Treasury auctions' influence on mortgage rates and the S&P 500

Treasury auctions likely aren't the first factor that comes to mind when considering mortgage rates or the S&P 500, but they deserve closer scrutiny.

A relevant case takes place later today, with the US Treasury planning to sell $39 billion in 10-year notes at 1700 GMT through a reopening of an existing issue. The key question is why this matters beyond the bond market.

First, consider what the auction actually entails.

To finance its borrowing, the US government periodically sells Treasury debt to investors, who then determine the required return for lending.

If today's 10-year auction sees strong demand, it indicates buyers are content with current yields and the sale proceeds smoothly.

Conversely, weak demand means the auction might need to clear at a higher yield for investors to take all the supply. That higher yield often ripples beyond Treasuries.

The 10-year Treasury yield is a key global benchmark. Mortgage rates tend to move with long-term Treasury yields since mortgage-backed securities vie with Treasuries for investment. Sustained increases in Treasury yields can thus lead to higher mortgage costs.

What about equities?

Treasury yields provide a low-risk return benchmark. When they rise, stocks face greater competition for investor capital.

A valuation effect also comes into play: higher Treasury yields reduce the present value of future corporate earnings. This especially impacts stocks trading at high multiples based on distant profits.

Of course, a single sluggish auction won't immediately spike mortgage rates or crash the S&P 500. The trend over time is what matters.

If investors consistently require higher yields to take on additional Treasury supply, those borrowing costs spread through the financial system.

For today's 10-year auction, key metrics to watch include the bid-to-cover ratio (demand strength), the clearing price relative to the market, and the amount of debt primary dealers end up with.

This may seem like arcane bond market mechanics, but following the connections shows that Treasury auctions ultimately influence everything from household mortgage rates to stock valuations like the S&P 500.