I have to agree with everyone who says that if you are interested in the position, you need to formally apply. I also have to say that that your statement that you will have to leave if this individual gets hired in above you does not serve you well.
In my department, we have Senior Specialists and we have Specialists. I was hired in as a Senior. I had not previously worked in this industry (let's call it widget making), but have extensive experience in other industries. One of the two Specialists who was here when I was hired was very, very unhappy that she wasn't promoted to a Senior position and that I was hired from outside the company. She deeply resented my lack of widget experience. Like you, she'd been here a year before the Senior position came up. She viewed promotion to the Senior position as simply a function of time at the company, and she treated me miserably when I arrived. Eventually, our manager and our vice president had to sit her down and have a talk with her about the fact that there is a difference between the two positions, even though we both do essentially the same work.
At my company, the Senior is expected, in addition to knowing all about widget-making, to have stronger project management skills and the ability to run larger, more complex widget projects. The Senior is expected to be able to represent the company externally, if necessary, and to interface more with senior management outside our department. The Senior is expected to be able to serve as acting manager if the manager is out of the office, to have a greater ability to see the big picture, to be able to manage the department workflow, and to be able to solve problems without involving management.
So, even though I had no widget experience before I came here, I was hired because I had all those other, more intangible, skills and abilities. My co-worker was not promoted into the Senior Specialist because she had not demonstrated to management that she had those skills. It took me three months to learn how to make widgets. In the year that she was here before me, she had not shown to management that she had learned those other skills.
If this individual has worked in an another position at the company, she may have more big-picture skills or other intangible attributes. So, before you go all stompy boots about someone being hired in above you, I would suggest that you ask management what is the difference in expectations between an Associate and a Senior Associate and what you need to do to get to the Senior Associate position. There may be some other skills that you need to develop.