The influence of orbital parameters on the North American Monsoon system during the Last Interglacial Period

Abstract

The response of summer precipitation in the western United States to climate variability remains a subject of uncertainty. For example, palaeoclimate records indicate the North American Monsoon (NAM) was stronger and spatially more extensive during the Holocene, whereas recent modelling suggests a weakened NAM response to increasing temperatures. These illustrate diverging pictures of the NAM response to warming. Here, we examine summer precipitation in the southwestern US related to Last Interglacial insolation forcing. Using a high-resolution climate model, we find that Eemian insolation forcing results in overall wetter conditions throughout most of the southwestern US, but significantly drier than present conditions over Arizona. The overall wetter conditions are associated with a northward shift of the anticyclonic circulation aloft and increased moisture in the lower and mid-troposphere during the Eemian. Increased advection of Gulf of Mexico moisture is responsible for increasing precipitation in New Mexico and the northern edges of the NAM region. Drier conditions over Arizona are likely related to reduced local convection associated with reduced vertical moisture transport. These results highlight the spatial complexity of the NAM response to increasing radiative forcing and allow a better understanding of monsoon dynamics and variability in response to a warming climate.

Wetter Summers Mitigated Temperature Stress on Rocky Mountain Forests During the Last Interglacial Warm Period

“…In consideration of the age uncertainty in the samples, we hereafter refer to the samples as interglacial but refer to results from the climate simulations as Eemian. While the climate of the interglacial was not homogeneously warm, we note that the increased summer temperatures and precipitation that are the key climate features of the Eemian simulations were also present in 120 ka simulations (Insel & Berkelhammer, 2021;Scussolini et al, 2019). The wood samples were preserved with sufficient integrity to identify ring boundaries and key anatomical features (Figures S1-S5).…”

“…The coarse climate model data provides constraints on the climate conditions (humidity, air temperature, wind, and radiation) whereas the canopy model translates those conditions to the leaf-level where isotopic fractionation occurs. The climate data for the modern and Eemian (125 ka) periods were drawn from downscaled 50 km 10-year model simulations run with the RegCM model (Figure S9) (Insel & Berkelhammer, 2021).…”

“…The ecohydrological significance of the different clusters was derived using a mechanistic modeling approach that closely follows that in Berkelhammer et al (2020) and is further illustrated in Figure S8. Briefly, climate data from a downscaled (50 km) GCM (Insel & Berkelhammer, 2021) was used to force a canopy model which, fed a biogeochemical model of fractionation associated with photosynthate formation and cellulose metabolism. The coarse climate model data provides constraints on the climate conditions (humidity, air temperature, wind, and radiation) whereas the canopy model translates those conditions to the leaf-level where isotopic fractionation occurs.…”

“…Second, utilization of summer rain in modern trees tends to increase during periods of high snowpack because shallow roots are able to stay active later in the season (Martin et al, 2018). Model simulations suggest the interglacial was associated with winter precipitation rates that were comparable to modern (Insel & Berkelhammer, 2021) and snowpack melted off at a similar time in the Eemian relative to modern (Figure S10). Based on the results from the climate simulations of the Eemian, there is limited evidence that snowpack persisted later allowing shallow roots to stay active longer.…”

“…The last interglacial period ( E 116-130 ka) was the most recent period in Earth's history that was persistently warmer than today and thus paleoecological records from this time presents a potential window into the ecological response to rising temperatures (Lunt et al, 2013;Otto-Bliesner et al, 2013;Pedersen et al, 2017). Recent downscaled climate model simulations for North America indicate that during the warmest periods of the last interglacial (i.e., the Eemian period at 125 ka), growing season (JJAS) temperatures were ∼1- 3 E C warmer than today in the southern Rocky Mountains (Figure 1) (Insel & Berkelhammer, 2021). The warmer summer temperatures and increased incoming shortwave radiation manifested in growing season canopy temperatures that were 2- 4 E C warmer during June and July and an increase in canopy VPD of up to 0.5 kPa ( E 30% increase) relative to modern (Figure 2).…”