# calc dt and dt^2 (first and second derivative)
def calc_adv(cv, u, v):

    dxcv = cv.differentiate(coord='lon')  # optional: add edge_order=2. default is 1.
    dycv = cv.differentiate(coord='lat')  # optional: add edge_order=2. default is 1.

    adv = -( u *dxcv) - ( v *dycv)           # adv = u*d/dx(cv) + v*d/dy(cv)

    dxadv = adv.differentiate(coord='lon')
    dyadv = adv.differentiate(coord='lat')

    advadv = ( u *dxadv) + ( v *dyadv)  # to constrain adv to locations where there is a local min.
    return (adv, advadv)