Figure I.6. Time series of the tropical Atlantic gradient index, formed by the difference between SSTA area indices for the tropical North (TNA) and South Atlantic (TSA). Colored dots are monthly averaged data and the smooth black curve is the triple 13-month running mean of the data. Red dots mark monthly averages where TNA and TSA were of opposite sign and their magnitudes exceeded 0.2 °C (dipole configurations). Blue dots indicate months when the tropical Atlantic was not in a dipole configuration. Note that dipoles are less frequent and tend to form at the decadally alternating extremes of the smoothed curve.