North Atlantic oscillation response to anomalous Indian Ocean SST in a coupled GCM

The dominant pattern of atmospheric variability in the North Atlantic sector is the North Atlantic Oscillation (NAO). Since the 1970s the NAO has been well characterized by a trend toward its positive phase. Recent atmospheric general circulation model studies have linked this trend to a progressive warming of the Indian Ocean. Unfortunately, a clear mechanism responsible for the change of the NAO could not be given. This study provides further details of the NAO response to Indian Ocean sea surface temperature (SST) anomalies. This is done by conducting experiments with a coupled ocean–atmosphere general circulation model (OAGCM). The authors develop a hypothesis of how the Indian Ocean impacts the NAO

Mediterranean climate change and Indian Ocean warming

General circulation model (GCM) responses to 20th century changes in sea surface temperatures (SSTs) and greenhouse gases are diagnosed, with emphasis on their relationship to observed regional climate change over the Mediterranean region. A major question is whether the Mediterranean region’s drying trend since 1950 can be understood as a consequence of the warming trend in tropical SSTs. We focus on the impact of Indian Ocean warming, which is itself the likely result of increasing greenhouse gases. It is discovered that a strong projection onto the positive polarity of the North Atlantic Oscillation (NAO) index characterizes the atmospheric response structure to the 1950-1999 warming of Indian Ocean SSTs. This influence appears to be robust in so far as it is reproduced in ensembles of experiments using three different GCMs. Both the equilibrium and transient responses to Indian Ocean warming are examined. Under each scenario, the latitude of prevailing midlatitude westerlies shifts poleward during the November-April period. The consequence is a drying of the Mediterranean region, whereas northern Europe and Scandinavia receive increased precipitation in concert with the poleward shift of storminess. The IPCC (TAR) 20th century coupled ocean-atmosphere simulations forced by observed greenhouse gas changes also yield a post-1950 drying trend over the Mediterranean. We argue that this feature of human-induced regional climate change is the outcome of a dynamical feedback, one involving Indian Ocean warming and a requisite adjustment of atmospheric circulation systems to such ocean warming

Advancing Indian monsoon rainfall predictions

Despite great strides made in seasonal climate forecasting using dynamical models, skill in predicting the Indian monsoon is woefully poor. Our analysis of the reasons for failure exposes a flaw in the popular design of dynamical prediction systems. The approach of driving atmospheric models with a projected ocean surface temperature presupposes Indian monsoon variability to be a consequence solely of the atmosphere reacting to the ocean. We demonstrate significant improvements in the skill of Indian monsoon predictions when atmospheric models are coupled to, and fully interactive with the ocean. The additional feedback of the atmosphere onto the ocean is thus deemed critical for harvesting skilful monsoon predictions

Tropical Origins for Recent North Atlantic Climate Change

Evidence is presented that North Atlantic climate change since 1950 is linked to a progressive warming of tropical sea surface temperatures, especially over the Indian and Pacific Oceans. The ocean changes alter the pattern and magnitude of tropical rainfall and atmospheric heating, the atmospheric response to which includes the spatial structure of the North Atlantic Oscillation (NAO). The slow, tropical ocean warming has thus forced a commensurate trend toward one extreme phase of the NAO during the past half-century

Atmosphere and ocean origins of North American droughts

The atmospheric and oceanic causes of North American droughts are examined using observations and ensemble climate simulations. The models indicate that oceanic forcing of annual mean precipitation variability accounts for up to 40% of total variance in northeastern Mexico, the southern Great Plains, and the Gulf Coast states but less than 10% in central and eastern Canada. Observations and models indicate robust tropical Pacific and tropical North Atlantic forcing of annual mean precipitation and soil moisture with the most heavily influenced areas being in southwestern North America and the southern Great Plains. In these regions, individual wet and dry years, droughts, and decadal variations are well reproduced in atmosphere models forced by observed SSTs. Oceanic forcing was important in causing multiyear droughts in the 1950s and at the turn of the twenty-first century, although a similar ocean configuration in the 1970s was not associated with drought owing to an overwhelming influence of internal atmospheric variability. Up to half of the soil moisture deficits during severe droughts in the southeast United States in 2000, Texas in 2011, and the central Great Plains in 2012 were related to SST forcing, although SST forcing was an insignificant factor for northern Great Plains drought in 1988. During the early twenty-first century, natural decadal swings in tropical Pacific and North Atlantic SSTs have contributed to a dry regime for the United States. Long-term changes caused by increasing trace gas concentrations are now contributing to a modest signal of soil moisture depletion, mainly over the U.S. Southwest, thereby prolonging the duration and severity of naturally occurring droughts

Causes and Predictability of the 2012 Great Plains Drought

Central Great Plains precipitation deficits during May-August 2012 were the most severe since at least 1895, eclipsing the Dust Bowl summers of 1934 and 1936. Drought developed suddenly in May, following near-normal precipitation during winter and early spring. Its proximate causes were a reduction in atmospheric moisture transport into the Great Plains from the Gulf of Mexico. Processes that generally provide air mass lift and condensation were mostly absent, including a lack of frontal cyclones in late spring followed by suppressed deep convection in summer owing to large-scale subsidence and atmospheric stabilization. Seasonal forecasts did not predict the summer 2012 central Great Plains drought development, which therefore arrived without early warning. Climate simulations and empirical analysis suggest that ocean surface temperatures together with changes in greenhouse gases did not induce a substantial reduction in summertime precipitation over the central Great Plains during 2012. Yet, diagnosis of the retrospective climate simulations also reveals a regime shift toward warmer and drier summertime Great Plains conditions during the recent decade, most probably due to natural decadal variability. As a consequence, the probability for severe summer Great Plains drought may have increased in the last decade compared to the 1980s and 1990s, and the so-called tail-risk for severe drought may have been heightened in summer 2012. Such an extreme drought event was nonetheless still found to be a rare occurrence within the spread of 2012 climate model simulations. Implications of this study's findings for U.S. seasonal drought forecasting are discussed

Impact of tropical Pacific variability on the mean North Atlantic thermohaline circulation

A series of 500 years long coupled general circulation model simulations has been performed, in which the sea surface temperatures (SSTs) in different tropical oceans have been prescribed from climatology. A statistically significant reduction by about one Sverdrup of the meridional overturning circulation (MOC) in the North Atlantic was found when the tropical Pacific SSTs do not vary interannually. Anomalously low salinities originating in the tropical Atlantic due to increased precipitation drive the reduction of the MOC. Climatological SSTs in the tropical Pacific lead to a “La Niña”-like state due to the nonlinear response of the atmosphere to SST anomalies. The shift of the mean atmospheric circulation in the tropical Pacific leads to a cyclonic anomaly over the eastern tropical Atlantic with a corresponding precipitation increase. The results suggest that changes in the SST variability of the tropical Pacific can drive changes in the mean state of remote regions

Climatology and interannual variability of boreal spring wet season precipitation in the eastern Horn of Africa and implications for its recent decline

The 1981-2014 climatology and variability of the March-May eastern Horn of Africa boreal spring wet season are examined using precipitation, upper- and lower-level winds, low-level specific humidity, and convective available potential energy (CAPE), with the aim of better understanding the establishment of the wet season and the cause of the recent observed decline. At 850 mb, the development of the wet season is characterized by increasing specific humidity and winds that veer from northeasterly in February to southerly in June and advect moisture into the region, in agreement with an earlier study. Equally important, however, is a substantial weakening of the 200-mb climatological easterly winds in March. Likewise, the shutdown of the wet season coincides with the return of strong easterly winds in June. Similar changes are seen in the daily evolution of specific humidity and 200-mb wind when composited relative to the interannual wet season onset and end, with the easterlies decreasing (increasing) several days prior to the start (end) of the wet season. The 1981-2014 decrease in March-May precipitation has also coincided with an increase in 200-mb easterly winds, with no attendant change in specific humidity, leading to the conclusion that, while high values of specific humidity are an important ingredient of the wet season, the recent observed precipitation decline has resulted mostly from a strengthening of the 200-mb easterlies. This change in the easterly winds appears to be related to an increase in convection over the Indonesian region and in the associated outflow from that enhanced heat source

Northeast Colorado Extreme Rains Interpreted in a Climate Change Context

The probability for an extreme five-day September rainfall event over northeast Colorado, as was observed in early September 2013, has likely decreased due to climate change