Was the North Pacific Wintertime Climate Less Stormy during the Mid-Holocene?

Model evidence is presented to make the case that the midlatitude North Pacific wintertime transient eddy activity may have been significantly weaker during the mid-Holocene (∼6000 yr BP). A simulation of the mid-Holocene climate in an atmospheric general circulation model coupled to a reduced gravity ocean model showed significant reduction to transient eddy activity, up to 30% in the main storm-track region. The reduced baroclinic eddy activity is associated with basinwide climate changes over the northern and tropical Pacific, including a deepening of the Aleutian low, colder SSTs in the western and central North Pacific, a strengthening and southward shift of the subtropical jet, and a strengthened South Pacific convergence zone. These associated climate changes are consistently simulated across a range of Paleoclimate Modeling Intercomparison Project Phase II (PMIP2) coupled models forced with mid-Holocene climate forcings, suggesting they are a robust response to mid-Holocene orbital forcing. The authors link the mid-Holocene climate changes to two related modern-day analogs: (i) interannual variations in wintertime North Pacific storminess and (ii) the phenomenon of midwinter suppression whereby North Pacific transient eddy activity in today’s climate is reduced in midwinter. In both instances, the associated North Pacific climate conditions resemble those seen in the mid-Holocene simulations. While it remains to be seen which analog is dynamically more appropriate, the latter link—midwinter suppression—offers the simple physical interpretation that the mid-Holocene reduction in storminess is a consequence of a “more winterlike” climate resulting from the mid-Holocene precessional forcing.