SPHERECURL is the jSphere module of jLab.

 SPHERECURL  Curl of a vector field on the surface of a sphere.
    CURL=SPHERECURL(LAT,LON,U,V) computes the vertical component of the 
    curl of the vector field (U,V) on the surface of the sphere.
    U and V are zonal and meridional velocities in meters per second, and
    CURL is in inverse seconds.
    LAT and LON are vectors specifing an evenly-spaced grid, and U and V
    are arrays of size LENGTH(LAT) x LENGTH(LON) x M, where M is greater
    than or equal to one. LAT and LON are in degrees.
    The radius of the Earth as specified by RADEARTH is used by default.
    SPHERECURL(...,R) uses a sphere of radius R, in kilometers, instead.
    Derivatives are computed using the first central difference.
    [CURL,UY,VX]=SPHERECURL(LAT,LON,U,V) optionally returns the zonal and
    meridional contributions UY and VX, such that CURL=VX-UY.
    First and last points
    SPHERECURL can use different boundary conditions in the numerical
    computation of derivatives for the first and last points.
    SPHERECURL(...,'periodic') uses a periodic derivative with respect to
    longitude.  This is the default behavior.  
    SPHERECURL(...,'endpoint') uses the forwards / first backwards 
    difference at the first and last longitude, respectively. 
    For both of these options, the endpoint condition is used for 
    differentiation with respect to latitude.
    SPHERECURL(...,'nans')  fills in the first and last values on all four
    sides of the region with NANs. 
    See VDIFF for more information.  
    'spherecurl --t' runs a test.
    Usage: curl=spherecurl(lat,lon,u,v);
    This is part of JLAB --- type 'help jlab' for more information
    (C) 2009--2017 J.M. Lilly --- type 'help jlab_license' for details

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