TITLE: Changes in water mass distribution and biogeochemical activity between 1993 and 2009 in the eastern tropical South Pacific
AUTHORS (FIRST NAME, LAST NAME): Pedro Jose Llanillo[1,2], Josep Lluis Pelegri[1,2], Lothar Stramma[3], Johannes Karstensen[3]
INSTITUTIONS (ALL): 1. Physical oceanography, ICM, CSIC, Barcelona, Spain.
2. International Lab in Global Change (LINCGlobal), CSIC - PUC, Barcelona - Santiago de Chile, Spain.
3. Physical oceanography, GEOMAR Helmholtz Centre for Ocean Research, Kiel, Germany.
ABSTRACT BODY: Two ship occupations of a meridional section along about 86°W in the tropical eastern South Pacific, one during El Niño/El Viejo conditions (March 1993) and the other one during La Niña/La Vieja conditions (February 2009), are used to investigate the distribution and changes in water masses and biogeochemical cycling in the upper ocean (top 1200 m). Special attention is placed to the changes in the distribution of those water masses involved in the ventilation and maintenance of the oxygen minimum zone (OMZ). The extended Optimum Multi-Parameter (OMP) analysis is applied to the data in order to estimate the water mass contributions and the amounts of remineralized organic matter, respired oxygen and denitrified nitrate. The El Niño-Southern Oscillation (ENSO) chiefly determines the water properties and water mass distribution in the upper 300 m of the water column, affecting the location of the Shallow Salinity Minimum (SSM) originated by the subsurface intrusion of subantarctic waters and leading to substantial changes in biogeochemical processes. During the weak 1993 El Niño event, the intrusion of relatively oxygenated waters from the west displaced and deepened the OMZ, the northward progression of the SSM was reduced and the denitrification was dampened. At the time of the 2009 La Niña conditions, the reinforced trade winds drove an intense upwelling and the upper part of the OMZ was found at relatively shallower depths, replacing the more oxygenated surface waters, promoting denitrification and reinforcing and extending the SSM further north. The influence of the Pacific decadal oscillation (PDO) may be appreciated in the deep portion of the water column but it is not easily discernible in the upper layers as a result of the superimposed ENSO signature, also changing from a warm phase in 1993 to a cold phase in 2009.
Friday afternoon: poster OS53B
TITLE: Physical processes governing summer Chl-a in the Southern Ocean
AUTHORS (FIRST NAME, LAST NAME): Magdalena M Carranza[1], Sarah T Gille[1]
INSTITUTIONS (ALL): 1. Climate, Atmospheric Sciences and Physical Oceanography, Scripps Institution of Oceanography - UCSD, La Jolla, CA, United States.
ABSTRACT BODY: The Southern Ocean plays a crucial role in sequestering atmospheric CO2 by exporting fixed carbon into the deep ocean through the biological pump. It is a High Nitrate Low Chlorophyll (HNLC) region where phytoplankton growth is primarily Fe-limited. Several sources of Fe have been identified in the Southern Ocean, and phytoplankton blooms occur annually. Many of these blooms persist through the summer and even peak in the summer months. The mechanisms that explain spring blooms are well known, and the shoaling of the mixed-layer depth (MLD) plays a critical role in terms of light availability. A question that remains unanswered is what sustains blooms through the summer when presumably light conditions are optimal and nutrients in the mixed layer have been depleted. One source of Fe is the deep ocean, and models and observations suggest blooms in the Southern Ocean are largely driven by ocean dynamics. The evaluation of the input of Fe from subsurface waters into the euphotic zone depends in part on mixed-layer dynamics and Ekman-induced upwelling. The turbulent mixing caused by winds, along with the buoyancy forcing, prescribe the MLD and deepening of the MLD may facilitate the entrainment of Fe. The curl of the wind stress, by Ekman pumping, provides a mechanism of upwelling that can also contribute to the supply of Fe. Sea surface temperature (SST) responds to both of these processes, as well as to surface heat fluxes at the ocean-air interphase. In this work we explore the potential influence of nutrient entrainment due to MLD deepening and Ekman-induced upwelling on summer phytoplankton blooms. Using weekly satellite estimates and ocean reanalysis we correlate anomalies of Chlorophyll-a (Chl-a) with anomalies of physical variables such as wind speed intensity (W), Ekman pumping velocities (WEK), SST and surface heat fluxes (Qnet). Winds and SST can have strong feedbacks through Qnet and instabilities in the atmospheric boundary layer, and therefore in our approach we use partial correlations to remove the atmospheric-driven component in the SST anomalies. We find oceanic-driven SST fluctuations have the largest influence on Chl-a variability with statistically significant correlations over coherent areas, showing patterns tightly linked to the position of major oceanic fronts. Two distinct regimes are observed. To the north of the Subtropical and Polar Fronts (STF and PF), summer blooms are associated with cold SSTs suggesting the importance of nutrient entrainment/upwelling. Over several of these regions positive correlations between Chl-a and W and negative correlations between SST and W support the entrainment hypothesis. South of the STF and PF, blooms are linked to warm SSTs suggesting a light-limited environment. The influence of W on Chl-a variability is more influential over open ocean waters where blooms are not intense. Nonetheless, at daily scales, significant correlations suggest strong winds have a measurable influence in sustaining phytoplankton blooms in the summer over broad areas. The influence of Ekman pumping on Chl-a shows more complex patterns suggesting that Ekman-induced upwelling is less influential. Argo profiles are not dense enough in the Southern Ocean to test the direct influence of MLD deepening on Chl-a. We aim to use MLD output from high-resolution data assimilation models to further test the entrainment hypothesis due to MLD deepening.
Wednesday afternoon poster OS43E-1859
TITLE: Equilibrium Beach Profiles on the East and West U.S. Coasts
AUTHORS (FIRST NAME, LAST NAME): Bonnie C Ludka[1], Robert T Guza[1], Jesse E McNinch[2], William O'Reilly[1]
INSTITUTIONS (ALL): 1. Scripps Institution of Oceanography, La Jolla, CA, United States.
2. Field Research Facility, Duck, NC, United States.
ABSTRACT BODY: Beach elevation change observations from the United States west and east coasts are used to identify statistically the dominant cross-shore patterns in sand level fluctuations, and these changes are related to equilibrium beach profile concepts. Three to seven years of observations at four beaches in Southern California include monthly surveys of the subaerial (near MSL) beach, and quarterly surveys from the backbeach to about 8m depth. At Duck, North Carolina, observations include 31 years of monthly surveys from the dunes to about 8m depth.
On the Southern California beaches, the dominant seasonal pattern is subaerial erosion in winter and accretion in summer. Seasonal fluctuations of 3m in shoreline vertical sand levels, and 50m in subaerial beach width, are not uncommon. The sand eroded from the shoreline in winter is stored in an offshore sand bar and returns to the beach face in summer. Wave conditions in Southern California also vary seasonally, with energetic waves arriving from the north in winter, and lower energy, longer period southerly swell arriving in summer. A spectral refraction model, initialized with a regional network of directional wave buoys, is used to estimate hourly wave conditions, in 10m water depth. Using an equilibrium hypothesis, that the shoreline (defined as the cross-shore location of the MSL contour) change rate depends on the wave energy and the wave energy disequilibrium, Yates (2009) modeled the time-varying shoreline location at several Southern California beaches with significant skill. The four free model parameters were calibrated to fit observations. Following Yates (2009), we extend the equilibrium shoreline model to include the horizontal displacement of other elevation contours. At the Southern California sites, the modeled contour translation depends on the incident wave energy, the present contour configuration, and observation-based estimates of the contour behavior (based on EOF spatial amplitudes).
At Duck, seasonal variations of the wave field (measured immediately offshore) are large, but shoreline changes (usually <30cm) are smaller than in Southern California. Maximum vertical variations occur just seaward of the shoreline and the nearshore bathymetry is often barred. Plant (1999) show that bar crest position at Duck has equilibrium-like behavior. We will present the results of equilibrium shoreline and profile modeling at Duck. At both sites, we diagnose sources (e.g. grain size and incident waves) of the sometimes strong observed alongshore variations in sand level change patterns.
Funding was provided by the US Army Corps of Engineers and the California Department of Boating and Waterways.
Tuesday morning poster, OS21B-1724
TITLE: Understanding the annual cycle of subsurface ocean temperature and salinity using Argo's first million profiles.
AUTHORS (FIRST NAME, LAST NAME): Donata Giglio[1], Dean H Roemmich[1], Philip Sutton[2]
INSTITUTIONS (ALL): 1. Scripps Institution of Oceanography, La Jolla, CA, United States.
2. National Institute of Water and Atmospheric Research, Wellington, New Zealand.
ABSTRACT BODY: The Argo Program, now into its 9th year of global subsurface ocean coverage, has grown from a sparse global array of about 1000 profiling floats in early 2004 to more than 3500 floats today, obtaining about 120,000 profiles per year. The array's global coverage and sustained sampling improved observations most significantly in the southern hemisphere. In this study, we take advantage of Argo's unprecedented spatial resolution, to better understand the annual cycle of ocean temperature and salinity.
Temperature and salinity variability includes both surface layer changes due to air-sea heat and freshwater fluxes, and subsurface changes due to ocean dynamics. The latter can result from either vertical or horizontal displacements of isopycnal surfaces. Thermosteric and halosteric variability are both considered. In the surface layers, this variability reveals heat and freshwater exchanges , as well as the signatures of water mass formation processes. In deeper layers, the separation into temperature and salinity contributions helps to resolve ambiguities in subsurface horizontal versus vertical displacements. Also, Argo profiles extend to depths as great as 2000 m: the depth-dependence of annual isopycnal displacement provides a basis for estimating the vertical scale of the annual cycle.
Finally, the relationship between subsurface ocean variability and the annual cycle in wind forcing is investigated.
Friday 5:25-5:40 pm, Moscone West 3011