循环出错，想请教下大家

halexxxxx

我想算这个指数“取（10-50°N）每隔2.5°纬圈上，用110°E处夏季平均的500hPa位势高度减去150°E处500hPa夏季平均位势高度的数值的总和”
这个是我的代码
f   = addfile("hgt.nc","r")
ff1  = short2flt(f->z(0:467,5,{10:50},{110:150}))
ff2  = month_to_season(ff1,"JJA")
ff3  = ff2(:,:,{110})
ff4  = ff2(:,:,{150})
ff5  = ff2/9.8
ff6  = ff3/9.8
do i = 0,i.le.38
    do j = 0, j.le.17
       a = ff5(i,j) - ff6(i,j)
       j := j+2.5
    end do
    fa:=dim_cumsum_n_Wrap(a, 0, 0)
    fi = fa
    i := i+1
end do
b = (/(/f0,f1,f2,f3,f4,f5,f6,f7,f8,f9,f10,f11,f12,f13,f14,f15,f16,f17,f18,f19,f20,f21,f22,f23,f24,f25,f26,f27,f28,f29,f30,f31,f32,f33,f34,f35,f36,f37,f38/),(/1979.,1980.,1981.,1982.,1983.,1984.,1985.,1986.,1987.,1988.,1989.,1990.,1991.,1992.,1993.,1994.,1995.,1996.,1997.,1998.,1999.,2000.,2001.,2002.,2003.,2004.,2005.,2006.,2007.,2008.,2009.,2010.,2011.,2012.,2013.,2014.,2015.,2016.,2017./)/)
b!0 = "index"
b!1 = "year"
time_pts =(/1979.,1980.,1981.,1982.,1983.,1984.,1985.,1986.,1987.,1988.,1989.,1990.,1991.,1992.,1993.,1994.,1995.,1996.,1997.,1998.,1999.,2000.,2001.,2002.,2003.,2004.,2005.,2006.,2007.,2008.,2009.,2010.,2011.,2012.,2013.,2014.,2015.,2016.,2017./)
wks = gsn_open_wks("png","pjy")
res = True
res@vpHeightF = 0.35
res@vpWidthF  = 0.6
res@trXMinF   = 1979
res@trXMaxF   = 2017
res@gsnYRefLine   = 0
res@tiXAxisString = "year"
res@tiYAxisString = "index"
res@xyDrashPattern = 0
res@xyLineThicknesses= 2
plot = gsn_csm_xy(wks,time_pts,b,res)
然后他就出了这个错