Gary Meyers

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.

What causes southeast Australia's worst droughts?

Since 1995, a large region of Australia has been gripped by the most severe drought in living memory, the so-called ‘‘Big Dry’’. The ramifications for affected regions are dire, with acute water shortages for rural and metropolitan areas, record agricultural losses, the dryingout of two of Australia’s major river systems and farreaching ecosystem damage. Yet the drought’s origins have remained elusive. For Southeast Australia, we show here that the ‘‘Big Dry’’ and other iconic 20th Century droughts, including the Federation Drought (1895–1902) and World War II drought (1937–1945), are driven by Indian Ocean variability, not Pacific Ocean conditions as traditionally assumed. Specifically, a conspicuous absence of Indian Ocean temperature conditions conducive to enhanced tropical moisture transport has deprived southeastern Australia of its normal rainfall quota. In the case of the ‘‘Big Dry’’, its unprecedented intensity is also related to recent higher temperatures. Citation: Ummenhofer, C. C., M. H. England, P. C. McIntosh, G. A. Meyers, M. J. Pook, J. S. Risbey, A. S. Gupta, and A. S. Taschetto (2009), What causes southeast Australia’s worst droughts?,

The Years of El Niño, La Niña, and Interactions with the Tropical Indian Ocean

Abstract The Indian Ocean zonal dipole is a mode of variability in sea surface temperature that seriously affects the climate of many nations around the Indian Ocean rim, as well as the global climate system. It has been the subject of increasing research, and sometimes of scientific debate concerning its existence/nonexistence and dependence/independence on/from the El Nino–Southern Oscillation, since it was first clearly identified in Nature in 1999. Much of the debate occurred because people did not agree on what years are the El Nino or La Nina years, not to mention the newly defined years of the positive or negative dipole. A method that identifies when the positive or negative extrema of the El Nino–Southern Oscillation and Indian Ocean dipole occur is proposed, and this method is used to classify each year from 1876 to 1999. The method is statistical in nature, but has a strong basis on the oceanic physical mechanisms that control the variability of the near-equatorial Indo-Pacific basin. Early in ...

Variation of Indonesian throughflow and the El Niño‐Southern Oscillation

The expendable bathythermograph (XBT) line Fremantle-Sunda Strait transects the eastern Indian Ocean between northwestern Australia and Java. It was established in 1983 with low-density sampling and upgraded to a frequently repeated line (>18 times per year) in 1987 to monitor currents. The observations during 1983 to 1994 are described and related to the field of wind stress. Variation of thermal structure shows a rich mixture of annual, semiannual, and interannual timescales. Empirical orthogonal function (EOF) analysis of anomalies of sea surface temperature (SST), dynamic height, and depth of the 20°C isotherm D20 identifies two distinctive signals. The El Nino - Southern Oscillation (ENSO) signal (EOF 1) appears throughout the region and is strongest off the coast of Australia. A modulation of the annual signal (EOF 2) appears off the coast of Java. EOF 2 has a shorter timescale than the ENSO signal, and its temporal coefficients are correlated to zonal winds over the equatorial Indian Ocean. For both EOFs, anomalously low SST and dynamic height occur at the same time as anomalously shallow D20 and vice versa for opposite anomalies. The XBT data are used with a climatological temperature-salinity relationship to calculate net, relative (0/400 dbar) geostrophic transports T through the section. For long timescales, T is representative of Indonesian throughflow. The variations associated with ENSO show a maximum during the La Nina of 1988–1989 and minima during the El Ninos of 1986–1987 and 1991–1994. The peak-to-trough amplitude of the ENSO signal is 5 Sv. The ENSO signal in throughflow is discussed in terms of earlier studies. For the shorter timescales, T is representative of currents from the Indian Ocean flowing in and out of the region between northwestern Australia and Indonesia, changing the volume of upper layer water stored there. Associated with EOF 2, a sharp peak in westward transport developed during May to October 1994.

RAMA: The research moored array for African-Asian-Australian monsoon analysis and prediction

The Indian Ocean is unique among the three tropical ocean basins in that it is blocked at 25°N by the Asian landmass. Seasonal heating and cooling of the land sets the stage for dramatic monsoon wind reversals, strong ocean–atmosphere interactions, and intense seasonal rains over the Indian subcontinent, Southeast Asia, East Africa, and Australia. Recurrence of these monsoon rains is critical to agricultural production that supports a third of the worlds population. The Indian Ocean also remotely influences the evolution of El Nino–Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), North American weather, and hurricane activity. Despite its importance in the regional and global climate system though, the Indian Ocean is the most poorly observed and least well understood of the three tropical oceans. This article describes the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA), a new observational network designed to address outstanding scientific questions related to Indian Ocean variability and the monsoons. RAMA is a multinationally supported element of the Indian Ocean Observing System (IndOOS), a combination of complementary satellite and in situ measurement platforms for climate research and forecasting. The article discusses the scientific rationale, design criteria, and implementation of the array. Initial RAMA data are presented to illustrate how they contribute to improved documentation and understanding of phenomena in the region. Applications of the data for societal benefit are also described.

An Intersection of Oceanic Waveguides: Variability in the Indonesian Throughflow Region

Abstract Temperature and sea level variability within the Indonesian seas and southeast Indian Ocean are described based on expendable bathythermograph deployments along volunteer merchant shipping lines under way since 1983. These data resolve variability at time scales ranging from the intraseasonal to the interannual. A lagged partial regression technique reveals that anomalies from a mean seasonal cycle of temperature and sea level for seasonal to interannual time scales can be largely understood in terms of free Kelvin and Rossby waves generated by remote zonal winds along the equator of the Indian and Pacific Oceans, with local wind forcing appearing to play a minor role. About 60%–90% of sea level variability and 70% of thermocline temperature variability can be accounted for in this way. Variations in zonal Pacific equatorial winds force a response along the Arafura/ Australia shelf break through Pacific equatorial Rossby waves exciting coastally trapped waves off the western tip of New Guinea, wh...

Observed temperature trends in the Indian Ocean over 1960–1999 and associated mechanisms

The linear trends in oceanic temperature from 1960 to 1999 are estimated using the new Indian Ocean Thermal Archive (IOTA), a compilation of historical temperature profiles. Widespread surface warming is found, as in other data sets, and reproduced in IPCC climate model simulations for the 20th century. This warming is particularly large in the subtropics, and extends down to 800 m around 40-50 degrees S. Models suggest the deep-reaching subtropical warming is related to a 0.5 degrees southward shift of the subtropical gyre driven by a strengthening of the westerly winds, and associated with an upward trend in the Southern Annular Mode index. In the tropics, IOTA shows a subsurface cooling corresponding to a shoaling of the thermocline and increasing vertical stratification. Most models suggest this trend in the tropical Indian thermocline is likely associated with the observed weakening of the Pacific trade winds and transmitted to the Indian Ocean by the Indonesian throughflow.

Forced Rossby waves in the southern tropical Indian Ocean

Seasonal and interannual variation of the upper southern tropical Indian Ocean (STIO) is described by harmonic and empirical orthogonal function (EOF) analysis of the depth of the 20°C isotherm (D20) derived from expendable bathythermograph (XBT) data and from an ocean general circulation model (OGCM). The harmonic analysis shows a band of large annual amplitude between 8° and 20°S extending across the STIO with a steady westward propagation in both the XBT and model data. The generation of the annual wave is discussed in terms of Ekman pumping and the westward propagation of long, nondispersive, baroclinic Rossby waves. The Ekman pumping on a large scale over the open ocean strongly modifies waves radiating from the eastern boundary and generates much of the structure in the amplitude and the phase of the annual signal in the STIO. The EOF analysis shows strong interannual variation of the anomaly of D20 on two XBT lines covering the western and eastern sides of the basin. Large interannual anomalies are observed in a band between 6° and 14°S on the western side, with deep D20 in 1988, 1991, and 1994 and shallow D20 in 1990 and 1992. On the eastern side, large interannual anomalies are observed close to the coast of Java, and sign is opposite to that on the western side, suggesting an oscillation in the zonal tilt of the thermocline across the STIO. Those interannual variations of D20 are also well simulated in the model. The generation of the interannual anomalies in the model are also due mainly to the wind stress curl integrated along the Rossby wave trajectories in the STIO.

Geostrophic transport of Indonesian throughflow

Abstract The Indonesian throughflow was measured during a six year period, permitting estimates of the mean and mean annual cycle of volume transport. The measurements are based on regularly repeated XBT sections, which were used with the climatological T/S relationship to calculate geostrophic currents relative to 400 m. The mean relative throughflow-transport is found to be 5 × 10 6 m 3 /s. Hydrographic data is used to investigate deeper currents and the total throughflow. The mean annual cycle of throughflow-transport has strong annual and semiannual components. The maximum net, relative transport toward the west between Australia and Indonesia is 12 Sv, in August/September. The amplitude and phase of the annual signal vary considerably within the Indonesian region.

On the Annual Rossby Wave in the Tropical North Pacific Ocean

Abstract Annual variation in the depth of the 14°C isotherm (i.e., the main thermocline) throughout the tropical Pacific Ocean between 30°N and 30°S is studied on the basis of 156 000 bathythermographs. Large-amplitude variations are confined in the region between 4 and 15°N. Near 6°N the variations in depth propagate westward. Near 12°N they have almost the same phase across the ocean from the American coast to 145°E. These variations are approximately consistent with a simple model that permits an oceanic response to local Ekman pumping, modified by nondispersive, baroclinic Rossby waves forced by the wind. Near 12°N, the rate of change in thermocline depth is nearly in phase with the Ekman pumping velocity, with only a minor but significant contribution coming from Rossby wave propagation. This type of response depends critically on variations in the eastern boundary region. Near 6°N, the westward propagating variations are generated by relatively large variability in Ekman pumping in the eastern Pacif...