title: Negative power spectrum
body: <p>I've got a set of 780 monthly temperature anomalies over 65 years and I want to analyse it for frequencies that are driving the anomalies. I've used the spectrum package to do this, I've included pictures of the series before and after the analysis.</p>
  
  <pre><code>from spectrum import *
  
  p = Periodogram(anomalies, sampling=1/12)
  p.run()
  plt.title('Power Spectrum of Monthly Temperature Anomalies')
  p.plot(marker='.')
  plt.show()
  </code></pre>
  
  <p>The resulting spectrum has several clear negative spikes. Now I understand that a negative value in Db isn't actually a negative absolute value, but why is this happening? Does it imply there's some specific frequency missing from my data? Because a positive spike implies one is present.</p>
  
  <p>Also, why are most of the frequencies displayed negative? What is the reference value for Db to be an amplification of?</p>
  
  <p>A part of me thinks I should be taking the absolute value of this spectrum but I'd like to understand why if that's the case. Also I put in the value for sampling as 1/12 because the data points are monthly so hopefully the frequency scale is in per year?</p>
  
  <p>Many thanks, this is my first post here so let me know if I need to be clearer about anything.</p>
  
  <p>Negative Energies</p>
  
  <p><img src="https://i.stack.imgur.com/qkzcK.png" alt=""></p>
  
  <p>The Series being Analysed</p>
  
  <p><img src="https://i.stack.imgur.com/N5N1P.png" alt=""></p>