As far as South American service, Northwest has never had a scheduled flight to that continent. However, in the winter, NW operates Saturday-only service to Liberia, Costa Rica from MSP, which is their only Central American route.
NW has plenty of seasonal service to Mexico, with flights to SJD, ZIH, ACA, PVR, and CZM on a Saturday-only basis, with daily service to CUN.
Northwest flies year-round twice daily service to MEX from DTW, using the A319. They upgraded to twice daily 319s from a once daily A320 about a year ago. This is their only year round service to Mexico.
For the Caribbean, NW operates year round daily service to SJU from DTW, which upgrades to a DC-10 during the winter months, and year round Saturday/Sunday service from Memphis to SJU. NW also flies TPA-MBJ all year round.
Seasonal service to the Caribbean is extensive, albeit only on Saturdays (and in some cases Sundays). NW flies GCM-DTW, GCM-MSP, MSP-POP, MSP-Punta Cana, DTW-Punta Cana, and DTW-SXM. These are just the routes I'm able to find now, I'll check nwa.com and see if I can find a few more.
Hope this helps...
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