extratropical forcing
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2021 ◽  
Author(s):  
Moetasim Ashfaq ◽  
Shahid Mehmood ◽  
Sarah Kapnick ◽  
Subimal Ghosh ◽  
Muhammad Adnan Abid ◽  
...  

Abstract A robust understanding of the sub-seasonal cold season (November–March) precipitation variability over the High Mountains of Asia (HMA) is currently lacking. Here, we identify dynamic and thermodynamic pathways through which natural modes of climate variability establish their teleconnections over the HMA. First, we identify evaporative sources that contribute to the cold season precipitation over the HMA and surroundings areas. The predominant moisture contribution comes from the mid-latitude regions including Mediterranean/Caspian Seas and Mediterranean land. Second, we establish that several tropical and extratropical forcing display a sub-seasonally fluctuating influence on the cold season precipitation distribution over the region, and given that many of them varyingly interact with each other, their impacts cannot be explained exclusively or at seasonal timescales. Lastly, a single set of evaporative sources cannot always be identified as the only determinant in propagating a remote teleconnection, because nature of moisture anomalies and its sources depend on the pattern of sub-seasonally varying dynamical forcing in the atmosphere.


Author(s):  
Yuya Hamaguchi ◽  
Yukari N. Takayabu

AbstractIn this study, the statistical relationship between tropical upper-tropospheric troughs (TUTTs) and the initiation of summertime tropical-depression type disturbances (TDDs) over the western and central North Pacific is investigated. By applying a spatiotemporal filter to the 34-year record of brightness temperature and using JRA-55 reanalysis products, TDD-event initiations are detected and classified as trough-related (TR) or non-trough-related (non-TR). The conventional understanding is that TDDs originate primarily in the lower-troposphere; our results refine this view by revealing that approximately 30% of TDDs in the 10°N-20°N latitude ranges are generated under the influence of TUTTs. Lead-lag composite analysis of both TR- and non-TR-TDDs clarifies that TR-TDDs occur under relatively dry and less convergent large-scale conditions in the lower-troposphere. This result suggests that TR-TDDs can form in a relatively unfavorable low-level environment. The three-dimensional structure of the wave activity flux reveals southward and downward propagation of wave energy in the upper troposphere that converges at the mid-troposphere around the region where TR-TDDs occur, suggesting the existence of extratropical forcing. Further, the role of dynamic forcing associated with the TUTT on the TR-TDD-initiation is analyzed using the quasi-geostrophic omega equation. The result reveals that moistening in the mid-to-upper troposphere takes place in association with the sustained dynamical ascent at the southeast side of the TUTT, which precedes the occurrence of deep convective heating. Along with a higher convective available potential energy due to the destabilizing effect of TUTTs, the moistening in the mid-to-upper troposphere also helps to prepare the environment favorable to TDDs initiation.


2021 ◽  
pp. 1-34
Author(s):  
Soumi Chakravorty ◽  
Renellys C. Perez ◽  
Bruce T. Anderson ◽  
Sarah M. Larson ◽  
Benjamin S. Giese ◽  
...  

AbstractThe El Niño/Southern Oscillation (ENSO) has been recently linked with extratropical-Pacific atmospheric variability. The two key mechanisms connecting the atmospheric variability of extratropical-Pacific with ENSO are the heat-flux driven “seasonal footprinting mechanism” (SFM) and the ocean-dynamics driven “trade wind charging” (TWC) mechanism. However, their relative contributions to ENSO are still unknown. Here we present modeling evidence that the positive phase of the SFM generates a weaker, short-lived central Pacific El Niño-like warming pattern in the fall, whereas the TWC positive phase leads to a wintertime eastern Pacific El Niño-like warming. When both mechanisms are active, a strong, persistent El Niño develops. While both mechanisms can trigger equatorial wind anomalies that generate an El Niño, the strength and persistence of the warming depends on the subsurface heat content buildup by the TWC mechanism. These results suggest that while dynamical-coupling associated with extratropical forcing is crucial to maintain an El Niño, thermodynamical-coupling is an extratropical source of El Niño diversity.


2021 ◽  
Author(s):  
Ying Lung Liu ◽  
Chi-Yung Tam ◽  
Andie Yee Man Au-Yeung

Abstract The role of extratropical forcing on the summertime tropical synoptic-scale disturbances (TSDs) in the western north Pacific has been investigated, by conducting parallel integrations of the Regional Climate Model (RegCM). The suite of experiments consists of a control run (CTRL) with European Centre for Medium Range Forecasts (ECMWF) Reanalysis data as boundary conditions, and an experimental run (EXPT) with the same setting, except that signals with zonal wavenumber > 6 were suppressed at the northern boundary (located at 42°N) of the model domain. Comparison between CTRL and EXPT showed that, without extratropical forcing, there is weaker TSD activity in the June-to-August season, with reduced precipitation over the TSD pathway. Associated with suppressed TSD, southeastward-directed wave activity is also reduced, leading to less active mixed Rossby gravity (MRG) waves in the equatorial western Pacific area. Further analysis revealed that extratropical forcing and associated circulation changes can modulate the TSD wavetrain and its coherence structure, in relation to low-level vorticity in far western north Pacific. In CTRL, west of about 140°E, TSD-related circulation tends to be stronger; in EXPT, vorticity signals and vertical motions are found to be slightly more coherent in the more eastern portion of the TSD wavetrain. The latter enhanced coherency of TSD east of 140°E, from the EXPT simulations, might be due to changes in wave activity transport channelled by modulated upper-level mid-latitude westerlies in EXPT. Energetics indicate that changes in low-level background circulation itself can also influence TSD characteristics. Our results serve to quantify how extratropical forcing and related general circulation features influence western north Pacific summertime TSD activities. Implications on understanding the initiation of TSD, as well as their variability on longer time scales, are discussed.


2021 ◽  
pp. 1-53
Author(s):  
Yechul Shin ◽  
Sarah M. Kang ◽  
Ken Takahashi ◽  
Malte F. Stuecker ◽  
Yen-Ting Hwang ◽  
...  

AbstractThis study examines the temporal evolution of the extratropically forced tropical response in an idealized aquaplanet model under equinox condition. We apply a surface thermal forcing in the northern extratropics that oscillates periodically in time. It is shown that tropical precipitation is unaltered by sufficiently high-frequency extratropical forcing. This sensitivity to the extratropical forcing periodicity arises from the critical time required for sea surface temperature (SST) adjustment. Low-frequency extratropical forcing grants sufficient time for atmospheric transient eddies to diffuse moist static energy to perturb the mid-latitude SSTs outside the forcing region, as demonstrated by a one-dimensional energy balance model with a fixed diffusivity. As the transient eddies weaken in the subtropics, a further equatorward advection is accomplished by the Hadley circulation. The essential role of Hadley cell advection in connecting the subtropical signal to the equatorial region is supported by an idealized thermodynamical-advective model. Associated with the SST changes in the tropics is a meridional shift of the Intertropical Convergence Zone. Since the time needed for SST adjustment increases with increasing mixed layer depth, the critical forcing period at which the extratropical forcing can affect the tropics scales linearly with the mixed layer depth. Our results highlight the important role of decadal-and-longer extratropical climate variability in shaping the tropical climate system. We also raise the possibility that the transient behavior of a tropical response forced by extratropical variability may be strongly dependent on cloud radiative effects.


2020 ◽  
pp. 1-55
Author(s):  
Yen-Ting Hwang ◽  
Po-Chun Chung

AbstractThis study explores the seasonal sensitivity of tropical circulation responses to an idealized extratropical thermal forcing using Community Atmosphere Model version 5 coupled to a slab ocean. The thermal heating over the Southern Ocean is held constant, and the tropical responses in each month of the year are investigated. An anomalous cross-equatorial cell and a southward tropical rain belt shift occur every month. The anomalous cross-equatorial cell has a strong influence on the strengths of the Hadley cell and the subtropical jet in the winter hemisphere; in contrast, it has nearly no impact on the Hadley cell and the subtropical jet strengths in the summer hemisphere.The seasonal variation of the anomalous cross-equatorial cell is small (<∼ 30% of the annual mean change), and could be understood via the energetic and the sea surface temperature gradient perspectives. Both perspectives point to the seasonality of the anomalous ocean heat uptake within the deep tropics as the key factor explaining the weak seasonality of the anomalous cross-equatorial cell. We propose a hypothesis explaining about 75% of this seasonal variation via the climatological position of the ITCZ relative to the anomalous cross-equatorial cell.The results suggest a modest seasonality in tropical precipitation and circulation responses to extratropical forcing. Also, such seasonality may be partly predicted by the climatological seasonal cycle of the tropical circulations.


2019 ◽  
Vol 32 (8) ◽  
pp. 2329-2348 ◽  
Author(s):  
Bui Minh Tuan

Abstract An EOF analysis is applied to high-resolution Vietnam Gridded Precipitation anomalies to support the notion that the characteristics of intraseasonal oscillation (ISO) of rainfall in Vietnam are distinct from location to location and highly affected by topography. Power spectral analysis reveals that the ISO of rainfall in Vietnam is dominated by submonthly-scale ISO (SISO), which is most active in September–October. The rainfall SISO shows remarkable relationships with heavy rainfall days in the Red River Delta and Mid-Central and Central Highlands but relatively weak correlations with heavy rainfall days in the Northeast and Southern Plain. A composite technique applied to filtered OLR and ERA-Interim shows that the first four principal components (PCs) of the rainfall SISO involve four different processes that closely relate to extratropical systems. The rainfall SISO in the PC1 is governed by interaction between the pressure surge induced by the submonthly amplifications of the Siberian high and tropical depressions (TDs). Rainfall SISO in PC2 is modulated by the convergence of the southward excursion of the polar air mass and TD-type waves. Rainfall SISO in PC3 is generated by the quasigeostrophic lifting of the extratropical wave train associated with TD-type waves. The effect of upstream development of the wave train from the North Pacific and TD-type wave is the key process inducing the rainfall SISO in PC4.


Atmosphere ◽  
2018 ◽  
Vol 9 (11) ◽  
pp. 431 ◽  
Author(s):  
Flávia Rosso ◽  
Nathalie Boiaski ◽  
Simone Ferraz ◽  
Tiago Robles

The South Atlantic convergence zone (SACZ) is the main summer-typical atmospheric phenomenon occurring in South America, and it is of great interest because it regulates the rainy season in the most populated regions of Brazil. Frequency variability, persistence, and geographical position of the SACZ and its relationship with intraseasonal variability is well described in the literature. However, the influence of extratropical forcing on the SACZ is not well understood. Consequently, the aim of this study is to evaluate the role of the Antarctic Oscillation (AAO) in SACZ events. The persistence and frequencies of SACZ events, mean, standard deviation of total precipitation per event, lag composite of daily precipitation and geopotential height anomalies were obtained for each phase of the AAO. Therefore, frequency, persistence and total precipitation of SACZ events were higher in positive AAO (AAO+) than negative AAO (AAO−). A teleconnection mechanism between the extratropics and the SACZ region is evident in AAO+, through intensification of the polar and subtropical jets, in the days preceding SACZ. The same was not observed in the AAO−, where the anomalies were confined in the subtropical region and displaced to the South Atlantic Ocean.


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