What is a term for the long-term emotional "climate" of a relationship?

I am looking for a word or phrase to describe the overarching "mood" of a relationship across its existence, rather than at one specific time. For example, if the "mood" of the relationship at a given time is analogous to the "weather" in the sense that it describes the immediate, how can one describe the emotional "climate" that spans a longer time?

From NOAA:

Whereas weather refers to short-term changes in the atmosphere, climate describes what the weather is like over a long period of time in a specific area. Different regions can have different climates. To describe the climate of a place, we might say what the temperatures are like during different seasons, how windy it usually is, or how much rain or snow typically falls.

Example sentence: At that moment, the mood between them was tense, even though in general the ________ of the relationship was carefree.

Based on the description and example sentence, a good word is nature:

[Merriam-Webster]
1 a : the inherent character or basic constitution (see CONSTITUTION sense 2) of a person or thing : ESSENCE
       // the nature of the controversy
1 b : DISPOSITION, TEMPERAMENT
       // it was his nature to look after others
       — F. A. Swinnerton
       // her romantic nature
3 : a kind or class usually distinguished by fundamental or essential characteristics
    // documents of a confidential nature
    // acts of a ceremonial nature

Used in the example sentence in the question:

At that moment, the mood between them was tense, even though in general the nature of the relationship was carefree.

The word "nature" comes to mind.

According to Webster's New Dictionary of Synonyms p. 839:

nature may suggest inherent, essential characteristics rather than superficial, ostensible, or tentative ones

This means that you can use "nature" to describe the "climate" but "mood" for the "weather."