Inflation Carry and Seasonality in Linkers

Inflation-linked bonds accrue principal based on a CPI figure with a roughly two-and-a-half month reporting lag, and that CPI series isn't seasonally adjusted for this purpose — the raw, non-seasonally-adjusted index is what feeds the bond's index ratio. Non-seasonally-adjusted CPI has a well-documented rhythm: it tends to jump in January (annual price resets on things like rent contracts, insurance, tuition, and post-holiday retail pricing) and again around mid-year with summer travel and apparel patterns, while running softer in other months. A linker holder's near-term carry, the inflation accrual expected over the next month or quarter, depends heavily on which calendar months are about to roll into the bond's reference period.

Where the seasonality comes from

Non-seasonally-adjusted CPI captures raw price levels each month, including patterns that recur every year regardless of the underlying inflation trend: January often sees a burst from contract resets and January sales patterns reversing; certain months see predictable moves in apparel, education, and utility pricing tied to the calendar rather than to economic conditions. Because the reference CPI used for TIPS accrual is this non-seasonally-adjusted series, lagged by about two and a half months, an investor holding a TIPS in, say, November and December is really accruing based on the September and October raw CPI prints — and if those months are historically soft ones, near-term carry on the bond will look weak even if the underlying trend inflation rate hasn't changed at all.

Worked example

A TIPS investor is evaluating carry for December and January. Because of the roughly 2.5-month lag, December's accrual is driven by non-seasonally-adjusted CPI data from mid-September to mid-October, historically a moderate month with month-over-month NSA increases averaging around 0.25%. January's accrual, however, is driven by NSA data from mid-October to mid-November, still moderate. But February's accrual is driven by December-to-January data, and January NSA CPI has historically jumped more, averaging closer to 0.45% month over month due to contract resets. An investor who understands this seasonal pattern expects noticeably stronger linker carry heading into February than into December, independent of any view on the actual underlying inflation trend.

Worked example: a seasonality-adjusted relative value trade

A desk holds two TIPS with similar real yields and duration, differing mainly in which months dominate their near-term accrual due to slightly different dated dates and coupon schedules. If one bond's next quarter of accrual leans more heavily on historically hot months (like January) and the other leans on historically soft months, the first bond has structurally higher expected near-term carry for reasons having nothing to do with credit or duration — a desk pricing relative value between the two needs to strip out this seasonal effect before concluding one bond looks cheap or rich to the other.

What this means in practice

Linker desks build seasonal adjustment factors, essentially a historical average NSA CPI seasonal pattern, and net them out of quoted breakeven and carry figures so they're comparing bonds on trend inflation expectations rather than on an accident of which calendar months happen to roll into each bond's accrual window next.