Past climatic conditions on the Baja California peninsula
and choice of GCM
We consulted geological literature to determine which GCM was best
supported by paleoprecipitation proxies, which included alluvial fan,
pollen, lake core record, and midden data across southern California and
the BCP (Supporting Information). Unlike global climate, literature
indicates that the BCP was generally wetter than present at the LGM
(Lora, 2018; Lyle et al., 2012), culminating with an El Niño-like
wettest period during the Late Pleistocene-Holocene transition
~14.5-10 ka (Antinao & McDonald, 2013). This was driven
by strengthening East Pacific (westerly) precipitation that affected
mainly southern California and the northern BCP in the form of winter
rain and less seasonality (Antinao & McDonald, 2013; Antinao et al.,
2016). On the other hand, while the south would have been drier overall
after LGM, there is evidence from alluvial fan deposits and offshore
cores that despite less precipitation, the precipitation came as
stronger, and possibly more frequent, tropical cyclones reaching the
southern BCP (Antinao et al., 2016). The Pacific High weakened
~17-11 kya and allowed warm tropical waters to move
northward, partially strengthening the North American Monsoon (NAM) and
leading to increasingly wetter conditions in the south comparable to
what is observed today (Antinao et al., 2016). After that, the southerly
NAM was stable but weaker, contributing less precipitation at least in
part due to cooler Gulf of California sea surface temperatures 11–8 ka
and a limited northward progression of those storms due to presence of
the Pacific High.
Considering total annual precipitation (bio12), CCSM shows drying in the
southern peninsula and increased wetness in the north (Figure 1b), which
is in agreement with the paleoprecipitation data, whereas MIROC shows
substantially wetter conditions throughout the peninsula (Supporting
Information). Furthermore, the precipitation of the wettest month
(bio13) shows a drying in the CCSM models in the south and slightly
increased precipitation in the north, but the MIROC model shows wetter
conditions throughout (also evident in bio18). CCSM shows a slight
strengthening of seasonality (bio15), while MIROC shows patchy decrease
of moderate magnitude, the latter of which is more in line with the
paleoprecipitation proxy data. Despite this last observation, we chose
the CCSM GCM as the overall best model for our analyses because it
appears to be more consistent with paleoprecipitation data particularly
as it relates to the increased precipitation in the north and decreased
effect of NAM precipitation and tropical storms in the south.