Xcode, remove subView

I have an app that displays locations of families across time. I use a Timer to advance the date every 1 second, and at each year data is retrieved from Core Data and an object is drawn on the view with a subView, overlaying a map. What I cannot fathom is how to clear the subView each time the year advances. Suggestions would be most welcome.

Timer code:

  @IBAction func startCount(_ sender: Any) {

        if !counting {
        timer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(counter), userInfo: nil, repeats: true)
        counting = true
    }
}

Counter code:

  @objc func counter() -> Void {
    yearCount.text = String(countYear)
    compareYear = String(countYear)
    print(compareYear)
    drawCircles()
    countYear += 1
}

Draw Circles code:

  func drawCircles() {

         if markerView != nil {
        if let markerView = view.viewWithTag(countYear-1){
        markerView.removeFromSuperview()
        }
    }

    var markers = [PlaceNames]()
    let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "PlaceNames")
    let predicate = NSPredicate(format: "year == %@", compareYear)
    fetchRequest.predicate = predicate

    do {
        markers = try coreDataStack.context.fetch(fetchRequest) as! [PlaceNames]
        print(markers.count)

    } catch let error as NSError {
        print("Could not fetch \(error), \(error.userInfo)")
    }

        for marker in markers {

            placeEastings = marker.value(forKey: "eastings") as? Float
            placeNorthings = marker.value(forKey: "northings") as? Float
            print(placeEastings, placeNorthings)

            if (placeEastings != nil && placeNorthings != nil) {

            let tempXAxis = placeEastings
            placeXAxis = (tempXAxis! - 6.0)/1.5
            let tempYAxis = placeNorthings
            placeYAxis = (tempYAxis! - 6.0)/1.5
                print(placeXAxis, placeYAxis)
            }

            markerView = UIImageView(frame: CGRect(x: Double(placeXAxis), y: Double(placeYAxis), width: 12.0, height: 12.0))

            let renderer = UIGraphicsImageRenderer(size: CGSize(width: 12, height: 12))

            markerImage = renderer.image { ctx in
                let rectangle = CGRect(x: 0, y: 0, width: 12, height: 12)
                ctx.cgContext.setFillColor(UIColor.white.cgColor)
                ctx.cgContext.setStrokeColor(UIColor.black.cgColor)
                ctx.cgContext.setLineWidth(1)

                ctx.cgContext.addEllipse(in: rectangle)
                ctx.cgContext.drawPath(using: .fillStroke)
            }

            markerView.tag = countYear
            markerView.image = markerImage

            self.view.addSubview(markerView)
    }
}     

I think you can store all markerViews in array like var markerViews: [UIImageView] and then

@objc func counter() -> Void {
    yearCount.text = String(countYear)
    compareYear = String(countYear)
    print(compareYear)

    clearExistingMarkerViews()
    drawCircles()
    countYear += 1
}

private func clearExistingMarkerViews() {
    markerViews.forEach { $0.removeFromSuperView() }
   markerViews.removeAll()
}