Antonio J. Busalacchi

The Tropical Ocean‐Global Atmosphere observing system: A decade of progress

A major accomplishment of the recently completed Tropical Ocean-Global Atmosphere (TOGA) Program was the development of an ocean observing system to support seasonal-to-interannual climate studies. This paper reviews the scientific motivations for the development of that observing system, the technological advances that made it possible, and the scientific advances that resulted from the availability of a significantly expanded observational database. A primary phenomenological focus of TOGA was interannual variability of the coupled ocean-atmosphere system associated with El Nino and the Southern Oscillation (ENSO).Prior to the start of TOGA, our understanding of the physical processes responsible for the ENSO cycle was limited, our ability to monitor variability in the tropical oceans was primitive, and the capability to predict ENSO was nonexistent. TOGA therefore initiated and/or supported efforts to provide real-time measurements of the following key oceanographic variables: surface winds, sea surface temperature, subsurface temperature, sea level and ocean velocity. Specific in situ observational programs developed to provide these data sets included the Tropical Atmosphere-Ocean (TAO) array of moored buoys in the Pacific, a surface drifting buoy program, an island and coastal tide gauge network, and a volunteer observing ship network of expendable bathythermograph measurements. Complementing these in situ efforts were satellite missions which provided near-global coverage of surface winds, sea surface temperature, and sea level. These new TOGA data sets led to fundamental progress in our understanding of the physical processes responsible for ENSO and to the development of coupled ocean-atmosphere models for ENSO prediction.

A Hybrid Vertical Mixing Scheme and Its Application to Tropical Ocean Models

Abstract A novel hybrid vertical mixing scheme, based jointly on the Kraus–Turner-type mixed layer model and Prices dynamic instability model, is introduced to aid in parameterization of vertical turbulent mixing in numerical ocean models. The scheme is computationally efficient and is capable of simulating the three major mechanisms of vertical turbulent mixing in the upper ocean, that is, wind stirring, shear instability, and convective overturning. The hybrid scheme is first tested in a one-dimensional model against the Kraus–Turner-type bulk mixed layer model and the Mellor–Yamada level 2.5 (MY2.5) turbulence closure model. As compared with those two models, the hybrid model behaves more reasonably in both idealized experiments and realistic simulations. The improved behavior of the hybrid model can be attributed to its more complete physics. For example, the MY2.5 model underpredicts mixed layer depth at high latitudes due to its lack of wind stirring and penetrative convection, while the Kraus–Turn...

The Pirata Program: History, Accomplishments, and Future Directions

The Pilot Research Moored Array in the tropical Atlantic (PIRATA) was developed as a multinational observation network to improve our knowledge and understanding of ocean-atmosphere variability in the tropical Atlantic. PIRATA was motivated by fundamental scientific issues and by societal needs for improved prediction of climate variability and its impact on the economies of West Africa, northeastern Brazil, the West Indies, and the United States. In this paper the implementation of this network is described, noteworthy accomplishments are highlighted, and the future of PIRATA in the framework of a sustainable tropical Atlantic observing system is discussed. We demonstrate that PIRATA has advanced beyond a “Pilot” program and, as such, we have redefined the PIRATA acronym to be “Prediction and Research Moored Array in the Tropical Atlantic.”

An Improved Procedure for EI Niño Forecasting: Implications for Predictability

A coupled ocean-atmosphere data assimilation procedure yields improved forecasts of El Ni�o for the 1980s compared with previous forecasting procedures. As in earlier forecasts with the same model, no oceanic data were used, and only wind information was assimilated. The improvement is attributed to the explicit consideration of air-sea interaction in the initialization. These results suggest that EI Ni�o is more predictable than previously estimated, but that predictability may vary on decadal or longer time scales. This procedure also eliminates the well-known spring barrier to EI Ni�o prediction, which implies that it may not be intrinsic to the real climate system.

Origin of upper-ocean warming and El Nino change on decadal scales in the tropical Pacific Ocean

The cause of decadal-scale variability in the tropical Pacific Ocean—such as that marked by the 1976–77 shift in the El Niño/Southern Oscillation—is poorly understood. Unravelling the mechanism of the recent decade-long warming in the tropical upper ocean is a particularly important challenge, given the link to El Niño variability, but establishing the hypothesized interannual/decadal oceanic connections between middle latitudes and tropics has proved elusive. Here we present observational evidence that Pacific upper-ocean warming and decadal changes in the El Niño/Southern Oscillation after 1976 may originate from decadal mid-latitude variability. In the middle 1970s the North Pacific Ocean is observed to have undergone a clear phase-transition; a ‘see-saw’ subsurface temperature anomaly pattern that rotates clockwise around the subtropical gyre. At middle latitudes a subsurface warm anomaly formed in the early 1970s from subducted surface-waters and penetrated through the subtropics and into the tropics, thus perturbing the tropical thermocline and driving the formation of a warm surface-water anomaly that may have influenced El Niño in the 1980s. The identification of this teleconnection of extratropical thermal anomalies to the tropics, through a subsurface ocean ‘bridge’, may enable improved prediction of decadal-scale climate variability.

Environmental signatures associated with cholera epidemics

The causative agent of cholera, Vibrio cholerae, has been shown to be autochthonous to riverine, estuarine, and coastal waters along with its host, the copepod, a significant member of the zooplankton community. Temperature, salinity, rainfall and plankton have proven to be important factors in the ecology of V. cholerae, influencing the transmission of the disease in those regions of the world where the human population relies on untreated water as a source of drinking water. In this study, the pattern of cholera outbreaks during 1998–2006 in Kolkata, India, and Matlab, Bangladesh, and the earth observation data were analyzed with the objective of developing a prediction model for cholera. Satellite sensors were used to measure chlorophyll a concentration (CHL) and sea surface temperature (SST). In addition, rainfall data were obtained from both satellite and in situ gauge measurements. From the analyses, a statistically significant relationship between the time series for cholera in Kolkata, India, and CHL and rainfall anomalies was determined. A statistically significant one month lag was observed between CHL anomaly and number of cholera cases in Matlab, Bangladesh. From the results of the study, it is concluded that ocean and climate patterns are useful predictors of cholera epidemics, with the dynamics of endemic cholera being related to climate and/or changes in the aquatic ecosystem. When the ecology of V. cholerae is considered in predictive models, a robust early warning system for cholera in endemic regions of the world can be developed for public health planning and decision making.

A Pilot Research Moored Array in the Tropical Atlantic (PIRATA)

Abstract The tropical Atlantic Ocean is characterized by a large seasonal cycle around which there are climatically significant interannual and decadal timescale variations. The most pronounced of these interannual variations are equatorial warm events, somewhat similar to the El Nino events for the Pacific, and the so-called Atlantic sea surface temperature dipole. Both of these phenomena in turn may be related to El Nino-Southern Oscillation variability in the tropical Pacific and other modes of regional climatic variability in ways that are not yet fully understood. PIRATA (Pilot Research Moored Array in the Tropical Atlantic) will address the lack of oceanic and atmospheric data in the tropical Atlantic, which limits our ability to make progress on these important climate issues. The PIRATA array consists of 12 moored Autonomous Temperature Line Acquisition System buoy sites to be occupied during the years 1997-2000 for monitoring the surface variables and upper-ocean thermal structure at key location...

Effects of Penetrative Radiation on the Upper Tropical Ocean Circulation

Abstract The effects of penetrative radiation on the upper tropical ocean circulation have been investigated with an ocean general circulation model (OGCM) with attenuation depths derived from remotely sensed ocean color data. The OGCM is a reduced gravity, primitive equation, sigma coordinate model coupled to an advective atmospheric mixed layer model. These simulations use a single exponential profile for radiation attenuation in the water column, which is quite accurate for OGCMs with fairly coarse vertical resolution. The control runs use an attenuation depth of 17 m while the simulations use spatially variable attenuation depths. When a variable depth oceanic mixed layer is explicitly represented with interactive surface heat fluxes, the results can be counterintuitive. In the eastern equatorial Pacific, a tropical ocean region with one of the strongest biological activity, the realistic attenuation depths result in increased loss of radiation to the subsurface, but result in increased sea surface te...

Simulation of the Tropical Oceans with an Ocean GCM Coupled to an Atmospheric Mixed-Layer Model

Abstract A reduced gravity, primitive equation, ocean general circulation model (GCM) is coupled to an advective atmospheric mixed-layer (AML) model to demonstrate the importance of a nonlocal atmospheric mixed-layer parameterization for a proper simulation of surface heat fluxes and sea surface temperatures (SST). Seasonal variability of the model SSTs and the circulation are generally in good agreement with the observations in each of the tropical oceans. These results are compared to other simulations that use a local equilibrium mixed-layer model. Inclusion of the advective AML model is demonstrated to lead to a significant improvement in the SST simulation in all three oceans. Advection and diffusion of the air humidity play significant roles in determining SSTs even in the tropical Pacific where the local equilibrium assumption was previously deemed quite accurate. The main, and serious, model flaw is an inadequate representation of the seasonal cycle in the upwelling regions of the eastern Atlantic...

Salinity effects in a tropical ocean model

A reduced gravity, primitive equation, ocean general circulation model (GCM) with a variable depth mixed layer and a natural boundary condition for freshwater fluxes is employed to investigate the role of salinity in tropical ocean dynamics and thermodynamics. Surface heat fluxes are computed without any feedback to observations by an advective atmospheric mixed layer (AML) model which is coupled to the ocean GCM. We analyze the differences in the tropical Atlantic, Pacific, and the Indo-Pacific basins between control runs (simulations with complete hydrology) and simulations where (1) precipitation (P) is neglected, (2) salinity effects are neglected, or (3) salinity is held constant in each layer. Salinity contributes to pressure gradient forces, mixed layer processes, and vertical stability/mixing. Setting P = 0 in the tropical Atlantic produces larger sea surface temperature (SST) changes than previously estimated due to the realistic oceanic mixed layer model and surface flux formulation. Neglecting salinity effects leads to a different choice of mixing parameters, which feeds back into model dynamics and thermodynamics. Salinity anomalies produce an asymmetric response across the equator in the Atlantic due to differences in the air-sea interactions. Including salinity effects in the tropical Pacific leads to an improved cold tongue simulation. The result is a reduced SST gradient at the equator which will have significant feedback in a coupled system. The same experiment with a restoring surface heat flux leads to an increased SST gradient, indicating that the surface flux parameterization is crucial for interpreting the role of salinity. The Indonesian throughflow (ITF) is reduced when salinity is neglected or held constant. The NINO3 and NINO4 SST indices are almost identical for the control run and the simulations when climatological P is used. However, associated subsurface temperature differences are larger, and they may play a role on decadal timescales. It is thus shown with a comprehensive set of experiments that even in the tropics, salinity plays an important role in the model dynamics and thermodynamics.