def warm_spell_duration_index(tasmax, tx90, window=6, freq='YS'):
    r"""Warm spell duration index

    Number of days with at least six consecutive days where the daily maximum temperature is above the 90th
    percentile. The 90th percentile should be computed for a 5-day window centred on each calendar day in the
    1961-1990 period.

    Parameters
    ----------
    tasmax : xarray.DataArray
      Maximum daily temperature [℃] or [K]
    tx90 : float
      90th percentile of daily maximum temperature [℃] or [K]
    window : int
      Minimum number of days with temperature below threshold to qualify as a warm spell.
    freq : str, optional
      Resampling frequency

    Returns
    -------
    xarray.DataArray
      Count of days with at least six consecutive days where the daily maximum temperature is above the 90th
      percentile [days].

    References
    ----------
    From the Expert Team on Climate Change Detection, Monitoring and Indices (ETCCDMI).
    Used in Alexander, L. V., et al. (2006), Global observed changes in daily climate extremes of temperature and
    precipitation, J. Geophys. Res., 111, D05109, doi: 10.1029/2005JD006290.

    """
    if 'dayofyear' not in tx90.coords.keys():
        raise AttributeError("tx90 should have dayofyear coordinates.")

    # The day of year value of the tasmax series.
    doy = tasmax.indexes['time'].dayofyear

    # adjustment of tx90 to tasmax doy range
    tx90 = utils.adjust_doy_calendar(tx90, tasmax)

    # Create an array with the shape and coords of tasmax, but with values set to tx90 according to the doy index.
    thresh = xr.full_like(tasmax, np.nan)
    thresh.data = tx90.sel(dayofyear=doy)

    above = (tasmax > thresh)

    return above.resample(time=freq).apply(rl.windowed_run_count, window=window, dim='time')
r"""Warm spell duration index

    Number of days with at least six consecutive days where the daily maximum temperature is above the 90th
    percentile. The 90th percentile should be computed for a 5-day window centred on each calendar day in the
    1961-1990 period.

    Parameters
    ----------
    tasmax : xarray.DataArray
      Maximum daily temperature [℃] or [K]
    tx90 : float
      90th percentile of daily maximum temperature [℃] or [K]
    window : int
      Minimum number of days with temperature below threshold to qualify as a warm spell.
    freq : str, optional
      Resampling frequency

    Returns
    -------
    xarray.DataArray
      Count of days with at least six consecutive days where the daily maximum temperature is above the 90th
      percentile [days].

    References
    ----------
    From the Expert Team on Climate Change Detection, Monitoring and Indices (ETCCDMI).
    Used in Alexander, L. V., et al. (2006), Global observed changes in daily climate extremes of temperature and
    precipitation, J. Geophys. Res., 111, D05109, doi: 10.1029/2005JD006290.