Atlantic Multidecadal Oscillation Warm Phase Research Articles

Modulation of the link between the Hadley circulation and the meridional structure of tropical SSTs by the Atlantic multidecadal oscillation

This paper investigates the effects of the Atlantic multidecadal oscillation (AMO) on the relationship between the Hadley circulation (HC) and the different meridional structures of the tropical sea surface temperature (SST). The response of the HC to SSTs shows inconsistent variations between the warm and cold phases of the AMO. The response of the HC to SSTs during the cold phase of the AMO is similar to that seen in the long-term and seasonal cycles. However, during the warm AMO phase, the magnitude of this response is significantly reduced. This significant difference in the response is caused mainly by the weakened response amplitude of the equatorially asymmetric HC and SST. The potential mechanisms associated with this suppression are also investigated, and they relate primarily to differences in the SST meridional anomalies within the Indo-Pacific warm pool (IPWP). During the AMO warm phase, the profile of SST anomalies in the IPWP exhibits characteristics similar to an equatorially symmetric distribution, which weakens the equatorially asymmetric component of the tropical SST and then contributes to the suppression of the response ratio. However, the amplitudes of the insignificant negative SST anomalies in the northern and southern IPWP are similar during the cold AMO phase. This distribution of SST anomalies has a minimal impact on the variation of SST gradients. Therefore, the response ratio during the cold AMO phase is comparable to that observed in the long-term and seasonal cycles. Other atmospheric reanalysis data are used to further confirm these results.

ENSO–IOD Inter‐Basin Connection Is Controlled by the Atlantic Multidecadal Oscillation

AbstractThe interactions between El Niño‐Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are known to have great implications for global climate variability and seasonal climate predictions. Observational analysis suggests that the ENSO–IOD inter‐basin connection is time‐varying and related to the Atlantic Multidecadal Oscillation (AMO) with weakened ENSO–IOD relationship corresponding to AMO warm phases. A suite of Atlantic pacemaker simulations successfully reproduces the decadal fluctuations in ENSO–IOD relationship and its link to the AMO. The warm sea surface temperature (SST) anomalies associated with the AMO drive a series of Indo‐Pacific mean climate changes through tropical‐wide teleconnections, including the La Niña‐like mean SST cooling over the central Pacific and the deepening of mean thermocline depth in the eastern Indian Ocean. By modulating ocean–atmosphere feedback strength, those mean state changes decrease both ENSO amplitude and the Indian Ocean sensitivity to ENSO forcing, therefore decoupling the IOD from ENSO.

Exploring the circum-global teleconnection—Indian summer monsoon interactions in the interannual and multidecadal timescales

The Northern hemispheric circum global teleconnection (CGT) pattern is thought to be maintained by two main forcings—viz-diabatic heating associated with the Indian summer monsoon (ISM) and barotropic instability generation over the jet exit region over the North Atlantic. The CGT and ISM impacts one another through the circulation responses over West central Asia (WCA). In this study we revisit the CGT-ISM interactions focusing on the WCA region and try to understand whether the downstream impact of CGT on ISM dominates over the ISM feedback on CGT. Analysis indicates that the Atlantic forced CGT responses play a lead role in modulating the ISM in the interannual timescale, by modulating the upper-level anticyclones over WCA and in turn affecting the ISM easterly vertical wind shear. Atlantic multi-decadal oscillation (AMO) is a major driver of ISM variability in the multi-decadal time scale and the AMO is associated with an arching wave-train of teleconnection across Eurasia. Our analysis indicates significant modulation of WCA anomalies by the AMO in the multi-decadal time scale, implying that the Atlantic-CGT-WCA-ISM pathway of teleconnection has a low frequency counterpart. We further demonstrate that the observed out of phase relationship between AMO and ISM in the recent decades, may be attributed to the relatively stronger high latitude warming over the north Atlantic during the recent AMO warm phase. The equivalent barotropic responses to the extra-tropical north Atlantic sea surface temperature (SST) anomalies alter the entire downstream teleconnection pattern producing cyclonic anomalies over WCA and in turn weakening the ISM.

An inter-basin teleconnection from the North Atlantic to the subarctic North Pacific at multidecadal time scales

Observational evidence suggests that the sub-arctic North Pacific (SANP; 45°–60° N, 155° E–165° W) sea surface temperature (SST) shows pronounced multidecadal variability, which cannot be explained by the Pacific Decadal Oscillation (PDO). Here, we find that the SANP SST multidecadal variability is closely linked to the remote Atlantic Multidecadal Oscillation (AMO), indicating a multidecadal inter-basin teleconnection. The teleconnection can be well reproduced in a set of Atlantic Pacemaker experiments. An atmospheric bridge mechanism for the teleconnection is proposed by analyzing both observations and simulation data. The AMO warm phase generates anomalous ascent and upper-level divergence over the North Atlantic. The upper-level outflows converge towards the subarctic North Pacific, leading to compensating subsidence along with anomalous high pressure there. The enhanced adiabatic descent causes anomalous warming and moistening of the lower troposphere above the SANP basin and increases the downwelling longwave radiation. The warming of the SANP SST is further induced and amplified due to water vapor-longwave radiation-SST positive feedback. The anomalous high also weakens the climatological cyclonic flow of Aleutian low and suppresses the turbulent heat release from ocean to atmosphere, contributing to the SANP SST warming. Our findings suggest that the AMO plays a crucial role in the subarctic North Pacific SST multidecadal variability.

Mechanisms of Abrupt Extreme Precipitation Change Over the Northeastern United States

AbstractIn 1996, the northeastern United States experienced an abrupt increase in extreme precipitation, but the causal mechanisms driving this increase remain poorly understood. We find that 89% of the 1996–2016 increase relative to 1979–1995 is explained by only 273 unique extreme events affecting >5 stations and occurring in the months of February, March, June, July, September, and October. We use daily weather maps to classify the 273 extreme precipitation events by meteorological cause (tropical cyclones, fronts, and extratropical cyclones) and use reanalysis data to determine large‐scale changes in the atmosphere and ocean associated with increased extreme precipitation for each classification. Results show that tropical cyclones account for almost half (48%) of the post‐1996 extreme precipitation increase, while fronts and extratropical cyclones are responsible for 25% and 15% of the increase, respectively. The remaining 11% is from extreme events in the other 6 months of the year and extreme events that affected <5 stations. The increase in extreme precipitation from tropical cyclones after 1996 is associated with a shift to the Atlantic Multidecadal Oscillation warm phase, higher total column water vapor, and potentially weakened steering winds. September and October tropical cyclones caused significantly more extreme precipitation during the current Atlantic Multidecadal Oscillation warm phase (1996–present) than during the last warm phase (1928–1962), despite the same number of northeast tropical cyclones in both periods. Increased extreme precipitation from fronts is associated with a wavier (higher amplitude) jet stream, which likely facilitates the development of more frequent fronts through the advection of cool northern air into the American Midwest.

Strong Dependence of U.S. Summertime Air Quality on the Decadal Variability of Atlantic Sea Surface Temperatures.

We find that summertime air quality in the eastern U.S. displays strong dependence on North Atlantic sea surface temperatures, resulting from large‐scale ocean‐atmosphere interactions. Using observations, reanalysis data sets, and climate model simulations, we further identify a multidecadal variability in surface air quality driven by the Atlantic Multidecadal Oscillation (AMO). In one‐half cycle (~35 years) of the AMO from cold to warm phase, summertime maximum daily 8 h ozone concentrations increase by 1–4 ppbv and PM2.5 concentrations increase by 0.3–1.0 μg m−3 over much of the east. These air quality changes are related to warmer, drier, and more stagnant weather in the AMO warm phase, together with anomalous circulation patterns at the surface and aloft. If the AMO shifts to the cold phase in future years, it could partly offset the climate penalty on U.S. air quality brought by global warming, an effect which should be considered in long‐term air quality planning.

The influence of large-scale climate phenomena on precipitation in the Ordos Basin, China

Large-scale atmospheric circulations significantly affect regional precipitation patterns. However, it is not well known whether and how these phenomena affect regional precipitation distribution in northern China. This paper reported the individual and coupled effects of El Nino–Southern Oscillation (ENSO), Indian summer monsoon (ISM), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) on annual precipitation for the Ordos Basin, an arid and semi-arid basin, currently with major industries of coal, fossil oil, natural gas, and halite in north central China. Our results showed that ENSO and ISM exerted substantial impact on annual precipitation while the impact of PDO and AMO was relatively limited. There were 24 and 15 out of 33 stations showing significant differences (p < 0.1) in annual precipitation (from 1950 to 2013) for ENSO and ISM, respectively. The median precipitation across the basin during El Nino years was 21.49 % higher than that during La Nina years and 17.28 % higher during the positive phase of ISM years compared to the negative phase of the ISM years. The impacts of ENSO and ISM on precipitation were enhanced during a PDO cold phase but weakened in a PDO warm phase. The impact of ENSO was still enhanced by an AMO warm phase. The effects of climatic phenomena on precipitation showed a strong spatial difference in the Ordos Basin. The impact of ENSO was mostly evident around the edges of the basin while the impact of ISM decreases from south to north. The deserts (i.e., Hobq Desert and Mu Us Sandy Land) in the center of the basin were less affected by these large-scale climatic phenomena. An improved understanding of such relationships would be helpful in water resource planning and disaster management for the Ordos Basin.

Individual and coupled influences of AMO and ENSO on regional precipitation characteristics and extremes

AbstractUnderstanding the influences of Atlantic multidecadal oscillation (AMO) and El Niño southern oscillation (ENSO) on regional precipitation extremes and characteristics in the state of Florida is the focus of this study. Exhaustive evaluations of individual and combined influences of these oscillations using, descriptive indices‐based assessment of statistically significant changes in rainfall characteristics, identification of spatially varying influences of oscillations on dry and wet spell transition states, antecedent precipitation prior to extreme events, intraevent temporal distribution of precipitation and changes in temporal occurrences of extremes including dry/wet cycles are carried out. Rain gage and gridded precipitation data analysis using parametric hypothesis tests confirm statistically significant changes in the precipitation characteristics from one phase to another of each oscillation and also in coupled phases. Spatially nonuniform and uniform influences of AMO and ENSO, respectively, on precipitation are evident. AMO influences vary in peninsular and continental parts of Florida and the warm (cool) phase of AMO contributes to increased precipitation extremes during wet (dry) season. The influence of ENSO is confined to dry season with El Niño (La Niña) contributing to increase (decrease) in extremes and total precipitation. Wetter antecedent conditions preceding daily extremes are dominant in AMO warm phase compared to the cool and are likely to impact design floods in the region. AMO influence on dry season precipitation extremes is noted for ENSO neutral years. The two oscillations in different phases modulate each other with seasonal and spatially varying impacts and implications on flood control and water supply in the region.

The relationship between the Atlantic Multidecadal Oscillation and temperature variability in China during the last millennium

ABSTRACTUsing a large number of temperature‐sensitive proxy records, we investigated the relationship between the Atlantic Multidecadal Oscillation (AMO) and temperature variability in China during the last millennium. The results indicate a relatively homogeneous pattern of cold/warm anomalies in East, West and the whole of China (EC, WC and WOC). The six major AMO cold phases coincided with cold conditions over EC, WC and WOC, while warm conditions across China occurred during the two major AMO warm phases. This close similarity between the AMO and multi‐decadal temperature variability in China is supported by climate model simulations and emphasizes the critical role of surface heating over the Asian continent in linking the AMO and the Indian summer monsoon. The mid‐latitude westerly anomalies and the propagation of Rossby waves related to the AMO may be two channels linking sea surface temperature anomalies in the North Atlantic and temperature variability over East Asia. Additionally, the Atlantic Meridional Overturning Circulation may play an important role in transmitting the AMO signal over the globe. We recommend further climate model studies on the mechanisms responsible for the connection between the AMO and regional to global climate changes.

Influences of Atlantic multidecadal oscillation phases on spatial and temporal variability of regional precipitation extremes

Summary A major teleconnection, Atlantic multidecadal oscillation (AMO) under two phases (cool and warm) influencing precipitation extremes in Florida, USA, is the main focus of this study. Long-term extreme precipitation data from several rain gages from temporal windows that coincide with the AMO phases are evaluated for changes in spatial and temporal variability across the region. Assessments of precipitation extremes for nine durations in different meteorologically homogenous rainfall areas as well as in the entire region are carried out. Methods of assessment included parametric unpaired t -tests and nonparametric Mann–Whitney U tests, kernel density estimates using Gaussian kernel for distribution-free comparative analysis and bootstrap sampling-based confidence intervals. Depth-duration-frequency (DDF) curves are also developed using generalized extreme value (GEV) distributions characterizing the extremes. Analysis of data indicated increase in precipitation extremes in warm phases of AMO for durations greater than 24 h. The influence of warm or cool phases of AMO on precipitation extremes is not spatially uniform in the region. Temporal shifts in occurrences of extremes from the later part of the year in warm phase to earlier in the year for the cool phase are evident. These shifts will have implications on flooding events in different regions of Florida. Magnitudes of extremes for a 25 year return period based on DDF curves were higher for all nine durations when data from cool or warm phase alone were compared to those obtained from data from two phases. Precipitation extremes for durations longer than a day are associated with increased landfalls of hurricanes occurring in the region in the AMO warm phases.

Temporal energy partitions of Florida extreme sea level events as a function of Atlantic multidecadal oscillation

Abstract. An energy-conservative metric based on the discrete wavelet transform is applied to assess the relative energy distribution of extreme sea level events across different temporal scales. The metric is applied to coastal events at Key West and Pensacola Florida as a function of two Atlantic Multidecadal Oscillation (AMO) regimes. Under AMO warm conditions there is a small but significant redistribution of event energy from nearly static into more dynamic (shorter duration) timescales at Key West, while at Pensacola the AMO-dependent changes in temporal event behaviour are less pronounced. Extreme events with increased temporal dynamics might be consistent with an increase in total energy of event forcings which may be a reflection of more energetic storm events during AMO warm phases. As dynamical models mature to the point of providing regional climate index predictability, coastal planners may be able to consider such temporal change metrics in planning scenarios.

Ethiopian decadal climate variability

Ethiopian decadal climate variability is characterized by application of singular value decomposition to gridded rainfall data over the period 1901–2007. Two distinct modes are revealed with different annual cycles and opposing responses to regional and global forcing. The northern zone that impacts the Nile River and underlies the tropical easterly jet has a unimodal rainy season that is enhanced by Atlantic Multidecadal Oscillation warm phase. This rainfall mode is linked with the Atlantic zonal overturning circulation and exhibits 10–12-year cycles through much of the twentieth century. The southern zone has a bimodal rainy season that is enhanced by Pacific Decadal Oscillation cool phase and the southern meridional overturning circulation. Multiyear wet and dry spells are characterized by sympathetic responses in the near-equatorial trough extending from Central America across the African Sahel to Southeast Asia. The interaction of Walker and Hadley cells over Africa appears to be a key feature that modulates Ethiopian climate at decadal frequency through anomalous north–south displacement of the near-equatorial trough.

The Atlantic Multidecadal Oscillation and its relation to rainfall and river flows in the continental U.S.

North Atlantic sea surface temperatures for 1856–1999 contain a 65–80 year cycle with a 0.4 °C range, referred to as the Atlantic Multidecadal Oscillation (AMO) by Kerr [2000]. AMO warm phases occurred during 1860–1880 and 1940–1960, and cool phases during 1905–1925 and 1970–1990. The signal is global in scope, with a positively correlated co‐oscillation in parts of the North Pacific, but it is most intense in the North Atlantic and covers the entire basin there. During AMO warmings most of the United States sees less than normal rainfall, including Midwest droughts in the 1930s and 1950s. Between AMO warm and cool phases, Mississippi River outflow varies by 10% while the inflow to Lake Okeechobee, Florida varies by 40%. The geographical pattern of variability is influenced mainly by changes in summer rainfall. The winter patterns of interannual rainfall variability associated with El Niño‐Southern Oscillation are also significantly changed between AMO phases.