Marie Curie Actions / Marie Curie International Reintegration Grants
“EDDYLC”
STARTPAGE
HUMAN RESOURCES AND MOBILITY (HRM)
ACTIVITY
MARIE CURIE ACTIONS
Marie Curie International Reintegration Grants (IRG)
PART B
“EDDYLC”
Name of the Researcher : [Nili Harnik]
Email : [
Fax : [Researcher’s fax]
B1 SCIENTIFIC QUALITY OF THE PROJECT
B1.1. Research topic
Understanding the inner workings of the midlatitude atmosphere is a complex task that is at the heart of understanding weather and climate. Given the complexity, scientists look for dominant patterns and relations in the various atmospheric and oceanic fields, and try to understand the dynamics that cause them. The leading modes - those which explain most of the observed variance - are typically a north-south pressure oscillation, which is accompanied by a north-south shift of the jet streams and the eddies which propagate on them. The North Atlantic Oscillation (NAO) is an example of such a mode which dominates the North America-Atlantic-Europe sector. The common framework for understanding the observed circulation is that of separating the atmospheric flow into a basic state – the planetary scale flow which varies on seasonal or slower time scales - and the deviations from it. The atmospheric Jet Stream, and the weather systems that propagate along it, are a common example of such a separation.
There is a complex mutual interaction between the basic state (which is also referred to as the mean flow), and the waves or eddies, which are the deviations from it. Observational studies have shown that wave-mean flow interactions involving momentum fluxes play a central role for explaining the leading modes of variability in midlatitudes (see Thompson et al, 2003 and references therein). A leading theory is that the dominant modes arise from a positive feedback between the mean flow and eddies - a selection process in which those anomalies which give rise to such a feedback will prevail (Branstator 1992, 1995). A positive feedback between eddy momentum flux and mean flow anomalies has been shown to exist in observations and models (e.g. Lorenz and Hartmann, 2003 and many references therein), but it has not been explicitly demonstrated that the eddy response to mean flow anomalies is reinforcing. Studies have also shown that Eddy momentum fluxes play a central role in the response of mid latitudes to forcing external to that region. These include the response of midlatitudes to the tropical phenomena of El Niño (e.g. Seager et al 2004 and references therein), the response or the atmosphere to anomalies in midlatitude sea surface temperature (Kushnir et al, 2002), and response of the tropospheric circulation to anomalies in the stratosphere (e.g. **Robinson, Limpasuvan et al, 2004). Understanding the main factors which determine the momentum fluxes in waves, and how they change for a given basic state anomaly, is therefore central for understanding atmospheric variability and its possible role in maintaining different climate regimes. These processes are still not well understood. A potentially important mechanism is provided by the idealized eddy lifecycle experiments of Simmons and Hoskins (1980). A typical eddy life cycle consists of linear growth, followed by saturation, and typically a nonlinear decay (e.g. Edmon et al, 1980). While eddy heat fluxes are largest during the growth stage, the eddy momentum fluxes are largest during the decay stage. Simmons and Hoskins (1980, see also Thorncroft et al, 1993) found two types of eddy life cycles (referred to as life cycles LC1 and LC2) which are characterized by a very different wave-mean flow interaction during the eddy decay stage, and correspondingly very different momentum fluxes and a final mean flow. The two life cycles were obtained by changing the barotropic component of the mean flow. This suggests a mechanism for a robust positive feedback between mean flow and eddy anomalies, which might give rise to the observed leading modes. The existence of two different types of life cycles, which differ strongly in their effect on the mean flow, also suggests an important mechanism by which external forcing can affect the midlatitude atmospheric circulation. By making a small initial perturbation to the mean flow, external forcing can lead to a large change in the nature of weather systems, which can amplify the response of the atmosphere considerably.
The underlying hypothesis of the current proposal is that an important part of the variability of momentum fluxes can be understood as a response of the waves to changes in the mean flow refractive properties. This follows from the fact the meridional propagation of waves directly involves momentum fluxes (e.g. Edmon et al, 1980). We seek to test this hypothesis explicitly, and to understand how such wave-mean flow interaction affects eddy life cycles, and how this might shape the internal variability of the midlatitude circulation and its response to external forcing.
B1.2. Project objectives
Professional Reintegration:
The proposed project will be conducted at Tel Aviv University (TAU) where I will start a research and teaching faculty position in the Department of Earth and Planetary Sciences. I will establish an Atmospheric Dynamics and Modelling group which will complement those of other professors in the department, but will be unique in its emphasis on combining geophysical fluid dynamics theory, numerical modelling, and observational analysis, to explain observed large scale atmospheric phenomena. I will draw extensively from my experience of studying, conducting research, and teaching, in leading universities in the United States. During my years in the US, I have collaborated with several excellent scientists on various topics, often with different but complementary expertise. Upon returning to Israel, I will be bringing with me considerable knowledge that I acquired from these colleagues. Also, these collaborations will help import into my group various necessary analysis tools, numerical models, and knowledge, by helping to teach students and researchers in my group to use them.
Research objectives:
The goal of the proposed study is to examine the role of wave geometry and wave-mean flow interaction for observed midlatitude variability, both forced and unforced. We will do this in the context of eddy life cycles of linear growth and nonlinear maturation and decay. We will address a few key questions:
1. What determines the type of eddy life cycle?
While the existence of two kinds of eddy life cycles is well established, both in models and in observations, the factors which determine the life cycle, as well as the implications, are still not clear. One of the goals of this proposal is to examine the role of basic state wave geometry in shaping the evolution of waves as they undergo a lifecycle of growth and decay.
Thorncroft, Hoskins, and McIntyre (1993, hereafter, THM) showed that the differences between the two types of eddy life cycle are consistent with a difference in the ability of the waves to propagate in the meridional direction, as determined by the index of refraction of the upper level flow. Subsequent studies, however, reveal a more complex picture. In particular, studies have shown that the wave momentum fluxes, and correspondingly the type of life cycle, are most sensitive to the lower, rather than upper level winds (e.g. Balasubramanian and Garner, 1997; Hartmann, 2000; Methven et al 2004 *???*). Since the equatorward propagation and decay occur at the upper levels, these studies concluded that the refractive properties of the upper level flow are not the key feature to determine the type of eddy life cycle. The lower level flow, however, can affect the upper level refractive properties, by affecting the zonal phase speed of the waves (the Index of Refraction is a function of wave phase speed). Indeed, the phase speed in the two life cycles is quite different. Recent preliminary calculations, using a wave-geometry diagnostic that I developed (see following sections for details), seem to confirm this. We therefore seek to re-examine the role of wave geometry for wave life cycles.
Although the eddy life cycles differ in the nonlinear behaviour of the waves, one implication of finding a significant role for wave geometry will be the importance of quasi-linear dynamics, with pure non-linearities (wave-wave interactions) mostly acting to amplify the quasi linear tendencies. The role of non-linearities, however, needs further examination. Orlanski (2003) recently showed that the two types of life cycles can be obtained by initiating a mean flow with different initial amplitude waves. Orlanski went on to suggest that the life cycle change in THM was due to the difference in low level baroclinicity, via its effect on wave amplitude. In the classical life cycle studies, waves grow from infinitesimal perturbations as a result of the instability of the mean flow. Given that observed waves occur in localized wave packets, which propagate on a zonally varying basic state, finite amplitude seeding might actually be a more realistic setup than normal modes growing from infinitesimal perturbations. We therefore seek to examine the role of seeding amplitude. This will in some sense extend recent studies we have done in which the growth of finite amplitude upper level waves was examined, in the context of the Pacific storm track (Harnik and Chang, 2004). We expect to find the classical life cycle behaviour for initial seeding that is not too large, but probably a different behaviour for waves which are large enough to be nearly saturated. The relation to Orlnaski's results will also be examined.
2. Why do the midlatitudes cool during El Niño?
One of the ways in which the two life-cycle paradigm might apply to the observations is in explaining the observed midlatitude response to the tropical phenomenon of El Niño - Southern Oscillation (ENSO). Indeed, an observational study by Shapiro et al (2001), of wave life cycles during one strong El Niño and one strong La Niña, found a significant change in the form of wave breaking, which is one of the main differences between the life cycles. We propose to examine whether this is a realistic mechanism, and under what conditions it might work.
Studies have shown that transient eddies play a central role in the midlatitude response to ENSO (e.g. Held et al, 1989). In particular, Seager et al (2004) recently showed, using observations, that during El Niño (La Niña), there is a hemispherically symmetric cooling (warming) of midlatitudes, with a large zonally symmetric component. They showed the cooling is due to a change in eddy momentum fluxes, and further demonstrated, using a linear model, that it is consistent with a change in the refraction of waves in the vertical-meridional plane. Seager et al (2004) considered the effects of the basic state on the linear growth stage of wave life cycle. Given that momentum fluxes are larger during the nonlinear decay stage, a change in eddy life cycle, in response to the initial ENSO induced subtropical mean flow anomaly, might result in a larger midlatitude response.
We will examine this mechanism both by analyzing more observations, and by conducting modelling studies. Orlanski (2003) also suggested the response to El Niño involves a change in eddy life cycle, however, he suggested the cause for the change in life cycle is the increase in moisture and lower level baroclinicity during El Niño. Results of our study will help determine which mechanism is more relevant for observations.
3. Do the eddies feed back positively when perturbed by a mean flow anomaly?
The main process of wave-mean flow interaction suggests the main time scale of variations of the leading modes of variation should be about 10 days (the time scale of an eddy life cycle), which is indeed observed (e.g. Feldstein, 2002). The source of longer time scale variations, is still unclear. One possibility is the coupling to the much slower ocean (Kushnir et al, 2002) or stratosphere (e.g. Thompson et al, 2003). It has also been suggested, however, that some of the low frequency variability arises internally in the atmosphere, from a positive feedback between the eddies and the mean flow (Branstator, 1992, 1995). Such a feedback is also thought to be involved in the response to external forcing (e.g. Kushin et al, 2002). The goal of this part of the proposal is to explicitly examine whether the eddy response to characteristic mean flow anomalies can give rise to such a positive feedback. We will examine both the linear response (via a change in wave propagation characteristics and the corresponding normal mode structure), and the nonlinear response (via a change in eddy life cycle). In the second stage of this work we will use our results to construct a dynamically based parameterization of the effects of the waves on the mean flow, for use in simplified models of low frequency variability. This will be done by constructing a mean flow anomaly equation, with an eddy flux term, which will be constructed directly from our linear quasi-geostrophic model for eddy anomalies. We will examine the importance of a positive feedback by comparing results for initial basic states which do give rise to a positive eddy feedback, to initial basic states that don’t.
Added value:
In order to carry out the proposed work, I will establish an atmospheric dynamics research group, and supervise graduate students. This is an important and exciting new stage in my academic career. In my last position, I held a lectureship, which entailed teaching an undergraduate course in climate dynamics, but did not involve any official advising of graduate students. My new position will allow a much more significant role as a teacher and mentor for students. In addition, this project entails importing a primitive-equation numerical model of the atmosphere. Since the installation, modification, and use of such a model takes up considerable time and resources, I have so far preferred to use such models only indirectly, by collaborating with others. This project will allow me to setup such a model in my own group, and thus to expand the type of work I have done.