Posted in Articles, Mesoscale Eddies, VOCALS

Using Lagrangian Coherent Structures with atmospheric data

 

I recently read this article on the use of Lagrangian coherent structures to inform on turbulence in the atmosphere near an airport.

Accurate extraction of Lagrangian coherent structures over finite domains with application to flight data analysis over Hong Kong International Airport.

W Tang and PW Chan.

Traditional approaches in the extraction of Lagrangian coherent structures (LCS) using finite-time Lyapunov exponents (FTLE) stop trajectories at the domain boundaries when data are only available in a limited region. This turns the domain boundaries into attractors and deteriorates the quality of the extraction. Direct application of this approach to practical problems is troublesome as spurious ridges appear and true ridges are suppressed, causing errors in the evaluation of a nonlinear flow field.

In this paper, they develop a new method that deals with this difficulty by extending the flow field into a linear global flow that best matches the data in the finite do- main, and allows trajectories to continue to separate once they cross the domain boundaries. With the new scheme trajectory, separations previously stopped prematurely are now recovered with the separation rate locked at their exit values and the artificial attractors are removed. Continue reading “Using Lagrangian Coherent Structures with atmospheric data”

Advertisements
Posted in MJO

Reorganization of Tropical Climate during El Niño

 

Benjamin R Lintner and John C H Chiang.

The applicability of a weak temperature gradient (WTG) formulation for the reorganization of tropical climate during El Niño–Southern Oscillation (ENSO) events is investigated. This idealized dynamical framework solves for the divergent portion of the tropical circulation by assuming a spatially homogeneous perturbation temperature profile and a mass balance constraint applied over the tropical belt. An inter- mediate-level complexity model [the Quasi-Equilibrium Tropical Circulation Model (QTCM)] configured with the WTG assumptions is used to simulate El Niño conditions and is found to yield an appropriate level of tropospheric warming, a plausible pattern of precipitation anomalies in the tropical Pacific source region of El Niño, and a gross precipitation deficit over the Tropics outside the Pacific (hereafter the “remote Tropics”). Continue reading “Reorganization of Tropical Climate during El Niño”

Posted in Articles, MJO

Madden, R. A. and P. R. Julian – Historical perspective of the MJO

Introduction

  • Quasibiennial Oscillation was “discovered” or described first in the in 1963 as steady features of the equatorial stratosphere by Ebdon(1963).
  • Yanai and Maruyama (1966) – wave like disturbances in the meridional wind, v, in tropical lower stratosphere with a timescale of five days, length scale of 10,000 km and westward phase speed of about 23 ms-1.
  • It was thought that such large-scale waves in the equatorial stratosphere was necessary for the momentum convergence needed to explain the QBO.
  • Matsuno (1966) published theoretical derivations for equatorial waves. Maruyama (1967) published results showing that the meridional signal was a mixed-Rossby gravity wave.
  • Wallace and Kousky (1968) found synoptic-scale 15-day period waves with length scales of 20,000-40,000 km (zonal wavenumber 2 and 1) and 6-10 km vertical scales. These were identified as Kelvin waves as predicted by Matsuno’s theory. Continue reading “Madden, R. A. and P. R. Julian – Historical perspective of the MJO”
Posted in Articles, MJO

Maloney, E. “The moist static energy budget of a composite tropical intraseasonal oscillation in a climate model.” Journal of Climate (2009).

The intraseasonal moist static energy (MSE) budget is analyzed in a climate model that produces realistic eastward-propagating tropical intraseasonal wind and precipitation variability. Consistent with the recharge– discharge paradigm for tropical intraseasonal variability, a buildup of column-integrated MSE occurs within low-level easterly anomalies in advance of intraseasonal precipitation, and a discharge of MSE occurs during and after precipitation when westerly anomalies occur. The strongest MSE anomalies peak in the lower troposphere and are, primarily, regulated by specific humidity anomalies. Continue reading “Maloney, E. “The moist static energy budget of a composite tropical intraseasonal oscillation in a climate model.” Journal of Climate (2009).”

Posted in MJO

Annamalai et al. Impact of Indian Ocean sea surface temperature on developing El Niño. J Climate (2005) vol. 18 (2) pp. 302-319

Impact of Indian Ocean sea surface temperature on developing El Niño.

H Annamalai, SP Xie, JP McCreary, and R Murtugudde.

        Prior to the 1976–77 climate shift (1950–76), sea surface temperature (SST) anomalies in the tropical Indian Ocean consisted of a basinwide warming during boreal fall of the developing phase of most El Niños, whereas after the shift (1977–99) they had an east–west asymmetry—a consequence of El Niño being associated with the Indian Ocean Dipole/Zonal mode. In this study, the possible impact of these contrasting SST patterns on the ongoing El Niño is investigated, using atmospheric reanalysis products and solutions to both an atmospheric general circulation model (AGCM) and a simple atmospheric model (LBM), with the latter used to identify basic processes. Specifically, analyses of reanalysis products during the El Niño onset indicate that after the climate shift a low-level anticyclone over the South China Sea was shifted into the Bay of Bengal and that equatorial westerly anomalies in the Pacific Ocean were considerably stronger. The present study focuses on determining influence of Indian Ocean SST on these changes.
        A suite of AGCM experiments, each consisting of a 10-member ensemble, is carried out to assess the relative importance of remote (Pacific) versus local (Indian Ocean) SST anomalies in determining precipi- tation anomalies over the equatorial Indian Ocean. Solutions indicate that both local and remote SST anomalies are necessary for realistic simulations, with convection in the tropical west Pacific and the subsequent development of the South China Sea anticyclone being particularly sensitive to Indian Ocean SST anomalies. Prior to the climate shift, the basinwide Indian Ocean SST anomalies generate an atmo- spheric Kelvin wave associated with easterly flow over the equatorial west-central Pacific, thereby weak- ening the westerly anomalies associated with the developing El Niño. In contrast, after the shift, the east–west contrast in Indian Ocean SST anomalies does not generate a significant Kelvin wave response, and there is little effect on the El Niño–induced westerlies. The Linear Baroclinic Model (LBM) solutions confirm the AGCM’s results.

Journal of Climate, 2005 vol. 18 (2) pp. 302-319