Local and remote wind stress forcing of the seasonal variability of the Atlantic Meridional Overturning Circulation (AMOC) transport at 26.5°N

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 120 (2015): 2488–2503, doi:10.1002/2014JC010317.The transport of the Atlantic Meridional Overturning Circulation (AMOC) varies considerably on the seasonal time scale at 26.5°N, according to observations made at the RAPID-MOCHA array. Previous studies indicate that the local wind stress at 26.5°N, especially a large wind stress curl at the African coast, is the leading contributor to this seasonal variability. The purpose of the present study is to examine whether nonlocal wind stress forcing, i.e., remote forcing from latitudes away from 26.5°N, affects the seasonal AMOC variability at the RAPID-MOCHA array. Our tool is a two-layer and wind-driven model with a realistic topography and an observation-derived wind stress. The seasonal cycle of the modeled AMOC transport agrees well with RAPID-MOCHA observations while the amplitude is in the lower end of the observational range. In contrast to previous studies, the seasonal AMOC variability at 26.5°N is not primarily forced by the wind stress curl at the eastern boundary, but is a result of a basin-wide adjustment of ocean circulation to seasonal changes in wind stress. Both the amplitude and phase of the seasonal cycle at 26.5°N are strongly influenced by wind stress forcing from other latitudes, especially from the subpolar North Atlantic. The seasonal variability of the AMOC transport at 26.5°N is due to the seasonal redistribution of the water mass volume and is driven by both local and remote wind stress. Barotropic processes make significant contributions to the seasonal AMOC variability through topography-gyre interactions.This study has been supported by the National Science Foundation (OCE 0927017).2015-10-0

Seasonal and interannual variability of downwelling in the Beaufort Sea

Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 114 (2009): C00A14, doi:10.1029/2008JC005084.In this paper, we examine the seasonal and interannual to decadal variability of oceanic downwelling in the Beaufort Sea. The surface wind stress is the primary driver for variability in the upper Arctic Ocean and sea ice. The seasonal variability of the surface wind over the western Arctic is strongly influenced by a high sea level pressure center that emerges in the fall and diminishes in the summer. The wind stress and sea ice velocity are both anticyclonic from fall to spring and thus force an upwelling along the Alaskan and Canadian coast and downwelling in the interior Beaufort Sea. The upwelling and downwelling varied significantly on the interannual to decadal time scales from 1979 to 2006. There was no significant correlation between the upwelling/downwelling rate in the Beaufort Sea and the Arctic Oscillation index over this 28 year period. The coastal upwelling and interior downwelling in the Beaufort Sea had gradually intensified from 1979 to 2006. This change was almost entirely due to the increase in sea ice velocity according to three additional sensitivity calculations. The anticyclonic ice velocity over the western Arctic Ocean accelerated in the 28 year period, and the acceleration was not driven solely by the wind stress. The geostrophic wind condition was actually similar between 1979–1986 and 1997–2004. However, the ice velocity was much greater in the latter period. We hypothesize that the change in ice dynamics (thinner and less areal coverage) was responsible for the change of ice velocity.his study has been supported by the National Science Foundation’s Office of Polar Program (OPP0424074 and ARC-0902090), WHOI Arctic Initiative, and NASA’s Cryospheric Science Program (NNG05GN93G)

The role of wind stress in driving the along-shelf flow in the Northwest Atlantic Ocean

Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 126(4), (2021): e2020JC016757, https://doi.org/10.1029/2020JC016757.The along-shelf circulation in the Northwest Atlantic (NWA) Ocean is characterized by an equatorward flow from Greenland's south coast to Cape Hatters. The mean flow is considered to be primarily forced by freshwater discharges from rivers and glaciers while its variability is driven by both freshwater fluxes and wind stress. In this study, we hypothesize and test that the wind stress is important for the mean along-shelf flow. A two-layer model with realistic topography when forced by wind stress alone simulates a circulation system on the NWA shelves that is broadly consistent with that derived from observations, including an equatorward flow from Greenland coast to the Mid-Atlantic Bight (MAB). The along-shelf sea-level gradient is close to a previous estimate based on observations. The along-shelf flows exhibit strong seasonal variations with along-shelf transports being strong in fall/winter and weak in spring/summer, consistent with available observations. It is found that the NWA shelf circulation is affected by both wind-driven gyres through their western boundary currents and wind-stress forcing on the shelf especially along the coasts of Newfoundland and Labrador. The local wind stress forcing has more direct impacts on flows in shallower waters along the coast while the open-ocean gyres tend to affect the circulations along the outer shelf. Our conclusion is that wind stress is an important forcing of the main along-shelf flows in the NWA. One objective of this study is to motivate further examination of whether wind stress is as important as freshwater forcing for the mean flow.Both Yang and Chen are also supported by NOAA Climate Program Office's Climate Variability and Prediction Program under grant NA20OAR4310398. JY is supported by Woods Hole Oceanographic Institution (WHOI) W. V. A. Clark Chair for Excellence in Oceanography and NSF Ocean Science Division under grant OCE1634886. Chen is supported by WHOI Independent Research and Development award.2021-09-3

Dynamical response of the Arctic atmospheric boundary layer process to uncertainties in sea-ice concentration

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Atmospheres 118 (2013): 12,383–12,402, doi:10.1002/2013JD020312.Impact of sea-ice concentration (SIC) on the Arctic atmospheric boundary layer (ABL) is investigated using a polar-optimized version of the Weather Research and Forecasting (Polar WRF) model forced with SIC conditions during three different years. We present a detailed comparison of the simulations with historical ship and ice station based data focusing on September. Our analysis shows that Polar WRF provides a reasonable representation of the observed ABL evolution provided that SIC uncertainties are small. Lower skill is obtained, however, with elevated SIC uncertainties associated with incorrect seasonal evolution of sea ice and misrepresentation of ice thickness near the marginal ice zone (MIZ). The result underscores the importance of accurate representation of ice conditions for skillful simulation of the Arctic ABL. Further, two dynamically distinctive effects of sea ice on the surface wind were found, which act on different spatial scales. Reduced SIC lowers ABL stability, thereby increasing surface-wind (W10) speeds. The spatial scale of this response is comparable to the basin scale of the SIC difference. In contrast, near-surface geostrophic wind (Wg) shows a strong response in the MIZ, where a good spatial correspondence exists among the Laplacian of the sea level pressure (SLP), the surface-wind convergence, and the vertical motion within the ABL. This indicates that SIC affects Wg through variation in SLP but on a much narrower scale. Larger-amplitude and broader-scale response in W10 implies that surface-wind stress derived from Wg to drive ice-ocean models may not fully reflect the effect of SIC changes.The authors acknowledge the support from WHOI Arctic Research Initiative and National Science Foundation’s Office of Polar Program. H.S. thanks Andrey Proshutinsky (WHOI), Sang- Hun Park (NCAR), Keith Hines (BPRC/OSU), and Jun Inoue (JAMSTEC) for insightful comments.2014-05-2