low latitude

Abstract

EviDENCE of rapid climate oscillations during the last glacial period has been identified in climate records from Greenland ice cores 1 ' 2 and ocean sediments in the North Atlantic 3 ' 4 • These records show that periods of gradual cooling are terminated by abrupt warming events 5 , with the coldest periods coinciding with the deposition of ice-rafted debris (so-called Heinrich events) throughout the North Atlantic. Heinrich events are thought to be a signature of massive iceberg discharges owing to collapse of the Laurentide ice sheet; Bond et al. 5 have proposed that the decrease in meltwater flux following collapse and retreat of the ice sheet enhanced the ocean's thermohaline circulation 6 , thereby increasing advection of heat from the tropics and giving rise to abrupt climate warming. Here we test this idea using a simple ocean model coupled to a model of a periodically surging ice sheet. We find that massive discharges of icebergs first stop the thermohaline circulation because of the consequent freshwater influx, cooling the North Atlantic region. This is followed by a rapid restart of the circulation, leading to abrupt warming. Thus our model can reproduce the qualitative features of the climate oscillations seen in the ice-core and ocean records. FIG. 1 The ocean model is composed of three boxes: 1, high-latitude waters; 2, \ow-latitude surface; and 3, intermediate plus deep ocean. The fourth reservoir is the ice sheet of height H. The diffusive mixings k,2, k, 3 , k2 3 are constant, and the thermohaline circulation m is assumed to be proportional to the density difference between the two surface boxes, using a linear equation of state for sea water m=Jl[a(T2-T,)-,8(S2-S,)], where T, and S, are the temperature and salinity of box i. In the atmosphere , the heat transport is L = Cc(T 2-T 1) and the water vapour is V=Cv(T 2-T 1). The snow accumulation is A= CA(T2-T 1), the melting is M = CM(T 1-T,) if T 1 > T,. Where T, is the melting temperature of ice (here 0 °C}, M=O otherwise. Iceberg calving is I= 0 when the ice sheet is growing and/=-V/r during iceberg discharges. The net radiation fluxes are Q, = Q~-q, r, and 02 = Q~-Q2 T2. The temperature profile in the ice sheet is parametrized by T(z) = T(H) + 8 2 (1-zjH)/(8 + (y-8)z/H), where 0 = T(O)-T(H), T(H) = T 1 + Cr-rH, and y is the temperature gradient at the base of the ice sheet imposed by the geothermal flux and, during surges, friction.