Enclosed aisle data centers with bypass recirculation have previously been shown to reduce significantly the cooling infrastructure's power consumption compared to a typical open-aisle configuration. Enclosed aisle configurations also facilitate IT load placement through so-called thermally aware, energy-optimized virtualization. The formidable problem of optimal workload placement, which has received increased interest by the IT industry and has been the subject of numerous investigations, becomes much simpler, almost trivial, when the aisles are enclosed. This is because there is no preference among identical servers in an enclosed aisle based on the thermal environment, which will be uniform in this case, making for a wide range of equivalent load placement possibilities within the data center, or at least those racks in the data center that share enclosed cold aisles. However, the reduction in IT load can have a significant impact on the energy consumption of the cooling infrastructure due to the off-design operation of equipment. The work presented in this paper uses an experimentally validated thermo-hydraulic model of the data center's cooling infrastructure, previously developed by the authors, to explore optimization possibilities in air-cooled, enclosed aisle data centers. The model is used to evaluate the total energy consumption of the data center's cooling infrastructure for data centers operated at reduced IT load. The analysis highlights the importance of reducing the total power required for moving the air within the CRACs, the plenum, and the servers, rather than focusing primarily or exclusively on reducing the refrigeration system's power consumption and shows the benefits of bypass recirculation in enclosed aisle configurations. Energy efficient operating strategies for data centers with enclosed aisles are given for a range of IT loads and ambient conditions.