INVITATION LETTER

The following letter was sent by the C20C Organizing Committee to prospective C20C participants.

Subj: Proposed 'International Climate of the Twentieth Century Project'

Dear Colleagues,

The purpose of this letter is to invite your participation in the International Climate of the Twentieth Century Project (C20C) which is jointly proposed by the Hadley Centre at the UK Meteorological Office and the Center for Ocean-Land-Atmosphere Studies (COLA). The basic goal of the project is to simulate climate variations since the late-nineteenth century using observed SST and sea ice forcing, and all other relevant forcings, and to determine the extent to which atmospheric models are able to simulate the observed climate variations against the background of natural variability.

The current century has a rich collection of variations (ENSO, monsoon floods and droughts, dust bowl, African drought, etc.) that can be assumed to be unrelated to solar and anthropogenic forcings and thus can serve as the basis for validating atmospheric GCMs.

Background: The C20C project was originally created by the Hadley Centre. This project has involved the use of both ocean-forced AGCMs and observed data, to study climate variations and changes over the last 130 years, in particular the period since 1949. The project took on an international dimension when a number of groups became interested in using the same SST and sea-ice data for simulating climate variability and change. So far the project has involved many informal bilateral collaborations between the Hadley Centre and groups in the USA, Canada, Australia, New Zealand, and UK universities, along with a few smaller collaborations elsewhere. Many other groups also have copies of the Global sea-Ice and Sea Surface Temperature data set (GISST), which is the central link in C20C. In late 1994, a very successful workshop was held at the Hadley Centre (Folland and Rowell, 1995, "Workshop on Simulations of the Climate of the Twentieth Century using GISST, 28-30 November 1994, Hadley Centre", CRTN 56), and in 1995 a C20C session was included in the 1st International AMIP Conference. The C20C idea is also mentioned in the CLIVAR plan. Some scientists hoped the project might expand in a more formal way, but unfortunately the requisite staffing was not available at the Hadley Centre to support this. There was also a concern that C20C should not just become a 'super AMIP', but must be very much focused on the key scientific issues which Hadley Centre funders would support in the longer term.

However, recent discussions between COLA and the Hadley Centre have now led to the idea of a re-invigorated and more formal International C20C Project. In particular, this has been made possible by the willingness of COLA to supply the necessary infrastructure. The purpose of this letter is simply to outline our proposals, and invite your participation.

The Proposed Project:

Our basic idea is that up to three sets of ensemble simulations would be run by each participating institute:

  1. The first set would be the 'classic' C20C/extended-AMIP type runs, using GISST3.0 to simulate the period 1949-1997, with a minimum ensemble size of four members. All participating institutes would run these experiments, although some would start them earlier; GISST3.0 begins in 1871.
  2. The second ensemble would also include most known atmospheric and external forcings, both natural and anthropogenic (see below). Again, we envisage ensemble sizes of at least four members, with runs starting from 1871 or 1949. We hope most (or all) participants will be able to carry out these runs, which we consider to be a major goal of the project, particularly as they should provide a useful way of further studying seasonal predictability. A few groups might also carry out other ensembles with fewer forcings (although we do not see 'climate change detection' as an aim of the coordinated project).
  3. The third (also optional) ensemble of C20C experiments would explore the additional impact of land surf ace changes on recent climate change and variability, particularly regional climates. These integrations would probably start from 1970.

We would provide the following datasets, for use by all participants, to be distributed by COLA:

  1. GISST3.0, the latest version of the Hadley Centre Global sea-Ice and Sea Surface Temperature data set. This was created using three analysis methods: for 1871-1981, by augmenting an EOF reconstruction of anomalies with gridded observations; for 1982-1994, by blending AVHRR SST with in-situ data; and for 1995-1997, using a blend of Reynolds OISST SST and in-situ SST. Observed, interannually varying, sea-ice concentrations were also used as far as possible.
  2. Various atmospheric and external forcing data: CO2, trace gases, ozone (stratospheric and tropospheric), sulfate aerosols (direct and indirect effects), volcanic aerosols, and solar variations. These have been developed or collected by various groups at the Hadley Centre, for use in both coupled and atmospheric GCM runs.
  3. A new soil moisture data set, the creation of which is under way at COLA. This uses atmospheric analysis data, observed rainfall, and an appropriate soil moisture model.
  4. A global land surface cover data set which includes major land surface changes over the last few decades, for example deforestation over West Africa, and tropical rainforests. Work on the creation of this data set has yet to begin, but it is hoped that this data can be organized by COLA, in collaboration with IGBP scientists.
  5. An observed snow dataset with maximal coverage and temporal resolution will also be assembled at COLA.

Our intention is then that an initial workshop would be held later to compare results and scientific findings from the first set of experiments. This would be organized by COLA. We then expect further workshops perhaps every 18-24 months, possibly after the completion of each of the second and third ensembles. The exchange of ideas and findings at these workshops is one of our major motivations in widening and re-invigorating the C20C project. We look forward to hearing from you.

Yours,

C20C Organizing Committee