Low Seesaw Research Articles

Influence of the Tibetan Plateau on the coupling of the North Pacific–North Atlantic pressure systems

The Tibetan Plateau (TP) exerts a profound influence on global climate change. In the boreal winter, the TP has a notable impact on the Asian winter monsoon. Nevertheless, the relationship between the TP and the climate of the remote Arctic region is not yet fully understood. The North Pacific and North Atlantic pressure systems exhibit dynamic linkages through the Aleutian Low–Icelandic Low seesaw in the boreal winter season. However, the extent to which large mountains, such as those in the TP, influence this coupling remains unclear. In this study, we conducted an orographically sensitive experiment using the Whole Atmosphere Community Climate Model Version 6. The results obtained from the model indicate that the TP influences Arctic pressure anomalies specifically via the modulation of the Aleutian Low–Icelandic Low seesaw. Specifically, the TP favors the eastward extension of a pattern resembling the Pacific–North American pattern, achieved by enhancing the horizontally propagating wave trains originating from the eastern Pacific, thus establishing a clear Aleutian Low–Icelandic Low seesaw. In addition, the TP was a critical factor in the stratosphere–troposphere coupling as it facilitated the shift of the stratospheric polar vortex towards Eurasia in a poleward direction, potentially leading to the extension of the wave train emanating from the North Pacific across North America. These results suggest that the TP significantly contributes to Arctic climate variability. This influence is observed via the TP-mediated modulation of the teleconnection between the North Pacific and North Atlantic regions and stratosphere–troposphere coupling.

Low Seesaw Scale in the Grimus–Neufeld Model

Low Seesaw Scale in the Grimus–Neufeld Model

Modulation of the Aleutian–Icelandic low seesaw and its surface impacts by the Atlantic Multidecadal Oscillation

Early studies suggested that the Aleutian–Icelandic low seesaw (AIS) features multidecadal variation. In this study, the multidecadal modulation of the AIS and associated surface climate by the Atlantic Multidecadal Oscillation (AMO) during late winter (February–March) is explored with observational data. It is shown that, in the cold phase of the AMO (AMO|−), a clear AIS is established, while this is not the case in the warm phase of the AMO (AMO|+). The surface climate over Eurasia is significantly influenced by the AMO’s modulation of the Aleutian low (AL). For example, the weak AL in AMO|− displays warmer surface temperatures over the entire Far East and along the Russian Arctic coast and into Northern Europe, but only over the Russian Far East in AMO|+. Similarly, precipitation decreases over central Europe with the weak AL in AMO|−, but decreases over northern Europe and increases over southern Europe in AMO|+. The mechanism underlying the influence of AMO|− on the AIS can be described as follows: AMO|− weakens the upward component of the Eliassen–Palm flux along the polar waveguide by reducing atmospheric blocking occurrence over the Euro–Atlantic sector, and hence drives an enhanced stratospheric polar vortex. With the intensified polar night jet, the wave trains originating over the central North Pacific can propagate horizontally through North America and extend into the North Atlantic, favoring an eastward-extended Pacific–North America–Atlantic pattern, and resulting in a significant AIS at the surface during late winter.

The spring relationship between the Pacific-North American pattern and the North Atlantic Oscillation

The Pacific-North American pattern (PNA) and North Atlantic Oscillation (NAO) are two dominant modes of low-frequency variability in the Northern Hemisphere winter. Generally these two patterns are separable and uncorrelated in both space and time. However, dating back to the 1970s there have been studies which have linked the Aleutian and Icelandic low intensities, and shown that a significant temporal correlation does, on occasion, occur. Each of these semi-permanent low pressure systems is a part of the PNA and NAO patterns, respectively, and therefore, given a link between the two semi-permanent low pressure systems, we explore whether a link between the PNA and NAO also appears. Recently studies have found such a link during the winters of certain decades. The present study, documents an observed relationship between the PNA and NAO which is consistently present during the spring and early summer. This relationship is shown to be due to the fact that the PNA and NAO are spatially overlapping projections of the same pattern of variability, given by the first EOF of 500 hPa geopotential height. Both the PNA and NAO patterns correlate more strongly to the first EOF’s spatial pattern, resembling the Aleutian low-Icelandic low seesaw pattern, than the rotated EOF (REOF) loading patterns used to compute their respective indices. Furthermore, when the two patterns are correlated, the effect of El-Nino on the Northern Hemisphere is separate from the pattern associated with the PNA. Finally, we examine a 21-year period in the winter for which the PNA and NAO are significantly correlated, and find that the conclusions drawn from the spring hold true. The results presented herein demonstrate that defining the PNA and NAO based upon typical loading patterns may not be the ideal approach; especially given that the patterns are spatially similar and covary in time.

Decadal Variation of the Aleutian Low-Icelandic Low Seesaw Simulated by a Climate System Model (CAS-ESM-C)

AbstractBased on a simulation using a newly developed climate system model (Chinese Academy of Sciences-Earth System Model-Climate system component, CAS-ESM-C), the author investigated the Aleutian Low-Icelandic Low Seesaw (AIS) and its decadal variation. Results showed that the CAS-ESM-C can reasonably reproduce not only the spatial distribution of the climatology of sea level pressure (SLP) in winter, but also the AIS and its decadal variation. The period 496–535 of the integration by this model was divided into two sub-periods: 496–515 (PI) and 516–535 (P2) to further investigate the decadal weakening of the AIS. It was shown that this decadal variation of the AIS is mainly due to the phase transition of the Pacific Decadal Oscillation (PDO), from its positive phase to its negative phase. This transition of the PDO causes the sea surface temperature (SST) in the equatorial eastern (northern) Pacific to cool (warm), resulting in the decadal weakening of mid-latitude westerlies over the North Pacific and...

Multi-decadal modulations in the Aleutian-Icelandic Low seesaw and the axial symmetry of the Arctic Oscillation signature, as revealed in the 20th century reanalysis

Seesaw relationship in intensity between the surface Aleutian and Icelandic Lows (AIS) is a manifestation of atmospheric teleconnection that bridges the interannual variability over the Pacific and Atlantic in particular winter months. Analysis of the 20th Century Reanalysis data reveals that the strength and timing of AIS have undergone multi-decadal modulations in conjunction with those in structure of the Arctic Oscillation (AO) signature, extracted in the leading mode of interannual sea-level pressure (SLP) variability over the extratropical Northern Hemisphere. Specifically, events of what may be called ‘pure AO’, in which SLP anomalies exhibit a high degree of axial symmetry in association with in-phase SLP variability between the midlatitude Atlantic and Pacific, tended to occur during multi-decadal periods in which the inter-basin teleconnection through AIS was active under the enhanced interannual variability of the Aleutian Low. In contrast, the axial symmetry of the AO pattern was apparently reduced during a multi-decadal period in which the AIS teleconnection was inactive under the weakened interannual variability of the Aleutian Low. In this period, the leading mode of interannual SLP variability represented a meridional seesaw between the Atlantic and Arctic, which resembles SLP anomaly pattern associated with the cold-ocean/warm-land (COWL) temperature pattern. These multi-decadal modulations in interannual AIS signal and the axial symmetry of the interannual AO pattern occurred under multi-decadal changes in the background state that also represented the polarity changes of the COWL-like anomaly pattern.

Decadal Variation of the Aleutian Low-Icelandic Low Seesaw Simulated by a Climate System Model (CAS-ESM-C)

Decadal Variation of the Aleutian Low-Icelandic Low Seesaw Simulated by a Climate System Model (CAS-ESM-C)

Mechanism of the wintertime Aleutian Low–Icelandic Low seesaw

The driving mechanism for the wintertime (December–March) Aleutian Low–Icelandic Low (AL‐IL) seesaw is investigated with National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data for 1948–2009. It is shown that the AL and the IL are dynamically linked through the eastern Pacific wave train (EPW) and that both the EPWs and the stratospheric polar vortex are found to work cooperatively to produce a significant AL‐IL seesaw. In general, it is found that wave reflection by the polar vortex is crucial for the formation of the AL‐IL seesaw. However, when the EPWs are extremely strong, the AL‐IL seesaw appears to be caused primarily by horizontal wave propagation. It is further shown that the Pacific center of the traditional Arctic Oscillation pattern is present when the AL‐IL seesaw is active, but it disappears when the AL‐IL seesaw is absent.

Cascade seesaw for tiny neutrino mass

The accessibility to physics responsible for tiny neutrino mass suggests that the mass should better originate from certain higher dimensional operators. The conventional three types of seesaw operate at dimension five with the help of either a new fermion or scalar multiplet. Here we propose a seesaw that generates neutrino mass through a dimension-(5+4n) operator. The seesaw is functioned by a fermion of isospin n+1 and zero hypercharge and a sequence of scalar multiplets that share unity hypercharge but have isospin from 3/2 to n+1/2 at a step of unity. Only the scalar of the highest isospin can couple to the relevant fermions while only the scalar of the lowest isospin can directly develop a naturally small vacuum expectation value (VEV). The VEV is then transmitted to scalars of higher isospin through a cascading process. No global symmetry is required to forbid lower dimensional operators. A neutrino mass of desired order can thus be induced with a relatively low seesaw scale without demanding too small couplings.

Reconciling leptogenesis with observable μ → eγ rates

We perform a detailed analysis of thermal leptogenesis in the framework of seesaw models which approximately conserve lepton number. These models are known to allow for large Yukawa couplings and a low seesaw scale in agreement with neutrino mass constraints, and hence to lead to large lepton flavour violating rates that can be probed experimentally. Although large Yukawa couplings lead to (inverse) decay rates much larger than the Hubble expansion rate, we show that the leptogenesis washout induced is generically small if the mass splitting between the right-handed neutrinos is small enough. As a result, large lepton flavour violating rates are compatible with successful leptogenesis. We emphasize that this scenario does not require any particular flavour structure. A small splitting is natural and radiatively stable in this context because it is protected by the lepton number symmetry.

Role of Eurasian snow cover in wintertime circulation: Decadal simulations forced with satellite observations

We investigate the impact of the Eurasian snow cover extent on the Northern Hemisphere winter circulation by performing a suite of ensemble simulations with the Météo‐France “Arpege Climat” atmospheric general circulation model, spanning 2 decades (1979–2000). Observed snow cover derived from satellite infrared and visible imagery has been forced weekly into the model. Variability in autumn‐early winter snow cover extent over eastern Eurasia is linked to circulation anomalies over the North Pacific that are influencing the North Atlantic sector in late winter through the development of the Aleutian‐Icelandic Low Seesaw teleconnection. The forcing of realistic snow cover in the model augments potential predictability over eastern Eurasia and the North Pacific and improves the hindcast skill score of the Aleutian‐Icelandic Low Seesaw teleconnection. Enhanced eastern Eurasia snow cover is associated with an anomalous upper‐tropospheric wave train across Eurasia, anomalously high upward wave activity flux, and a displaced stratospheric polar vortex.

Observed decadal tropical Pacific–North Atlantic teleconnections

Observations of the last century are considered to investigate the decadal scale Pacific/North Atlantic teleconnections. By using wavelet analysis we find a significant low‐frequency coherency between major indices of climate variability. In particular, at periods between 10 and 20 yr, the North Atlantic Oscillation (NAO) tends to be out of phase with the El Niño/Southern Oscillation (ENSO) (strong Icelandic low during La Niña) and the Pacific Decadal Oscillation (PDO), but in phase with the North Pacific Index (NPI). Hence, the Icelandic low is strong during La Niña, but the Aleutian low is weak. The band‐pass SST pattern shows a close relationship between the “decadal‐ENSO” mode in the Pacific and the sea surface temperature anomaly tripole in the Atlantic, consistent with the low‐frequency global teleconnections. The spatial sea level patterns corresponding to the band‐pass filtered indices suggest that the Aleutian‐Icelandic Low seesaw is a main interbasin link on decadal time scale, consistent with the larger coherency of the NPI with the NAO than with ENSO.

Light Higgs mass bound in supersymmetric left-right models

We show that in supersymmetric left-right models (SUSYLR), the upper bound on the lightest neutral Higgs mass can be appreciably higher than that in the minimal supersymmetric standard model (MSSM). The exact magnitude of the bound depends on the scale of parity restoration and can be 10--20 GeV above the MSSM bound if the mass of the right-handed ${W}_{R}$ is in the TeV range. An important implication of our result is that since SUSYLR models provide a simple realization of the seesaw mechanism for neutrino masses, measurement of the Higgs boson mass could provide an independent probe of a low seesaw scale.

Influence of the Aleutian-Icelandic Low Seesaw and ENSO onto the Stratosphere in Ensemble Winter Hindcasts

Using an ensemble of wintertime hindcasts with a high-resolution (T106L60) Atmospheric General Circulation Model (AGCM) forced by observed sea surface temperatures (SSTs) and extending into the stratosphere, we investigate the formation and lifecycle of the Aleutian-Icelandic low Seesaw (AIS) during the 1978 to 1993 period. The AIS has been newly proposed to be an important mode of variability, linking the major wintertime surface lows, the Icelandic Low and the Aleutian Low, in late winter, and thereby linking climate variability over the North Pacific and the North Atlantic. We demonstrate for the first time with a stratosphere-troposphere model, that a coherent, ensemble-mean AIS extension into the stratosphere exists, where its presence modulates ultra-long planetary wave propagation and the polar night jet intensity. The model AIS peaks in February, when the Aleutian and Icelandic Low anti-correlation maximizes at -0.59. The AIS provides a new way to describe the El Niño-Southern Oscillation (ENSO) phenomenon influence into the stratosphere. For example, El-Nino conditions correspond to a deeper than normal Aleutian Low, extending its influence into the Icelandic sector as an AIS negative phase (weakened Icelandic Low), hence enhanced planetary wave vertical propagation and a weakened stratospheric polar vortex. This maturation of the AIS in late winter explains the intra-seasonal variability of the stratospheric response to ENSO, which peaks in late winter. Internal model variability is large and enhanced potential predictability is found primarily in the western hemisphere, with a western Atlantic maxima being more pronounced in the stratosphere than in the upper troposphere.

μ−τsymmetry and radiatively generated leptogenesis

We consider a $\ensuremath{\mu}\ensuremath{-}\ensuremath{\tau}$ symmetry in neutrino sectors realized at the GUT scale in the context of a seesaw model. In our scenario, the exact $\ensuremath{\mu}\ensuremath{-}\ensuremath{\tau}$ symmetry realized in the basis where the charged lepton and heavy Majorana neutrino mass matrices are diagonal leads to vanishing lepton asymmetries. We find that, in the minimal supersymmetric extension of the seesaw model with large $\mathrm{tan}\ensuremath{\beta}$, the renormalization group (RG) evolution from the GUT scale to seesaw scale can induce a successful leptogenesis even without introducing any symmetry breaking terms by hand, whereas such RG effects lead to tiny deviations of ${\ensuremath{\theta}}_{23}$ and ${\ensuremath{\theta}}_{13}$ from $\ensuremath{\pi}/4$ and zero, respectively. It is shown that the right amount of the baryon asymmetry ${\ensuremath{\eta}}_{B}$ can be achieved via so-called resonant leptogenesis, which can be realized at rather low seesaw scale with large $\mathrm{tan}\ensuremath{\beta}$ in our scenario so that the well-known gravitino problem is safely avoided.

Impacts of the Aleutian–Icelandic Low Seesaw on Surface Climate during the Twentieth Century

Abstract An interannual seesaw between the intensities of the Icelandic and Aleutian lows and its impact on surface climate observed during the twentieth century are investigated. In a recent period from the late 1960s to the early 1990s, their seesaw relationship was particularly apparent in late winter. The associated anomalies in surface air temperature were significant in many regions over the extratropical Northern Hemisphere except in central portions of the continents. The seesaw also modified the ocean–atmosphere exchange of heat and moisture extensively over the North Atlantic and North Pacific by changing evaporation and precipitation. Since the seesaw formation was triggered by eastward propagation of stationary Rossby wave trains from the North Pacific into the North Atlantic, anomalous circulation over the North Pacific in January was identified as a good precursor for February surface air temperatures in the Euro–Atlantic sector during that period. The seesaw relationship between the two lows underwent multidecadal modulations during the twentieth century. It was weak in the mid-1950s through the mid-1960s, while it was particularly strong during the preceding period from the 1920s to the 1940s with its impact on surface temperatures as extensive as in the recent period. Although it reached maturity in January, the precursory signal of the seesaw in that early period was also found in the North Pacific one month earlier, which suggests that the formation was through essentially the same mechanisms as in the recent period.

Formation, Mechanisms, and Predictability of the Aleutian–Icelandic Low Seesaw in Ensemble AGCM Simulations

Abstract The potential predictability associated with the remote influence of midlatitude tropospheric anomalies over the North Pacific or the North Atlantic, via a seesawlike interannual oscillation between the surface Aleutian and Icelandic lows (AL and IL, respectively) is investigated. Data from a 24-member ensemble of 50-yr atmospheric general circulation model simulation forced with observed sea surface temperature (SST) conditions are analyzed by separating the total simulated fluctuations into the external component forced by the prescribed SST and the internal component generated by atmospheric internal dynamics. The AL–IL seesaw can be identified in both the external and internal components of the variability. In the external variability, determined through the ensemble mean, the seesaw is gradually formed from December to March through the development of a Pacific–North American (PNA) pattern–like wave train, remotely forced by the El Niño–Southern Oscillation. The amplitudes of the externally forced North Atlantic anomalies are only about half as large as the North Pacific anomalies. The potential predictability of the Atlantic anomalies, defined as the ratio of the SST-forced variance to the total variance, does not exceed the 20% level. In the internal component of the variability, determined from the deviations of each ensemble member from the ensemble mean, the negative correlation between the AL and IL anomalies is modest but persistent through winter. It is confirmed that, regardless of the polarity of the AL–IL seesaw, the IL anomalies are formed through eastward wave activity propagation of the stationary Rossby wave train emanating from the AL region in the form of what may be called a “PNAA pattern,” the extension of the PNA-like wave train into the Atlantic. Thus, the midwinter development of North Pacific anomalies is found to be a necessary, though not sufficient, condition for the seesaw formation. The persistence of the North Pacific anomalies beyond a 1-month time span appears to augment the probability of the seesaw formation by sustaining eastward wave activity propagation to the North Atlantic.