PRISM3 boundary conditions as absolutes rather than as anomalies from present-day

[Alan Haywood]

Question emailed by Nanne Weber regarding why PRISM3 boundary
conditions are provided as absolutes rather than anomalies from
present-day.

Weber: I did have one question from my earlier browsing of the
webpage: you give all BCs as absolute values for the mid Pliocene,
rather than as differences between mid Plio and now. Is this on
purpose? in PMIP the BCs were used as differences, so I took Peltiers
LGM minus now orography and added that to my models' present-day
orography. It depends on your emphasis obviously (do you want to look
at simulated differences or simulated past climate), but was just
wondering about your rationale.

Haywood: Your question is a very good one. I understand the rationale
for working with anomalies for the LGM but obviously the further back
in time you go the more problematic this becomes. The Eocene would be
impossible because of the altered land/sea mask. In that sense the
Pliocene is in between, the continents had reached very nearly their
modern position but there are important regional differences (i.e.
west Antarctica, Hudson Bay etc) that need to be taken into account
which is why we peruse the method of prescribing the absolutes.