Dipoles, temperature gradients, and tropical climate anomalies

An examination into misleading terminlolgy.

Empirical diagnostic analyses over the past three decades have progressively elucidated important circulation mechanisms of tropical climate anomalies. In the process, certain meteorological elements in limited domains were found to be particularly indicative, and correspondingly some index series have come to have wide application. The intent of the present paper is to address the validity and usefulness of the underlying concepts.

PRESSURE DIPOLES. Long-term inverse variations of pressure at distant locations have been known for a century (review and references in Hastenrath 1985, 253-258). For the pressure seesaw between the eastern and western extremities of the tropical Pacific basin, Walker coined the term "Southern Oscillation" (SO); various indices such as the normalized pressure anomaly difference Tahiti minus Darwin have been used to advantage in the study of climate anomalies to describe the phases of the SO. Similarly, Walker introduced the term "North Atlantic Oscillation" (NAO) to denote a long-term pressure seesaw between the Icelandic low and the Azores high, and indices such as the normalized pressure anomaly difference Akureyri minus Ponta Delgada well capture the phenomenon. In both examples, there are significant negative correlations between the two pressure series, so there is a seesaw in the pressure variations, and the locations of the two series are appropriately called the "dipoles" of the oscillation. This reminder may serve as a background reference for the considerations in the following sections.

TROPICAL ATLANTIC TEMPERATURE.

Work at the University of Wisconsin in the 1970s (Hastenrath and Heller 1977; Hastenrath 1976, 1978; Lamb 1978a,b; Hastenrath 1995, 302-326) first identified the sea surface temperature (SST) patterns in the tropical Atlantic associated with climatic disasters in key land areas around that basin. We found that rainfall in northeast Brazil is more strongly correlated with the contrast between hemispheres than with anomalies in either the north or south (Hastenrath and Heller 1977). Later work (Hastenrath and Druyan 1993; Hastenrath and Greischar 1993a) elucidated the ways in which the interhemispheric SST gradient hydrostatically controls the pressure and hence the wind field and latitude position of the intertropical convergence zone, and the basinwide meridional SST gradient in the tropical Atlantic became a major predicfor in the decade-long real-time forecasting for the rainy season of northeast Brazil (Hastenrath and Greischar 1993b; Greischar and Hastenrath 2000; Folland et al. 2001).

Although our work had thus identified the interhemispheric SST gradient in the tropical Atlantic as a major causal factor in regional climate anomalies, and SST in the tropical North and South Atlantic are uncorrelated on a long-term basis (Hastenrath and Greischar 1993a), the term "Atlantic SST dipole" emerged more than a decade later, with the implication of a north-south seesaw. Distracting from the functionally relevant SST gradient, this entailed a protracted controversy (Servain 1991; Houghton and Tourre 1992; Enfield et al. 1999). The term found its way into planning documents at the international level (Intergovernmental Oceanographic Commission-World Meteorological Organization 1990, p. 25; International CLIVAR Project Office 1999, p. 31), although a later report (International CLIVAR Project Office 2000, p. 7) has this rectified to "cross-- equatorial SST gradient." People unfamiliar with the subject may be misled into thinking of a north-south seesaw instead of the interhemispheric SST gradient.

INDIAN OCEAN TEMPERATURE. For the equatorial Indian Ocean we discovered close associations between zonal SST gradient, equatorial surface westerlies, East African coastal rainfall, and the SO during boreal autumn (Hastenrath et al. 1993), when the equatorial surface westerlies are part of a powerful zonal circulation cell, which is likewise closely associated with the SO (Hastenrath 2000). There is no zonal SST seesaw, in fact SSTs at the western and eastern extremities of the equatorial Indian Ocean are correlated positively. At the January 2001 Annual Meeting of the AMS in Albuquerque, New Mexico, the absence of a dipole was acknowledged (Nicholls and Drosdowsky 2001). Although the absence of a zonal SST seesaw and a close association with the SO has thus long been demonstrated (Hastenrath et al. 1993), a recent paper (Saji et al. 1999) claims an Indian Ocean SST dipole and the independence of ENSO. That paper along with the term "Indian Ocean dipole"are mentioned in another article in the same journal issue (Anderson 1999). The notion of an Indian Ocean dipole and its independence of ENSO was reiterated at the Indian Meteorological Society (IMS)India Meteorological Department (IMD)-World Meteorological Organization (WMO) Monsoon Conference in Delhi, India, in March 2001 (Yamagata 2001), with conjecture that it may have a role to play in the climatic variability on a larger scale. Prompted by such experience, within three weeks the present paper was submitted to BAMS.

Since then, misconceptions about, and preoccupation with the subject continue in program committees, research proposals, and literature. A June 2001 paper (Li and Mu 2001), unaware of the discoveries a decade earlier, assumes an Indian Ocean SST dipole and seeks influences on the Asian monsoon circulation. Half a year after submission of the present paper an article appeared, authored by nine colleagues from three continents (Allan et at. 2001) that reviews a host of papers, some aware (Webster et al. 1999; Mutai and Ward 2000; Reason et al. 2000), but many ignorant (Behera et al. 1999, 2000; Chambers et al. 1999; Iizuka et al. 2000; Murtugudde et al. 2000; Li and Mu 2001) of the findings a decade ago (Hastenrath et al. 1993). In a valiant effort to clear up confusion, Allan et al. (2001) examined the Saji et al. (1999) paper in particular and refuted the notions of an equatorial Indian Ocean SST dipole pattern and the independence of ENSO. With good reason they expressed concern about misconceptions being perpetuated. A May 2001 editorial by Glantz (2001) was apposite: "we are doing the same research on the same topics that others are doing, and we are often unaware of it. This is truly a waste of time, resources, and human potential."

CONCLUDING REMARKS. Of great relevance to interannual climate variability, the SO and NAO are characterized by inverse pressure variations at the extremities (or "dipoles") of the standing oscillation system. By contrast, for the tropical Atlantic we found in the 1970s that the basinwide interhemispheric SST gradient is closely associated with regional climate anomalies, but there is no seesaw. Despite this, a distraction developed later around the term "Atlantic SST dipole," which lingered on over a quarter century. For the equatorial Indian Ocean, we discovered in the early 1990s a close association between zonal SST gradient, equatorial westerlies, East African rainfall, and the SO, but again there is no SST seesaw between the western and eastern extremities of the equatorial Indian Ocean. A deja vu, a decade later the term dipole appears again in the literature. In context this is misleading and should be avoided.

ACKNOWLEDGMENTS. For exchanges of thought I thank Peter Lamb, John McBride, Peter Webster, and Neville Nicholls. I also appreciate the good suggestions of an anonymous reviewer. Our work is supported by NSF Grants ATM-0110061 and ATM-9732673.

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[Author Affiliation]

AFFILIATION: HASTENRATH-University of WisconsinMadison, Madison, Wisconsin

CORRESPONDING AUTHOR. Dr. Stefan Hastenrath, Dept of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison, Madison, Wl 53706

E-mail: barafu@macc.wisc.edu