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I grew up and live in the Northern Hemisphere. In high school, I first learned that the northern and southern hemispheres have opposite seasons. I thought it interesting how this might imply that the mean surface temperature on Earth doesn’t change much throughout the year.

I am also aware of concepts such as the second law of thermodynamics and thermohaline circulation, which imply that heat dissipates throughout the Earth, approaching an equilibrium.

Due to all these factors, it seems logical to me that there exist some kind of feedback mechanism where weather conditions in one hemisphere might affect or be connected to weather conditions in the other hemisphere. Is there any merit to this idea? If my state or country is going through a heat wave, is it likely that somewhere in the Southern Hemisphere there is or will be a cold wave?

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    $\begingroup$ just a small note: the opposite seasons come from the fact that with earth's axial tilt, during northern summer the northern parts are pointed towards the sun (getting more sunlight, and thus more heat) while the southern parts are pointed away from it. And the other way around during northern winter. $\endgroup$
    – Syndic
    Commented Oct 24 at 14:01
  • $\begingroup$ As the answers note, there is little relation between the weather of the Northern and Southern hemisphere outside of a few things like ENSO and, obviously, having opposite seasons due to axial tilt. However, the premise of the question is a bit off in that "heat waves" don't really happen in an entire hemisphere at once in most cases, but rather different parts of that hemisphere will be colder or warmer than seasonal average. See, for example, the effects of the Arctic Oscillation in the Northern hemisphere. $\endgroup$
    – reirab
    Commented Oct 25 at 15:21

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Is a heat wave in the Northern Hemisphere likely to be accompanied by a cold wave in the Southern Hemisphere?

That is not very likely. There is very little cross coupling between the weather in the Northern and Southern Hemispheres. There are exceptions such as the El Nino Southern Oscillation (ENSO); more below. It is almost as if the Earth has two atmospheres, a Northern Hemisphere atmosphere and a Southern Hemisphere atmosphere, with mild coupling and some admixture at the Intertropical Convergence Zone (ITZC).

Regarding the aforementioned El Nino Southern Oscillation (ENSO), while that does have worldwide effects, those are seasonal as opposed to short term effects. Your heat wave in the Northern Hemisphere and cold front in the Southern Hemisphere are short term events. The mild cross coupling between the two atmospheres means those short term events are uncoupled.

That said, Does the flap of a butterfly's wings in Brazil set off a tornado in Texas?

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With weather and climate there is rarely, if ever, a zero sum game, particularly between the northern and southern hemispheres.

A heat wave in one hemisphere does not imply or require a corresponding "cold wave" in the other hemisphere. If a "cold wave" were to occur in one hemisphere it may to be totally unrelated to the heat wave in the other hemisphere.

One of the tenets of global warming is that, allowing for localized variability, the general trend is that temperatures are increasing everywhere at the same time.

If one hemisphere has a heat wave during summer, the other hemisphere may have a winter that is less cold than one would expect from an established base line derived from historical records.

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    $\begingroup$ How can it not be a zero sum game? Does the overall energy stored in the atmosphere oscillate that strongly? I would have expected it to stay constant (minus global warming recently, volcano eruptions etc.), which would obviously imply a zero-sum game. If things like cloud cover, albedo changes, aerosol concentration etc. lead to moving the energy content away from its long-term mean, that should be fairly predictable, shouldn't it? $\endgroup$
    – Peter - Reinstate Monica
    Commented Oct 23 at 9:42
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    $\begingroup$ @Peter-ReinstateMonica Take away global warming, volcanic eruptions, etc., and it's still not a zero sum game, at least not over the short or medium term. The Earth's atmosphere, oceans, and solid surface store energy. How much and where varies. This energy storage can be seen over the short term in the fact that the daily high typically occurs hours after solar noon, and over the medium term in that the warmest and coldest days of the year occur well after the solstices. Over the very long term it might be a zero sum game. There aren't many teleconnections that cross the equator. $\endgroup$
    – David Hammen
    Commented Oct 23 at 13:23
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    $\begingroup$ @Peter-ReinstateMonica the atmosphere stores roughly (Fermi estimate) a thermal energy of $10^{24}\,\mathrm{J}$. The earth receives roughly $10^{18}\,\mathrm{W}$ of solar irradiance. So it takes only $10^6\,\mathrm{s}$, about a week, to replentish the entire energy from sun. On average, irradiance and Stefan-Boltzmann emissions are balanced, but changes in albedo have an effect indeed and it's not as "long-term" as you think. $\endgroup$
    – leftaroundabout
    Commented Oct 23 at 16:36
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    $\begingroup$ The oceans, for context, have much more thermal inertia, storing about $10^{27}\,\mathrm{J}$. $\endgroup$
    – leftaroundabout
    Commented Oct 23 at 16:47

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