Date of Award
Doctor of Philosophy in Oceanography
Kathleen A. Donohue
Sea level (SL) is an informative index of climate and a serious concern for coastal communities. Understanding the observational SL record is important from scientific and societal points of view. We consider the tide gauge SL record, focusing on data along the North American northeast coast, aiming to identify relevant geophysical processes responsible for observed SL changes.
SL changes reflect dynamic and isostatic ocean effects. Recent works have interpreted accelerated and extreme SL changes along the northeast coast of North America primarily in terms of dynamic changes. In manuscript 1, we consider the influence of the ocean's isostatic response to surface atmospheric pressure loading| the inverted barometer (IB) effect - on annual mean SL from tide gauge records. The IB effect explains ~25% of interannual SL variance and accounts for ~50% of the magnitude of a recent extreme event of SL rise along Atlantic Canada and New England. Estimated IB effects also amount to ~10-30% of recent multidecadal SL accelerations over the Mid-Atlantic Bight and Southern New England. These findings reiterate the need for careful estimation and removal of isostatic effects for studies of dynamic SL.
In manuscript 2, we continue our investigation of east coast tide gauge SL, seeking to better understand the relation between coastal SL and the variable ocean circulation. Annual SL records (adjusted for the IB effect) from tide gauges along the North American northeast coast over 1980-2010 are compared to a set of data-assimilating \ocean reanalysis" products as well as a global barotropic model solution forced with wind stress and barometric pressure. Correspondence between models and data depends strongly on model and location. At sites north of Cape Hatteras, the barotropic model shows as much (if not more) skill than ocean reanalyses, explaining ~ 50% of the variance in the adjusted annual tide gauge SL records. Additional numerical experiments show that annual SL changes along this coast from the barotropic model are driven by local wind stress over the continental shelf and slope. This result is interpreted in the light of a simple dynamic framework, wherein bottom friction balances surface wind stress in the alongshore direction and geostrophy holds in the across-shore direction. Results highlight the importance of barotropic dynamics on coastal SL changes on interannual and decadal time scales; they also have implications for diagnosing errors in ocean reanalysis, using tide gauge records to infer past changes in ocean circulation, and identifying mechanisms responsible for projected regional SL rise.
Finally, in manuscript 3, three global gridded reanalysis products are used alongside a carefully curated set of station records and tide gauges to consider IB changes over the global ocean and their relation to SL changes over the Twentieth Century. Centennial IB trends from reanalysis products show meridional structure consistent with the IB response expected under global warming. Annual IB variations show stronger amplitudes at higher latitudes, as in past studies focusing on higher frequencies or shorter periods. Discrepancies between gridded IB products tend to be smaller (larger) for more recent (earlier) periods and ocean regions with good (poor) historical data coverage. Comparisons between reanalysis products and station data reveal evidence for common errors across reanalyses over a wide range of time scales from annual to centennial. Notwithstanding their errors, the gridded reanalysis products are useful for interpreting tide gauge records: subtracting IB from SL records reduces both temporal variance within tide gauge records and spatial variance across tide gauge sites. Results advocate for making the IB correction to tide gauge SL data using reanalysis products in studies of ocean circulation and climate on centennial time scales.
Piecuch, Christopher Gilbert, "Understanding Tide Gauge Mean Sea Level Changes on the East Coast of North America" (2016). Open Access Dissertations. Paper 472.