scholarly journals Transient Atmospheric Response to Interactive SST Anomalies

2008 ◽  
Vol 21 (3) ◽  
pp. 576-583 ◽  
Author(s):  
David Ferreira ◽  
Claude Frankignoul
Keyword(s):  
North Atlantic ◽  
Initial Conditions ◽  
Maximum Amplitude ◽  
Atmospheric Response ◽  
The North ◽  
Winter Conditions ◽  
Initial Evolution ◽  
Sst Anomaly ◽  
The North Atlantic ◽  
Sst Anomalies

Abstract The transient atmospheric response to interactive SST anomalies in the midlatitudes is investigated using a three-layer QG model coupled in perpetual winter conditions to a slab oceanic mixed layer in the North Atlantic. The SST anomalies are diagnosed from a coupled run and prescribed as initial conditions, but are free to evolve. The initial evolution of the atmospheric response is similar to that obtained with a prescribed SST anomaly, starting as a quasi-linear baroclinic and then quickly evolving into a growing equivalent barotropic one. Because of the heat flux damping, the SST anomaly amplitude slowly decreases, albeit with little change in pattern. Correspondingly, the atmospheric response only increases until it reaches a maximum amplitude after about 1–3.5 months, depending on the SST anomaly considered. The response is similar to that at equilibrium in the fixed SST case, but it is 1.5–2 times smaller, and then slowly decays away.

scholarly journals Atmospheric Responses to North Atlantic SST Anomalies in Idealized Experiments. Part II: North American Precipitation

2016 ◽  
Vol 29 (2) ◽  
pp. 659-671 ◽  
Author(s):  
Qi Hu ◽  
Michael C. Veres
Keyword(s):  
North America ◽  
North Atlantic ◽  
North American ◽  
North Atlantic Ocean ◽  
Warm Season ◽  
The North ◽  
Precipitation Anomalies ◽  
Sst Anomaly ◽  
The North Atlantic ◽  
Sst Anomalies

Abstract This is the second part of a two-part paper that addresses deterministic roles of the sea surface temperature (SST) anomalies associated with the Atlantic multidecadal oscillation (AMO) in variations of atmospheric circulation and precipitation in the Northern Hemisphere, using a sequence of idealized model runs at the spring equinox conditions. This part focuses on the effect of the SST anomalies on North American precipitation. Major results show that, in the model setting closest to the real-world situation, a warm SST anomaly in the North Atlantic Ocean causes suppressed precipitation in central, western, and northern North America but more precipitation in the southeast. A nearly reversed pattern of precipitation anomalies develops in response to the cold SST anomaly. Further examinations of these solutions reveal that the response to the cold SST anomaly is less stable than that to the warm SST anomaly. The former is “dynamically charged” in the sense that positive eddy kinetic energy (EKE) exists over the continent. The lack of precipitation in its southeast is because of an insufficient moisture supply. In addition, the results show that the EKE of the short- (2–6 day) and medium-range (7–10 day) weather-producing processes in North America have nearly opposite signs in response to the same cold SST anomaly. These competing effects of eddies in the dynamically charged environment (elevated sensitivity to moisture) complicate the circulation and precipitation responses to the cold SST anomaly in the North Atlantic and may explain why the model results show more varying precipitation anomalies (also confirmed by statistical test results) during the cold than the warm SST anomaly, as also shown in simulations with more realistic models. Results of this study indicate a need to include the AMO in the right context with other forcings in an effort to improve understanding of interannual-to-multidecadal variations in warm season precipitation in North America.

Why Does a Colder (Warmer) Winter Tend to Be Followed by a Warmer (Cooler) Summer over Northeast Eurasia?

2020 ◽  
Vol 33 (17) ◽  
pp. 7255-7274
Author(s):  
Shangfeng Chen ◽  
Renguang Wu ◽  
Wen Chen ◽  
Kai Li
Keyword(s):  
Atmospheric Circulation ◽  
North Atlantic ◽  
Wave Train ◽  
The Arctic ◽  
The North ◽  
Atmospheric Wave ◽  
Anomaly Pattern ◽  
Sst Anomaly ◽  
The North Atlantic ◽  
Sst Anomalies

AbstractThis study reveals a pronounced out-of-phase relationship between surface air temperature (SAT) anomalies over northeast Eurasia in boreal winter and the following summer during 1980–2017. A colder (warmer) winter over northeast Eurasia tends to be followed by a warmer (cooler) summer of next year. The processes for the out-of-phase relation of winter and summer SAT involve the Arctic Oscillation (AO), the air–sea interaction in the North Atlantic Ocean, and a Eurasian anomalous atmospheric circulation pattern induced by the North Atlantic sea surface temperature (SST) anomalies. Winter negative AO/North Atlantic Oscillation (NAO)-like atmospheric circulation anomalies lead to continental cooling over Eurasia via anomalous advection and a tripolar SST anomaly pattern in the North Atlantic. The North Atlantic SST anomaly pattern switches to a dipolar pattern in the following summer via air–sea interaction processes and associated surface heat flux changes. The summer North Atlantic dipolar SST anomaly pattern induces a downstream atmospheric wave train, including large-scale positive geopotential height anomalies over northeast Eurasia, which contributes to positive SAT anomalies there via enhancement of downward surface shortwave radiation and anomalous advection. Barotropic model experiments verify the role of the summer North Atlantic SST anomalies in triggering the atmospheric wave train over Eurasia. Through the above processes, a colder winter is followed by a warmer summer over northeast Eurasia. The above processes apply to the years when warmer winters are followed by cooler summers except for opposite signs of SAT, atmospheric circulation, and SST anomalies.

Extratropical Atmospheric Response to Equatorial Atlantic Cold Tongue Anomalies

2007 ◽  
Vol 20 (10) ◽  
pp. 2076-2091 ◽  
Author(s):  
Reindert J. Haarsma ◽  
Wilco Hazeleger
Keyword(s):  
Rossby Wave ◽  
North Atlantic ◽  
Maximum Amplitude ◽  
Vertical Shear ◽  
Boreal Summer ◽  
Atmospheric Response ◽  
Cold Tongue ◽  
The North ◽  
Subtropical Jet ◽  
The North Atlantic

Abstract The extratropical atmospheric response to the equatorial cold tongue mode in the Atlantic Ocean has been investigated with the coupled ocean–atmosphere model, Speedy Ocean (SPEEDO). Similar to the observations, the model simulates a lagged covariability between the equatorial cold tongue mode during late boreal summer and the east Atlantic pattern a few months later in early winter. The equatorial cold tongue mode attains its maximum amplitude during late boreal summer. However, only a few months later, when the ITCZ has moved southward, it is able to induce a significant upper-tropospheric divergence that is able to force a Rossby wave response. The lagged covariability is therefore the result of the persistence of the cold tongue anomaly and a favorable tropical atmospheric circulation a few months later. The Rossby wave energy is trapped in the South Asian subtropical jet and propagates circumglobally before it reaches the North Atlantic. Due to the local increase of the Hadley circulation, forced by the cold tongue anomaly, the subtropical jet over the North Atlantic is enhanced. The resulting increase in the vertical shear of the zonal wind increases the baroclinicity over the North Atlantic. This causes the nonlinear growth of the anomalies due to transient eddy feedbacks to be largest over the North Atlantic, resulting in an enhanced response over that region.

scholarly journals Impacts of Summer North Atlantic Sea Surface Temperature Anomalies on the East Asian Winter Monsoon Variability

2019 ◽  
Vol 32 (19) ◽  
pp. 6513-6532 ◽  
Author(s):  
Zhang Chen ◽  
Renguang Wu ◽  
Zhibiao Wang
Keyword(s):  
North Atlantic ◽  
Wave Train ◽  
Asian Winter Monsoon ◽  
The North ◽  
Atmospheric Wave ◽  
Anomaly Pattern ◽  
Dipole Pattern ◽  
Sst Anomaly ◽  
The North Atlantic ◽  
Sst Anomalies

Abstract The present study investigates the impacts of the North Atlantic sea surface temperature (SST) anomalies on the East Asian winter monsoon (EAWM) variability. It is found that the northern component of the EAWM variability is associated with a dipole pattern of preceding summer North Atlantic SST anomalies during 1979–2016. The processes linking preceding summer North Atlantic SST to EAWM include the North Atlantic air–sea interactions and atmospheric wave train triggered by the North Atlantic SST anomalies. Atmospheric wind anomalies in the preceding spring–summer result in the formation of a dipole SST anomaly pattern through surface heat flux changes. In turn, the induced SST anomalies provide a feedback on the atmosphere, modifying the location and intensity of anomalous winds over the North Atlantic. The associated surface heat flux anomalies switch the North Atlantic SST anomaly distribution from a dipole pattern in summer to a tripole pattern in the following winter. The North Atlantic tripole SST anomalies excite an atmospheric wave train extending from the North Atlantic through Eurasia to East Asia in winter, resulting in anomalous EAWM. However, the relationship of the northern component of EAWM to preceding summer North Atlantic SST anomalies is weak before the late 1970s. During 1956–76, due to weak air–sea interaction over the North Atlantic, no obvious tripole SST anomaly pattern is established in winter. The atmospheric wave train in winter is located at higher latitudes, leading to a weak connection between the northern component of EAWM and the preceding summer North Atlantic dipole SST anomaly pattern.

scholarly journals Atmospheric Responses to North Atlantic SST Anomalies in Idealized Experiments. Part I: Northern Hemispheric Circulation

2015 ◽  
Vol 28 (15) ◽  
pp. 6204-6220 ◽  
Author(s):  
Michael Veres ◽  
Qi Hu
Keyword(s):  
North America ◽  
North Atlantic ◽  
Lower Troposphere ◽  
Precipitation Anomaly ◽  
Atmospheric Response ◽  
Model Experiments ◽  
The North ◽  
Sst Anomaly ◽  
The North Atlantic ◽  
Geopotential Anomaly

Abstract Idealized model experiments using the NCAR CESM1.0.5 under equinox conditions are designed and performed to address two fundamental questions about the effects of the sea surface temperature (SST) variation associated with the Atlantic multidecadal oscillation (AMO) on circulation and precipitation in North America and Europe: 1) Is the observed relationship between the AMO SST and the warm-season precipitation in North America a statistical coincidence? and 2) Why is the response of negative precipitation anomaly to warm SST in the AMO fairly uniform across most of North America, whereas the positive precipitation anomaly in the cold SST rather spotty? Model experiments are done with either a warm or cold SST anomaly in an aquaplanet, a planet with idealized continents, and a planet with both idealized continents and orography. Major results show that the atmospheric response to warm SST anomaly in the North Atlantic is fairly similar among the three sets of experiments. In the lower troposphere, the response has a significant negative geopotential anomaly from the SST anomaly center to the east and a positive geopotential anomaly in upstream North America. However, the response to the cold SST anomaly changes considerably among these experiments, particularly in North America. These results provide a foundation to answer the abovementioned two questions. First, they show that there is physical connection of the AMO SST and atmospheric circulation anomalies in North America. Moreover, the rather stable atmospheric response to the warm SST may explain the observed largely consistent response to the warm SST anomaly. The varying responses of the atmosphere to the cold SST indicate a strong sensitivity of the atmosphere to other forcings during the cold SST anomaly in the North Atlantic. This sensitivity could explain the varying and less stable response of the atmosphere to the cold SST during the AMO.

scholarly journals The Changing Relationship between Interannual Variations of the North Atlantic Oscillation and Northern Tropical Atlantic SST

2015 ◽  
Vol 28 (2) ◽  
pp. 485-504 ◽  
Author(s):  
Shangfeng Chen ◽  
Renguang Wu ◽  
Wen Chen
Keyword(s):  
Atmospheric Circulation ◽  
North Atlantic ◽  
Interannual Variations ◽  
Tropical Atlantic ◽  
The North ◽  
Anomaly Pattern ◽  
Sst Anomaly ◽  
The North Atlantic ◽  
Interdecadal Changes ◽  
Sst Anomalies

Abstract The relationship between interannual variations of boreal winter North Atlantic Oscillation (NAO) and northern tropical Atlantic (NTA) sea surface temperature (SST) experienced obvious interdecadal changes during 1870–2012. Similar interdecadal changes are observed in the amplitude of NTA SST anomalies. The mean NTA SST change may be a plausible reason for several changes in the NAO–NTA SST connection. Under a higher mean NTA SST, NTA SST anomalies induce larger wind anomalies over the North Atlantic that produce a tripole SST anomaly pattern and amplify NTA SST anomalies. Comparison of the evolution of anomalies between 1970–86 and 1996–2012 unravels changing roles of El Niño–Southern Oscillation (ENSO) and extratropical atmospheric disturbances in the formation of NTA SST anomalies. During 1970–86, ENSO events play a key role in initiating NTA SST anomalies in the preceding spring through atmospheric circulation changes. With the decay of ENSO, SST anomalies in the midlatitude North Atlantic weaken in the following summer, whereas NTA SST anomalies are maintained up to winter. This leads to a weak NAO–NTA SST connection in winter. During 1996–2012, the preceding spring atmospheric circulation disturbances over the midlatitude North Atlantic play a dominant role in the genesis of a North Atlantic horseshoe (NAH)-like SST anomaly pattern in the following summer and fall. This NAH-like SST anomaly pattern contributes to the development of the NAO in late fall and early winter. The atmospheric circulation anomaly, in turn, is conducive to the maintenance of NTA SST anomalies to winter via changing surface latent heat flux and shortwave radiation. This leads to a close NAO–NTA SST connection in winter.

scholarly journals The Influence of Atlantic Variability on Asian Summer Climate Is Sensitive to the Pattern of the Sea Surface Temperature Anomaly

2020 ◽  
Vol 33 (17) ◽  
pp. 7567-7590 ◽  
Author(s):  
Satyaban B. Ratna ◽  
Timothy J. Osborn ◽  
Manoj Joshi ◽  
Jürg Luterbacher
Keyword(s):  
North Atlantic ◽  
Wave Train ◽  
Climate Anomalies ◽  
Summer Climate ◽  
The North ◽  
Asian Summer ◽  
Sst Anomaly ◽  
Rossby Wave Train ◽  
The North Atlantic ◽  
Sst Anomalies

AbstractWe simulate the response of Asian summer climate to Atlantic multidecadal oscillation (AMO)-like sea surface temperature (SST) anomalies using an intermediate-complexity general circulation model (IGCM4). Experiments are performed with seven individual AMO SST anomalies obtained from CMIP5/PMIP3 global climate models as well as their multimodel mean, globally and over the North Atlantic Ocean only, for both the positive and negative phases of the AMO. During the positive (warm) AMO phase, a Rossby wave train propagates eastward, causing a high pressure and warm and dry surface anomalies over eastern China and Japan. During the negative (cool) phase of the AMO, the midlatitude Rossby wave train is less robust, but the model does simulate a warm and dry South Asian monsoon, associated with the movement of the intertropical convergence zone in the tropical Atlantic. The circulation response and associated temperature and precipitation anomalies are sensitive to the choice of AMO SST anomaly pattern. A comparison between global SST and North Atlantic SST perturbation experiments indicates that East Asian climate anomalies are forced from the North Atlantic region, whereas South Asian climate anomalies are more strongly affected by the AMO-related SST anomalies outside the North Atlantic region. Experiments conducted with different amplitudes of negative and positive AMO anomalies show that the temperature response is linear with respect to SST anomaly but the precipitation response is nonlinear.

Strengthened Connection between Springtime North Atlantic Oscillation and North Atlantic Tripole SST Pattern since the Late 1980s

2020 ◽  
Vol 33 (5) ◽  
pp. 2007-2022 ◽  
Author(s):  
Shangfeng Chen ◽  
Renguang Wu ◽  
Wen Chen
Keyword(s):  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Atmospheric Response ◽  
Synoptic Scale ◽  
Mean Flow ◽  
The North ◽  
Anomalous Anticyclone ◽  
Anomaly Pattern ◽  
Sst Anomaly ◽  
The North Atlantic

AbstractThis study reveals a marked enhancement in the relationship between the North Atlantic Oscillation (NAO) and North Atlantic tripole (NAT) sea surface temperature (SST) anomaly pattern during boreal spring since the late 1980s. A comparative analysis is conducted for two periods before and after the late 1980s to understand the reasons for the above interdecadal change. During both periods, SST cooling in the northern tropical Atlantic during the positive phase of the NAT SST pattern results in an anomalous anticyclone over the subtropical western North Atlantic via a Rossby wave–type atmospheric response. The westerly wind anomalies along the north flank of the anomalous anticyclone are accompanied by a marked decrease in synoptic-scale eddies over the midlatitudes as well as cyclonic (anticyclonic) vorticity forcings at the north (south) side. As such, an NAO-like dipole atmospheric anomaly is induced over the North Atlantic, which in turn helps to maintain the NAT SST anomaly via modulating surface heat fluxes. The intensity of the synoptic-scale eddy feedback to mean flow is stronger after than before the late 1980s, which is related to interdecadal increase in the intensity of North Atlantic synoptic-scale eddies. This is followed by a stronger NAO-like atmospheric response to the NAT SST anomaly since the late 1980s. Further analysis shows that changes in the spatial structure of the spring NAO may also partly contribute to changes in the spring NAO–NAT SST connection around the late 1980s. In particular, spring NAO-related atmospheric anomalies are weaker and shift northward before the late 1980s, which reduces the contribution of the NAO to a tripole SST anomaly pattern in the North Atlantic.

scholarly journals The Persistence of Winter Sea Surface Temperature in the North Atlantic

2003 ◽  
Vol 16 (9) ◽  
pp. 1364-1377 ◽  
Author(s):  
Gaëlle de Coëtlogon ◽  
Claude Frankignoul
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Mixed Layer ◽  
North Atlantic ◽  
Sea Surface ◽  
The North ◽  
Sst Anomaly ◽  
The North Atlantic ◽  
The Impact ◽  
Sst Anomalies

Abstract The impact of the seasonal variations of the mixed-layer depth on the persistence of sea surface temperature (SST) anomalies is studied in the North Atlantic, using observations. A significant recurrence of winter SST anomalies during the following winter occurs in most of the basin, but not in the subtropical area of strong subduction. When taking reemergence into account, the e-folding timescale of winter SST anomalies generally exceeds 1 yr, and is about 16 months for the dominant SST anomaly tripole. The influence of advection by the mean oceanic currents is investigated by allowing for a displacement of the maximum recurrent correlation and, alternatively, by considering the SST anomaly evolution along realistic mean displacement paths. Taking into account the nonlocality of the reemergence generally increases the wintertime persistence, most notably in the northern part of the domain. The passive response of the mixed layer to the atmospheric forcing thus has a red spectrum down to near-decadal frequencies.

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