Chemical and biological cycling in the ocean varies both spatially and temporally. While there is a broad-scale understanding of spatial variability on the scale of an ocean basin, significant variability may occur at smaller scales, particularly due to eddies. Additionally, biological and chemical cycling may vary over time scales ranging from interannual to millions of years due to changes in climate and tectonics. I've studied a range of such processes with collaborators. Since coming to Johns Hopkins particularly important projects include
1. Examination of biological variability in the Continuous Plankton Recorder dataset. Since the 1930s, researchers at the Sir Alistair Hardy Foundation for Ocean Science have sampled oceanic plankton using ships of opportunity. Graduate student Sara Rivero-Calle, now at University of Southern California, reanalysed this dataset to look at how major phytoplankton and zooplankton functional groups vary over time. Her first paper, published last year in Science, found an order-of-magnitude increase in the presence of the main calcifying phytoplankton group, coccolithophorids, which we hypothesize is due to alleviation of carbon limitation. A second paper recently published at Global Biogeochemical Cycles finds the the main open-ocean nitrogen fixer, Trichodesmium, showed an order of magnitude increase during the late 1980s and 1990s, which we hypothesize could be due to atmospheric conditions favoring the transport of iron-rich dust from the Sahara. A final paper in preparation examines drivers for interdecadal changes multiple phytoplankton groups and suggests that explaing such changes in terms of a single "regime shift" may be overly simplistic.
2. Characterization of biogeochemical variability associated with interannual physical variability in climate models. This work examines how bloom conditions vary from year to year in Earth System Models. We find that in most locations the total productivity does not vary substantially from one year to the next, but that the timing of when this occurs does. Analysis of how this works in the Pacific and North Atlantic Oceans reveals different physical regimes. In some locations where nutrients are limiting, changes in wintertime mixing play a key role in driving spring productivity. In others, where light is more limiting, it is more the timing of the shallowing of the mixed layer that varies from year to year. In the Arabian Sea, guest student Safoura Sedigh Marvasti showed that eddy modulation of mixing may play an important role, but that this process is difficult to simulate in global models within this region (Sedigh-Marvasti et al., 2016).
Additionally, as part of a new project funded by the National Oceanographic and Atmospheric Administration we examine whether these biogeochemical tracers can be used to better constrain and monitor the overturning circulation.
Gnanadesikan, John P. Dunne and Rym Msadek, 2014: Connecting Atlantic temperature variability and ocean ecosystems in two Earth System Models, Link
Sedigh Marvasti, S., A. Gnanadesikan, A.A. Bidokhti, J.P. Dunne,and S. Ghader, 2016: Challenges in modelling spatiotemporally varying phytoplankton blooms in the Northwestern Arabian Sea,Biogeosciences, 13,1049-1069. Link
Rivero-Calle, Sara, Anand Gnanadesikan, Carlos del Castillo, William Balch and Seth Guikema, 2015: Multidecadal increase in North Atlantic coccolithophores and the potential role of rising CO2 , 350,1533-1537, doi:10.1126/science.aaa8026. Link
Rivero-Calle, Sara, Carlos del Castillo, 2016: Anand Gnanadesikan, Amin Dezfuli, Ben Zaitchik and David Johns, Interdecadal Trichodesmium variability in cold North Atlantic waters, Global Biogeochemical Cycles ,30, 1620-1638. Link