Japan Spent $59 Billion on the Yen. The Plumbing Question Is Whether It Can Keep Doing It

Things started getting a bit ridiculous toward the end of last week. Japan may have spent as much as $59 billion in a single day to prop up the yen. That number is worth sitting with for a moment, because it's not a typo - and it's not the end of the story. The real question isn't how much Tokyo threw at the currency. It's how they're funding it, who's helping them pay for it, and what happens when the reserves run thin.

Here's what happened. Bank of Japan data showed an 8.2 trillion yen net outflow from the money market on July 31 - far larger than broker forecasts that ranged from a 1.4 trillion yen surplus to a 1.73 trillion yen shortfall. That gap is the tell. When the BOJ soaks up yen from the market to buy it back, the resulting drain on bank reserves shows up as an outsized shortfall in the next day's money market projection. Translated: Japan was buying yen at an enormous scale, and the plumbing left a receipt.

But here's the part most commentary missed. The United States joined them. The Treasury, working through the Federal Reserve Bank of New York, sold euros to buy yen via Goldman Sachs and Morgan Stanley. This was Washington's first yen-buying intervention alongside Tokyo in more than a decade - the last time was 2011, after the earthquake and tsunami. A Reuters photo of Treasury Secretary Scott Bessent's notepad from a cabinet meeting at Camp David read "Buy Japanese Yen (JPY) $5-10 bil." It wasn't aspirational. The purchase happened.

Yes, you could argue that a one-time intervention of any size is just a speed bump. The dollar/yen pair has been drifting higher for months, pushed by the interest rate gap between the US and Japan. The BOJ is at 1%. The Fed is still far higher. That differential makes the yen the natural funding currency for carry trades - where investors borrow cheaply in yen and deploy those funds into higher-yielding dollar assets. Until that gap closes, the carry trade keeps running. Intervention doesn't fix the rate differential. It just puts a bid underneath.

But the mechanism for this bid is different than the one most people are imagining, and that's what matters. Japan's foreign exchange reserves totaled $1.287 trillion at the end of June, down from $1.375 trillion at the end of March. That's a drain of roughly $88 billion in three months, with the steepest single-month drop coming in May - $77 billion - after a record intervention that cost 11.7 trillion yen. A single-day $59 billion intervention is nearly 4.6% of total reserves. If Japan keeps hitting the market at that scale, the reserves get thin fast.

So the Finance Ministry flagged something called the FIMA Repo Facility. That's the standing Foreign and International Monetary Authorities Repo Facility, a tool the Federal Reserve operates that lets foreign central banks borrow dollars by posting US Treasuries as collateral. It was created during the 2020 pandemic as an emergency liquidity backstop. The practical benefit for Japan is that it can get the dollars it needs to intervene without actually selling Treasuries - which would spike US yields and create a political problem with the Fed and Treasury alike. Japan's top currency bureaucrat, Vice Finance Minister Atsushi Mimura, said publicly that US support goes beyond "moral support." He put the FIMA facility right on the table.

Understand the mechanism: Japan holds one of the world's largest portfolios of US Treasury bonds. Selling them to buy yen would inject a massive supply of Treasuries into the market, pushing yields up. That undermines the Fed's own balance sheet management and the Treasury's own borrowing costs. The FIMA facility sidesteps the entire problem. Japan keeps the Treasuries on its books, borrows dollars against them, and uses those dollars to buy yen. The plumbing works both ways.

That said, the FIMA facility isn't infinite. It's a repo - meaning the Treasuries are collateral, not a gift. Japan has to return the dollars and unwind the collateral eventually. It's a bridge, not a permanent solution. And it raises its own question: if Japan needs to keep drawing on FIMA repeatedly, at what point does the Fed get nervous about its own counterparty exposure?

Bessent called the yen "very undervalued" on Fox Business and said "excess volatility" isn't healthy. Understanding what I understand about spreads and economics, the US having a skin in this game makes sense. A collapsing yen creates import inflation in Japan, which weakens the Japanese economy, which pulls down global growth. The US doesn't want a weaker Japan - it wants a stable partner. But the US also doesn't want to watch Japan dump Treasuries and send yields higher. So the coordinated intervention with the FIMA backstop is the compromise that lets both sides manage the problem.

The yen has recovered some ground since its 40-year low near 164 yen per dollar in late June. As of early August, it's trading around 156. But the underlying forces haven't changed. The BOJ held rates steady at 1% on July 31. Governor Kazuo Ueda kept a September rate hike in play but didn't commit to one. The Fed's next decision isn't until September 16. The interest rate gap - the engine behind the carry trade - remains wide open.

Here's the conditional chain. If the Fed signals it's done with tightening and the BOJ then hikes, the rate gap narrows, the carry trade unwinds, and the yen strengthens on its own. Intervention then becomes the insurance that wasn't needed. If the Fed holds and the BOJ stays put, the pressure continues. Japan then has to keep intervening - at which point reserves become the limiting factor and FIMA becomes the crutch. Either way, the plumbing tells you what to watch.

What to watch next: Japan's reserve levels each month, any actual FIMA usage (the Fed doesn't report that with full transparency), and the BOJ's September meeting. The BOJ's decision follows two days later on September 18, which makes the two-day gap between decisions the critical window. If the Fed signals it's done with tightening on September 16 and the BOJ then hikes on September 18, the rate gap narrows and the yen strengthens. If the BOJ stays put, the intervention machine has to keep running.

Same rate gap. Same carry trade. Different impact this time - because the US is standing on the other side of the desk, and the plumbing between the two central banks is actually being used. The question isn't whether Japan can afford one more $59 billion day. The question is how many it can afford before the FIMA facility stops looking like a backstop and starts looking like a lifeline.

The views expressed here are the author's personal analysis and are not investment advice.