About five to four million years ago, in the early Pliocene epoch, Earth had a warm, temperate climate. The gradual cooling that followed led to the establishment of modern temperature patterns, possibly in response to a decrease in atmospheric CO2 concentration, of the order of 100 parts per million, towards preindustrial values. Here we synthesize the available geochemical proxy records of sea surface temperature and show that, compared with that of today, the early Pliocene climate had substantially lower meridional and zonal temperature gradients but similar maximum ocean temperatures. Using an Earth system model, we show that none of the mechanisms currently proposed to explain Pliocene warmth can simultaneously reproduce all three crucial features. We suggest that a combination of several dynamical feedbacks underestimated in the models at present, such as those related to ocean mixing and cloud albedo, may have been responsible for these climate conditions.

(Fig. 1a). Similarly, the meridional temperature gradient from the equator to the mid-latitudes was significantly reduced 21 . In fact, in the early Pliocene the SST distribution was virtually flat between the equator and the subtropics, indicating a poleward expansion of the ocean warm pool (Fig. 1b) modelling the large-scale atmospheric circulation with a General Circulation Model, GCM; and (c) using the GCM data to drive a Statistical DownScaling Model, SDSM, which computes synthetic hurricane tracks and intensity (Methods Summary; Methods).The ocean temperature field is the starting point for our analysis, since the most reliable climatic information for the early Pliocene comes as SST data. Following ref.(21), we take the SST distribution in Fig How would these changes in hurricane activity influence the tropical climate? To answer this question, we first need to look at the upper ocean circulation in the tropics (Fig. 3a). This wind-driven circulation connects the regions of subduction off the coasts of California and Chile with the equatorial undercurrent (EUC) and eventually with the eastern equatorial Pacific. The circulation is associated with a shallow ocean meridional overturning, with penetration depths not greater than ~200m, typically referred to as the shallow Subtropical Cells, STC 7,28 .The effect of hurricanes on the ocean can be measured with the annual average Power Dissipation Index (PDI), which approximates the amount of energy per unit area that tropical cyclones generate each year 1 . A significant fraction of this energy is available to mix the upper ocean. In general, the PDI distribution coincides with the areas of strong hurricane activity, both in terms of frequency of occurrence and strength. For the modern climate, the simulated mean PDI exhibits a strong maximum in the western tropical Pacific north of 10 o N (Fig. 3b). Water parcels can travel towards the equator after subduction through two "windows" of the modern PDI distribution in the central Pacific without any interference from tropical storms (also see Fig. S1). In the West Pacific, the parcels travel at the fringe of the PDI maximum, where the mixing can affect the Pacific Subtropical gyre, but not the STC.For the early Pliocene, two bands of high PDI now cover the entire zonal extent of the Pacific (Fig. 3c), allowing interaction between the hurricanes and ocean circulation. Fig. S3). For comparison, we display SST changes in the same model (Fig. 4c) when only CO 2 concentrations are raised. In this case, the temperature increase is moderate (below 1 o C) and almost uniform, indicating a near radiative-equilibrium response.The warming of the equatorial cold tongue in response to increased extra-tropical mixing remains robust in a broad parameter range. Its magnitude depends on the specified diffusivity and the width of the equatorial gap between the two bands of enhanced mixing. The smaller the gap, the greater the warming that is achieved. The gap being fully closed wipes out the zonal SST gradient almost completely ...

[1] During the early Pliocene (roughly 4 Myr ago), the ocean warm water pool extended over most of the tropics. Subsequently, the warm pool gradually contracted toward the equator, while midlatitudes and subpolar regions cooled, establishing a meridional sea surface temperature (SST) gradient comparable to the modern about 2 Myr ago (as estimated on the eastern side of the Pacific). The zonal SST gradient along the equator, virtually nonexistent in the early Pliocene, reached modern values between 1 and 2 Myr ago. Here, we use an atmospheric general circulation model to investigate the relative roles of the changes in the meridional and zonal temperature gradients for the onset of glacial cycles and for Pliocene-Pleistocene climate evolution in general. We show that the increase in the meridional SST gradient reduces air temperature and increases snowfall over most of North America, both factors favorable to ice sheet inception. The impacts of changes in the zonal gradient, while also important over North America, are somewhat weaker than those caused by meridional temperature variations. The establishment of the modern meridional and zonal SST distributions leads to roughly 3.2°C and 0.6°C decreases in global mean temperature, respectively. Changes in the two gradients also have large regional consequences, including aridification of Africa (both gradients) and strengthening of the Indian monsoon (zonal gradient). Ultimately, this study suggests that the growth of Northern Hemisphere ice sheets is a result of the global cooling of Earth's climate since 4 Myr rather than its initial cause. Thus, reproducing the correct changes in the SST distribution is critical for a model to simulate the transition from the warm early Pliocene to a colder Pleistocene climate.Citation: Brierley, C. M., and A. V. Fedorov (2010), Relative importance of meridional and zonal sea surface temperature gradients for the onset of the ice ages and Pliocene-Pleistocene climate evolution, Paleoceanography, 25, PA2214,

Abstract. The favourability of the mid-Pliocene, Last Glacial Maximum (LGM) and mid-Holocene for tropical cyclone formation is investigated in five climate models. This is measured by a genesis potential index, derived from large-scale atmospheric properties known to be related to storm formation. The mid-Pliocene and Last Glacial Maximum (LGM) were periods where carbon dioxide levels were higher and lower than preindustrial levels respectively, while the mid-Holocene differed primarily in its orbital configuration. The cumulative global genesis potential is found to be fairly invariant across the palaeoclimates in the multi-model mean. Despite this all ensemble members agree on coherent responses in the spatial patterns of genesis potential change.During the mid-Pliocene and LGM, changes in carbon dioxide led to sea surface temperature changes throughout the tropics, yet the potential intensity (a measure associated with maximum tropical cyclone strength) is calculated to be relatively insensitive to these changes. Changes in tropical cyclone genesis potential during the mid-Holocene are found to be asymmetric about the Equator: being reduced in the Northern Hemisphere but enhanced in the Southern Hemisphere. This is clearly driven by the altered seasonal insolation. Nonetheless, the enhanced seasonality drove localised changes in genesis potential, by altering the strength of monsoons and shifting the intertropical convergence zone. Trends in future tropical cyclone genesis potential are consistent neither between the five models studied nor with the palaeoclimate results. It is not clear why this should be the case.