1
Supplementary materials
2
S.1 The ENSO frequency in ICE
3
The ENSO simulated by CCSM3 tends to have a dominant frequency of ~2 year
4
(Deser et al., 2006; Liu et al., 2014). In ICE, the frequency of ENSO during the entire
5
simulation is also close to quasi-biennial (Fig. S1). The power spectrum is increased
6
after 14 ka BP with a larger peak, consistent with the evolution of amplitude of ENSO
7
as shown in Fig. 1e.
8
9
S.2 The energy balance
10
The interhemispheric asymmetry of atmospheric heat budget that the ice-sheet retreat
11
induces (as in our case, the substantial decay of the LIS at 14 ka BP) can also be
12
reviewed in perspective of energy balance. Recent studies (e.g. Frierson et al., 2013;
13
Marshall et al., 2013) have argued that the mean position of ITCZ north of the equator
14
is a result of northward oceanic heat transport across the equator which leads to a
15
more heated NH atmosphere, despite the slight interhemispheric difference of
16
radiative forcing top of atmosphere. The compensating southward cross-equatorial
17
atmospheric heat transport is thus established, while it requires a ITCZ mean
18
displacement and the collocated ascending branch of Hadley cell north of the equator.
19
Here the role of ice cover as an extratropical thermal forcing that changes the
20
atmospheric heat budget, namely the competing of the declining ice sheet and the fast
21
sea ice expansion, is investigated. The global zonal mean anomalous heat fluxes
22
absorbed by the atmospheric column are depicted in Fig. S2 (Fig .S3/Fig. S4 is over
23
land/ocean). It is calculated as the sum of anomalous shortwave and longwave heat
24
fluxes top of atmosphere (TOA) and anomalous shortwave, longwave, latent and
25
sensible heat fluxes at the surface. The competing effects can be seen in the
26
substantial dipole anomaly in the NH extratropics (Fig. S2f, blue line). It shows a
27
peak of atmospheric heating over the southern edge of LIS (~40oN, confirmed by Fig.
28
S3f), and a comparable peak of cooling over the region of sea ice expansion (~55oN,
29
confirmed by Fig. S4f). Above the sea ice where was open ocean before, the
30
anomalous reflective shortwave heat flux mostly just goes through the atmospheric
31
column (Fig. S2a, Fig. S4a). The much cooler temperature of sea ice than ocean
32
reduces emitted longwave radiation at the surface that heats the atmosphere, and this
33
surface anomaly is stronger than outgoing longwave radiation (OLR) anomaly at TOA
34
as the cooling of air temperature that dominates OLR is weaker (Fig. S2b, Fig. S4b).
35
As a result the top minus surface anomalous longwave radiative cooling is not
36
negligible. The sea ice further cuts off the sensible and latent heat fluxes warming at
37
the surface, both inducing cooling with a magnitude larger than the longwave
38
radiative cooling (Fig. S2c,d, Fig. S4c,d). A combination of all the terms (TOA and
39
surface) of anomalous heat fluxes suggests the atmospheric column loses heat over
40
the sea ice (Fig. S2f, Fig. S4f). With almost all the terms opposite, except for
41
negligible latent heat flux anomaly (Fig. S2, Fig. S3), the heat budget suggests that the
42
atmospheric column gains heat above the decaying ice sheet. Therefore, in concept, at
43
14 ka BP the NH ice sheet retreat favors a southward shift of ITCZ by warming the
44
NH while the comparable (in terms of heat budget) sea ice expansion favors a
45
northward shift of ITCZ by cooling the NH. The role of sea ice in determining the
46
interhemispheric asymmetry is thus confirmed.
47
48
The anomaly of heat budget of atmospheric column in either hemisphere is
49
compensated by the cross-equatorial atmospheric heat transport. At 14 ka BP, there is
50
an energy redistribution with an increase (0.015 PW) in the NH and a decrease
51
(-0.013 PW) in the SH, implying a small northward atmospheric heat transport across
52
the equator. It is more than one order smaller than the change of heat transport by
53
removing imposed ice without the compensating sea ice in NH high latitude (e.g.
54
~0.35 PW, from Chiang and Bitz, 2005). The global mean ITCZ shifts southward at
55
14 ka BP (Figure not shown) rather than northward following the energy balance
56
mechanism, probably due to the relative small perturbation. Indeed the anomalous
57
interhemispheric asymmetry is subtle in the tropics (Fig. 11a-c), except for the region
58
of ENSO/annual cycle action—eastern Pacific. We argue that only under robust
59
extratropical forcing (e.g. Chiang and Bitz, 2005; Dong and Sutton, 2002; Lee et al.,
60
2014) can the energy balance mechanism work.
61
62
63
64
65
References
66
67
Deser, C., Capotondi, A., Saravanan, R., and Phillips, A. S., 2006: Tropical Pacific and
Atlantic climate variability in CCSM3. J. Clim., 19(11), 2451-2481.
68
69
70
Frierson, D. M., Hwang, Y. T., Fučkar, N. S., Seager, R., Kang, S. M., Donohoe, A., and
Battisti, D. S., 2013: Contribution of ocean overturning circulation to tropical rainfall
peak in the Northern Hemisphere. Nature Geoscience, 6(11), 940-944.
71
72
Marshall, J., Donohoe, A., Ferreira, D., and McGee, D., 2014: The ocean’s role in setting the
mean position of the Inter-Tropical Convergence Zone. Clim. Dyn., 42(7-8), 1967-1979.
73
74
75
76
77
78
79
Fig. S1 Power spectra of Nino3.4 monthly SST variability in ICE (after removing the annual
cycle) in five 1,000-year windows: 1–2, 5–6, 10–11, 15–16 and 18–19 ka BP. For each
spectrum, the 95% cut-off level and the corresponding red noise curve are also plotted (in
dotted lines). The black bar at the bottom shows the 1.5–7-year band used for the calculation
of ENSO variance.
80
81
82
83
84
85
86
87
88
89
Fig. S2 Heat fluxes (W/m2) averaged over latitude bands on area weighted axis, green is for
mean state at 14 ka BP and blue is for anomalies (50-yr average) after minus before 14 ka BP.
Solid lines show heat fluxes at the surface and dashed lines show heat fluxes at the top of
atmosphere (TOA). All positive (negative) values suggest heating (cooling) of the
atmospheric column (e.g. it is gaining heat with downward fluxes at the TOA and upward
fluxes at the surface). (a)-(f): shortwave heat fluxes, longwave heat fluxes, latent heat fluxes,
sensible heat fluxes, net heat fluxes at the TOA and the surface, net heat fluxes absorbed by
the atmospheric column. The numbers in (f) are heat budget anomalies absorbed by the
atmospheric column summed over the NH and SH.
90
91
Fig. S3 The same as Fig. S2 but averaged over land.
92
93
94
Fig. S4 The same as Fig. S2 but averaged over ocean. Also note that the y-axis scale of heat
flux anomalies are changed.