[1] While subsidence is widely recognized as a driver of geomorphic change in the northern Gulf of Mexico (GOM), there is considerable disagreement over the rates of subsidence and the interpreted variability in these rates, which leads to controversies over the impacts of subsidence on surface land area change. Here we present a new method to calculate subsidence rates from the tide gauge record that is based on an understanding of the meteorological drivers of inter-annual sea-level change. In Grand Isle, LA and Galveston, TX, we explicitly show that temporal patterns of subsidence are closely linked to subsurface fluid withdrawal and coastal land loss, and suggest changes in withdrawal rates can both increase and decrease rates of subsidence and wetland loss. Our results also imply that the volume of sediment needed to rebuild GOM wetlands may currently fall within the low end of some restoration scenarios. Citation: Kolker, A. S., M. A. Allison, and S. Hameed (2011), An evaluation of subsidence rates and sea-level variability in

No abstract

We simulate the global cycle of reactive nitrogen in a three‐dimensional model of chemistry, transport, and deposition. Our model is based on the Lagrangian tracer model described by Walton et al. [1988] and uses winds and precipitation fields calculated by the Livermore version of the NCAR Community Climate Model. The model includes the basic chemical reactions of NO, NO2, and HNO3. For this study, we use prescribed OH and O3 concentrations and calculate the concentrations of NO, NO2, and HNO3 for a perpetual January and a perpetual July. The sources of reactive nitrogen due to fossil‐fuel combustion (22 Mt N/yr), lightning discharges (3 Mt N/yr), soil microbial activity (10 Mt N/yr), biomass burning (6 Mt N/yr), and the oxidation of N2O in the stratosphere (1 Mt N/yr) are included. Model‐predicted concentrations of NO, NO2, and HNO3 are compared to available measurements. In general, we find reasonable agreement between model predictions and measurements except for concentrations of HNO3 in the remote Pacific. At these latter locations, we require a larger source of reactive nitrogen to fit the observations. This may be supplied by lightning discharges, although increasing this source degrades our agreement with measured HNO3 abundances in the free troposphere. Alternatively, a local marine source could contribute to the measured abundances. Predictions for nitrate deposition by precipitation are within a factor of 2 of measured deposition rates in the northern hemisphere in the summer and in both seasons at remote locations. The model underpredicts nitrate deposition in winter in Europe, due primarily to the excessively strong winds generated by the general circulation model. Model simulations for NOx and HNO3 surface mixing ratios from calculations including only the fossil‐fuel source, only natural sources, and all sources acting together, are compared. Anthropogenic sources have substantially increased the concentrations of NOx and HNO3 throughout all continents during both January and July. Fossil‐fuel sources are responsible for most of this increase in the northern hemisphere, while both biomass burning and fossil‐fuel combustion contribute in the southern hemisphere.

Over the last few decades, rising greenhouse gas emissions have promoted poleward expansion of the large-scale atmospheric Hadley circulation that dominates the Tropics, thereby affecting behavior of the Intertropical Convergence Zone (ITCZ) and North Atlantic Oscillation (NAO). Expression of these changes in tropical marine ecosystems is poorly understood because of sparse observational datasets. We link contemporary ecological changes in the southern Caribbean Sea to global climate change indices. Monthly observations from the CARIACO Ocean Time-Series between 1996 and 2010 document significant decadal scale trends, including a net sea surface temperature (SST) rise of ∼1.0 ± 0.14°C (±SE), intensified stratification, reduced delivery of upwelled nutrients to surface waters, and diminished phytoplankton bloom intensities evident as overall declines in chlorophyll a concentrations (ΔChla = −2.8 ± 0.5%·y −1 ) and net primary production (ΔNPP = −1.5 ± 0.3%·y −1 ). Additionally, phytoplankton taxon dominance shifted from diatoms, dinoflagellates, and coccolithophorids to smaller taxa after 2004, whereas mesozooplankton biomass increased and commercial landings of planktivorous sardines collapsed. Collectively, our results reveal an ecological state change in this planktonic system. The weakening trend in Trade Winds (−1.9 ± 0.3%·y −1 ) and dependent local variables are largely explained by trends in two climatic indices, namely the northward migration of the Azores High pressure center (descending branch of Hadley cell) by 1.12 ± 0.42°N latitude and the northeasterly progression of the ITCZ Atlantic centroid (ascending branch of Hadley cell), the March position of which shifted by about 800 km between 1996 and 2009. ecosystem state change | oceanography | plankton productivity P hytoplankton support over 95% of marine food webs and are responsible for about half of the Earth's conversion of CO 2 to biomass through net primary production (NPP) (1). Long-term declines in phytoplankton biomass and production in over 70% of the global ocean have been inferred recently from satellite imagery and century-long shipboard records of water clarity (2, 3). These reports of large-scale changes are at odds with trends directly observed at specific locations within the same ocean domains.

The location where the Gulf Stream separates from the American coast and turns eastward is called its northwall. The interannual fluctuations of the northwall are significantly correlated with the North Atlantic Oscillation with a lag of two years. When the Azores High and the Icelandic Low pressures are taken as independent variables, the latter dominates the relationship with the northwall, and the influence of the Azores High pressure is insignificant. This is consistent with the hypothesis that the major oceanic control of the northwall is in the southward flow of Labrador Sea Water into the Slope Sea. The alternative mechanism that the interaction of westward propagating Rossby waves with the American coast is responsible for northwall fluctuations is considered less likely because its initiation requires perturbations of the eastward winds in the mid‐Atlantic region, and they are very likely dependent on the Azores High. The analysis suggests that the time lag between perturbations of the Icelandic Low and the northwall varies between one and three years.