You can use the client-side ColumnSort event to fire a custom javascript event on sorting. For example:

$grid->setGridEvent('onSortCol', $sorcol);
(setGridEvent is a common method for definition of jqGrid events)
The event handler receives three parameters - (columnName, columnIndex, sortOrder). 

You can also force sorting of a certain column by calling the javascript jqGrid method sortGrid(columnName, reload).

For details, please review the PHP tab for code specifics.