Imprint of the Atlantic multi-decadal oscillation and Pacific decadal oscillation on southwestern US climate: past, present, and future

Abstract

The surface air temperature increase in the southwestern United States was much larger during the last few decades than the increase in the global mean. While the global temperature increased by about 0.5°C from 1975 to 2000, the southwestern US temperature increased by about 2°C. If such an enhanced warming persisted for the next few decades, the southwestern US would suffer devastating consequences. To identify major drivers of southwestern climate change we perform a multiple-linear regression of the past 100 years of the southwestern US temperature and precipitation. We find that in the early twentieth century the warming was dominated by a positive phase of the Atlantic multi-decadal oscillation (AMO) with minor contributions from increasing solar irradiance and concentration of greenhouse gases. The late twentieth century warming was about equally influenced by increasing concentration of atmospheric greenhouse gases (GHGs) and a positive phase of the AMO. The current southwestern US drought is associated with a near maximum AMO index occurring nearly simultaneously with a minimum in the Pacific decadal oscillation (PDO) index. A similar situation occurred in mid-1950s when precipitation reached its minimum within the instrumental records. If future atmospheric concentrations of GHGs increase according to the IPCC scenarios (Solomon et al. in Climate change 2007: working group I. The Physical Science Basis, Cambridge, 996 pp, 2007), climate models project a fast rate of southwestern warming accompanied by devastating droughts (Seager et al. in Science 316:1181-1184, 2007 Williams et al. in Nat Clim Chang, 2012). However, the current climate models have not been able to predict the behavior of the AMO and PDO indices. The regression model does support the climate models (CMIP3 and CMIP5 AOGCMs) projections of a much warmer and drier southwestern US only if the AMO changes its 1,000 years cyclic behavior and instead continues to rise close to its 1975-2000 rate. If the AMO continues its quasi-cyclic behavior the US SW temperature should remain stable and the precipitation should significantly increase during the next few decades.

Observed and Projected Precipitation Changes over the Nine US Climate Regions

“…In particular, they have predicted that the US southwest will transition to a more arid regime [14,15]. In contrast, regression models have suggested a return of more precipitation in the US southwest [16], which is in agreement with recent data. In this report we analyze the temperature and precipitation data in nine separate US climate regions (Figure 1) and use the CMIP5 climate models outputs, together with statistical regression models, to identify causes of the different climate trajectories over the eastern and western US.…”

“…The future projections of regression models for the southwestern US temperature and precipitation were reported in an earlier publication [16]. Perhaps the lagged PDO/AMO correlation combined with a possible quasi-periodicity of AMO may be worth pursuing.…”

Recent Progress in Understanding and Predicting Atlantic Decadal Climate Variability

“…The twentieth century AMV has been linked to observed variability in (see also references within these recent papers): seasonal climate (SAT and precipitation) over North America and Europe [21][22][23]; rainfall over India, northeast Brazil, and the Sahel [24,25]; Atlantic hurricane activity [24][25][26] . Proxy records are also crucial for characterizing AMV and its impacts over multiple cycles prior to the instrumental record.…”

Climate Change and Physical Disturbance Manipulations Result in Distinct Biological Soil Crust Communities

“…In addition, climate and land use change can act in concert to affect these regions. For example, drylands across the southwestern United States are currently experiencing shrub expansion at the expense of local grasses, a situation attributed to a combination of increased herbivory and rising temperatures (5,6). Precipitation patterns in the region, particularly the summer monsoons that deliver up to 35% of annual precipitation, are also changing (7,8).…”