In the mid-Pleistocene, the period of glacial cycles changed from 41kyr to 100kyr.

The zonal SST gradient increased during MPT due to the cooling in the EEP and relatively stable WEP.

Correlation of interannual SST anomalies between the site of Hole806B and other regions of the tropics.

1. Hole 806B (0°19.1’N, 159°21.7’E)2. ODP Hole 846 (3°5’S, 90°49’W), 2. MD97 (2°02’N, 141°46’E)3, ODP Hole 677 (1°12’N, 83°44’W).

Ice expansion + Regional freshening 72.088.0 No hydrological changes

[WEP records]S

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Thibault, Yair, Franck and Luc

MPT has been attributed to global cooling for which evidence

is restricted to cool upwelling regions.

SST in the WEP is stable. Little long-term change in the

tropical net radiation budget.

The increased temperature contrast and stronger Walker

circulation (transition to La Nina-like condition) across the

equatorial Pacific might be important in climate transition.

Martin and David

The thermocline in WEP is deep, so that SSTs are less likely to be

affected by thermocline depth changes. Thus changes in

thermocline depth is unlikely to explain the observed warm-pool

SST variability.

The lead of SST over continental ice volume rules out the

hypothesis that tropical SST variability is controlled by the direct

radiative influence of Northern Hemisphere continental ice sheets.

They suggest that MPT is the result of changes in greenhouse

forcing as mediated by the radiative effect caused by variability in

atmospheric CO2. This can be supported by CO2 being in phase

with Antarctic atmospheric temperature, tropical SST and bottom

water temperature.

• The early Pliocene (~4.5 to 3.0 Ma)- Warmer climate than today (~3K higher global SST)

- Similar boundary conditions to today

- Deep EEP thermocline

- Smaller west-to-east SST difference across the equatorial Pacific

• The modern strong SST gradient across the equatorial Pacific is not a stable and permanent feature.

• Sustained El Nino-like conditions could play an important role in determining global warmth.

• Trade winds westward currents

WEP: thick and warm mixed layer, deep thermocline

EEP: thin and warm mixed layer, shallow thermocline

• El Nino events (reduction in the zonal SST gradient and Walker circulation) redistribute heat stored in the tropical Pacific to extratropics

5.3~1.7 Ma

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deepshallow

• This tells that the Pliocene warm period was not

characterized by the typical west-to-east asymmetric

conditions of the modern equatorial Pacific. Rather it was

more like permanent El Nino condition with more

symmetric conditions across the tropical Pacific.

• Theoretically, reduced subtropical SST or surface salinity

gradients could have resulted in a warmer and deeper

thermocline. Thus the El Nino-like mean state could be

related to changes in the mean state of extratropics. Or

the mean state could have been influenced by processes

within the tropics.

• G. tuimida has a depth ecology at ~100m, so it may not have been sensitive to additional shoaling above 100m.

• Decrease in salinity of subsurface water could masked the effect of an increase in delta_T after 4 Ma.

• Thermocline depth and SST are not linearly related.