Central Bank FX Intervention

In September 2022, the Bank of Japan bought yen with dollars for the first time in 24 years, after the currency slid past 145 per dollar. It wasn't defending a peg — Japan doesn't have one — it was intervention: a central bank trading in the open market to move an exchange rate it judges has moved too far or too fast, without any prior promise about where the rate "should" be. Intervention sits between a hard peg and doing nothing, and how a central bank does it — loudly or quietly, alone or with allies — changes how much it actually moves the rate.

Two channels, not one

The portfolio-balance channel works through pure supply and demand: selling dollars and buying yen genuinely shifts the relative amounts of each currency available in the market, pushing the price. The signaling channel works through expectations: a visible intervention tells the market "we think this rate is wrong and we're willing to act," which changes what traders expect the central bank — or its interest-rate policy — to do next, and traders trade on that expectation well beyond the size of the actual trade.

$$\Delta S \approx f(\text{intervention size}) + g(\text{credibility of future action})$$

In words: the exchange rate moves partly because of the trade itself, and partly — often mostly — because of what the trade signals about future policy. A small intervention from a central bank with a credible track record can move markets more than a huge one from a bank the market doesn't believe will follow through.

Worked example

USD/JPY trades at 152. The Bank of Japan sells $20 billion, buying yen, pushing the rate to 147 within hours — a clear, immediate portfolio-balance effect. But the BOJ simultaneously sterilizes the intervention: it sells short-term bonds to soak up the yen it created, keeping Japan's domestic money supply unchanged. Because the underlying rate differential between the US and Japan hasn't moved, the yen drifts back toward 150 over the following weeks as the signal fades and traders resume selling yen for the same interest-rate reasons as before. A larger, unsterilized intervention — or one accompanied by an actual rate hike — would have left a more durable mark, because it would have changed the fundamentals the market was pricing, not just the spot supply for one afternoon.

What this means in practice

Traders price intervention risk directly into "one-way" currency trends: the further and faster a currency has moved, the higher the odds a central bank steps in, which shows up as fatter tails and sudden reversals in the options market rather than a smooth continuation of the trend.