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”

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).”

Upper-Ocean Processes under the Stratus Cloud Deck in the Southeast Pacific Ocean.

Yangxing Zheng, George N Kiladis, Toshiaki Shinoda, E Joseph Metzger, Harley E Hurlburt, Jialin Lin, and Benjamin S Giese.

Abstract The annual mean heat budget of the upper ocean beneath the stratocumulus/stratus cloud deck in the southeast Pacific is estimated using Simple Ocean Data Assimilation (SODA) and an eddy-resolving Hybrid Coordinate Ocean Model (HYCOM). Both are compared with estimates based on Woods Hole Oceanographic Institution (WHOI) Improved Meteorological (IMET) buoy observations at 20°S, 85°W. Net surface heat fluxes are positive (warming) over most of the area under the stratus cloud deck. Upper-ocean processes responsible for balancing the surface heat flux are examined by estimating each term in the heat equation. In contrast to surface heat fluxes, geostrophic transport in the upper 50 m causes net cooling in most of the stratus cloud deck region. Ekman transport provides net warming north of the IMET site and net cooling south of the IMET site. Although the eddy heat flux divergence term can be comparable to other terms at a particular location, such as the IMET mooring site, it is negligible for the entire stratus region when area averaged because it is not spatially coherent in the open ocean. Although cold-core eddies are often generated near the coast in the eddy-resolving model, they do not significantly impact the heat budget in the open ocean in the southeast Pacific.

http://dx.doi.org/10.1175/2009JPO4213.1

http://journals.ametsoc.org/doi/full/10.1175/2009JPO4213.1

Abstract

  • Net surface heat fluxes are positive (warming) over most of the area under the stratus cloud deck.
  • Upper ocean processes balance this net warming from the surface fluxes.
  • Geostrophic transport in the top 50 m causes net cooling in most of the surface under the stratus deck.
  • Ekman transport causes net warming north of the IMET buoy site and net cooling south of the buoy site.
  • Eddy heat flux divergence is not coherent over the open ocean.

Introduction

  • SSTs in the Southeast Pacific near the coast of Peru and Chile are colder than at any other comparable latitude around the globe.
  • These cold SSTs are believed to play an important role in the formation and maintenance of the persistent stratus deck in this region.
  • Most coupled global ocean GCM models have a systematic warm SST bias and low cloud cover in this region.
  • Colbo and Weller (2007) estimate the upper budget from 6 year time series of temperature, salinity, velocity and surface meteorological variables.
  • They conclude that in the top 250 m, the surface warming (positive surface heat fluxes) is balanced by geostrophic heat transport and eddy heat flux divergence.
  • They deduce from these analysis that cold core eddies from the coast propagate westward transporting heat offshore and cooling the SST.
  • The main drawback of their calculation is that the eddy heat flux divergence in the heat equation is calculated as a residual of all other terms.
  • Also, 250 m depth advection and eddy heat flux divergence may not affect the surface SSTs as the winter time mixed layer in most of this region is only 150 m.
  • This study uses the SODA dataset and the HYCOM global ocean model for the ocean state variables.
  • The data from the mooring site are used to validate the model.

Model and Datasets

  • SODA dataset is used. Its based on the LANL POP model simulations with a resolution of 0.4 deg x 0.25 deg (longitude x latitude) with 40 vertical levels. Time period of data available – 1958-2001 forced by ECMWF and 2002 to 2005 forced by QuikSCAT winds.
  • Surface fluxes are computed from bulk formulae.
  • HYCOM model is used for the analysis.
  • Observation data set used in this study is from the WHOI IMET buoy deployed at 20S 85W in Oct 2000.
  • Surface heat fluxes are calculated from SST and surface met. data from the buoy using the TOGA-COARE bulk air-sea flux algorithm. (Bradley,2000).
  • AVISO SSH is used to validate the model’s ability to simulate eddy activity.

wpid-j-phys_-oceanogr-2010zheng-pdfpage4of18-2011-08-17-10-342.jpg

Comparisons with IMET obs.

  • The seasonal cycle in the surface heat fluxes of the model tracks that from the IMET buoy.
  • Yet, during Jan-Feb and Sep-Oct-Nov, there is a difference of 80 Wm-2
  • Spatial distribution of NOGAPS sfc fluxes are positive in most areas discussed which is consistent with IMET estimates.
  • wpid-j-phys_-oceanogr-2010zheng-pdfpage5of18-2011-08-17-10-342.jpg
  • SODA and HYCOM have smoother fields compared to insitu IMET measurements.
  • Yet both of them are able capture the seasonal cycle of the mixed layer.
  • The mixed layer depth is 20-30 m in the austral summer (Feb-Mar) and becomes deepest (~150m) after the austral winter (Sept-Oct)

Mean Heat Budget

wpid-j-phys_-oceanogr-2010zheng-pdfpage6of18-2011-08-17-10-342.jpg

  • Qnet is the net sfc heat flux, Cp is the specific heat of seawater at constant pressure, rho is the density of seawater, w is the vertical velocity, V is horizontal velocity vector, T is temperature and V’ and T’ are deviations from the seasonal mean.
  • The divergence of the eddy heat flux is given by the V’T’ term in the above equation.
  • The seasonal average of V and T are used as the mean values in this study.

wpid-j-phys_-oceanogr-2010zheng-pdfpage5of18-1-2011-08-17-10-342.jpg

  • Table 1 shows that the dominant terms in the heat balance in the IMET buoy observations are Qnet (+), geostrophic transport(-) and Ekman transport(-).
  • For the HYCOM model, the Ekman transport(-), geostrophic transport(-) are balanced by the eddy flux divergence(+). The Qnet(+) is of a smaller magnitude.
  • Geostrophic currents are computed from the model temperature and salinity.
  • Ekman currents are the difference between the total velocity and the geostrophic velocity.
  • Eddy flux divergence is large in the IMET observations and in the models. In HYCOM, it has a positive sign unlike in SODA and IMET.
  • Area average values are derived for the model outputs to compare with IMET estimates in the region 30-10S, 100-80 W.
  • wpid-j-phys_-oceanogr-2010zheng-pdfpage7of18-2011-08-17-10-342.jpg
  • Also values for just the upper 50 m were derived as in shallow mixed layer conditions eddy flux divergence and horizontal heat advection at 250 m might not impact the SST.

Spatial distribution of the upper ocean heat budget

  • Net surface heat flux is positive over most of the region under the stratus deck.
  • Strong warming from the net sfc heat flux neat the coast south of 15S.

wpid-j-phys_-oceanogr-2010zheng-pdfpage7of18-1-2011-08-17-10-342.jpg

  • Geostrophic advection of heat causes significant upper-ocean cooling over the stratus cloud deck region.
  • Northwestward geostrophic currents in most of the stratus deck region cross the isotherms nearly perpendicularly and thus geostrophic currents transport cold water to the stratus cloud region.

wpid-j-phys_-oceanogr-2010zheng-pdfpage8of18-2011-08-17-10-342.jpg

  • In some locations (south of 30S between 100 and 90 W , geostrophic currents are parallel to the isotherms) heat advection by geostrophic currents is negligible compared to the Ekman transport.
  • Yet in most of the locations, from the model analysis and IMET buoy studies, the heat advection due to geostrophic currents is significant.

Ekman transport of heat

  • In SODA, Ekman heat transport causes warming north of the IMET site.

wpid-j-phys_-oceanogr-2010zheng-pdfpage9of18-2011-08-17-10-342.jpg

  • Ekman heat transport at the IMET site is negligible as it is at the boundary of the positive and negative heat advection.
  • Ekman mass transport is given by:

wpid-j-phys_-oceanogr-2010zheng-pdfpage9of18-1-2011-08-17-10-342.jpg

  • Ekman advective heat flux is given by :

wpid-j-phys_-oceanogr-2010zheng-pdfpage10of18-2011-08-17-10-342.jpg

Mean advective heat flux in the mixed layer resulting from Ekman transport, mixed layer temperature and Ekman transport can be seen in the figure below.

wpid-j-phys_-oceanogr-2010zheng-pdfpage10of18-1-2011-08-17-10-342.jpg

  • Ekman heat transport is positive north of the IMET site and negative south of this site.
  • Ekman transport is nearly parallel to the mean SST isotherms in the offshore region near the IMET site.

Vertical Heat advection

  • Vertical advection causes weak warming in the open ocean but not in the coastal region where strong upwelling occurs.
  • The cooling resulting from the upwelling near the coast is balanced by the positive net sfc heat flux.
  • Cold water upwelled near the coast is advected offshore by the mean flow in the upper 40m.
  • Because of convergence of sfc currents in the open ocean, weak subsurface warming occurs due to vertical heat advection in most of the areas under the stratus region.

wpid-j-phys_-oceanogr-2010zheng-pdfpage11of18-2011-08-17-10-342.jpg

Eddy Heat flux divergence

  • Eddy Kinetic energy
    • Eddies are more active near the coastal region than in the open ocean.
    • HYCOM EKE is higher than in AVISO derived EKE.
    • Peru-Humboldt current can interact with the coastline and cause higher eddy activity close to the coast.
    • Also strong upwelling fronts in spring and summer can cause strong baroclinic instabilities than can enhance mesoscale variability.
    • Downwelling coastal Kelvin waves can strongly intensify the Peru-Chile Undercurrent system which may destabilize the near-sfc coastal circulation generating eddies (Shaffer et al., 1997).
    • wpid-j-phys_-oceanogr-2010zheng-pdfpage12of18-2011-08-17-10-342.jpgwpid-j-phys_-oceanogr-2010zheng-pdfpage12of18-1-2011-08-17-10-342.jpg
    • Vertical component of the vorticity and the Okubo-Weiss parameter (OWP) at 30 m depth were computed.

wpid-j-phys_-oceanogr-2010zheng-pdfpage13of18-2011-08-17-10-342.jpg

  • OWP is defined as:

wpid-j-phys_-oceanogr-2010zheng-pdfpage13of18-1-2011-08-17-10-342.jpg

  • Eddies can be defined as patched of negative OWP values surrounded by positive rings.
  • Eddies are seen to be more active near the coast and weaker in the open ocean.

Example of cold-core eddies

  • wpid-j-phys_-oceanogr-2010zheng-pdfpage14of18-2011-08-17-10-342.jpg
  • Anticyclonic eddies are warm core and cyclonic eddies are cold core eddies.

wpid-j-phys_-oceanogr-2010zheng-pdfpage14of18-1-2011-08-17-10-342.jpg

  • Mean eddy heat flux divergence in upper 50 m is seen in fig 12.
  • Mean eddy heat flux divergence in time is noted to be negligible as it is not spatially coherent.
  • Subsurface cold waters trapped in the eddies are more visible offshore than cold water at the surface due to warming at the surface from sfc heat flux.

Discussion

  • Heat advection dues to Ekman transport is significant to the North of 20 deg S. It gives stronger warming to the North of 20S.
  • Eddy heat flux divergence term is negligible both North and South of 20S.
  • Vertical diffusion is a term neglected in this study due to the difficulty in estimating it in models.
  • HYCOM model output shows eddy heat flux divergence is negligible compared to the horizontal heat advection.

Summary

  • Net warming of the surface is due to the surface heat flux.
  • This is balanced by the cooling due to horizontal geostrophic transport.
  • Ekman transport is comparable to the geostrophic currents, but the heat transport due to geostrophic currents is significant as these currents are perpendicular to the isotherms.
  • Ekman currents are parallel to the isotherms of SST.
  • Ekman transport generates warming north of 20S and cooling south of there.

Zheng, Y. et al., Upper-Ocean Processes under the Stratus Cloud Deck in the Southeast Pacific Ocean.