1.1 Observations of the Convectively Coupled Equatorial Waves

by Joe Archive on May 6, 2012

*Now that my thesis is accepted, and most of the hard work is done, I want to try writing a series of posts that will cover what I did with my time the last few years.*

In this project, I’ve mostly been thinking about convection and some of the large-scqle organized circulations associated with it. Specifically, we’re interested in several types of propagating signals that form a large part of the variability in the tropical atmosphere that consist of coupled systems of convection and circulation: The Convectively coupled Equatorial waves and the MJO

-These structure serve to organize convection, which is a small/fast process on much larger and longer scales.
-Global Climate Models do not, in general, represent these waves well – It seems this is because they do not represent the interaction between the large scale and the convection very well.
-Studying these waves seems like a good place to approach the question of how convection interacts with the large scale, as they are very well observed, to some extent linear.

The MJO and CCEW dynamics involve planetary scale circulations interacting with small scale convective systems making it a challenging phenomenon to understand. However, understanding the MJO phenomena is important to our grasp of the tropical atmosphere and climate for many reasons.

MJO and CCEW related variations are among the dominant intraseasonal variabilities of the tropical ocean-­‐atmosphere system, spanning the timescales between climate and weather. For example, the MJO and the Convectively coupled waves are seen to influence the rainfall over virtually all regions of the tropics, the subtropics and into the mid-­latitudes: the Asian and  Australian Monsoons; over Indonesia; along the west coast of North and South America and Africa.

The MJO has also been associated with El Nino-­‐La Nina transitions, with the suggestion that the strong westerly winds associated with the MJO may set off the flows in the Pacific Ocean that transition between the two states.

The MJO&CCEW has also been observed to modulate the genesis of tropical cyclones in the Pacific, Atlantic, and Gulf of Mexico. It has been observed that improved forecasts of MJO and wave dynamics may help improve short term tropical cyclone prediction.

The MJO affects the global medium and long range weather forecasts

1.1 Observations of the Convectively Coupled Equatorial Waves

One way we observe convection is in Outgoing Long-wave Radiation (OLR) – satellite measurements of the infra-red radiation coming up out of the atmosphere. The satellite essentially measures the infra-red radiated from the uppermost opaque surface. OLR is mostly a measure of the cloud top temperature – clouds act as reasonably good black bodies, so the OLR goes like temperature to the fourth power through the Stefan-Boltzmann law. Because temperature in the troposphere is closely related to height, high clouds are colder than low clouds, so OLR gives us a measure of cloud top height.

The deepest clouds give the lowest OLR, and places where the sky is clear give a higher OLR.

NOAA Satellite Outgoing Long Wave Radiation

NOAA Satellite Outgoing Long Wave Radiation averaged over three days near the end of 2007

 

 

 

 

 

 

 

 

 

 

In the figure, you can see a large area of blue-cold-high clouds indicating the deep convection over Indonesia and in the South-Pacific Convergence Zone (SPCZ). The Inter-Tropical Convergence Zone (ITCZ) is also visible, although the convection there is a little weaker (probably due to the averaging over several days of the more intermittent/variable convection in that region). In contrast, in places like Western Australia and over parts of Africa, the radiating surface is hot – in that region the satellites are seeing the surface.

In the satellite record of outgoing longwave radiation (OLR; Liebmann and Smith 1996)—a good proxy for deep tropical convection (see, e.g., Arkin and Andanuy 1989)—there are patterns of enhanced convection and precipitation organized on planetary scales. The waves are easily visualized in Hovmöller diagrams of the equatorial OLR (figure 1-1). This figure shows the OLR signal averaged between S and N for approximately a year of the OLR record (early October 1990 to early November 1991). Along with this is shown the OLR filtered to the Kelvin Wave (KW) and MJO spectral regions (discussed below), showing more clearly the presence of coherent waves of convection traversing the tropics.

The strongest and largest of these convective structures propagate eastwards at about 5 m/s with periods in the range of 30 to 90 days from the Indian Ocean to the central Pacific, coupled to planetary scale wind, temperature, and moisture anomalies. This disturbance is the MJO, which plays an important role in the global weather and climate (see below), so correctly simulating its behavior is an important goal for climate models. However, the MJO seen in most models is too weak and propagates too fast (e.g. Hayashi and Sumi 1986, Lau and Lau. 1986, Slingo et al. 1996, Maloney and Hartman 2001, Waliser et al. 2003, Zhang et al. 2006, Lin et al. 2006). The quality of the simulated MJO seems to be very sensitive to the details of the representation of convection (e.g. Wang and Schlesinger 1999, Maloney and Hartmann 2001, Zhu et al. 2009), indicating that the deficiencies in the modeling of the MJO may stem from a lack of understanding how convection interacts with the larger scale flows in reality.

OLR Hovmoller, averaged over the equator 5S-5N
OLR averaged between 5 North and 5 South, for about a years worth of the satellite record (april 2004 – Feb 2005) Large Eastward propagating signals appear to be the MJO, with smaller, faster Kelvin waves. Other things are visible propagating westwards
 The waves can be seen in Hovmoller-type diagrams. In the figure above, the OLR averaged from 5 North to 5 South is contoured against longitude and time. The deep, organized, convection can been seen as extended areas of cooler colors. There are several large, slow moving, signals that are the Madden-Julian Oscillation, and many faster, smaller, Kelvin Waves embedded within them.

 

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