Plan for S-RIP chapter on QBO and tropical variability

1. Introduction

As with other S-RIP chapters, the basic scientific goal of the QBO and tropical variability chapter is to compare the representation of the QBO in different global reanalyses. Much as climate model intercomparisons are now commonplace due to the large number of available climate models, the number of global reanalyses that are presently available (eight) or soon to be available (four) motivates an intercomparison of these datasets. Reanalyses are used in a variety of contexts to provide a best-guess estimate of the observed atmospheric state, yet differences between them exist due to the choice of observations assimilated, the assimilation method employed, and the characteristics of the forecast model. Characterizing these differences and understanding their origins is essential, especially so that future users can be steered away from making inappropriate use of reanalysis datasets. Specifically in the case of the QBO, the sparsity of tropical radiosonde wind observations, as well as the difficulty experienced by most atmospheric general circulation models (AGCMs) in representing the QBO, suggest that a careful assessment of the fidelity of reanalyses QBOs would be useful. Put simply: how well do we know the state of the QBO?

2. Overview of scientific issues

    2.1 How well observed is the QBO?

A basic goal of the chapter is to characterize inter-reanalysis differences in the structure of the QBO in zonal-mean tropical winds and temperatures. Diagnostics will address the vertical and meridional structure of the QBO, its amplitude and period, and its partial synchronization with the annual cycle. Modellers trying to represent the QBO in free-running AGCMs may tune their models to match reanalysis, so it is of interest to determine how well constrained are the reanalysis QBOs. Direct comparison with observations will be made where possible. In particular, IGRA and IGRA2 tropical radiosondes, and SABER and HIRDLS satellite data, are expected to be useful.

    2.2 Zonal asymmetry

Due to the paucity of near-equatorial radiosonde stations, it has long been the case that the "Singapore winds" provided by FUB are usually interpreted as being representative of the zonal-mean QBO. This is a good assumption provided that zonal asymmetries (in the monthly means) are genuinely small. The FUB time series is compiled from the consecutive records of three near-equatorial stations. The availability of more tropical radiosonde records, especially from the soon-to-be-available IGRA2, should allow a closer examination of possible zonal asymmetries of the QBO. While persistent zonal asymmetries are generally expected to be small, we may be able to test this assumption more thoroughly than has been possible in the past.

    2.3 Diagnosing model errors

Atmospheric GCMs that are free-running (i.e. without data assimilation) are well known to have problems exhibiting spontaneous QBOs, and the forecast models are likely to share similar problems. We anticipate that careful diagnosis of the QBO momentum budget in reanalyses may yield insight into model errors that are applicable to both reanalyses and free-running AGCMs. For this purpose we hope to make use of the analysis increments, where available, since they indicate where the free-running (forecast) model would err in reproducing the observed QBO if it were not constrained by data assimilation.

    2.4 Extratropical influence

The QBO influences the winter stratospheric polar vortices of both hemispheres. Other teleconnections also affect the polar vortex, such as ENSO and the 11-year solar cycle. To the extent that these influences co-vary over the course of the observed record, diagnostic separation of their effects is difficult, and their interactions may be important. We will compare these teleconnections across reanalyses, so as to determine their robustness and hopefully gain insight into their interactions. One important aspect involves detemining how robust are these teleconnections to the choice of extratropical metric - e.g. whether one focuses on major sudden warmings, annular mode anomalies, temperature at the pole, etc. Since the Stratosphere-troposphere coupling (STC) chapter of S-RIP will be producing a variety of such metrics, close collaboration with the STC chapter will facilitate a comprehensive analysis of teleconnections.

    2.5 Tropical dynamics

The behaviour of tropical waves in the reanalyses is of interest: since reanalysis QBOs tend to be closer to observed tropical winds than free-running model QBOs, tropical waves propagate through more realistic background wind shears in the reanalyses than they do in most climate models. Since wave dissipation forces the zonal-mean QBO, spectral analysis of tropical waves may give insight into the partitioning of QBO forcing amongst different wave types. Effects of the QBO may also extend downward below the region of the stratosphere in which the QBO is the dominant mode of variability. There is evidence that QBO modulation of the tropical tropopause height affects deep convection, suggesting a QBO feedback on the generation of the waves that force it. If robust, this behaviour may be evident in reanalyses.

3. Organization and schedule

The long-term goal of S-RIP is to produce a SPARC report. For the QBO chapter, we intend for analyses to be paper-driven - i.e. contributors will lead and co-author papers to be published in the peer-reviewed literature, and results from these papers will contribute to the final S-RIP report. Note, however, that the S-RIP report is intended to serve as a comprehensive reference that will benefit the users of reanalysis products, and as such it is feasible to include a greater level of detail in the report than is generally possible in papers. We nevertheless anticipate that the most interesting analyses are driven by science questions, and hence should be directed toward the writing of papers. Submitting papers prior to the publication of the S-RIP report may also prevent potential future problems involving publication of papers that include material already published in the report.

The schedule of the chapter is to produce at least the basic QBO characterizations within the next 1-2 years, consistent with the schedule of the "basic" S-RIP chapters (i.e. chapters 1-4). More complex diagnostics can take slightly longer, consistent with the schedule of the other "advanced" S-RIP chapters. We aim to keep pace with developments in the stratosphere-troposphere coupling (STC) chapter, since this facilitates analysis of teleconnections (as described above). A short interim progress report is planned for each year, as described in the S-RIP SPARC newsletter article. However, we envisage that the main vehicle for up-to-date information re. this chapter will be the website.

New contributors to the chapter are most welcome. If you are interested, and have expertise in a research field related to the QBO and tropical variability, please get in touch with the chapter co-leads, James and Lesley (contact info on main page). Do please first check the list of current projects (Section 5, below) to see how you could:

    (1) Contribute to a current project;
    (2) Initiate a new one (by identifying gaps in the list of current projects).

Regarding (1), inclusion of new contributors to current projects is at the discretion of those people already taking the lead on those projects. Regarding (2), inclusion of new projects within the QBO chapter is at the discretion of the chapter co-leads. We aim to be as flexible and open as possible, but of course some coordination of efforts is needed to ensure that a coherent final chapter can be written, and also that contributors don't unwittingly duplicate or intrude on others' work.

4. Potential links with other chapters

The following links with other chapters of the S-RIP report seem clear at this early stage of the project. Possibly other links will emerge as the S-RIP project progresses.

Stratosphere-troposphere coupling (STC) chapter: Diagnosis of teleconnections will employ extratropical metrics provided by the STC chapter contributors, enabling a more comprehensive analysis than would be possible otherwise.

Tropical tropopause layer (TTL) chapter: QBO modulations of tropical tropopause height, tropical deep convection, and the water vapour tape recorder may be diagnosed.

Upper stratosphere and lower mesosphere (USLM) chapter: Above the QBO-dominated altitudes of the tropical stratosphere is found a semi-annual oscillation (SAO) near the stratopause. Since waves that force the SAO must first propagate through QBO wind shears, interpretation of inter-reanalysis SAO differences will likely require QBO diagnostics. The structure of stratospheric polar vortices at very high altitudes may also be relevant to QBO teleconnections.

5. Current projects

This list describes papers/projects currently in progress by the chapter contributors. For each paper/project, the following info is indicated if it is available:

    1) Who is involved (not just the lead author, but others if known)
    2) What diagnostics are used (will be useful info for other projects, to know what is already being computed by someone)
    3) Roughly, what is the timeline (the schedules of projects will define the schedule of the chapter)
    4) Summary of most interesting results, and/or publication status (when this becomes available)

As analyses evolve, there may be some question as to which chapter of the S-RIP report most appropriately includes a given analysis. E.g., analysis of QBO teleconnections with extratropical surface climate may ultimately be a better fit in the stratosphere-troposphere coupling chapter than the QBO chapter. Our view is that this will become clearer as the final report-writing stage draws nearer. Because the work is organized by papers/projects, it doesn't matter so much where an analysis ends up in the final report, as long as it's done somewhere.

[Note, this list is only in very rough form right now. We're still in the process of gathering this info from the contributors.]

    Lesley Gray - Regression analysis of solar cycle and other teleconnections

    Corwin Wright, James Anstey - Gravity wave forcing of the QBO using HIRDLS and SABER data

    Dann Mitchell - Combined QBO-solar cycle modulation of the NH polar vortex

    James Anstey, Lesley Gray - Review of existing literature on QBOs in reanalyses
        Ongoing job; will be required for final write-up of the QBO chapter. Hence seems appropriate for co-leads to do this.
        Not intended as a stand-alone paper, just as part of the S-RIP report.
        In-progress version will likely be available on website, since this is likely to be a useful for collaborators.

Here also is an indication of the areas of interest of the contributors, organized by person rather than by project.

    Lesley Gray                Regression analysis of teleconnections, especially solar cycle
    Masatomo Fujiwara        Regression analysis of teleconnections, especially volcanoes
    Corwin Wright            HIRDLS + SABER comparison
    Hua Lu                    Holtan-Tan effect, including mechanisms and interference with the solar cycle
    Yoshio Kawatani            Comparison of IGRA zonal wind, in addition to FUB zonal wind, with reanalysis data
    Matt Toohey                Volcanic teleconnections (and their relation to other teleconnections)
    James Anstey            Basic QBO characterization, seasonal synchronization of the QBO, teleconnections (Holton-Tan effect)
    Dann Mitchell            Teleconnections (Labitzke-van Loon effect, etc)

Latest update: 6 Mar 2014 by James Anstey