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A unique set of physical, bio‐optical, and meteorological observations were made from a mooring located in the open ocean south of Iceland (59°29.5′N, 20°49.8′W) from April 13 to June 12, 1989. The present measurements are apparently the first to resolve the rapid transition to springtime physical and biological conditions at such a high latitude site. Our data were collected with bio‐optical and physical moored systems every few minutes. The abrupt onset of springtime stratification was observed with the mixed layer shoaling from ∼550 m to ∼50 m in ∼5 days. During this period a major phytoplankton bloom occurred with a tenfold increase in near‐surface chlorophyll concentration in less than 3 weeks. Our statistical analysis indicates that the velocity shear in the upper layer is driven primarily by local wind stress. Mesoscale variability is also apparent from these and concurrent airborne oceanographic lidar observations. Our complementary modeling results suggest that the near‐surface layer may be reasonably well described by a one‐dimensional model and that the spring bloom was initiated during incipient near‐surface restratification.

Bio-optical data recorded from April 30 to July 19, 1991, using a mooring located in the open ocean (59ø35.6'N, 20ø57.9'W) are described and interpreted. Five multi-variable moored systems (MVMS) were deployed in the upper 90 m to obtain concurrent, co-located measurements of horizontal currents, water temperature, photosynthetically available radiation (PAR), transmission of light at 660 nm (c660), and stimulated chlorophyll fluorescence. In addition, meteorological and subsurface temperature data (12 depths from 80 to 310 m) were collected. When the mooring was deployed, surface waters were weakly stratified and there was little evidence of a phytoplankton bloom. Soon after the deployment, a marked increase in phytoplankton concentration occurred simultaneously with an increase of near-surface water temperature. The most striking observation was a period (year days 128-140) of strong mixed layer depth variability (daily amplitude of about 40 m) during which phytoplankton standing stock reached its maximum. During this period, phytoplankton biomass was mixed down to deeper waters at nighttime. As a result, the variability of the bio-optical parameters was extremely high, and deepwater phytoplankton concentration was much greater than would have been expected from the productivity estimates. Later, phytoplankton concentrations declined sharply in response to extremely stormy weather around year day 140. Once the storm passed (after day 143), surface waters stratified and the phytoplankton stock increased again, but the depth integrated biomass concentration did not reach as high values as before the storm. During this strong thermal stability period, fluorescence and c660 signals in near-surface waters were much higher than at depth, and displayed a diel cycle which was well correlated with PAR. Introduction Phytoplankton blooms in the North Atlantic have been investigated in the past by various means, including coarse resolution time series obtained from ocean weather station ships and continuous plankton recorders deployed from ships of opportunity [e.g., Williams, 1975; Colebrook, 1979]. Later, much attention was directed toward Coastal Zone Color Scanner (CZCS) data for the North Atlantic. These data revealed that the open ocean can sustain intense blooms over vast areas and extended time periods [Esaias et al., 1986]. Recently, a few international research programs have been conducted to cover relatively large spans of time in the North Atlantic area [Ducklow and Harris, 1993]. Included among these efforts was the Marine Light -Mixed Layer (MLML) program which conducted field experiments in 1989 and 1991. One of the important MLML strategies was to use moorings which allow samoline of ohvsical and bio-ootical variables at a fixed position in space, over time periods of several months, and with resolution of the order of minutes [Dickey, 1991; Dickey et al., 1991, 1993, 1994]. Bio-optical variables can reveal abundances of phytoplankton, which play a key role for Copyright 1995 by the American Geophysical Uni...

Observations and models show that sudden changes in the magnitude of the wind stress which occur within a time interval of one‐half inertial period are most effective in increasing surface current speeds and mixing the upper layers of the ocean. The purpose of the present study is to quantify the effects of concurrent time dependent wind direction. The Mellor‐Yamada level 2 1/2 turbulence closure model is used. A series of model runs was executed in order to determine the relative sensitivity of mixed layer depth and sea surface temperature to wind speed as compared with the rate of change of wind direction. The results indicate that the accuracy and time resolution of wind direction should be given special consideration in the design and interpretation of field experiments which will be used for testing prognostic mixed layer models.

Waves with near‐inertial frequencies were observed along a front associated with a large mesoscale feature in the Sargasso Sea during the late summer of 1987. High subsurface chlorophyll concentrations occurred on the edge of this front, coincident with the wave packets. The amplitude of the waves increased with time, and kinetic energy propagated downward, reducing 20‐m Richardson numbers in the thermocline to 1 or less. Chlorophyll levels were episodic, showing no periodicity coincident with wave dynamics. However, on two occasions, chlorophyll concentration increased from <0.5 to >1 mg Chl m−3, several hours after the waves penetrated the thermocline. It was hypothesized that mixing associated with shear instabilities stimulated new production. A diffusivity model combined with nutrient data produced a phytoplankton bloom that accounted for only one of the maxima. The other increase in chlorophyll may have been the result of horizontal advection of the wave packets near the front.

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Abstract. Optical properties were collected along a transect across cyclonic eddy Opal in the lee of Hawaii during the E-Flux III field experiment (10-27 March 2005). The eddy was characterized by an intense doming of isopycnal surfaces, and by an enhanced Deep Chlorophyll Maximum Layer (DCML) within its core. The phytoplankton bloom was diatom dominated, evidencing an eddy-induced shift in ecological community. Four distinct regions were identified throughout the water column at Opal's core: a surface mixed layer dominated by small phytoplankton; a layer dominated by "senescent" diatoms between the bottom of the upper mixed layer and the DCML; the DCML; and a deep layer characterized by decreasing phytoplankton activity. We focused on two parameters, the ratio of chlorophyll concentration to particulate beam attenuation coefficient, [chl]/c p , and the backscattering ratio (the particle backscattering to particle scattering ratio),b bp , and tested their sensitivity to the changes in particle composition observed through the water column at the eddy center. Our results show that [chl]/c p is not a good indicator. Despite the shift in ecological community, the ratio remains controlled primarily by the variation in chlorophyll concentration per cell with depth (photoadaptation), so that its values increase throughout the DCML. Steeper increase of [chl]/c p below the DCML suggest that remineralization might be another important controlling factor. On the other hand,b bp clearly indicates a shift from a small phytoplankton to a diatom dominated community. Below an upper layer characterized by constant values, thẽ b bp showed a rapid decrease to a broad minimum within the DCML. The higher values below the DCML are consistent with enhanced remineralization below the eddy-induced bloom. Both the "senescent" and the "healthy" diatom layers are characterized by similar optical properties, indicatingCorrespondence to: F. Nencioli (francesco.nencioli@opl.ucsb.edu) some possible limitations in using optical measurements to fully characterize the composition of suspended material in the water column. The inverse relationship betweenb bp and [chl]/c p , reported by others for Case II waters, is observed neither for the background conditions, nor in the presence of the eddy-induced diatom bloom. Between the two parameters, only the backscattering ratio showed the potential to be a successful indicator for changes in particle composition in Case I waters.