 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 Hole 806B 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 No hydrological changes [WEP records] Surface salinity

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 CO 2. This can be supported by CO 2 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 (Surface – base of photic zone) 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.