This study presents an analysis of observed Atlantic Meridional Overturning Circulation (AMOC) variability at 26.5°N on submonthly to interannual time scales compared to variability characteristics produced by a selection of five high- and low-resolution, synoptically and climatologically forced OGCMs. The focus of the analysis is on the relative contributions of ocean mesoscale eddies and synoptic atmospheric forcing to the overall AMOC variability. Observations used in this study were collected within the framework of the joint U.K.-U.S. Rapid Climate Change (RAPID)-Meridional Overturning Circulation & Heat Flux Array (MOCHA) Program. The RAPID-MOCHA array has now been in place for nearly 6 years, of which 4 years of data (2004-2007) are analyzed in this study. At 26.5°N, the MOC strength measured by the RAPID-MOCHA array is 18.5 Sv. Overall, the models tend to produce a realistic, though slightly underestimated, MOC. With the exception of one of the high-resolution, synoptically forced models, standard deviations of model-produced MOC are lower than the observed standard deviation by 1.5 to 2 Sv. A comparison of the MOC spectra at 26.5°N shows that model variability is weaker than observed variability at periods longer than 100 days. Of the five models investigated in this study, two were selected for a more in-depth examination. One model is forced by a monthly climatology derived from 6-hourly NCEP/NCAR winds (OFES-CLIM), whereas the other is forced by NCEP/NCAR reanalysis daily winds and fluxes (OFES-NCEP). They are identically configured, presenting an opportunity to explain differences in their MOCs by their differences in forcing. Both of these models were produced by the OGCM for the Earth Simulator (OFES), operated by the Japan Agency for Marine-Earth Science & Technology (JAMSTEC). The effects of Ekman transport on the strength, variability, and meridional decorrelation scale are investigated for the OFES models. This study finds that AMOC variance due to Ekman forcing is distributed nearly evenly between the submonthly, intraseasonal, and seasonal period bands. When Ekman forcing is removed, the remaining variance is the result of geostrophic motions. In the intraseasonal period band this geostrophic AMOC variance is dominated by eddy activity, and variance in the submonthly period band is dominated by forced geostrophic motions such as Rossby and Kelvin waves. It is also found that MOC variability is coherent over a meridional distance of ~8° throughout the study region, and that this coherence scale is intrinsic to both Ekman and geostrophic motions. A Monte Carlo-style evaluation of the 27-year-long OFES-NCEP timeseries is used to investigate the ability of a four year MOC strength timeseries to represent the characteristics of lengthier timeseries. It is found that a randomly selected four year timeseries will fall within ~1 Sv of the true mean 95% of the time, but long term trends cannot be accurately calculated from a four year timeseries. Errors in the calculated trend are noticeably reduced for each additional year until the timeseries reaches ~11 years in length. For timeseries longer than 11-years, the trend's 95% confidence interval asymptotes to 2 Sv/decade.

The Southern Caribbean Sea experiences a strong upwelling process along the coast from about 61°W to 75.5°W and 10-13°N. In this dissertation three aspects of this upwelling system are examined: (A) A mid-year secondary upwelling that was previously observed in the southeastern Caribbean Sea between June-July, when land based stations show a decrease in wind speed. The presence and effects of this upwelling along the whole southern Caribbean upwelling system were evaluated, as well as the relative forcing contribution of alongshore winds (Ekman Transport, ET) and wind-curl (Ekman Pumping, EP). (B) Stronger upwelling occurs in two particular regions, namely the eastern (63-65°W) and western (70-73°W) upwelling areas. However, the eastern area has higher fish biomass than the western area (78% and 18%, respectively, of the total small pelagic biomass of the southern Caribbean upwelling system). The upwelling dynamics along the southern Caribbean margin was studied to understand those regional variations on fish biomass. (C) The most important fishery in the eastern upwelling area off Venezuela is the Spanish sardine (Sardinella aurita). The sardine artisanal fishery is protected and only takes place up to ~10 km offshore. The effects of the upwelling cycle on the spatial distribution of S. aurita were studied. The main sources of data were satellite observations of sea surface temperature (SST), chlorophyll-a (Chl) and wind (ET and EP), in situ observations from the CARIACO Ocean Time-Series program, sardine biomass from 8 hydroacoustics surveys (1995-1998), and temperature profiles from the World Ocean Atlas 2005 used to calculate the depth of the Subtropical Underwater core (traced by the 22°C isotherm). The most important results of the study were as follows: (A) The entire upwelling system has a mid-year upwelling event between June-August, besides the primary upwelling process of December-April. This secondary event is short-lived (~5 weeks) and ~1.5°C warmer than the primary upwelling. Together, both upwelling events lead to about 8 months of cooler waters (-3, averaged from the coast to 100 km offshore) in the region. Satellite nearshore wind (~25 km offshore) remained high in the eastern upwelling area (> 6 m s-1) and had a maximum in the western area (~10 m s-1) producing high offshore ET during the mid-year upwelling (vertical transport of 2.4 - 3.8 m3 s-1 per meter of coastline, for the eastern and western areas, respectively). Total coastal upwelling transport was mainly caused by ET (~90%). However, at a regional scale, there was intensification of the wind curl during June as well; as a result open-sea upwelling due to EP causes isopycnal shoaling of deeper waters enhancing the coastal upwelling. (B) The eastern and western upwelling areas had upwelling favorable winds all year round. Minimum / maximum offshore ET (from weekly climatologies) were 1.52 / 4.36 m3 s-1 per meter, for the western upwelling area; and 1.23 / 2.63 m3 s-1 per meter, for the eastern area. The eastern and western upwelling areas showed important variations in their upwelling dynamics. Annual averages in the eastern area showed moderate wind speeds (6.12 m s-1), shallow 22°C isotherm (85 m), cool SSTs (25.24°C), and phytoplankton biomass of 1.65 mg m-3. The western area has on average stronger wind speeds (8.23 m s-1) but a deeper 22°C isotherm (115 m), leading to slightly warmer SSTs (25.53°C) and slightly lower phytoplankton biomass (1.15 mg m-3). We hypothesize that the factors that most inhibits fish production in the western upwelling area are the high level of wind-induced turbulence and the strong offshore ET. (C) Hydroacoustics values of Sardinella aurita biomass (sAsardine) and the number of small pelagics schools collected in the eastern upwelling region off northeast Venezuela were compared with environmental variables (satellite products of SST, SST gradients, and Chl -for the last two cruises-) and spatial variables (distance to upwelling foci and longitude-latitude). These data were examined using Generalized Additive Models. During the strongest upwelling season (February-March) sAsardine was widely distributed in the cooler, Chl rich upwelling plumes over the wide (~70km) continental shelf. During the weakest upwelling season (September-October) sAsardine was collocated with the higher Chl (1-3 mg m-3) found within the first 10 km from the upwelling foci; this increases Spanish sardine availability (and possibly the catchability) for the artisanal fishery. These results imply that during prolonged periods of weak upwelling the environmentally stressed (due to food scarceness) Spanish sardine population would be closer to the coast and more available to the fishery, which could easily turn into overfishing. After two consecutive years of weak upwelling (2004-2005) Spanish sardine fishery crashed and as of 2011 has not recovered to previous yield; however during 2004 a historical capture peak occurred. We hypothesize that this Spanish sardine collapse was caused by a combination of sustained stressful environmental conditions and of overfishing, due to the increased catchability of the stock caused by aggregation of the fish in the cooler coastal upwelling cells during the anomalous warm upwelling season.