Investigating the causes of increased 20th-century fall precipitation over the southeastern United States

Abstract

Much of the eastern United States (US) experienced increased precipitation over the 20^th century. Characterizing these trends and their causes is critical for assessing future hydroclimate risks. Here, US precipitation trends are analyzed during 1895-2016, revealing that fall precipitation in the southeastern region north of the Gulf of Mexico (SE-Gulf) increased by nearly 40%, primarily increasing after the mid-1900s. As fall is the climatological dry season in the SE-Gulf and precipitation in other seasons changed insignificantly, the seasonal precipitation cycle diminished substantially. The increase in SE-Gulf fall precipitation was caused by increased southerly moisture transport from the Gulf of Mexico, which was almost entirely driven by stronger winds associated with enhanced anticyclonic circulation west of the North Atlantic Subtropical High (NASH) and not by increases in specific humidity. Atmospheric models forced by observed SSTs and fully-coupled models forced by historical anthropogenic forcing do not robustly simulate 20^th-century fall wetting in the SE-Gulf. SST-forced atmospheric models do simulate an intensified anticyclonic low-level circulation around the NASH, but the modeled intensification occurred farther west than observed. CMIP5 analyses suggest an increased likelihood of positive SE-Gulf fall precipitation trends given historical and future GHG forcing. Nevertheless, individual model simulations (both SST-forced and fully-coupled) only very rarely produce the observed magnitude of the SE-Gulf fall precipitation trend. Further research into model representation of the western ridge of the fall NASH is needed, which will help us better predict whether 20^th-century increases in SE-Gulf fall precipitation will persist into the future.

Keywords: drought; hydroclimate; moisture transport; pluvial; subtropical High.

Figure 1.

NOAA Climgrid total precipitation trends (1895–2016) over the contiguous US for a) annual, b) winter, c) spring, d) summer, and e) fall. Boxed area represents selected hydroclimate region across the southeastern US (SE-Gulf = Southeast US-Gulf of Mexico). Cross-hatching indicates insignificant (p>0.05) trends using a standard two-sided t-test.

Figure 2.

SE-Gulf mean fall precipitation from GHCN, Climgrid, CRU, GPCC, and PRISM. Coverage is 1895–2016 for GHCN, PRISM, GPCC, and Climgrid; and 1901–2016 for CRU. Dotted lines represent linear trends; horizontal dashed line represents mean Climgrid fall precipitation for 1895–2016.

Figure 3.

Spearman’s rank correlation (1948–2016) between SE-Gulf fall precipitation and a) 200 hPa geopotential height, b) SLP, and c) vertically-integrated (SLP to 700hPa) moisture flux (colors=magnitude, arrows=direction and magnitude) from the NCEP-NCAR Reanalysis. Dots in (a and b) indicate significant (p<0.05) correlation coefficients. In (c), all correlation vectors are shown as black arrows to aid interpretability of the circulation patterns, while the correlation magnitude is also shown and calculated as the square root of the sum of squared correlations for the meridional and zonal directions. Black (a-b) and red (c) boxes bound the SE-Gulf region. The maximum length arrow vector in a horizontal or vertical direction represents a correlation magnitude of 0.85.

Figure 4.

Drivers of the increase in SE-Gulf fall precipitation from 1948–2016. Linear estimates of precipitation anomalies based on a) Gulf of Mexico meridional surface-700hPa moisture flux (blue) and b) wind-driven meridional moisture flux (red) with observed NOAA Climgrid fall precipitation anomalies (black). c) Percent contributions to the observed trend from unaccounted factors (white), Gulf of Mexico meridional moisture flux (blue), and wind-driven moisture flux (pink), specific humidity-driven moisture flux (green), and the residual interaction between specific humidity and wind (grey). Whiskers bound 95% confidence intervals of each trend contribution derived from the predicted fall precipitation regression coefficients.

Figure 5.

NCEP-NCAR (1948–2016) reanalysis fall trends in standardized a) vertically-integrated (surface to 700 hPa) moisture flux, b) wind velocity, and c) sea-level pressure (SLP). Colors indicate trend magnitude, arrows indicate direction and magnitude of trend (a,b). Black box bounds the SE-Gulf region.

Figure 6.

Ensemble mean relative change (1901–2015) in fall precipitation for a) CCM3 (16 members), b) CAM5 (16 members), and c) relative change for individual member runs with CCM3 and CAM5 simulations, including 1901–2015 observed GHCN, Climgrid, CRU, GPCC, and PRISM relative change for reference. Map colors in (a,b) indicate relative percent change per decade calculated via linear regression and dots indicate grid cells where greater than 75% of model simulations agree in the sign of change. Vertical colored lines in (c) indicate 95% confidence intervals for observed linear trends from 12-year block bootstrapping, and horizontal colored lines indicate ensemble means of simulated change.

Figure 7.

CMIP5 fall season precipitation trends (relative linear change per decade). Maps show trends in ensemble means for (a) historical (1901–2005) and (b) RCP8.5 (2006–2099) radiative forcing scenarios. These ensemble means are based on the 43 models for which monthly precipitation data were available for both scenarios. Swarm plots in (c) show (horizontal lines; colored by simulation type) trends in ensemble means and (dots; colored by simulation type) each individual model simulation for the SE-Gulf region for multiple CMIP5 experiments: historical, historical aerosol-only (11 models), historical GHG-only (20 models), historical natural-only (20 models), and RCP8.5. Modeled trends are compared to 1901–2005 observed trends calculated from multiple datasets: GHCN, Climgrid, CRU, GPCC, and PRISM relative change for reference (vertical bars around trend values bound the 95% confidence intervals from 12-year block bootstrapping; colored by dataset). Trends are quantified by taking the total percent change as calculated by a linear regression. In maps, dots indicate grid cells where greater than 75% of model simulations agree on the sign of change.

Figure 8.

Maximum simulated trends (relative change per decade) in fall SE-Gulf precipitation for (a) SST-forced and (b) CMIP5 Historical simulations. Dots indicate maximum trend values among all (a) 115-year and (b) 105-year periods in each model simulation and horizontal bars span the full range of all simulated trends. Dots and bars at the bottom represent trends and 95% confidence intervals from 12-year block bootstrapping for the four observational datasets during (a) 1901–2015 and (b) 1901–2005. Trends are quantified by taking the total percent change as calculated by a linear regression. In (a), blue dots indicate trends after removal of each model’s multi-simulation mean to isolate trends due to internal atmospheric variability.

Figure 9.

1948–2005 trends in fall sea-level pressure (colors) and 850 hPa winds (arrows) for a) NCEP-NCAR Reanalysis and ensemble mean b) CCM3, c) CAM5, and d) CMIP5 historical simulations. Contours show climatological mean sea-level pressure. Black box bounds the SE-Gulf region.