The role of the Pacific Decadal Oscillation and ocean-atmosphere interactions in driving United States heatwaves

2021 ◽  
Author(s):  
Sem Vijverberg ◽  
Dim Coumou
Keyword(s):  
Rossby Wave ◽  
Rossby Waves ◽  
Low Frequency ◽  
Causal Link ◽  
Causal Mechanism ◽  
Temperature Forecast ◽  
The Pacific ◽  
Ocean Atmosphere ◽  
The Waves

<p>Heatwaves can have devastating impact on society and reliable early warnings at several weeks lead time are needed. Heatwaves are often associated with quasi-stationary Rossby waves, which interact with sea surface temperature (SST). Previous studies showed that north-Pacific SST can provide long-lead predictability for eastern U.S. temperature, moderated by an atmospheric Rossby wave. The exact mechanisms, however, are not well understood. Here we analyze Rossby waves associated with heatwaves in western and eastern US. Causal inference analyses reveal that both waves are characterized by positive ocean-atmosphere feedbacks at synoptic timescales, amplifying the waves. However, this positive feedback on short timescales is not the causal mechanism that leads to a long-lead SST signal. Only the eastern US shows a long-lead causal link from SSTs to the Rossby wave. We show that the long-lead SST signal derives from low-frequency PDO variability, providing the source of eastern US temperature predictability. We use this improved physical understanding to identify more reliable long-lead predictions. When, at the onset of summer, the Pacific is in a pronounced PDO phase, the SST signal is expected to persist throughout summer. These summers are characterized by a stronger ocean-boundary forcing, thereby more than doubling the eastern US temperature forecast skill, providing a temporary window of enhanced predictability.</p>

scholarly journals Changes in the role of Pacific decadal oscillation on sea ice extent variability across the mid-1990s

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
Hyerim Kim ◽  
Sang-Wook Yeh ◽  
Soon-Il An ◽  
Se-Yong Song
Keyword(s):  
Sea Ice ◽  
Pacific Decadal Oscillation ◽  
Far East ◽  
Low Frequency ◽  
Early Summer ◽  
Sea Ice Extent ◽  
The Pacific ◽  
Before And After ◽  
Relationship Of

Abstract Characteristics of sea ice extent (SIE) have been rapidly changing in the Pacific Arctic sector (PAS) in recent years. The SIE variability in PAS during the late spring and early summer (i.e., April–May–June, AMJ) plays a key role in determining the SIE during the following fall when SIE is at a minimum. We find that the Pacific Decadal Oscillation (PDO), which is the most dominant variability of sea surface temperature (SST) on the low-frequency timescales, differently influences the SIE in PAS during AMJ before and after the mid-1990s. While a positive phase of PDO during the previous winter acts to increases SIE during AMJ before the mid-1990s, it acts to decrease SIE during AMJ after the mid-1990s. Further analysis indicates that atmospheric circulation associated with PDO differently influences the variability of SIE in the PAS during AMJ by modulating poleward moisture transport across the Alaska or the Far East Asia peninsula. This results in the change in the relationship of PDO and SIE in the PAS before and after the mid-1990s.

scholarly journals Propagation properties of Rossby waves for latitudinal β-plane variations of <I>f</I> and zonal variations of the shallow water speed

2012 ◽  
Vol 30 (5) ◽  
pp. 849-855 ◽  
Author(s):  
C. T. Duba ◽  
J. F. McKenzie
Keyword(s):  
Shallow Water ◽  
Group Velocity ◽  
Rossby Wave ◽  
Rossby Waves ◽  
Low Frequency ◽  
Wave Number ◽  
Coriolis Parameter ◽  
Propagation Properties ◽  
Wave Normal ◽  
Water Speed

Abstract. Using the shallow water equations for a rotating layer of fluid, the wave and dispersion equations for Rossby waves are developed for the cases of both the standard β-plane approximation for the latitudinal variation of the Coriolis parameter f and a zonal variation of the shallow water speed. It is well known that the wave normal diagram for the standard (mid-latitude) Rossby wave on a β-plane is a circle in wave number (ky,kx) space, whose centre is displaced −β/2 ω units along the negative kx axis, and whose radius is less than this displacement, which means that phase propagation is entirely westward. This form of anisotropy (arising from the latitudinal y variation of f), combined with the highly dispersive nature of the wave, gives rise to a group velocity diagram which permits eastward as well as westward propagation. It is shown that the group velocity diagram is an ellipse, whose centre is displaced westward, and whose major and minor axes give the maximum westward, eastward and northward (southward) group speeds as functions of the frequency and a parameter m which measures the ratio of the low frequency-long wavelength Rossby wave speed to the shallow water speed. We believe these properties of group velocity diagram have not been elucidated in this way before. We present a similar derivation of the wave normal diagram and its associated group velocity curve for the case of a zonal (x) variation of the shallow water speed, which may arise when the depth of an ocean varies zonally from a continental shelf.

scholarly journals Role of Low-Frequency Wind Variability in Inducing WWBs during the onset of super El Niños

2021 ◽  
Author(s):  
Yi-Kai Wu ◽  
Chi-Cherng Hong ◽  
Tim Li ◽  
An-Yi Huang
Keyword(s):  
El Niño ◽  
North Pacific ◽  
El Nino ◽  
Low Frequency ◽  
Wind Fields ◽  
Wind Variability ◽  
The Pacific ◽  
Sst Anomaly ◽  
Wind Bursts

Abstract In this study, the effect of multiple timescale wind fields on the westerly wind bursts (WWBs) was investigated during the onset of super (1982, 1997, and 2015) and moderate El Niño events. The results revealed that extreme WWBs during the onset of the super El Niño group were attributed to low-frequency westerly (≥90 days, LFW), medium-frequency westerly (20–90 days, MFW, or intraseasonal) and high-frequency westerly (≤10 days, HFW) components, accounting for approximately 51%, 33% and 16%, respectively. Thus, the extreme WWBs during the onset of super El Niños were primarily contributed by LFWs and MFWs. By contrast, the WWBs during the onset of moderate El Niños were determined primarily by MFWs (38%) and HFWs (35%), whereas the LFW contribution is relatively small (27%). A further analysis indicated that LFWs during the onset of the super El Niños were primarily a response to a positive SST anomaly in the tropical to eastern North Pacific resembling the Pacific Meridional Mode (PMM), which had persisted during the preceding 9–12 months in the extratropical eastern North Pacific. A significant lagged correlation between the tropical and extratropical North Pacific SST was identified, and their correlation has become stronger since the late 1980s. MFWs during the onset of the super El Niños were primarily associated with the Madden-Julian Oscillation.

scholarly journals Generation of Rossby waves off the Cape Verde Peninsula: the role of the coastline

Ocean Science ◽  
2019 ◽  
Vol 15 (6) ◽  
pp. 1667-1690 ◽  
Author(s):  
Jérôme Sirven ◽  
Juliette Mignot ◽  
Michel Crépon
Keyword(s):  
Rossby Wave ◽  
Rossby Waves ◽  
Numerical Study ◽  
Cape Verde ◽  
Water Model ◽  
Kelvin Wave ◽  
Reduced Gravity ◽  
The North ◽  
Cartesian Coordinate

Abstract. In December 2002 and January 2003 satellite observations of chlorophyll showed a strong coastal signal along the west African coast between 10 and 22∘ N. In addition, a wavelike pattern with a wavelength of about 750 km was observed from 20 December 2002 and was detectable for 1 month in the open sea, south-west of the Cape Verde Peninsula. Such a pattern suggests the existence of a locally generated Rossby wave which slowly propagated westward during this period. This hypothesis was confirmed by analysing sea surface height provided by satellite altimeter during this period. To decipher the mechanisms at play, a numerical study based on a reduced-gravity shallow-water model has first been conducted. A wind burst, broadly extending over the region where the offshore oceanic signal is observed, is applied for 5 d. A Kelvin wave quickly develops along the northern edge of the cape, then propagates and leaves the area in a few days. Simultaneously, a Rossby wave whose characteristics seem similar to the observed pattern forms and slowly propagates westward. The existence of the peninsula limits the extent of the wave to the north. The spatial extent of the wind burst determines the extent of the response and correspondingly the timescale of the phenomenon (about 100 d in the present case). When the wind burst has a large zonal and small meridional extent, the behaviour of a wave to the north of the peninsula differs from that to the south. These results are corroborated and completed by an analytical study of a linear reduced-gravity model using a non-Cartesian coordinate system. This system is introduced to evaluate the potential impact of the coastline shape. The analytical computations confirm that a period of around 100 d can be associated with the observed wave considering the value of the wavelength; they also show that the role of the coastline remains moderate at such timescales. By contrast, when the period becomes shorter (smaller than 20–30 d), the behaviour of the waves is modified because of the shape of the coast. South of the peninsula, a narrow band of sea isolated from the rest of the ocean by two critical lines appears. Its meridional extent is about 100 km and Rossby waves could propagate there towards the coast.

The Role of Air–Sea Interaction in Controlling the Optimal Perturbations of Low-Frequency Tropical Coupled Ocean–Atmosphere Modes

2003 ◽  
Vol 16 (6) ◽  
pp. 951-968 ◽  
Author(s):  
Andrew M. Moore ◽  
Jérôme Vialard ◽  
Anthony T. Weaver ◽  
David L. T. Anderson ◽  
Richard Kleeman ◽  
...  
Keyword(s):  
Low Frequency ◽  
Ocean Atmosphere

First-order resonance in the reflection of baroclinic Rossby waves

1993 ◽  
Vol 251 ◽  
pp. 515-532 ◽  
Author(s):  
Federico Graef
Keyword(s):  
Boundary Condition ◽  
Rossby Wave ◽  
Linear Growth ◽  
Rossby Waves ◽  
Transient Flow ◽  
Multiple Scales ◽  
Graphical Method ◽  
First Order ◽  
Mode Amplitude ◽  
The Waves

The nonlinear interaction between an incident and a reflected Rossby wave produces a steady flow parallel to the (non-zonal) reflecting wall and a transient flow oscillating at twice the frequency of the incident-reflected pair. If the transient forcing is resonant, i.e. a free Rossby wave, the resonant response must have zero amplitude at the wall in order to fulfil the boundary condition there; a straightforward expansion predicts a linear growth of its amplitude in the offshore direction y. Resonance is possible only if 0 < |sin α| ≤ $\frac13$, where α is the angle between the wall and the easterly direction. This requirement is met by several boundaries in the ocean. A simple graphical method to find a resonant triad is described.Using the method of multiple scales, it is shown that the wave amplitudes of the triad are slowly varying periodic functions of y, such that the energy flux of the triad through any plane parallel to the wall vanishes, as required by energy conservation. The waves participating in the resonant triad become wave packets. The three waves do not exchange energy in time due to the additional constraint on the motion imposed by the boundary condition at the wall. It is shown that the wave amplitudes cannot be slowly varying functions of v and time.As a possible oceanic application of the theoretical findings, the distance from the wall where one would expect to find large semi-annual amplitudes if annual Rossby waves are impinging on the boundary is of the order of 100 km. Motivated by similar studies (Plumb 1977; Mysak 1978), there are speculations on what would happen if three incident-reflected Rossby wave pairs (or modes) are taken, allowing each mode amplitude to be slowly varying in time.

The impact of ENSO and the Atlantic Multidecadal Variability (AMV) on the upper tropospheric large scale flow in the PRIMAVERA models.

2020 ◽  
Author(s):  
Paolo Ghinassi ◽  
Federico Fabiano ◽  
Virna L. Meccia ◽  
Susanna Corti
Keyword(s):  
Rossby Wave ◽  
Large Scale ◽  
Rossby Waves ◽  
Wave Activity ◽  
Transient Wave ◽  
Weather Regime ◽  
Weather Regimes ◽  
The Pacific ◽  
Large Scale Flow ◽  
The Impact

&lt;p&gt;Rossby waves play a fundamental role for both climate and weather. They are in fact associated with heat, momentum and moisture transport across large distances and with different types of weather at the surface. Assessing how they are represented in climate models is thus of primary importance to understand both predictability and the present and future climate. In this study we investigate how ENSO and the AMV affect the large scale flow pattern in the upper troposphere of the Northern Hemisphere, using reanalysis data and data from the PRIMAVERA simulations.&lt;/p&gt;&lt;p&gt;The upper tropospheric large scale flow is investigated in terms of the Rossby wave activity associated with persistent and recurrent patterns over the Pacific-North American and Euro-Atlantic regions during winter, the so called weather regimes. In order to quantify the vigour of Rossby wave activity associated with each weather regime we make use of a recently developed diagnostic based on Finite Amplitude Local Wave Activity in isentropic coordinates, partitioning the total wave activity into the stationary and transient components. The former is associated with quasi-stationary, planetary Rossby waves, whereas the latter is associated with synoptic scale Rossby wave packets. This allows one to quantify the contribution from stationary versus transient eddies in the total Rossby wave activity linked to each weather regime.&lt;/p&gt;&lt;p&gt;In this study we explore how ENSO and the AMV affect both the weather regimes frequencies and the upper tropospheric waviness in the Pacific and Atlantic storm tracks, respectively. Furthermore we analyse how both the stationary and transient wave activity component modulate the onset and transition between different regimes.&lt;/p&gt;

scholarly journals Generation of Rossby waves off the Cape Verde peninsula; role of the coastline

2019 ◽  
Author(s):  
Jérôme Sirven ◽  
Juliette Mignot ◽  
Michel Crépon
Keyword(s):  
Rossby Wave ◽  
Time Scale ◽  
Rossby Waves ◽  
Numerical Study ◽  
Cape Verde ◽  
Water Model ◽  
Kelvin Wave ◽  
Reduced Gravity ◽  
The North

Abstract. In December 2002 and January 2003 satellite observations of Chlorophyll showed a strong coastal signal along the west african coast between 10° and 22° N. In addition, a wavelike pattern with a wavelength of about 750 kms was observed from December 20th 2002 and was detectable for one month in the open sea, south west to the Cape Verde peninsula. Such a pattern suggests the existence of a locally generated Rossby wave which slowly propagated westward during this period. To verify this hypothesis a numerical study based on a reduced gravity shallow water model has been conducted. A wind burst, broadly extending over the region where the offshore oceanic signal is observed, is applied during 5 days. A Kelvin wave quickly develops along the northern edge of the cape, then propagates and leaves the area in a few days. Simultaneoulsly, a Rossby wave whose characterisics seem similar to the observed pattern forms and slowly propagates westward. The existence of the peninsula limits the extent of the wave to the north. The spatial extent of the wind burst determines the extent of the response and correspondingly the time scale of the phenomenon (about 100 days in the present case). When the wind burst has a large zonal and small meridional extent, the behaviour of a wave to the north of the peninsula differs from that to the south. These results are corroborated and completed by an analytical study of a linear reduced gravity model using a non-Cartesian coordinate system. This system is introduced to evaluate the potential impact of the coastline shape. The analytical computations confirm that, considering the value of the wavelength, a time scale around 100 days can be associated with the observed wave. They also show that the role of the coastline remains moderate at such time scales. On the contrary, when the period becomes shorter (smaller than 20–30 days), the behaviour of the waves is modified because of the shape of the coast. South of the peninsula, a narrow band of sea isolated from the rest of the ocean by two critical lines appears. Its meridional extent is about 100 km and Rossby waves could propagate there towards the coast.

scholarly journals Role of the Rossby Waves in the Broadening of an Eastward Jet

2012 ◽  
Vol 42 (3) ◽  
pp. 476-494 ◽  
Author(s):  
Genta Mizuta
Keyword(s):  
Potential Vorticity ◽  
Rossby Waves ◽  
Equation Model ◽  
Transient Region ◽  
Interior Flow ◽  
The Mean ◽  
The Difference ◽  
Vorticity Flux ◽  
The Waves

Abstract To investigate the effect of the Rossby waves on an eastward jet such as the Kuroshio or Gulf Stream Extensions, a series of numerical experiments is conducted using a primitive equation model. In these experiments, an inflow and an outflow imposed on the western and eastern boundaries drive an unstable narrow jet and a broad interior flow in the western and eastern regions of the model domain, respectively. The barotropic Rossby waves are radiated from the transient region between the two regions. The eddy potential vorticity flux by the waves tends to compensate for the difference in the mean potential vorticity along mean streamlines between both sides of the transient region. Instability of the jet is insufficient for this compensation and weakens the mean potential vorticity gradient too much. Moreover, as the potential vorticity of the outflow is increased, the Rossby waves are intensified in order to compensate for the increase in the difference in the mean potential vorticity. These features strongly suggest that the Rossby waves are substantial in matching a jet with an interior flow. The speed of the waves and properties of eddies in recirculations of the jet are consistent with a two-layer analytic model, which indicates that the Rossby waves are radiated from eddies in recirculations. These eddies as well as the Rossby waves increase in amplitude with the transport of the recirculation near the surface presumably because of mean advection. Therefore, the mean potential vorticity of the interior flow, the intensity of the Rossby waves, and the transport of the recirculation change consistently with one another.

scholarly journals Do High-Frequency Eddies Contribute to Low-Frequency Teleconnection Tendencies?*

2010 ◽  
Vol 67 (2) ◽  
pp. 419-433 ◽  
Author(s):  
Panos J. Athanasiadis ◽  
Maarten H. P. Ambaum
Keyword(s):  
Low Frequency ◽  
Rate Of Change ◽  
The North ◽  
Budget Analysis ◽  
The Pacific ◽  
Isentropic Potential Vorticity ◽  
Eddy Transport ◽  
Eddy Fluxes ◽  
The North Atlantic Oscillation

Abstract An isentropic potential vorticity (PV) budget analysis is employed to examine the role of synoptic transients, advection, and nonconservative processes as forcings for the evolution of the low-frequency PV anomalies locally and those associated with the North Atlantic Oscillation (NAO) and the Pacific–North American (PNA) pattern. Specifically, the rate of change of the low-frequency PV is expressed as a sum of tendencies due to divergence of eddy transport, advection by the low-frequency flow (hereafter referred to as advection), and the residual nonconservative processes. The balance between the variances and covariances of these terms is illustrated using a novel vector representation. It is shown that for most locations, as well as for the PNA pattern, the PV variability is dominantly driven by advection. The eddy forcing explains a small amount of the tendency variance. For the NAO, the role of synoptic eddy fluxes is found to be stronger, explaining on average 15% of the NAO tendency variance. Previous studies have not assessed quantitively how the various forcings balance the tendency. Thus, such studies may have overestimated the role of eddy fluxes for the evolution of teleconnections by examining, for example, composites and regressions that indicate maintenance, rather than evolution driven by the eddies. The authors confirm this contrasting view by showing that during persistent blocking (negative NAO) episodes the eddy driving is relatively stronger.

Sign in / Sign up

Export Citation Format

Share Document