Role of Ocean Dynamics in the Seasonal Hadley Cell: A Response to Idealized Arctic Amplification

How atmospheric and oceanic circulations respond to Arctic warming at different timescales are revealed with idealized numerical simulations. Induced by local forcing and feedback, Arctic warming appears and leads to sea-ice melting. Deep-water formation is inhibited, which weakens the Atlantic Meridional Overturning Circulation (AMOC). The flow and temperature in the upper layer does not respond to the AMOC decrease immediately, especially at mid-low latitudes. Thus, nearly uniform surface warming in mid-low latitudes enhances (decreases) the strength (width) of the Hadley cell (HC). With the smaller northward heat carried by the weaker AMOC, the Norwegian Sea cools significantly. With strong warming in Northern Hemisphere high latitudes, the long-term response triggers the “temperature-wind-gyre-temperature” cycle, leading to colder midlatitudes, resulting in strong subsidence and Ferrel cell enhancement, which drives the HC southward. With weaker warming in the tropics and stronger warming at high latitudes, there is a stronger HC with decreased width. A much warmer Southern Hemisphere appears due to a weaker AMOC that also pushes the HC southward. Our idealized model results suggest that the HC strengthens under both warming conditions, as tropical warming determines the strength of the HC convection. Second, extreme Arctic warming led by artificially reduced surface albedo decreases the meridional temperature gradient between high and low latitudes, which contracts the HC. Third, a warmer mid-high latitude in the Northern (Southern) Hemisphere due to surface albedo feedback (weakened AMOC) in our experiments pushes the HC northward (southward). In most seasons, the HC exhibits the same trend as that described above.