Flash drought is characterized by a period of rapid drought intensification with impacts on agriculture, water resources, ecosystems, and the human environment. Addressing these challenges requires a ...fundamental understanding of flash drought occurrence. This study identifies global hotspots for flash drought from 1980-2015 via anomalies in evaporative stress and the standardized evaporative stress ratio. Flash drought hotspots exist over Brazil, the Sahel, the Great Rift Valley, and India, with notable local hotspots over the central United States, southwestern Russia, and northeastern China. Six of the fifteen study regions experienced a statistically significant increase in flash drought during 1980-2015. In contrast, three study regions witnessed a significant decline in flash drought frequency. Finally, the results illustrate that multiple pathways of research are needed to further our understanding of the regional drivers of flash drought and the complex interactions between flash drought and socioeconomic impacts.
Recent advances in tropical Pacific climate variability have focused on understanding the development of El Niño–Southern Oscillation (ENSO) events, specifically the types or “flavors” of ENSO (i.e., ...central versus eastern Pacific events). While precursors to ENSO events exist, distinguishing the particular flavor of the expected ENSO event remains unresolved. This study offers a new look at ENSO predictability using South Pacific atmospheric variability during austral winter as an indicator. The positive phase of the leading mode of South Pacific sea level pressure variability, which we term the South Pacific Oscillation (SPO), exhibits a meridional dipole with with a(n) (anti)cyclonic anomaly dominating the subtropics (extratropics/high latitudes). Once energized, the cyclonic anomalies in the subtropical node of the SPO weaken the southeasterly trade winds and promote the charging of the eastern equatorial Pacific Ocean, giving rise to eastern Pacific ENSO events. Indeed, the type of ENSO event can be determined accurately using only the magnitude and phase of the SPO during austral winter as a predictor (17 out of 23 cases). The SPO may also play a role in explaining the asymmetry of warm and cold events. Collectively, our findings present a new perspective on ENSO‐South Pacific interactions that can advance overall understanding of the ENSO system and enhance its predictability across multiple timescales.
Key Points
South Pacific atmospheric variability, namely, the South Pacific Oscillation (SPO), plays a key role in the prediction of ENSO flavors
The sources of variability of the SPO are both intrinsic and also forced by anomalous convection in the central tropical Pacific
The SPO potentially plays a significant role in climate variability on multiple timescales: seasonal, interannual, and multidecadal
Warm Arctic, Cold Continents COHEN, JUDAH; JONES, JUSTIN; FURTADO, JASON C. ...
Oceanography,
12/2013, Letnik:
26, Številka:
4
Journal Article
Recenzirano
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Arctic sea ice was observed to be at a new record minimum in September 2012. Following this summer minimum, northern Eurasia and much of North America experienced severe winter weather during the ...winter of 2012/2013. A statistical model that used Eurasian snow cover as its main predictor successfully forecast the observed cold winter temperatures. We propose that the large melting of Arctic sea ice may be related to the rapid advance of snow cover, similar to the connection made in studies of past climates between low Arctic sea ice and enhanced continental snowfalls and glacial inception via ice sheet growth. Regressions between autumnal sea ice extent and Eurasian snow cover extent and Northern Hemisphere temperatures yield the characteristic "warm Arctic/cold continents" pattern. This pattern was observed during winter 2012/2013, and it is common among years with observed low autumn sea ice, rapid autumn snow cover advance, and a negative winter Arctic Oscillation. Dynamical models fail to capture this pattern, instead showing maximum warming over the Arctic Ocean and widespread winter warming over the adjacent continents. We suggest that the simulated widespread warming may be due to incorrect sea ice-atmosphere coupling, including an incorrect triggering of positive feedback between low sea ice and atmospheric convection, resulting in significant model errors that are evident in seasonal predictions and that potentially impact future climate change projections.
Asymmetric seasonal temperature trends Cohen, Judah L.; Furtado, Jason C.; Barlow, Mathew ...
Geophysical research letters,
February 2012, Letnik:
39, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Current consensus on global climate change predicts warming trends driven by anthropogenic forcing, with maximum temperature changes projected in the Northern Hemisphere (NH) high latitudes during ...winter. Yet, global temperature trends show little warming over the most recent decade or so. For longer time periods appropriate to the assessment of trends, however, global temperatures have experienced significant warming trends for all seasons except winter, when cooling trends exist instead across large stretches of eastern North America and northern Eurasia. Hence, the most recent lapse in global warming is a seasonal phenomenon, prevalent only in boreal winter. Additionally, we show that the largest regional contributor to global temperature trends over the past two decades is land surface temperatures in the NH extratropics. Therefore, proposed mechanisms explaining the fluctuations in global annual temperatures should address this apparent seasonal asymmetry.
Key Points
Global warming has not stopped
The cessation of an observed warming trend is isolated to winter
The coupled climate models poorly simulate the observed winter trend
The most up to date consensus from global climate models predicts warming in the Northern Hemisphere (NH) high latitudes to middle latitudes during boreal winter. However, recent trends in observed ...NH winter surface temperatures diverge from these projections. For the last two decades, large-scale cooling trends have existed instead across large stretches of eastern North America and northern Eurasia. We argue that this unforeseen trend is probably not due to internal variability alone. Instead, evidence suggests that summer and autumn warming trends are concurrent with increases in high-latitude moisture and an increase in Eurasian snow cover, which dynamically induces large-scale wintertime cooling. Understanding this counterintuitive response to radiative warming of the climate system has the potential for improving climate predictions at seasonal and longer timescales.
Previous research has linked wintertime Arctic Oscillation (AO) variability to indices of Siberian snow cover and upward wave activity flux in the preceding fall season. Here, daily data are used to ...examine the surface and tropospheric processes that occur as the link between snow cover and upward forcing into the stratosphere develops. October Eurasian mean snow cover is found to be significantly related to sea level pressure (SLP) and to lower-stratosphere (100 hPa) meridional heat flux. Analysis of daily SLP and 100-hPa heat flux shows that in years with high October snow, the SLP is significantly higher from approximately 1 November to 15 December, and the 100-hPa heat flux is significantly increased with a two-week lag, from approximately 15 November to 31 December. During November–December, there are periods with upward wave activity flux extending coherently from the surface to the stratosphere, and these events occur nearly twice as often in high snow years compared to low snow years. The vertical structure of these events is a westward-tilting pattern of high eddy heights, with the largest normalized anomalies near the surface in the same region as the snow and SLP changes. These results suggest that high SLP develops in response to the snow cover and this higher pressure, in turn, provides part of the structure of a surface-to-stratosphere wave activity flux event, thus making full events more likely. Implications for improved winter forecasts exist through recognition of these precursor signals.
Despite substantial progress made in the theoretical understanding and practical prediction of the El Niño‐Southern Oscillation (ENSO), accurate predictions of particular ENSO characteristics (e.g., ...evolution, intensity, and spatial pattern) remain challenging. Using two models from the North American Multimodel Ensemble (NMME) Phase‐II hindcasts, we find that the austral winter atmospheric internal variability is a key determinant of how the South Pacific atmospheric circulation responds to concurrent tropical Pacific sea surface temperature anomalies. While this internal variability may not trigger ENSO onsets, it regulates the southeasterly trades and contributes to thermodynamic feedbacks that grow into an ENSO‐like structure during the following austral summer. The difference in the simulation of South Pacific atmospheric variability amongst ensemble members appears to be a significant source of the inter‐member spread in ENSO predictions. Monitoring South Pacific atmospheric variability provides an opportunity to improve the prediction of ENSO intensity and flavor with about a two‐season lead time.
Plain Language Summary
The El Niño‐Southern Oscillation (ENSO) phenomenon substantially affects weather and climate conditions worldwide. While these influences are sensitive to the eventual intensity and spatial pattern of the ENSO sea surface temperature anomalies, it remains challenging for operational forecast models to predict accurately these ENSO characteristic months in advance. This study finds that although austral winter (i.e., June–August) South Pacific sea level pressure patterns may not initiate ENSO events, they can influence the evolution of ENSO events thereafter by affecting the strength of the South Pacific subtropical high and the associated trade winds, which communicate with the tropical Pacific. Since these South Pacific circulation patterns occur when ENSO is entering its growth phase, the way that models simulate these patterns is important for ENSO forecasts and could potentially advance the predictability and prediction skill of specific ENSO characteristics by almost 6 months in advance.
Key Points
The austral winter South Pacific atmospheric internal variability regulates the response of local atmospheric circulation to tropical SSTA
The South Pacific atmospheric internal variability is a primary factor driving the intermember spread of ENSO predictions
The goal of the Pacific Ocean Boundary Ecosystem and Climate Study (POBEX) was to diagnose the large-scale climate controls on regional transport dynamics and lower trophic marine ecosystem ...variability in Pacific Ocean boundary systems. An international team of collaborators shared observational and eddy-resolving modeling data sets collected in the Northeast Pacific, including the Gulf of Alaska (GOA) and the California Current System (CCS), the Humboldt or Peru-Chile Current System (PCCS), and the Kuroshio-Oyashio Extension (KOE) region. POBEX investigators found that a dominant fraction of decadal variability in basin- and regional-scale salinity, nutrients, chlorophyll, and zooplankton taxa is explained by a newly discovered pattern of ocean-climate variability dubbed the North Pacific Gyre Oscillation (NPGO) and the Pacific Decadal Oscillation (PDO). NPGO dynamics are driven by atmospheric variability in the North Pacific and capture the decadal expression of Central Pacific El Niños in the extratropics, much as the PDO captures the low-frequency expression of eastern Pacific El Niños. By combining hindcasts of eddy-resolving ocean models over the period 1950–2008 with model passive tracers and long-term observations (e.g., CalCOFI, Line-P, Newport Hydrographic Line, Odate Collection), POBEX showed that the PDO and the NPGO combine to control low-frequency upwelling and alongshore transport dynamics in the North Pacific sector, while the eastern Pacific El Niño dominates in the South Pacific. Although different climate modes have different regional expressions, changes in vertical transport (e.g., upwelling) were found to explain the dominant nutrient and phytoplankton variability in the CCS, GOA, and PCCS, while changes in alongshore transport forced much of the observed long-term change in zooplankton species composition in the KOE as well as in the northern and southern CCS. In contrast, cross-shelf transport dynamics were linked to mesoscale eddy activity, driven by regional-scale dynamics that are largely decoupled from variations associated with the large-scale climate modes. Preliminary findings suggest that mesoscale eddies play a key role in offshore transport of zooplankton and impact the life cycles of higher trophic levels (e.g., fish) in the CCS, PCCS, and GOA. Looking forward, POBEX results may guide the development of new modeling and observational strategies to establish mechanistic links among climate forcing, mesoscale circulation, and marine population dynamics.
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