OS44A-01 INVITED
The Andes, the SPCZ and the eastern Pacific ITCZ
Idealized coupled ocean-atmosphere model experiments indicate that the mechanical effect of the Andes mountains on the atmospheric flow and local air-sea interactions are enough to modify a zonally and equatorially symmetric distribution of tropical sea surface temperature (SST) and rainfall to become qualitatively similar to the observed in the Pacific. The main mechanism responsible is the advection by the southeasterly trade winds of low moist static energy air masses brought in contact with the surface of the southeast Pacific by subsidence induced by the presence of the Andes, which results in reduced SST (through surface evaporation) and precipitation in a region extending equatorward and westward from the southeast Pacific, leading to a precipitation distribution featuring a northward displaced intertropical convergence zone (ITCZ) and a south Pacific convergence zone (SPCZ) oriented in a southeast-northwest direction. Positive air-sea feedback processes are important for the magnitude of the response, particularly the interaction between low cloud albedo and SST in the southeast Pacific. The spatial extent of the dry and cool region is a function of the speed of advection by the trade winds, the entrainment rate into the atmospheric boundary layer and the surface drag coefficient, while its southeast- northwest orientation results mainly from the direction of the southeasterly trades. The latter promotes the formation of the SPCZ, which is organized further by local air-sea feedback.
OS44A-02
Tropical-Extratropical Interaction and Intraseasonal Rainfall Variability within the SPCZ
The South Pacific Convergence Zone (SPCZ) is within a region characterized by strong atmospheric tropical-extratropical interactions. The northwest-southeast oriented portion of the SPCZ shares many of the same characteristics as other diagonal convergence zones across the globe, namely, a dominance of transient precipitation events linked to frontal and Rossby wave activity originating within the midlatitude westerlies. We show that deep convection within the diagonal portion of the SPCZ is controlled primarily by the impingement of upper level troughs embedded within Rossby wave trains propagating into the western Pacific from the extratropical Indian Ocean. This interaction is favored by the existence of a wave guiding due to the large scale basic state flow over the region. As a contrast, the dominant mode of precipitation variability in the more zonally-oriented tropical portion of the SPCZ, equatorward of around 10S latitude, is linked to equatorial Rossby (ER) wave activity propagating westward. This equatorial wave activity is in turn frequently triggered by extratropical wave activity propagating into the east Pacific from the Northern Hemisphere. Both types of synoptic scale activity is significantly modulated by the Madden-Julian Oscillation (MJO), through its intraseasonal alteration of the lower frequency background basic state flow.
OS44A-03
An Overview of Sea Surface Salinity Variability in the South Pacific Convergence Zone
Sea Surface Salinity (SSS) includes relevant information about climate change and the earth's water cycle at different time and space scales. Its importance for the climate motivated the development of existing in situ observing systems and future dedicated satellite missions (SMOS and Aquarius) to enhance global observations. In this presentation, we present an overview of recent published results obtained from the analysis of SSS data collected from voluntary observing ships and derived from coral skeleton in the South Pacific Convergence Zone (SPCZ) for the period 1970-2006. Emphasis is on the seasonal cycle, the El Nino Southern Oscillation (ENSO) phenomenon, the PDO (Pacific Decadal Oscillation)-like decadal signal and long-term salinity trends. We first quantify the regional manifestations of these basin scale signals in the SPCZ, and then interpret the signals in terms of changes in precipitation, evaporation and/or horizontal salt advection. The implications of the observed SSS variability regarding SPICE (South PacIfic Ocean Circulation and Climate Experiment) objectives are discussed. http://www.legos.obs- mip.fr/observations/sss/
OS44A-04 INVITED
Formation and Circulation of Thermocline Waters in the South Pacific
Using available high-resolution CTD observations, complemented by a large number of recently available Argo floating profiles, this study provides a brief description of the formation and circulation of thermocline waters in the South Pacific. Based on a new, reliable climatological dataset on mixed layer properties, an annual subduction rate of about 50 Sv (1 Sv=106 m3 s-1) is obtained. Two peaks stand out in this subduction rate sorted by winter mixed layer density. One corresponds to the formation of Eastern Subtropical Mode Water and the other to the formation of Sub-Antarctic Mode and Antarctic Intermediate Water. After being subducted into the thermocline, these waters circulate about the subtropical gyre and make their ways to the equator. Both interior and western boundary pathways are revealed, but the latter becomes increasingly important with depth. Some preliminary results on the origin and pathway of Equatorial 13°C Water will also be presented, which we believe will initiate a more complete understanding of how the shallow subtropical cells relate to thermocline waters originating in the South Pacific and how variabilities in these waters contribute to climate variability.
OS44A-05
Circulation in the Southwest tropical Pacific for the year 2000 from an assimilated model
A regional eddy-permitting model of the tropical Pacific has been adjusted to fit observations for the year 2000 by controlling initial conditions, boundary conditions, and forcing. The model domain covers 26S to 26N and has uniform 1/3 degree resolution. The fit includes most observations: SSH and SST from satellites, in-situ T and S profiles from floats and TAO moorings, and u and v from drifters and TAO moorings. Except for the forcing, initial conditions, and boundary conditions adjustments, the model dynamics were assumed to be perfect. The resultant circulation obeys the model equations, and could in principle be reproduced by another model using the same controls. The restricted set of controls and the finite number of iterations (approximately 140) means that exact matches to all observations are not expected. The mean and time-evolving circulation in the Southwest (SW) tropics will be discussed and compared to that inferred from Argo profiles and other observations. In addition, the sensitivity of circulation features to surface forcing, both local and remote, will be explored.
OS44A-06
Monitoring the East Australian Current - a historical perspective and future plans
We present a review of results from a 15-year time-series of quarterly eddy-resolving XBT surveys of the East Australian Current in the southwestern Pacific. The region contains the South Pacific's subtropical western boundary current system, the East Australian Current (EAC), with its associated intense mesoscale variability. It provides both the western termination of the South Pacific Gyre and a linking element between the Pacific and Indian Ocean gyres. Western Boundary Current systems such as the EAC are important elements of the ocean circulation heat engine, removing excess heat from the tropics and releasing it to the mid-latitude atmosphere. The outflow of the EAC also provides a pathway for mode and intermediate waters to move from the Pacific to the Indian Oceans. This outflow represents a further component of the global thermohaline circulation. The XBT transects are supplemented with observations from research cruises, broad-scale XBT and satellite altimetric height data, and air-sea fluxes from ECMWF and NCEP analyses. Time series of EAC flow from 1993-2004 are generated by merging XBT and altimeter data. Net water mass conversions in the upper ocean reflect the net evaporation and heat loss in the formation of South Pacific Subtropical Mode Water. A multi-year variation in EAC transport is documented and related to decadal SST signals around Tasmania. Connections are made between the Tasman Box observations and both the decadal spin-up of the South Pacific Gyre and the southward penetration of EAC waters to Tasmania. Plans for future observation activity within the SPICE program are outlined.
OS44A-07 INVITED
A Coupled GCM Intercomparison Study of the South Pacific Convergence Zone
The South Pacific Convergence Zone (SPCZ) is an important component of seasonal climate variations in the Southern Hemisphere. Though several associated processes are already discussed using observational data, the SPCZ is yet to be resolved properly in global general circulation models (GCMs). Particularly, the ocean- atmosphere coupled GCMs often fail to simulate the correct orientation and the zonal extent of the SPCZ. Most of these models replicate an east-west zonally oriented ITCZ similar to that in the Northern Hemisphere giving rise to the so-called double ITCZ problem. In this study simulation results from a variety of models are used to understand model biases in resolving the temporal and spatial distribution of the SPCZ. These models range from standalone atmospheric GCMs to the state of the art ocean-atmosphere coupled GCMs. It is found that the seasonal SPCZ in standalone atmospheric GCM results is better represented than that in the coupled GCM with an identical atmospheric component and a spatial resolution of about 100 km. The dry zone to the east of the SPCZ is not well-formed in coupled GCMs, particularly in austral summer when the SPCZ is pronounced. This is related to the model biases of the sea surface temperature, which is warmer in eastern Pacific in coupled GCM than the observation. The bias is not as clearly manifested in a spatially higher resolution coupled GCM in which the eastern Pacific SST is better simulated. The increase in model horizontal resolution helps in resolving the local air-sea interactions, the cross- equatorial winds and the local circulation cells. The dry zone is also improved in another experiment in which the improvement in model coupling physics improved the bias in equatorial cold tongue. It is also found that the SPCZ simulation is not improved by just increasing the vertical resolution in coupled GCM. This also imply that a proper representation of the boundary layer and the associated physics is more important. The meridional tilt, which is distinctly observed in the southern part of the SPCZ, is not well reproduced in any of the coupled GCM results. The biases in coupled GCM storm tracks seem to cause the biases in the SPCZ tilt. Interestingly, a standalone atmospheric GCM, with realistic boundary forcings, captures the tilt and the overall pattern closer to observation. The interaction of SPCZ with neighboring land processes is not very clear in that model results though. The seasonal variation in the spatial rainfall pattern of the SPCZ did not change much when the Australian continent was replaced by seasonal SSTs in a standalone atmospheric GCM experiment. On interannual time-scale, the link between the SPCZ and the El Nino/Southern Oscillation (ENSO) is not very clear. The observed data in austral summer shows a significant correlation between the rainfall index derived from the SPCZ region and the central Pacific SST anomalies. Therefore, it is possible that the newly identified El Nino Modoki (or pseudo El Nino) could influence the interannual variability of SPCZ. During an El Nino Modoki event, the warm central tropical Pacific is flanked by clod anomalies on either side. The SPCZ rainfall index is also seen to be significantly correlated with the Indian Ocean Dipole at one season lag. Apparently, the SPCZ plays an important role in the information exchanges between the two neighboring basins.
OS44A-08
The South Pacific Convergence Zone and Southern Hemisphere Teleconnections
The South Pacific Convergence Zone (SPCZ) delineates the main region of quasi-persistent convection in the tropical southwest Pacific. Interannual variability in its location has a significant impact on precipitation and to a lesser extent on temperature variability across many southwest Pacific Island states. SPCZ variability is significantly linked to variations in the El Niņo-Southern Oscillation (ENSO) cycle, and to longer-scale variability in the Pacific characterised by the Interdecadal Pacific Oscillation (IPO, also known as the Pacific Decadal Oscillation). There also appear to be links between the SPCZ and higher latitude Southern Hemisphere circulation variability and the Southern Annular Mode. This presentation will review the nature of ENSO/IPO relationships with the SPCZ, and will discuss mechanisms for teleconnections with the SAM and the extratropical Southern Hemisphere circulation.