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• 1. [Article] Analysis of a Method to Estimate Chlorophyll-a Concentration from Irradiance Measurements at Varying Depths

  A model to estimate chlorophyll-a concentration and yellow substance absorption at 440 nm from irradiance measurements made at varying depths is examined. The derivation of the model, requiring irradiance ...

  Citation × Citation Citation Abstract Copy Title: Analysis of a Method to Estimate Chlorophyll-a Concentration from Irradiance Measurements at Varying Depths Author: Abbott, Mark R., Pegau, W. Scott, Nahorniak, Jasmine S., Letelier, Ricardo M. A model to estimate chlorophyll-a concentration and yellow substance absorption at 440 nm from irradiance measurements made at varying depths is examined. The derivation of the model, requiring irradiance measurements at three wavebands, is presented and tested on data collected in oligotrophic (with low chlorophyll concentrations) waters and in coastal waters (with both high and low chlorophyll concentrations). The results indicate excellent quantitative agreement with chlorophyll-a concentration and yellow substance absorption measurements. A sensitivity analysis of the model shows it to be highly sensitive to pressure sensor precision, the accuracy of the value used for the mean cosine of the downwelling radiance distribution, and the irradiance sensor measurement error. However, provided that these factors are taken into account, accurate estimates of chlorophyll-a concentrations in case I (phytoplankton-dominated) waters using a single (or multiple) irradiance sensor with three wavebands can be derived. This model can be applied to irradiance data from a variety of deployment methods including profilers, bottom-tethered moorings, subsurface drifters, and towed platforms. Title: Analysis of a Method to Estimate Chlorophyll-a Concentration from Irradiance Measurements at Varying Depths Author: Abbott, Mark R., Pegau, W. Scott, Nahorniak, Jasmine S., Letelier, Ricardo M. A model to estimate chlorophyll-a concentration and yellow substance absorption at 440 nm from irradiance measurements made at varying depths is examined. The derivation of the model, requiring irradiance measurements at three wavebands, is presented and tested on data collected in oligotrophic (with low chlorophyll concentrations) waters and in coastal waters (with both high and low chlorophyll concentrations). The results indicate excellent quantitative agreement with chlorophyll-a concentration and yellow substance absorption measurements. A sensitivity analysis of the model shows it to be highly sensitive to pressure sensor precision, the accuracy of the value used for the mean cosine of the downwelling radiance distribution, and the irradiance sensor measurement error. However, provided that these factors are taken into account, accurate estimates of chlorophyll-a concentrations in case I (phytoplankton-dominated) waters using a single (or multiple) irradiance sensor with three wavebands can be derived. This model can be applied to irradiance data from a variety of deployment methods including profilers, bottom-tethered moorings, subsurface drifters, and towed platforms. Close

• 2. [Article] Seasonal and non-seasonal variability of satellite-derived surface pigment concentration in the California Current

  Satellite-derived pigment concentrations from the west coast time series (WCTS) are averaged into monthly mean fields over the California Current system (CCS) for the period July 1979 to June 1986. Errors ...

  Citation × Citation Citation Abstract Copy Title: Seasonal and non-seasonal variability of satellite-derived surface pigment concentration in the California Current Author: Abbott, Mark R., Thomas, A. C., James, Corinne, Strub, P. Ted Satellite-derived pigment concentrations from the west coast time series (WCTS) are averaged into monthly mean fields over the California Current system (CCS) for the period July 1979 to June 1986. Errors caused by the scattering algorithm used in the WCTS are reduced by an empirical correction function, although winter values (November–February) remain unreliable. For the March–October period we look at both the mean seasonal development and the nonseasonal monthly anomalies of pigment concentration. These are compared with fields of alongshore wind stress, mixing power of the wind (u(3/*)) and wind stress curl. Outside of the Southern California Bight there is a strong seasonal cycle with a spring-summer maximum, a northward progression of high pigment concentrations from California to Oregon and a double maximum off Washington (spring and summer, with a lull in between). Within the Southern California Bight, seasonality is low, with a relative minimum in late summer. Off Baja California the pattern is similar to that off northern California. In regions where previous work has been done, there is general agreement with the seasonal cycles found here. Title: Seasonal and non-seasonal variability of satellite-derived surface pigment concentration in the California Current Author: Abbott, Mark R., Thomas, A. C., James, Corinne, Strub, P. Ted Satellite-derived pigment concentrations from the west coast time series (WCTS) are averaged into monthly mean fields over the California Current system (CCS) for the period July 1979 to June 1986. Errors caused by the scattering algorithm used in the WCTS are reduced by an empirical correction function, although winter values (November–February) remain unreliable. For the March–October period we look at both the mean seasonal development and the nonseasonal monthly anomalies of pigment concentration. These are compared with fields of alongshore wind stress, mixing power of the wind (u(3/*)) and wind stress curl. Outside of the Southern California Bight there is a strong seasonal cycle with a spring-summer maximum, a northward progression of high pigment concentrations from California to Oregon and a double maximum off Washington (spring and summer, with a lull in between). Within the Southern California Bight, seasonality is low, with a relative minimum in late summer. Off Baja California the pattern is similar to that off northern California. In regions where previous work has been done, there is general agreement with the seasonal cycles found here. Close

• 3. [Article] The structure of the transition zone between coastal waters and the open ocean off northern California, winter and spring 1987

  Physical and biological fields in the coastal transition zone off northern California were measured during February, March, May and June 1987 in an extended alongshore region between 60 km and 150 km offshore. ...

  Citation × Citation Citation Abstract Copy Title: The structure of the transition zone between coastal waters and the open ocean off northern California, winter and spring 1987 Author: Chavez, F. P., Barber, R. T., Strub, P. Ted, Smith, Robert L. (Robert Lloyd), 1935-, Abbott, Mark R., Cowles, Timothy J., Jessen, P., Ramp, S. R., Huyer, Adriana, 1945-, Kosro, P. Michael, 1951-, Small, Lawrence F. Physical and biological fields in the coastal transition zone off northern California were measured during February, March, May and June 1987 in an extended alongshore region between 60 km and 150 km offshore. The spring transition, as seen in coastal sea level and winds, occurred in mid-March. Surface variability during the two spring cruises was stronger and of larger scale than that seen during the two winter cruises. An equatorward-tending current, flowing along the boundary between low steric sea level inshore and high steric sea level offshore, dominated both the directly-measured (acoustic Doppler current profiler) and geostrophic current fields during spring. Current jets of comparable strength directed both offshore and onshore were seen off Cape Mendocino and Point Arena; these evolved significantly in the 3 weeks between cruises. Inshore of the current, properties associated with upwelled water were found near the surface, including low temperature and high salinity, nutrients and chlorophyll; offshore of the current, waters were warmer, less saline, lower in nutrients and more oligotrophic. Geostrophic and directly measured volume transports in the current were about 2–3 Sv. Isopycnals inshore of the spring upwelling front were displaced vertically by O(40–80 m) from their depths during the winter survey; these displacements extended deep into the water column and were largely independent of depth between 100 and 400 m. Surface mixed layers tended to be deep in winter and shallower inshore of the upwelling front in spring. A connection between the equatorward-tending frontal jet off northern California and the more well-studied California Current further south is suggested by the similarity of their transports and of their dynamic height values. Title: The structure of the transition zone between coastal waters and the open ocean off northern California, winter and spring 1987 Author: Chavez, F. P., Barber, R. T., Strub, P. Ted, Smith, Robert L. (Robert Lloyd), 1935-, Abbott, Mark R., Cowles, Timothy J., Jessen, P., Ramp, S. R., Huyer, Adriana, 1945-, Kosro, P. Michael, 1951-, Small, Lawrence F. Physical and biological fields in the coastal transition zone off northern California were measured during February, March, May and June 1987 in an extended alongshore region between 60 km and 150 km offshore. The spring transition, as seen in coastal sea level and winds, occurred in mid-March. Surface variability during the two spring cruises was stronger and of larger scale than that seen during the two winter cruises. An equatorward-tending current, flowing along the boundary between low steric sea level inshore and high steric sea level offshore, dominated both the directly-measured (acoustic Doppler current profiler) and geostrophic current fields during spring. Current jets of comparable strength directed both offshore and onshore were seen off Cape Mendocino and Point Arena; these evolved significantly in the 3 weeks between cruises. Inshore of the current, properties associated with upwelled water were found near the surface, including low temperature and high salinity, nutrients and chlorophyll; offshore of the current, waters were warmer, less saline, lower in nutrients and more oligotrophic. Geostrophic and directly measured volume transports in the current were about 2–3 Sv. Isopycnals inshore of the spring upwelling front were displaced vertically by O(40–80 m) from their depths during the winter survey; these displacements extended deep into the water column and were largely independent of depth between 100 and 400 m. Surface mixed layers tended to be deep in winter and shallower inshore of the upwelling front in spring. A connection between the equatorward-tending frontal jet off northern California and the more well-studied California Current further south is suggested by the similarity of their transports and of their dynamic height values. Close

• 4. [Article] Tidal and atmospheric forcing of the upper ocean in the Gulf of California, Part 2: Surface heat flux

  Satellite infrared imagery and coastal meteorological data for March 1984 through February 1985 are used to estimate the net annual surface heat flux for the northern Gulf of California. The average annual ...

  Citation × Citation Citation Abstract Copy Title: Tidal and atmospheric forcing of the upper ocean in the Gulf of California, Part 2: Surface heat flux Author: Abbott, Mark R., Winant, Clinton D., Paden, Cynthia A. Satellite infrared imagery and coastal meteorological data for March 1984 through February 1985 are used to estimate the net annual surface heat flux for the northern Gulf of California. The average annual surface heat flux for the area north of Guaymas and Santa Rosalia is estimated to be +74 W m-2 for the 1984–1985 time period. This is comparable to the +20–50 W m-2 previously obtained from heat and freshwater transport estimates made with hydrographic surveys from different years and months. The spatial distribution of the net surface heat flux shows a net gain of heat over the whole northern gulf. Except for a local maximum near San Esteban Island, the largest heat gain (+110–120 W m-2) occurs in the Ballenas and Salsipuedes channels, where strong tidal mixing produces anomalously cold sea surface temperatures (SSTs) over much of the year. The lowest heat gain occurs in the Guaymas Basin (+40–50 W m-2), where SSTs are consistently warmer. In the relatively shallow northern basin the net surface heat flux is fairly uniform, with a net annual gain of approximately +70 W m-2. A local minimum in heat gain (approximately +60 W m-2) is observed over the shelf in the northwest, where spring and summer surface temperatures are particularly high. A similar minimum in heat gain over the shelf was observed in a separate study in which historical SSTs and 7 years (1979–1986) of meteorological data from Puerto Penasco were used to estimate the net surface heat flux for the northern basin. In that study, however, the heat fluxes were higher, with a gain of +100 W m-2 over the shelf and +114 W m-2 in the northern basin. These larger values are directly attributable to the higher humidities in the 1979–1986 study compared to the 1984-1985 satellite study. Significant interannual variations in humidity appear to occur in the northern gulf, with relatively high humidities during El Niño years and low humidities during anti-El Niño years. High humidities reduce evaporation and the associated latent heat loss, promoting a net annual heat gain. In the northern Gulf of California, however, tidal mixing appears to play a key role in the observed gain of heat. Deep mixing in the island region produces a persistent pool of cold water which is mixed horizontally by the large-scale circulation, lowering surface temperatures over most of the northern gulf. These cold SSTs decrease evaporation by reducing the saturation vapor pressure of the overlying air. As a result, heat loss is substantially reduced, even when humidities are low. By removing heat from the surface, tidal mixing alters the time scale of air-sea interaction and reduces or possibly even inhibits the formation of deep water masses via convection. Over climatological timescales, it may be tidal mixing that ultimately maintains the estuarinelike circulation in the northern Gulf of California, differentiating it from the Mediterranean and Red seas, which lose heat to the atmosphere. Title: Tidal and atmospheric forcing of the upper ocean in the Gulf of California, Part 2: Surface heat flux Author: Abbott, Mark R., Winant, Clinton D., Paden, Cynthia A. Satellite infrared imagery and coastal meteorological data for March 1984 through February 1985 are used to estimate the net annual surface heat flux for the northern Gulf of California. The average annual surface heat flux for the area north of Guaymas and Santa Rosalia is estimated to be +74 W m-2 for the 1984–1985 time period. This is comparable to the +20–50 W m-2 previously obtained from heat and freshwater transport estimates made with hydrographic surveys from different years and months. The spatial distribution of the net surface heat flux shows a net gain of heat over the whole northern gulf. Except for a local maximum near San Esteban Island, the largest heat gain (+110–120 W m-2) occurs in the Ballenas and Salsipuedes channels, where strong tidal mixing produces anomalously cold sea surface temperatures (SSTs) over much of the year. The lowest heat gain occurs in the Guaymas Basin (+40–50 W m-2), where SSTs are consistently warmer. In the relatively shallow northern basin the net surface heat flux is fairly uniform, with a net annual gain of approximately +70 W m-2. A local minimum in heat gain (approximately +60 W m-2) is observed over the shelf in the northwest, where spring and summer surface temperatures are particularly high. A similar minimum in heat gain over the shelf was observed in a separate study in which historical SSTs and 7 years (1979–1986) of meteorological data from Puerto Penasco were used to estimate the net surface heat flux for the northern basin. In that study, however, the heat fluxes were higher, with a gain of +100 W m-2 over the shelf and +114 W m-2 in the northern basin. These larger values are directly attributable to the higher humidities in the 1979–1986 study compared to the 1984-1985 satellite study. Significant interannual variations in humidity appear to occur in the northern gulf, with relatively high humidities during El Niño years and low humidities during anti-El Niño years. High humidities reduce evaporation and the associated latent heat loss, promoting a net annual heat gain. In the northern Gulf of California, however, tidal mixing appears to play a key role in the observed gain of heat. Deep mixing in the island region produces a persistent pool of cold water which is mixed horizontally by the large-scale circulation, lowering surface temperatures over most of the northern gulf. These cold SSTs decrease evaporation by reducing the saturation vapor pressure of the overlying air. As a result, heat loss is substantially reduced, even when humidities are low. By removing heat from the surface, tidal mixing alters the time scale of air-sea interaction and reduces or possibly even inhibits the formation of deep water masses via convection. Over climatological timescales, it may be tidal mixing that ultimately maintains the estuarinelike circulation in the northern Gulf of California, differentiating it from the Mediterranean and Red seas, which lose heat to the atmosphere. Close

• 5. [Article] Phytoplankton pigment patterns and wind forcing off central California

  We use a 4-year time series of high-resolution coastal zone color scanner imagery to study mesoscale variability in phytoplankton pigment (as a surrogate for biomass) distributions off central California ...

  Citation × Citation Citation Abstract Copy Title: Phytoplankton pigment patterns and wind forcing off central California Author: Abbott, Mark R., Barksdale, B. We use a 4-year time series of high-resolution coastal zone color scanner imagery to study mesoscale variability in phytoplankton pigment (as a surrogate for biomass) distributions off central California during the spring–summer upwelling season. We use empirical orthogonal functions to decompose the time series of spatial images into its dominant modes of variability. Similarly, we analyze wind fields derived from the Fleet Numerical Oceanography Center pressure fields. The results from these analyses are used to investigate the coupling between wind forcing of the upper ocean and phytoplankton distributions on mesoscales. Prior to the spring transition, wind stress is dominated by strong northward conditions, and pigment is low and relatively uniform throughout the California Current domain. The first strong southward wind event soon results in a bloom of phytoplankton that is also uniformly distributed offshore. Nearshore values fluctuate relatively little in response to variations in wind forcing. After the spring transition, the wind becomes more steady (southward) and the fluctuations are dominated by changes in the strength of the curl. During southward events, the curl tends to become more positive. Depending on the fluctuations and strength of these curl episodes, filaments begin to form at particular locations. The overall pigment concentration in the California Current drops as a larger fraction of the pigment is distributed within the filaments. The filaments also tend to become shorter during this period, which starts soon after the spring transition and usually reaches its maximum intensity in early summer. Winds (and curl) become much weaker in the mid-July to August period, and the filaments become much less prominent in terms of concentration and their length increases. Isolated wind events occasionally reinvigorate the filaments. A final bloom in the fall occasionally appears with spatial structure similar to the spring bloom. We conclude that wind forcing, in particular the curl of the wind stress, plays an important role in the distribution of phytoplankton pigment in the California Current. Although the underlying dynamics, especially of the filaments, may be dominated by processes other than forcing by wind stress curl (such as eddies or topographically induced instabilities), it appears that curl may force the variability of the filaments and hence the pigment patterns. Title: Phytoplankton pigment patterns and wind forcing off central California Author: Abbott, Mark R., Barksdale, B. We use a 4-year time series of high-resolution coastal zone color scanner imagery to study mesoscale variability in phytoplankton pigment (as a surrogate for biomass) distributions off central California during the spring–summer upwelling season. We use empirical orthogonal functions to decompose the time series of spatial images into its dominant modes of variability. Similarly, we analyze wind fields derived from the Fleet Numerical Oceanography Center pressure fields. The results from these analyses are used to investigate the coupling between wind forcing of the upper ocean and phytoplankton distributions on mesoscales. Prior to the spring transition, wind stress is dominated by strong northward conditions, and pigment is low and relatively uniform throughout the California Current domain. The first strong southward wind event soon results in a bloom of phytoplankton that is also uniformly distributed offshore. Nearshore values fluctuate relatively little in response to variations in wind forcing. After the spring transition, the wind becomes more steady (southward) and the fluctuations are dominated by changes in the strength of the curl. During southward events, the curl tends to become more positive. Depending on the fluctuations and strength of these curl episodes, filaments begin to form at particular locations. The overall pigment concentration in the California Current drops as a larger fraction of the pigment is distributed within the filaments. The filaments also tend to become shorter during this period, which starts soon after the spring transition and usually reaches its maximum intensity in early summer. Winds (and curl) become much weaker in the mid-July to August period, and the filaments become much less prominent in terms of concentration and their length increases. Isolated wind events occasionally reinvigorate the filaments. A final bloom in the fall occasionally appears with spatial structure similar to the spring bloom. We conclude that wind forcing, in particular the curl of the wind stress, plays an important role in the distribution of phytoplankton pigment in the California Current. Although the underlying dynamics, especially of the filaments, may be dominated by processes other than forcing by wind stress curl (such as eddies or topographically induced instabilities), it appears that curl may force the variability of the filaments and hence the pigment patterns. Close