Why it matters

The dollar cycle is the single most important macro variable globally. Dollar strength tightens global financial conditions (via dollar-denominated debt servicing costs), compresses EM growth, and drains global liquidity. Dollar weakness does the reverse. The DXY is not just a price - it is a global financial conditions index. The Milkshake Theory (Brent Johnson): structural demand for dollars (reserve status, eurodollar system, dollar-denominated liabilities) creates persistent dollar strength that eventually destabilises the system.

Intuition

FX models are famously bad (Meese-Rogoff). In practice, the three factors that work: (1) relative rate expectations (2Y OIS differentials), (2) terms of trade shocks (commodity prices for resource currencies), and (3) positioning extremes (crowded carry trades unwind violently). PPP is a 10-year anchor with ~5-year half-life of deviations. Most FX alpha comes from timing the carry trade regime (risk-on vs risk-off) rather than predicting equilibrium rates.

Example

The yen carry trade unwind of August 2024 demonstrated the nonlinearity of FX positioning. An estimated $1tn in yen-funded carry positions unwound in days when the BOJ hiked and the Fed signalled cuts simultaneously - compressing the rate differential from both sides. USD/JPY moved 15% in three weeks, the Nikkei fell 12% in a day, and VIX spiked to 65. The carry trade is a slow escalator up and a fast elevator down.

What to watch

Cross-currency basis: the deviation from covered interest parity reveals dollar funding stress. Persistent negative basis = structural dollar shortage
FX reserve composition (IMF COFER): gradual diversification away from USD (~59% of reserves, down from 70%) is a slow structural headwind for the dollar
Real rate differentials: nominal rate differentials adjusted for inflation expectations - the true carry adjusted for purchasing power erosion
Vol-adjusted carry: carry/implied vol (the Sharpe ratio of the carry trade) determines when carry is attractive vs when the risk exceeds the reward

The nominal bilateral exchange rate. Key identity: covered interest parity (CIP) should hold by arbitrage: F/S = (1+r_d)/(1+r_f). CIP violations (non-zero cross-currency basis) indicate dollar funding market stress. UIP (uncovered) is systematically violated - the forward rate bias (carry premium) averages ~2-3% annually for G10 but exhibits negative skewness and excess kurtosis (crash risk).

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