Showing papers by "Kevin J. Anchukaitis published in 2010"

Asian monsoon failure and megadrought during the last millennium.

Abstract: The Asian monsoon system affects more than half of humanity worldwide, yet the dynamical processes that govern its complex spatiotemporal variability are not sufficiently understood to model and predict its behavior, due in part to inadequate long-term climate observations. Here we present the Monsoon Asia Drought Atlas (MADA), a seasonally resolved gridded spatial reconstruction of Asian monsoon drought and pluvials over the past millennium, derived from a network of tree-ring chronologies. MADA provides the spatiotemporal details of known historic monsoon failures and reveals the occurrence, severity, and fingerprint of previously unknown monsoon megadroughts and their close linkages to large-scale patterns of tropical Indo-Pacific sea surface temperatures. MADA thus provides a long-term context for recent monsoon variability that is critically needed for climate modeling, prediction, and attribution.

Climate as a contributing factor in the demise of Angkor, Cambodia

Abstract: The “hydraulic city” of Angkor, the capitol of the Khmer Empire in Cambodia, experienced decades-long drought interspersed with intense monsoons in the fourteenth and fifteenth centuries that, in combination with other factors, contributed to its eventual demise. The climatic evidence comes from a seven-and-a-half century robust hydroclimate reconstruction from tropical southern Vietnamese tree rings. The Angkor droughts were of a duration and severity that would have impacted the sprawling city’s water supply and agricultural productivity, while high-magnitude monsoon years damaged its water control infrastructure. Hydroclimate variability for this region is strongly and inversely correlated with tropical Pacific sea surface temperature, indicating that a warm Pacific and El Nino events induce drought at interannual and interdecadal time scales, and that low-frequency variations of tropical Pacific climate can exert significant influence over Southeast Asian climate and society.

Influence of volcanic eruptions on the climate of the Asian monsoon region

Abstract: [1] Several state-of-the-art general circulation models (GCMs) predict that large volcanic eruptions should result in anomalous dry conditions throughout much of monsoon Asia. Here, we use long and well-validated proxy reconstructions of Asian droughts and pluvials to detect the influence of volcanic radiative forcing on the hydroclimate of the region since the late Medieval period. Superposed epoch analysis reveals significantly wetter conditions over mainland southeast Asia in the year of an eruption, with drier conditions in central Asia. Our proxy and model comparison suggests that GCMs may not yet capture all of the important ocean-atmosphere dynamics responsible for the influence of explosive volcanism on the climate of Asia.

Tropical cloud forest climate variability and the demise of the Monteverde golden toad.

Abstract: Widespread amphibian extinctions in the mountains of the American tropics have been blamed on the interaction of anthropogenic climate change and a lethal pathogen. However, limited meteorological records make it difficult to conclude whether current climate conditions at these sites are actually exceptional in the context of natural variability. We use stable oxygen isotope measurements from trees without annual rings to reconstruct a century of hydroclimatology in the Monteverde Cloud Forest of Costa Rica. High-resolution measurements reveal coherent isotope cycles that provide annual chronological control and paleoclimate information. Climate variability is dominated by interannual variance in dry season moisture associated with El Nino Southern Oscillation events. There is no evidence of a trend associated with global warming. Rather, the extinction of the Monteverde golden toad (Bufo periglenes) appears to have coincided with an exceptionally dry interval caused by the 1986–1987 El Nino event.

A Homogeneous Record (1896–2006) of Daily Weather and Climate at Mohonk Lake, New York*

Abstract: Reliable, long-term records of daily weather and climate are relatively rare but are crucial for understanding long-term trends and variability in extreme events and other climate metrics that are not resolvable at the monthly time scale. Here, the distinct features of a continuous, long-term (1896‐2006) daily weather record from Mohonk Lake, New York, are highlighted. The site is optimal for daily climate analyses, since it has experienced negligible land-use change, no station moves, and has maintained methodological and instrumental consistency over the entire period of record. Unlike many sites, the site has always used maximum/minimum thermometers rather than shifting to the automated Maximum/Minimum Temperature Sensor. Notable results from the analysis of this record include 1) a warming trend driven largely by trends in maximum temperatures, especially during summer, 2) increasing diurnal temperature range during summer, and 3) a reduction in the number of freeze-days per year with little change in the length of the freeze-free season. These findings deviate from some regional level trends, suggesting there may be value in revisiting selected, consistently monitored, and maintained stations similar to Mohonk for focused analyses of regional climate change.

Forced and unforced variability of twentieth century North American droughts and pluvials

Abstract: Research on the forcing of drought and pluvial events over North America is dominated by general circulation model experiments that often have operational limitations (e.g., computational expense, ability to simulate relevant processes, etc). We use a statistically based modeling approach to investigate sea surface temperature (SST) forcing of the twentieth century pluvial (1905–1917) and drought (1932–1939, 1948–1957, 1998–2002) events. A principal component (PC) analysis of Palmer Drought Severity Index (PDSI) from the North American Drought Atlas separates the drought variability into five leading modes accounting for 62% of the underlying variance. Over the full period spanning these events (1900–2005), the first three PCs significantly correlate with SSTs in the equatorial Pacific (PC 1), North Pacific (PC 2), and North Atlantic (PC 3), with spatial patterns (as defined by the empirical orthogonal functions) consistent with our understanding of North American drought responses to SST forcing. We use a large ensemble statistical modeling approach to determine how successfully we can reproduce these drought/pluvial events using these three modes of variability. Using Pacific forcing only (PCs 1–2), we are able to reproduce the 1948–1957 drought and 1905–1917 pluvial above a 95% random noise threshold in over 90% of the ensemble members; the addition of Atlantic forcing (PCs 1–2–3) provides only marginal improvement. For the 1998–2002 drought, Pacific forcing reproduces the drought above noise in over 65% of the ensemble members, with the addition of Atlantic forcing increasing the number passing to over 80%. The severity of the drought, however, is underestimated in the ensemble median, suggesting this drought intensity can only be achieved through internal variability or other processes. Pacific only forcing does a poor job of reproducing the 1932–1939 drought pattern in the ensemble median, and less than one third of ensemble members exceed the noise threshold (28%). Inclusion of Atlantic forcing improves the ensemble median drought pattern and nearly doubles the number of ensemble members passing the noise threshold (52%). Even with the inclusion of Atlantic forcing, the intensity of the simulated 1932–1939 drought is muted, and the drought itself extends too far into the southwest and southern Great Plains. To an even greater extent than the 1998–2002 drought, these results suggest much of the variance in the 1932–1939 drought is dependent on processes other than SST forcing. This study highlights the importance of internal noise and non SST processes for hydroclimatic variability over North America, complementing existing research using general circulation models.