Lower terrestrial ice cover in the Northern Hemisphere and higher SST in the North Atlantic-North Pacific provide the initial forcing mechanism behind the predicted surface temperature increases over the USA at 3 Ma B.P. However, reductions in albedo, particularly during DJF generated by reduced snow cover and altered Pliocene vegetation distributions, would have caused an increase in the absorption of solar radiation promoting further warming. The snow depth decrease occurs over the USA despite a general increase in wintertime precipitation. An increased proportion of precipitation during the warmer middle Pliocene simulation falls as rain and not as snow. Comparison of absolute temperature patterns for present-day and the middle Pliocene over the annual cycle, DJF and JJA, reveal interesting differences (Figure 3). During the middle Pliocene, the freezing line over the annual period and DJF is positioned further to the north of the present-day position (~10ºN). Furthermore, the pattern of decreasing temperatures with increasing latitude during the middle Pliocene is less spatially variable than for present-day. This is a function of the prescribed reduction in elevation (by 50%) of the western cordillera of North America within the PRISM2 boundary conditions.

A clear signal of changing precipitation rates over the USA during the middle Pliocene is predicted by the HadAM3 model (Figure 4). Two zones of change are evident. Over the annual cycle and during winter (DJF), northern parts of the USA experience more rainfall (+ 4.00 to 8.00 mm/day). This increase in precipitation occurs as a broad belt across North America between 40º to 60ºN. This increase of precipitation is consistent with an enhancement of the moisture bearing westerly winds over the USA during the middle Pliocene. In the southern and central areas of the USA, particularly during JJA, total precipitation rates decrease by an average 2.00 mm/day.

Changes in the pattern and amount of evaporation and humidity over the USA are mainly a reflection of the altered precipitation distribution and warmer surface temperatures. Areas experiencing enhanced precipitation via moisture bearing westerly winds in the northern USA have elevated evaporation rates by as much as 0.50 to 1.00 mm/day. This reflects the greater moisture availability for evaporation and the warmer surface temperatures. Conversely, central and southern parts of the USA have lower evaporation rates relative to present-day (by a maximum of 4.00 mm/day during JJA) due to a decreased availability of moisture. Humidity values also show a corresponding decrease by as much as 30 percent in this area during JJA.

Precipitation minus evaporation (P-E) changes over the USA, for the Pliocene control simulation, indicates small anomalies compared to present-day model simulations during JJA. However, significant anomalies are observed for the winter (DJF) season (Figure 5). The model suggests that for the northern USA, during the middle Pliocene, a substantially greater (0.00 to 4.00 mm/day) precipitation flux existed compared to present-day. In contrast, the southeast of the USA during DJF has a greater evaporative flux (0.50 to 8.00 mm/day) compared to model simulations for the present, which extends further south into the Gulf of Mexico. These P-E anomalies are a product of the predicted Pliocene precipitation changes over the USA during DJF (see Figure 5).

The primary forcing mechanism, producing the altered precipitation pattern over the USA during the middle Pliocene, is a strong increase in westerly wind strength (Figure 6). These enhanced westerly winds aid the transport of moisture that falls mostly as rain because of the higher annual surface temperatures. The westerly wind increase is brought about by a complex set of climatic feedbacks. Higher SST in the North Atlantic-Pacific, combined with reduced ice cover in the high latitudes of the Northern Hemisphere, deepen the Aleutian and Icelandic low-pressure systems. This generates an enhanced pressure gradient between the high latitude low pressure cells (Aleutian and Icelandic low-pressure systems) and the subtropical high pressure cells (i.e., Azores high-pressure system) intensifying westerly wind strength in the Northern Hemisphere, particularly during winter. This aids in both the transport of heat and moisture and explains why the highest magnitude changes in surface temperature and precipitation during the middle Pliocene over the USA are predicted for the winter season.

The feedbacks generating the precipitation pattern in central and southern regions of the USA during the middle Pliocene are subtler. A possible explanation for the drying in these areas is the reduced SST at lower latitudes prescribed during certain months in the PRISM2 digital data set. This has the effect of decreasing southerly wind strength in the middle Pliocene simulation over the Gulf of Mexico while reducing the flow of moist maritime air into the continental interior of the USA, particularly during JJA.

The reduced height of the western cordillera of North America by 50% in the middle Pliocene simulation should have the effect of reducing the rain shadow effect to interior parts of the USA. Intuitively, this should also increase precipitation in this area. Discrimination of the effects of such a change from other factors, such as increased SST that also increase precipitation in the region, will be addressed in the following section.