycle is examined using a simple model for production and consumption, coupled to three different mixed layer models. The magnitude and timing of the peak concentration, the magnitude of the average concentration, and the air‐sea flux are considered. The models are run through a range of heating and wind stress and compared to experimental data reported by Kettle [1994]. The key to the dynamics is the relative size of four length scales; Dmix, the depth to which mixing occurs over the consumption time; L, the length scale over which production occurs; Lout, the depth to which the mixed layer is ventilated over the consumption time; and Lcomp, the depth to which the diurnal production can maintain a concentration in equilibrium with the atmosphere. If Dmix ≫ L, the actual model parameterization can be important. If the mixed layer is maintained by turbulent diffusion, Dmix can be substantially less than the mixed layer depth. If the mixed layer is parameterized as a homogeneous slab, Dmix is equivalent to the mixed layer depth. If Dmix > Lout, production is balanced by consumption rather than outgassing. The ratio between Dmix and Lcomp determines whether the ocean is a source or a sink for CO. The main thermocline depth H sets an upper limit for Dmix and hence Dmix/L, Dmix/Lout, and Dmix/Lcomp. The models are run to simulate a single day of observations. The mixing parameterization is shown to be very important, with a model which mixes using small‐scale diffusion, producing markedly larger surface concentrations than models which homogenize the mixed layer completely and instantaneously.

Salinity Variability and Its Role in the Barrier-Layer Formation during TOGA-COARE

Abstract During the intensive observation period of TOGA-COARE between November 1992 and February 1993, two R/V Franklin cruises, FR09/92 and FR01/93, were carried out to study the response of oceanic surface layer temperature and salinity to atmospheric inputs such as wind, radiation, precipitation, etc. A total of seven Seasonar surveys (each has a square of about 50 km) were designed and performed to monitor the variations in oceanic heat and freshwater content. This paper focuses on the observed salinity variation in spatial and time scales and its association with the barrier layer production. During the first and second 1992 surveys, a salinity front was observed. The crossing-front salinity difference was at least 0.16 from the surface down to below 40 m, implying a freshwater influx of about 20 cm. MIT Doppler radar located on R/V Vickers showed very heavy and persistent storm rains covered a very large area of a few hundred square kilometers west of the surveys for two days before the surveys sta...

Salinity variability in the surface layer of the tropical western Pacific Ocean

Observations of surface layer salinity and temperature, rainfall, wind and upper ocean currents from two surveys during the Tropical Ocean Global Atmosphere‐Coupled Ocean‐Atmosphere Response Experiment are presented. The surveys covered a region of 25×25 n. mi (46.3×46.3 km) with hydrographic sections of 5 n. mi (9.3 km) separation, using a Seasoar towed conductivity‐temperature‐depth profiler. One survey was dominated by the presence of several rain‐induced patches of low‐salinity water with slightly reduced temperature. These patches extended to the bottom of the mixed layer, which at the time, was 20–30 m deep, and appeared to be the result of convection driven by their reduced temperature. Below the mixed layer was a 15 to 20 m thick barrier layer. By distributing the freshwater over the mixed layer almost instantly, the convecting patches appeared to contribute to the maintenance of the barrier layer. This survey is typical for conditions found during most of the observation period. The second survey was dominated by a salinity front and advection of low‐salinity water with the large‐scale westerly wind that prevailed at the time. A single event of very heavy rain produced a shallow lens of freshened water; but the reduction in salinity was so strong that the associated cooling was insufficient to initiate convection. No barrier layer was seen ahead of the front but the observations indicate the development of a barrier layer after the passage of the front. The work demonstrates the difficulties in establishing a freshwater budget for the tropical ocean. Ship borne rain gauge measurements, even though becoming more accurate, seem not to be representative for an area much beyond a few kilometers or less. Sometimes the surface salinity distribution shows better visual correlation with the wind gust pattern than with the rainfall distribution, indicating that the wind gusts associated with atmospheric convection cells are less intermittent and more coherent in space than the rain.

Regional climate variability and ocean heat transport in the southwest Pacific Ocean

The winter of 1992 was the coldest on record in New Zealand since the 1940s. Temperatures in New Zealand were as much as 3°C below average, with heavy rain and unusual snow. The oceanic surface layer in the southwest Pacific was also anomalously cold over the same period. A World Ocean Circulation Experiment (WOCE) hydrographic section (P14C) between Auckland and Fiji during September 1992 found cold sea surface temperatures and deep mixed layers near New Zealand when compared to 8 years of high‐resolution expendable bathy thermograph (XBT) temperature measurements collected along the same transect. High nutrient and low dissolved oxygen concentrations in the surface layer indicated recent entrainment of thermocline waters. The Auckland to Fiji XBT section is one of three WOCE high‐resolution XBT survey lines in the “Tasman Box” region, whose boundaries are Auckland‐Fiji, Fiji‐Brisbane and Wellington‐Sydney transects plus the Australian coast. Geostrophic shear and transport were estimated from 10 realizations of the Tasman Box during the period 1991–1993. The time series of geostrophic transport shows that following a convergence in late 1991, early in 1992 there was a substantial divergence of mass in the upper waters, equivalent to a thinning of the warm water layer. The phase of this anomalous divergence is matched to an observed amplification of the seasonal oceanic heat storage cycle in 1991–1992. The top 200‐m average temperature was warmer in the 1992 summer than in 1991 or 1993, but the winter of 1992 was the coldest of the 8‐year record along the Auckland‐Fiji line. The divergence (thinning) of the warm water layer appears to have preconditioned the region for the exceptionally cold 1992 winter. The alternative, cool conditions from anomalous air‐sea heat exchange caused by variability in the wind field, is considered unlikely as large fluctuations in heat loss are not observed in the air‐sea flux data during this period. The severe weather conditions and anomalous ocean heat transport are most likely related to the prolonged El Niño‐Southern Oscillation episode that began in early 1991.

Oceanic transport and storage of carbon emissions

Using a global carbon cycle model (GLOCO) that considers seven terrestrial biomes, surface and deep ocean layers based on the HILDA model and a single mixed atmosphere, we analyzed the response of atmospheric CO2 concentration and oceanic DIC and DOC depth profiles to additions of carbon to the atmosphere and ocean. The rate of transport of carbon to the deepest oceanic layers is rather insensitive to the atmosphereic-ocean surface gas exchange coefficient over a wide range, hence discrepancies between researchers on the precise global average value of this coefficient do not significantly affect predictions of atmospheric response to anthropogenic inputs. Upwelling velocity, on the other hand, amplifies oceanic response by increasing primary production in the upper ocean layers, resulting in a larger flux into DOC and sediments and increased carbon storage; experiments to reduce the uncertainty in this parameter would be valuable.

Instrumentation for the Measurement of Sound Speed near the Ocean Surface

Abstract Air bubbles entrained by breaking waves in the ocean surface layer can dramatically alter the velocity and attenuation of acoustic waves. The development of an effective technique for directly measuring the sound speed near the ocean surface is reported. The method makes use of the travel time of short acoustic pulses between a transmitter and a receiver separated by 40 cm. Phase distortions caused by acoustic reflections from the surface or from nearby buoy structural elements are separated in time from the direct path signal. A DSP-based data processing system was implemented to cross correlate the transmitted and received acoustic pulses and thus yield sound-speed measurements in real time. Perhaps the most significant novelty of the present measurement technique is its ability to make simultaneous measurements of the sound speed at several depths, starting as close as 0.5 m to the surface, at frequencies down to 5 kHz, and at a sample rate of 4 Hz per channel. Furthermore, the technique is di...

Seasonal variation in the distribution and abundance of Oithona and Oncaea species (Copepoda, Crustacea) in the southeastern Weddell Sea, Antarctica

The coupling between the seasonal distribution of dominant Oithonidae and Oncaeidae and the prevailing environmental conditions in the eastern Weddell Sea was investigated. In samples from February, a community on the shelf and in the oceanic surface layer, consisting mainly of Oithona similis and Oncaea curvata and juveniles of both genera, could be differentiated from a community found in deeper oceanic waters, consisting of Oithona frigida, Oncaea parila, Oncaea antarctica and Oncaea englishi. In November the differences in the species distribution between the two regions were less pronounced and the abundances of all species were much lower than earlier in the year. The dominant species were Oithona similis, which reached abundances of up to 182/m3 in February, and Oncaea curvata with up to 215/m3. Deeper living species were comparatively rare. Oithona similis occurred mainly in the top 200 m during both seasons. The other species, by contrast, were generally deeper and more dispersed through the water column in late winter than in summer.