### I. Introduction

One of the major elements of CLIVAR's study of climate variability on seasonal to interannual time scales will be a study of the Variability of the American Monsoon Systems (VAMOS). VAMOS is identified as element G3 in CLIVAR planning documents (WCRP, 1998 - the CLIVAR Implementation Plan). By early 1998, the scientific goals of VAMOS and preliminary plans for approaching those objectives had been developed and were being written into the CLIVAR Implementation Plan. By that same time, the pilot phase of the U.S. Pan-American Climate Study (PACS) was in the field at 125°W near the equator, work had begun to plan the more comprehensive Eastern Pacific Investigation of Climate Processes in the Coupled Ocean-Atmosphere System (EPIC), and other climate-related work focusing on the American monsoon was being developed. There was both the opportunity and the need to coordinate and further develop these efforts and thus to begin to move forward in planning VAMOS.

As a result, a workshop on VAMOS and PACS Field Programmes was planned and held in São Paulo, Brazil on March 30-April 2, 1998. The intent was to further focus the development of plans for field work to be done as part of VAMOS, to co-ordinate monsoon research programmes under consideration by the nations in the Americas, and to establish links between investigators planning to work in the ocean, the atmosphere, and on the land. The workshop was supported by International CLIVAR and drew about 60 participants from 10 countries. This document is a report of that workshop.

### II. Summary

A joint VAMOS/PACS Workshop of Field Programmes and the first Meeting of the CLIVAR VAMOS Panel were held at the Instituto Oceanografico of the University of São Paulo. The chairman of the CLIVAR VAMOS Panel, Prof. C. R. Mechoso and the co-chairs of the organizing committee, Drs. C. Nobre and R. Weller, and the local host, Dr. E. Campos, welcomed about 60 participants. The major goals of this workshop were to identify the scientific problems for a study of the American monsoon systems, to review the existing and already planned field programmes and process studies, to identify opportunities for and encourage collaboration among monsoon-related programmes in the Americas, and to initiate the planning of field programmes and process studies under VAMOS.

After a review of the VAMOS objectives by its Panel Chair, the workshop started with a series of talks that reiterated the science issues and objectives of the programme from various perspectives. Three overview presentations about the American Monsoon Systems were given. For the North American Monsoon System (NAMS) the following basic questions were reviewed: What data is required to study the physical mechanisms for the systemís life cycle, and to what extent is this life cycle captured in global and regional models? What are the relative roles of internal atmospheric dynamics, remote boundary forcing, and local and regional land surface forcing in determining the interannual and intraseasonal variability of the system? What are the dominant factors responsible for determining the interannual variability of the monsoon onset?

The Central American Monsoon System (CAMS) is not as well defined as NAMS in terms of calendar dates and distinct transitions, though the seasonal variations are quite distinct on the Pacific side of Central America. A basic question related to the mechanism of rainfall variability is whether the SST variations in the eastern Pacific warm pool exert a fundamental control on the fluctuations in Inter-tropical Convergence Zone (ITCZ) precipitation over the region. The available observational datasets are not suitable to address these issues.

The South American Monsoon System (SAMS) develops over a land mass characterized by a large area at the equator, very high mountains to the west that effectively block air transport, and surface cover that varies from tropical forests over Amazonia to high altitude deserts over the Bolivian altiplano. Plentiful moisture supply from the Atlantic maintains a precipitation maxima over central Brazil. The combination of this heat source and the orography results in seasonal evolution of convection latitude systems in the organization of tropical precipitation. Many important questions remain on the relative roles of orography, the Bolivian altiplano, the Brazilian planalto, the Andes mountains and tropical heat sources on regulating circulation features over South America. Modelling and theoretical studies have given partial answers to these questions, but validation of these results require observational confirmation with more complete data sets.

These talks were followed by a number of presentations about the role and importance of different processes (e.g., in atmosphere, ocean, land). Thereafter, the relevant field work relevant to VAMOS was reviewed which enabled the participants to establish focal points for VAMOS field programmes. Thereafter the workshop split into two working groups, one focusing on the specific problems of NAMS and CAMS and the other on the aspects of SAMS followed by a plenary discussion of the monsoon systems.

In the following the ocean-atmosphere and land-atmosphere interactions were discussed in depth. In these sessions, attempts were made to step back from the specifics of individual experiments and to address linkages such as those among regions, programmes, and between the land, atmosphere, and ocean. For example, an effort was made to diagrammatically summarize the large scale moisture transports in the Americas and the adjacent oceans. This was a particularly fruitful discussion, as it laid the groundwork for developing hypotheses that could be tested by VAMOS field programmes. One example drawn from the session is the discussion of why the warm waters of the Gulf of Mexico and the Caribbean do not act more like a traditional warm pool and the hypothesis that the amazonian convection dominated the region. Further discussion of this topic raised questions about the contrast between the influence of the warm water found on the west coast of Central America and that found in the Gulf of Mexico and about the moisture fluxes into North America from these two regions. A second example is the discussion of the stratus clouds found off Peru and Chile and the possibilities that the convection over the altiplano in South America contributes to the atmospheric subsidence in the stratus deck region and that there is some transport through the Andes between the altiplano and the eastern Pacific. Because of links between ENSO variability and climate in South America, there may thus be a possible feedback to the equatorial eastern Pacific by the Altiplano's influence on the stratus region, which in turn influences the cold tongue-warm pool region.

In the last session a number action items were identified to keep the momentum up and the development of VAMOS field programmes moving forward. In the near term, completion of a workshop report and communication of the results of the workshop was flagged as important. Groups to facilitate writing a workshop report were identified. While the science thrusts of, and plans for, work on the North American monsoon and on the eastern tropical Pacific are maturing as the Pan American Climate Study (PACS) and the Eastern Pacific Investigation of Climate processes in the coupled ocean-atmosphere system (EPIC), the discussion identified the need to encourage further developments associated with the SAMS and the stratus. After a review of planned and proposed process studies issues of data sets and enhanced monitoring relevant to VAMOS were discussed. For the further planning of the VAMOS programme, the workshop recommended to establish five small working groups with a primary task to collect, assess and integrate the information about (a) process studies, (b) data sets, (c) sustained measurements, (d) stratus, and (e) the SAMS. With the exception of the SAMS Working Group, these groups have a limited lifetime to prepare the workshop report. The SAMS Working Group was tasked to organize a workshop about the specific aspects of the South American monsoon later in 1998 or early 1999.

The VAMOS Panel met for its first session after the workshop. There was general agreement on a planning approach based on phases, each of which will address the problems of capacity building in data-void regions of the Americas. The Panel reviewed the results of the workshop and made the following recommendations:

- to endorse EPIC as a VAMOS programme;
- to endorse a field programme on the South America low-level jet (LLJ);
- to encourage PIRATA planners to extend their programme to match the EPIC period;
- to encourage the planification of a field programme on the Altiplano heat source;
- to encourage exchanges between VAMOS and to other CLIVAR panel focusing on the Pacific and Atlantic Oceans.

Furthermore, the VAMOS Panel expressed the opinion that a programme on the Southern Atlantic can contribute significant to VAMOS. It was also decided to invite an IAI Application Scientist to the next panel meeting, which was tentatively fixed for March 1999.

C.R. Mechoso and R. Weller

### III. Review of VAMOS Science Objectives

Part 1: The American Monsoon Systems (J.Paegle, T.Ambrizzi)

This part of the workshop was devoted to discussion of the monsoon circulations over the Americas. There were three presentations on North-American, Central-American and South-American monsoon systems (NAMS, CAMS and SAMS, respectively). A common feature to all these three monsoonal circulations is the complexity of their time variability, with important modulations in the diurnal, intraseasonal and interannual scales.

a) NAMS (W. Higgins)

There are different research programmes (such as GCIP, PACS and the joint GCIP/PACS) that contribute to VAMOS. The joint GCIP/PACS programme is based on the premise that there is a deterministic element in the year to year variability of summertime precipitation and temperature over North-America. It is known that time scales of variability of the North-American hydrological cycle range from intraseasonal to centennial. Examples are the 1998 U.S. midWest drought, the 1993 floods and the Dustbowl era of the 30's and 40's.

The evolution of the NAMS appears to be well established. There is a development phase during the months of May and June, characterized by a weakening and northward shift of the extratropical storm track, with the initiation of the Mexican monsoon, increasing frequency of occurrence of low level jets east of the Rockies and strengthening of the upper troposphere monsoon high. This is followed by a mature phase (July and August) that sees the Mexican monsoon extending into Arizona and New Mexico, establishment of the warm season continental precipitation regime and an upper troposphere ridge located over the west/central U.S. The decay phase (September-October) sees a reversal of the development phase and slow weakening of the system. Dates of monsoon onset latitudinal progression were given based on precipitation data, from June 5 at 15°N to mid-July at 35°N. Monsoons with long periods of heavy rainfall after onset average out to anomalously wet monsoons. North-American monsoons exhibit interesting regional variations. For example, rainfall over SW Mexico is found to be in phase with rainfall in much of the monsoon region and out of phase with precipitation in the Great Plains. The onset date for the SW Mexico monsoon is highly correlated with interannual fluctuations in rainfall over the entire monsoon region. It is also found that wet/dry monsoons in the SW often follow winters characterized by dry/wet conditions in the Pacific northwest. Questions remain on the extent to which this latter association is due to season-to-season memory of the coupled ocean-atmosphere-land system.

b) CAMS (V. Magaña)

Sea surface temperatures (SSTs) have an important effect in modulating the seasonal cycle in the vicinity of Central America. The summer rainy season over Southern Mexico, Central America and the Caribbean occurs from May through October, with strong local modulations due to orography. The seasonal precipitation exhibits a double maximum with a relative minimum during July and August (referred to as the mid-summer drought). Possible explanations for this feature involve interactions between SSTs off the western coast of Mexico, solar insolation, convection and cooling effects due to intense trade winds. Questions remain on mechanisms that modulate convection over the Intra Americas Sea (IAS). In particular, reasons for weak convective activity and low precipitation rate over the IAS warm pool are not well known. There are well defined interannual variations in precipitation over Mexico and Central America. Drought over this region (associated with enhanced subsidence) is found with an equatorward shift in the mean position of the ITCZ during El Niño years.

c) SAMS (P. Silva-Dias)

The conventional definition of a monsoon circulation requires a full seasonal reversal of low level winds. This is nowhere observed over South America. Nevertheless, a high pressure system develops over elevated terrain over Bolivia (the "Bolivian high") during the austral summer. The South Atlantic Convergence Zone (SACZ) is a main feature of summer convection over this continent. Other north-west to south-east convergence zones, such as those over the South Pacific (SPCZ) and east of Africa, are conspicuous features during austral summer which do not appear to have counterparts in the Northern Hemisphere. The morphology of South America is unique with a large continental area at the equator, very high mountains to the west that effectively block air transport, and surface cover that varies from tropical forests over Amazonia to high altitude deserts over the Bolivian altiplano. Plentiful moisture supply from the Atlantic maintains precipitation maxima over central Brazil. The combination of this heat source and the orography results in seasonal evolution of convection unique to this region. Furthermore, there is an important influence of mid-latitude systems organizing tropical precipitation. Many important questions remain on the relative roles of orography, the Bolivian altiplano, the Brazilian planalto, the Andes mountains and tropical heat sources (both over South America and other continents) on regulating circulation features over South America. Modelling and theoretical studies have given partial answers to these questions, but of the results obtained in those studies require observational confirmation with more complete data sets.

Part 2: Processes of the Monsoon Systems (I. Wainer, P. Aceituno)

The annual cycle of the monsoon, or the onset and demise of the rainy season has led the population of the monsoon controlled regions to divide their lives between the rainy and the dry cycles. The physical processes that govern the onset and demise of the wet and dry cycles associated with the monsoons were once believed to be local in origin. The advancement of global observing systems and the understanding of large scale ocean-atmosphere interaction processes and its global impact brought a new perspective into the nature of the problem. The physical processes that govern the coupled ocean-atmosphere-land system are very complex. As pointed out by Webster et. al. (1998) the monsoon may be thought of as a circulation responding to the annual cycle of solar heating in an interactive ocean-atmosphere-land system. In addition to the interactive elements of the monsoon (discussed below), there are influences from other climate systems such as the El Niño Southern Oscillation (ENSO), the Atlantic dipole and the extratropical regime. The monsoon systems are driven by the differential heating of the land and the ocean and are affected by land-surface processes and orography and moist processes, which determine the strength, location and duration of the rainy season.

a) The role of atmospheric processes (V. B. Rao)

This presentation addressed the differences between the strength and duration of the SAMS and NAMS. SAMS behaves similarly to the East-Asian Monsoon. However, the land-sea contrasts leading to the differential heating which drives the Asian monsoon do not happen in South America. The importance of the SACZ, and its land versus ocean controls was noted. The determination of moisture budgets associated with monsoon process is fundamental to understand the variability of the system.

b) The role of ocean processes (R. Weller)

To advance modelling and prediction of the monsoon systems it is necessary to improve our knowledge of the seasonal to interannual variability of climate. To understand the monsoon system it is important to comprehend how the atmosphere and ocean communicate. This interaction is predominantly through the interfacial fluxes of heat, moisture and momentum.

This presentation emphasized that SST is a fundamental driver of the monsoon system and to access its variability one needs to know the complex interaction between the tropical SST and surface forcing on seasonal to interannual time-scales. To understand how tropical SSTs in the Americas affect climate variability in the American sector, one needs to improve our knowledge of the ocean processes that govern the SST variability: these include air-sea exchanges of heat, freshwater, momentum fluxes on broad ranges of time scales, plus three-dimensional advection processes from basin to local scale and horizontal and vertical mixing as well, on time-scales ranging from turbulent scale to interannual.

The presentation also addressed the ocean processes modelling questions within the PACS domain. The large scale questions associated with the connections of equatorial current system and the fluxes of heat, salt and freshwater between the ITCZ and the pacific cold-tongue were discussed. The discussion included the complexity of air-sea interaction and the role of high frequency forcing, coastal processes that modify SST and coupled processes studies such as the interaction of ocean currents with nearby orography forcing.

c) The role of land processes (R. Koster)

The basic elements of land-atmosphere interaction are the exchanges of moisture and energy between these two systems. Fluxes of moisture and heat from the land surface help determine the overlying distributions of atmospheric temperature, water vapour, precipitation, cloud properties, and hence even the downward radiative fluxes at the surface. This presentation emphasized the importance of interactive land surface parameterizations in a General Circulation Models (GCM). Inclusion of such processes leads to substantially greater interannual variability of precipitation over both tropical and mid-latitude land than does the inclusion of observed ocean temperature variations. The implication is that the understanding of continental precipitation variability ó including that associated with monsoon systems ó requires an understanding of how land surface energy and water balance anomalies influence local water recycling and the general circulation itself.

The nature of this feedback in a modelling system depends on the chosen representation of land and atmospheric processes and on the manner in which they are coupled. While some of these coupling issues are common to the study of ocean-atmosphere interaction, a unique difficulty in the treatment of land-atmosphere interaction is the extensive spatial heterogeneity found in land surface characteristics. Much current research is aimed at developing parsimonious representations of this heterogeneity that will allow operational land surface models to go beyond the standard one-dimensional representations of surface physics currently available.

d) The role of coupled atmosphere-ocean processes (P. Chang)

The importance of coupled ocean-atmosphere variability over the tropical Atlantic sector in monsoon rainfall over the neighbouring continents was already recognized in the early 1980s. In contrast to the Pacific, where equatorial SST anomalies dominate, off-equatorial SST anomalies in the tropical Atlantic appear to be most energetic on interannual-to-decadal time scales. These SST anomalies influence the temperature gradient near the equator, which has a strong impact on the position of ITCZ and consequently rainfall over the neighbouring continents. Changes in the SST gradient are accompanied by spatially coherent cross-equatorial surface wind anomalies and changes in the trade winds on both sides of the equator. The pattern of SST and wind anomaly suggests a positive air-sea feedback through surface heat flux. Although some modelling studies suggest that the air-sea interaction could lead to decadal oscillation of SST in the tropical Atlantic, major uncertainties remain in our understanding of the fundamental processes of air-sea interactions in the tropical Atlantic. For example, because of lack of reliable observations, we do not know the exact relationships between the surface fluxes and SSTs. We also do not understand how important the tropical Atlantic SSTs are in driving the atmospheric circulation, and how much tropical Atlantic SST variability can be attributed to local air-sea interactions and how much can be attributed to remote influence of Pacific ENSO. An improved understanding of the coupled processes in the tropical Atlantic is essential for the development of satisfactory predictive models for the region. Advances are most likely through multidisciplinary process studies that connect the upper ocean and lower atmosphere.

P art 3: Predictability and Prediction (A. Moura)

The International Research Institute for climate prediction (IRI) has been established through a co-operative agreement between NOAA/Office of Global Programs, Columbia University/Lamont-Doherty Earth Observatory, and the University of California, San Diego/Scripps Institution of Oceanography. The mission of the IRI is to assess and continually develop seasonal-to-interannual climate predictions; to produce the best and most useful climate forecast and prediction information on a routine basis; and to apply such information to the benefit of affected societies throughout the word. The IRI will address all aspects of end-to-end prediction, including model and forecast system development, experimental prediction, climate monitoring and dissemination, applications research, and training, in coordination and collaboration with the international climate research and growing network of applications centres and activities. The IRI involvement in the CLIVAR/VAMOS programme is meant to support the development of the scientific agenda as well as providing a framework for modelling and climate simulation and data assimilation studies to justify the observational systems such as GOOS, GCOS, PIRATA, LBA among others. The "end-to-end" approach taken by the IRI provides stimulus for national resources allocated to the above mentioned programmes and projects, via practical application demonstration projects in crucial areas such as agriculture, public health, water resources, as related to climate fluctuations and their prediction.

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