Black horizontal patches in the plot

I am plottting a simple sst difference plot. My lon data is in the range -180 to 180. I have converted it to 0 to 360. After plotting I am geeting these black patches (attached).

clear
clc

% Define the file paths for WeakTEJYears
WeakTEJYears = {
    'F:\SST_MA\1979.nc'
    'F:\SST_MA\1983.nc'
    'F:\SST_MA\1987.nc'
    'F:\SST_MA\1997.nc'
    'F:\SST_MA\2002.nc'
    'F:\SST_MA\2015.nc'
    'F:\SST_MA\2023.nc'
};

% Define the file paths for StrongTEJYears
StrongTEJYears = {
    'F:\SST_MA\1985.nc'
    'F:\SST_MA\1988.nc'
    'F:\SST_MA\1999.nc'
    'F:\SST_MA\2009.nc'
    'F:\SST_MA\2013.nc'
};

% Initialize variables to store the sum of SST data
sst_sum_weak = 0;
sst_sum_strong = 0;

num_weak_files = length(WeakTEJYears);
num_strong_files = length(StrongTEJYears);

lat_min = -25;
lat_max = 70;
lon_min = 0;   
lon_max = 360;

for i = 1:num_weak_files
    ncfile = WeakTEJYears{i};

    latitude = ncread(ncfile, 'latitude');
    longitude = ncread(ncfile, 'longitude');
    
    longitude(longitude < 0) = longitude(longitude < 0) + 360;
    
    [longitude, sort_idx] = sort(longitude);
    
    sst = ncread(ncfile, 'sst');
    sst_sorted = sst(sort_idx, :, :);  % Re-sort SST data according to longitude sorting

    lat_idx = find(latitude >= lat_min & latitude <= lat_max);
    lon_idx = find(longitude >= lon_min & longitude <= lon_max);

    sst_region = sst_sorted(lon_idx, lat_idx, :);

    sst_sum_weak = sst_sum_weak + mean(sst_region, 3);  % Mean over time
end

for i = 1:num_strong_files
    ncfile = StrongTEJYears{i};

    latitude = ncread(ncfile, 'latitude');
    longitude = ncread(ncfile, 'longitude');
    
    longitude(longitude < 0) = longitude(longitude < 0) + 360;
    
    [longitude, sort_idx] = sort(longitude);
    
    sst = ncread(ncfile, 'sst');
    sst_sorted = sst(sort_idx, :, :);  

    lat_idx = find(latitude >= lat_min & latitude <= lat_max);
    lon_idx = find(longitude >= lon_min & longitude <= lon_max);

    sst_region = sst_sorted(lon_idx, lat_idx, :);

    sst_sum_strong = sst_sum_strong + mean(sst_region, 3);  % Mean over time
end

sst_avg_weak = sst_sum_weak / num_weak_files;
sst_avg_strong = sst_sum_strong / num_strong_files;

sst_diff = sst_avg_strong - sst_avg_weak;

figure;
[lon_mesh, lat_mesh] = meshgrid(longitude(lon_idx), latitude(lat_idx));

contourf(lon_mesh, lat_mesh, sst_diff', 20, 'EdgeColor', 'none');
colormap jet;
colorbar;
caxis([-2, 2]);
c.Label.String = 'Temperature (K)';
c.Label.FontSize = 18;
c.Label.FontWeight = 'bold';

coast = load('coastlines');  

coastlon = coast.coastlon;
coastlat = coast.coastlat;
coastlon(coastlon < 0) = coastlon(coastlon < 0) + 360;  

hold on;
plot(coastlon, coastlat, 'k', 'LineWidth', 1.5);

xlim([lon_min, lon_max]);  
ylim([lat_min, lat_max]);  

title('Difference: Strong - Weak TEJ Average SST', 'FontSize', 14);

Answer 1

[Star Strider]

Those sorts of horizontal liines are usuallly caused by unsorted data. Experiment with sorting the latitude as well:

[longitude, sort_idx] = sort(longitude);

latitude = latitude(sort_index);

Without your data, I cannot be certaiin.

imshow(imread('Diff_Plot.png'))

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