Response of Meridional Wind to Greenhouse Gas Forcing, Arctic Sea-Ice Loss, and Arctic Amplification

Under increasing greenhouse gases, the Arctic warms about twice as fast as elsewhere, known as Arctic amplification (AA). AA weakens meridional temperature gradients and is hypothesized to weaken zonal wind and cause wavier circulation with stronger meridional wind (upsilon) over northern mid-to-high latitudes. Here model simulations are analyzed to examine the upsilon response to increased CO2 and AA alone. Total upsilon changes are found to be dominated by the effect of increased CO2 without AA, with a zonal wavenumber-4 (wavenumber-3) change pattern over the northern (southern) extratropics that generally enhances current upsilon and results partly from changes in zonal temperature gradients. The extratropical upsilon change patterns are quasi-barotropic and are more pronounced during boreal winter. The CO2 forcing also causes baroclinic upsilon changes over the tropics tied to convection changes. The impact of AA on upsilon is mainly over the northern extratropics and is opposite to the effect of increased CO2 but with smaller magnitude. An eastward shift (similar to 5 degrees longitude) and an amplitude increase (similar to 1 m s(-1)) in the climatology of the northerlies over Europe caused mainly by CO2 forcing contribute to the drying in southern Europe, while both AA and CO2 forcing enhance the climatology of the northerlies over East Asia. Over the northern mid-to-high latitudes, Arctic sea ice loss and AA enhance the land-ocean thermal contrast in winter, while increased CO2 alone weakens it, resulting in opposite changes in zonal temperature gradients and thus upsilon. Different warming rates over land and ocean also contribute to the intermodel spread in upsilon response patterns among climate models. Significance StatementMeridional wind (upsilon) greatly contributes to thermal and moisture advection due to large meridional gradients in these fields. It is hypothesized that the enhanced Arctic warming under anthropogenic global warming could weaken meridional temperature gradients, decelerate westerly jets, and cause wavier circulation with stronger upsilon over northern extratropics. Using novel climate model simulations, we found that the effect of increased CO2 without AA determines the total upsilon changes. AA generally weakens the climatological upsilon, contrary to the direct effect of increased CO2. The upsilon changes are small relative to its climatology but may have large impacts on regional climate over central Europe, East Asia, and interior North America. More research is needed to examine the mechanisms causing regional upsilon changes.