Given a triangle $ABC$. Point $M$ moves along the side $BA$ and point $N$ moves along the side $AC$ beyond point $C$ such that $BM=CN$. Find the geometric locus of the centers of the circles circumscribed around the triangle $AMN$.
To find the geometric locus of the centers of the circles circumscribed around the triangle \(AMN\), we need to analyze the given conditions and derive the necessary geometric properties.

1. **Given Conditions and Setup:**
   - Triangle \(ABC\) with points \(M\) on \(BA\) and \(N\) on \(AC\) such that \(BM = CN\).
   - We need to find the locus of the circumcenters of \(\triangle AMN\).

2. **Circumcenter and Circumradius:**
   - Let \(O\) be the circumcenter of \(\triangle ABC\) and \(R\) be its circumradius.
   - Let \(X\) be a point on the line \(\ell\) passing through \(O\) and parallel to the internal angle bisector of \(\angle A\).

3. **Circle and Intersection Points:**
   - Consider the circle with center \(X\) and radius \(XA\). This circle intersects \(AB\) at \(M\) and \(AC\) at \(N\).

4. **Geometric Relationships:**
   - We need to show that \(BM = CN\) implies a specific relationship between the segments \(AM\) and \(AN\).

5. **Angle and Segment Calculations:**
   - Let \(\theta = \angle (\ell, XA)\).
   - The length \(XA\) can be expressed as:
     \[
     XA = \frac{\cos(C + \frac{A}{2})}{\sin \theta} R
     \]
   - Using the Law of Cosines in \(\triangle AMN\), we find:
     \[
     AM = 2XA \cos \left(\frac{A}{2} + \theta\right)
     \]
     \[
     AN = 2XA \cos \left(\frac{A}{2} - \theta\right)
     \]

6. **Difference of Segments:**
   - The difference \(AM - AN\) is:
     \[
     AM - AN = 2XA \cos \left(\frac{A}{2} + \theta\right) - 2XA \cos \left(\frac{A}{2} - \theta\right)
     \]
   - Using the trigonometric identity for the difference of cosines:
     \[
     \cos \alpha - \cos \beta = -2 \sin \left(\frac{\alpha + \beta}{2}\right) \sin \left(\frac{\alpha - \beta}{2}\right)
     \]
     we get:
     \[
     AM - AN = 2XA \left[-2 \sin \left(\frac{A}{2}\right) \sin \theta\right]
     \]
     \[
     AM - AN = -4XA \sin \left(\frac{A}{2}\right) \sin \theta
     \]

7. **Simplifying the Expression:**
   - Substitute \(XA\) from step 5:
     \[
     AM - AN = -4 \left(\frac{\cos(C + \frac{A}{2})}{\sin \theta} R\right) \sin \left(\frac{A}{2}\right) \sin \theta
     \]
     \[
     AM - AN = -4R \cos \left(C + \frac{A}{2}\right) \sin \left(\frac{A}{2}\right)
     \]

8. **Using Trigonometric Identities:**
   - Using the identity \(\cos(C + \frac{A}{2}) = \cos C \cos \frac{A}{2} - \sin C \sin \frac{A}{2}\):
     \[
     AM - AN = -4R (\cos C \cos \frac{A}{2} - \sin C \sin \frac{A}{2}) \sin \frac{A}{2}
     \]
     \[
     AM - AN = -4R \cos C \cos \frac{A}{2} \sin \frac{A}{2} + 4R \sin C \sin^2 \frac{A}{2}
     \]
     \[
     AM - AN = 2R (\sin C - \sin (A + C))
     \]

9. **Final Relationship:**
   - Since \(BM = CN\), we have:
     \[
     AM - AN = c - b
     \]

Thus, the geometric locus of the centers of the circles circumscribed around \(\triangle AMN\) is the line \(\ell\) passing through the circumcenter \(O\) of \(\triangle ABC\) and parallel to the internal angle bisector of \(\angle A\).

The final answer is the line \( \boxed{ \ell } \) passing through the circumcenter \(O\) parallel to the internal angle bisector of \(\angle A\).