Please help in writing Thormthwaite Equation.

I have written the following code. But the code is not giving correct answer though it runs perfectly. Kindly help. I have attached the thormthwaite equation.

%Thornthwaite Equation
%ET=Evapotranspiration
%I=Thermal power index imposed by the local normal climatic temperature
clc; clear all;
x=importdata('F:\project\IMP files\Project_Data.xlsx');
m=x.data.Sheet2(:,:);
Ta=x.data.Sheet2(:,3);
year=[1980:2010]';
for i=1:length(year)
        n=find(m(:,1)==year(i,1)); 
        s=(Ta(n)/5).^1.514;
        sp(i,1)=sum(s);
end
for j=1:12
    aa(j,1)=6.75*10^(-7)*sp(j,1).^3-7.71*10^(-5)*sp(j,1).^2+1.7912*10^(-2)*sp(j,1)+0.49239;   
    ET(j,1)=16*(10*Ta(j,1)/sp(j,1)).^aa(j,1);
end