Jonghun Kam

Probabilistic Seasonal Forecasting of African Drought by Dynamical Models

AbstractAs a natural phenomenon, drought can have devastating impacts on local populations through food insecurity and famine in the developing world, such as in Africa. In this study, the authors have established a seasonal hydrologic forecasting system for Africa. The system is based on the Climate Forecast System, version 2 (CFSv2), and the Variable Infiltration Capacity (VIC) land surface model. With a set of 26-yr (1982–2007) seasonal hydrologic hindcasts run at 0.25°, the probabilistic drought forecasts are validated using the 6-month Standard Precipitation Index (SPI6) and soil moisture percentile as indices. In terms of Brier skill score (BSS), the system is more skillful than climatology out to 3–5 months, except for the forecast of soil moisture drought over central Africa. The spatial distribution of BSS, which is similar to the pattern of persistency, shows more heterogeneity for soil moisture than the SPI6. Drought forecasts based on SPI6 are generally more skillful than for soil moisture, an...

Water Balance in the Amazon Basin from a Land Surface Model Ensemble

AbstractDespite recent advances in land surface modeling and remote sensing, estimates of the global water budget are still fairly uncertain. This study aims to evaluate the water budget of the Amazon basin based on several state-of-the-art land surface model (LSM) outputs. Water budget variables (terrestrial water storage TWS, evapotranspiration ET, surface runoff R, and base flow B) are evaluated at the basin scale using both remote sensing and in situ data. Meteorological forcings at a 3-hourly time step and 1° spatial resolution were used to run 14 LSMs. Precipitation datasets that have been rescaled to match monthly Global Precipitation Climatology Project (GPCP) and Global Precipitation Climatology Centre (GPCC) datasets and the daily Hydrologie du Bassin de l’Amazone (HYBAM) dataset were used to perform three experiments. The Hydrological Modeling and Analysis Platform (HyMAP) river routing scheme was forced with R and B and simulated discharges are compared against observations at 165 gauges. Simu...

The Influence of Atlantic Tropical Cyclones on Drought over the Eastern United States (1980–2007)

AbstractTo assess the influence of Atlantic tropical cyclones (TCs) on the eastern U.S. drought regime, the Variable Infiltration Capacity (VIC) land surface hydrologic model was run over the eastern United States forced by the North American Land Data Assimilation System phase 2 (NLDAS-2) analysis with and without TC-related precipitation for the period 1980–2007. A drought was defined in terms of soil moisture as a prolonged period below a percentile threshold. Different duration droughts were analyzed—short term (longer than 30 days) and long term (longer than 90 days)—as well as different drought severities corresponding to the 10th, 15th, and 20th percentiles of soil moisture depth. With TCs, droughts are shorter in duration and of a lesser spatial extent. Tropical cyclones variously impact soil moisture droughts via late drought initiation, weakened drought intensity, and early drought recovery. At regional scales, TCs decreased the average duration of moderately severe short-term and long-term drou...

Changes in drought risk over the contiguous United States (1901–2012): The influence of the Pacific and Atlantic Oceans

We assess uncertainties in the influence of sea surface temperatures on annual meteorological droughts over the contiguous U.S. within a Bayesian approach. Observational data for 1901–2012 indicate that a negative phase of the Pacific Decadal Oscillation (PDO) and the El Nino–Southern Oscillation (ENSO) elevated annual drought risk over the southern U.S., such that the 4 year return period event becomes a 3 year event, while a positive phase of the Atlantic Multidecadal Oscillation has a weak influence. In recent decades, the impacts of the negative phases of the PDO and ENSO on U.S. drought have weakened and shifted toward the southwestern U.S. These changes indicate an increasing of role of atmospheric variability on the U.S. drought overall with implications for long-term changes in drought and the potential for seasonal forecasting.

Did a skillful prediction of sea surface temperatures help or hinder forecasting of the 2012 Midwestern US drought

The latest drought to hit the Midwestern (MW) US region, in 2012, was driven by the least summer precipitation for the last three decades with

A multiscale analysis of drought and pluvial mechanisms for the Southeastern United States

20 billion in agriculture losses. For 2012, the summer forecast skill for Pacific and Atlantic sea surface temperature (SST) anomalies and low MW precipitation is remarkably good for some National Multi-Model Ensemble (NMME) models, but this is not generally repeated for other drought years, with some models predicting extreme wet anomalies, despite skill in predicting Pacific and Atlantic SST anomalies. In order to diagnose the origins of the limited skill of the NMME models, we use singular value decomposition (SVD) for global SSTs and continental US (CONUS) precipitation from observational data and NMME hindcasts (1982–2012). Observational data indicate that there is an insignificant coupling between global SSTs and MW precipitation during summer over the last 30 years. However, the NMME climate forecast models show strong coupling and therefore predicted the 2012 drought fortuitously for the wrong reason (a strong pan-Pacific El Nino–Southern Oscillation (ENSO)-like pattern). The observational data indicate that the strength of ENSO teleconnections with CONUS precipitation has weakened and the precipitation footprint has shifted over the past decades, suggesting that the transient nature of teleconnections may play a role in poor model skill.

Increased Drought and Pluvial Risk over California due to Changing Oceanic Conditions

The Southeast (SE) U.S. has experienced several severe droughts over the past 30 years, with the most recent drought during 2006–2008 causing agricultural impacts of

Multimodel Assessment of Anthropogenic Influence on Record Global and Regional Warmth During 2015

1 billion. However, the mechanisms that lead to droughts over the region and their persistence have been poorly understood due to the regions humid coastal environment and its complex climate. In this study, we carry out a multiscale analysis of drought mechanisms for the SE U.S. over 1979–2008 using the North American Regional Reanalysis (NARR) to identify conditions associated with drought and contrast with those associated with pluvials. These conditions include land surface drought propagation, land-atmosphere feedbacks, regional moisture sources, persistent atmospheric patterns, and larger-scale oceanic conditions. Typical conditions for SE U.S. droughts (pluvials) are identified as follows: (1) weaker (stronger) southerly meridional fluxes and weaker (stronger) westerly zonal fluxes, (2) strong moisture flux divergence (convergence) by transient eddies, and (3) strong (weak) coupling between the land surface and atmosphere. The NARR demonstrates that historic SE droughts are mainly derived from a combination of a strong North Atlantic subtropical high (NASH) and Icelandic Low (IL) during summer and winter, respectively, which peak 1 month earlier than the onset of the drought. The land surface plays a moderate role in drought occurrence over the SE via recycling of precipitation, and the oceans show an asymmetric influence on droughts and pluvials depending on the season. This study suggests that the NASH and IL can be used as a predictor for SE droughts at 1 month lead despite the overall that it represents an atmospheric forcing.

Record Annual Mean Warmth Over Europe, the Northeast Pacific, and the Northwest Atlantic During 2014: Assessment of Anthropogenic Influence

AbstractThis study evaluates wintertime drought and pluvial risk over California through a Bayesian analysis of the upper and lower quartile of PRISM-based precipitation from 1901 to 2015. Risk is evaluated for different time windows to estimate the impact of interannual and decadal-to-multidecadal Pacific and Atlantic variability [positive and negative phases of ENSO, Pacific decadal oscillation (PDO), and Atlantic multidecadal oscillation (AMO)]. The impact of increasing trends in global sea surface temperature (SST) on drought and pluvial risk is also examined with idealized experimental runs from three climate models [GFDL Atmospheric Model version 2.1 (AM2.1), CCM3, and GFS]. The results show that the influence of oceanic conditions on drought risk in California is significant but has changed with higher risk in the last half century, especially in Southern California. The influence of oceanic conditions on pluvial risk has also been significant, especially during the warm phase of the Pacific Ocean,...

Climate Model Assessment of Changes in Winter–Spring Streamflow Timing over North America

Introduction. HadCRUT4v4 observed surface temperature data (Morice et al. 2012; 5° × 5° lat.–lon. grid boxes) indicates that 2015 was a clear recordbreaking year for global annual mean temperatures (Figs. 2.1a,b,e). In this analysis, we consider only grid boxes with at least 100 years of historical data, which narrows the focus mainly to the Atlantic and Indian Oceans, the North Pacific Ocean, Europe, the United States, southern Asia, and Australia (Fig. 2.1d). Sixteen percent of this analyzed area experienced record annual warmth during 2015 (Fig. 2.1d). Observed global temperatures over the past decade had been warming at a rate less than the ensemble mean warming in the Coupled Model Intercomparison Project phase 5 all-forcing historical runs (CMIP5–ALL; Taylor et al. 2012). However, the record global temperature of 2015 (Fig. 2.1e), including the influence of a strong El Niño event (Fig. 2.1f), was warmer globally than the mean of the CMIP5-ALL model ensemble levels for 2015, relative to their respective 1881–1920 means. Major regions with unprecedented annual mean warmth in 2015 included the northeast Pacific and northwest Atlantic, while during September– November (SON) 2015, southern India/Sri Lanka stood out with record seasonal warmth (Fig. 2.1g; our region of focus in southern India and Sri Lanka does include some SST influence, as we used the combined SST/Tair dataset; see Supplemental Material). Only a small region south of Greenland (0.2% of the globe) experienced record annual mean cold surface temperatures (Fig. 2.1d). We constructed our regions of focus based on areas highlighted in Fig. 2.1d. These regions had some irregular shapes and were constructed to be mostly covered by new record annual or seasonal temperatures in 2015. In addition to global mean temperatures, we focused on two main regions and temporal domains—the Niño-4 region (annual means) and a region including southern India and Sri Lanka (SON means). To demonstrate the robustness for annual mean record warmth in 2015 over the Niño-4 region (Fig. 2.1g), we also showed extended reconstructed sea surface temperature (ERSST.v4; Huang et al. 2016) and Hadley Centre sea ice and sea surface temperature (HadISST1.1; Rayner et al. 2003) data reconstructions and found that these also show unprecedented annual mean warmth during 2015. This study investigates the causes of these record warm events using an eight-model set of all-forcing (anthropogenic + natural) historical climate model runs, associated long-term control (unforced) runs, and natural forcing runs (CMIP5–ALL, –CONT, and –NAT, respectively). These eight models (listed in Supplemental Material) were selected, as they were the ones with CMIP5-NAT runs extending to 2012. Our methods follow the studies of Knutson et al. (2013 and 2014); some of the descriptive text below is drawn from those reports.