An Exploration of Factors that Affect Earth's Climate in Time

Abstract

This Earth’s climate is strongly influenced by both external and internal forcings. Several examples are the Earth’s rotation rate, variations in the Earth’s orbit, and the land-sea geometry. My PhD work specifically studies the impact of these three factors on the Earth’s climate system. We explore the question “what determines the meridional energy transport (MHT)?” by performing a series of rotation-rate experiments with an aquaplanet GCM coupled to a slab ocean. The change of MHT with rotation rate (Ω) falls into two regimes: in a “slow-rotating” regime ( Ω < 1/2 modern rotation), MHT decreases with increasing Ω; in a “fast-rotating” regime (Ω ≥ 1/2 modern rotation), MHT is nearly invariant. The two-regime feature of MHT is primarily related to the reduction in tropical clouds and increase in tropical temperature that are associated with the narrowing and weakening of the Hadley Cell with increasing Ω. In the slow-rotating regime, the Hadley Cell contracts and weakens as Ω is increased; the resulting warming causes a local increase in outgoing longwave radiation (OLR) which consequently decreases MHT. In the fast-rotating regime, the Hadley Cell continues to contract as Ω is increased, resulting in a decrease in tropical and subtropical clouds which increases the local absorbed shortwave radiation (ASR) by an amount that almost exactly compensates the local increases in OLR. In the fast-rotating regime, the model heat transport is approximately diffusive, with an effective eddy diffusivity that is consistent with eddy mixing-length theory. The effective eddy diffusivity decreases with increasing Ω. However, this decrease is nearly offset by a strong increase in the meridional gradient of moist static energy and hence results in a near-constancy of MHT. Our results extend previous work on the MHT by highlighting that the spatial patterns of clouds and the factors that influence them are leading controls on MHT. In Chapter 2, we examined two factors that contribute to the early Eocene climate of tropical South America: a narrower Atlantic ocean and enhanced CO_2. For this study, We used two atmospheric general circulation models (ECHAM 4.6 and CAM5) coupled to a slab ocean. Experiments show that, to first order, narrowing the Atlantic decreases the precipitation of tropical South America, whereas increasing atmospheric CO_2 tends to increase the precipitation. The early Eocene climate in our model is a near-linear contribution of change in atmospheric CO_2 concentration and change in Atlantic geometry, with a dominant contribution from the latter, resulting in a drier-than-today tropical South America during the early Eocene. Using water budget analysis, we find that the precipitation reduction induced by narrowing the Atlantic is mainly due to the reduction of water vapor flux entering South America across the northeast and east boundaries which, in turn, is due to a reduction in the amount of water evaporated from the ocean as air travels from Africa to the South American continent. It is not due to changes in atmospheric circulation. In fact, there is no dramatic atmospheric circulation change around South America even though the Atlantic Ocean is shrunk to less than half its modern width. Our study provides a step towards a dynamical understanding of how the Eocene climate of South America differs from today’s. Summertime insolation was more intense in the Northern Hemisphere during the mid-Holocene, resulting in enhanced monsoonal precipitation. In Chapter 3, we examine the changes in the annual mean tropical precipitation, as well as changes in atmospheric circulation and upper ocean circulation in the mid-Holocene compared to the pre-industrial climate, as simulated by 12 coupled climate models from PMIP3. In addition to the pre-dominant zonally-asymmetric changes in tropical precipitation, there is a small northward shift in the location of intense zonal mean precipitation (mean ITCZ) in the mid-Holocene in the majority (9 out of 12) of the coupled climate models. In contrast, the shift is southward in simulations using an atmospheric model coupled to a slab ocean. The northward mean ITCZ shift in the coupled simulations is due to enhanced northward ocean heat transport across the equator (OHT(EQ)) which demands a compensating southward atmospheric energy transport across the equator, accomplished by shifting the Hadley cell and hence move the zonal mean ITCZ northward. The increased northward OHT(EQ) is primarily accomplished by changes in the gyre circulation in the upper-ocean in the tropical Pacific acting on the zonally asymmetric climatological temperature distribution. The gyre intensification results from the intensification of the monsoonal winds in the Northern Hemisphere and the weakening of the winds in the Southern Hemisphere, both of which are forced directly by the insolation changes.