美元代表着美国在国际金融市场中的角色，然而，美元的涨与跌，并不直接与“强势美元”或“弱势美元”划上等号。在更多时候，美元象征着一个“名义锚”，美国的货币政策可以通过它从国内传达到国际市场上。

在次级债危机中，美元经历了抛售——大举购买——抛售的过程；短期内美元的贬值可能意味着国际资金流的减少，然而考虑巨额的财政赤字，美政府可能更庆幸负担减轻了不少。

在2006年到2007年的次级债危机中，美元的转变和作用不能忽视，在未来一段时间，我们相继会关注美元与资本流入、通货膨胀与财政赤字等相关问题。

以下是次级债中美元的表现：

The dollar fell on Wednesday as US equities continued to slide on concerns that problems in the US subprime mortgage market could spill over to the wider economy.

But unlike Tuesday, which saw the dollar’s fall limited to a sharp drop against the low-yielding yen, the US unit lost ground against European currencies on Wednesday.

By mid-afternoon in New York, the dollar was down 0.3 per cent to $1.3240 against the euro, 0.3 per cent to $1.9340 against sterling and 0.5 per cent to SFr1.2115 against the Swiss franc.

“There are signs that dollar is at the start of a broad-based decline that reflects a downward re-assessment of the US economy,” said Paul Mackel, currency strategist at HSBC.

According to Mr Mackel, recent market focus on the effect of the unwinding of carry trades on the yen has masked the true story of dollar weakness.

Most analysts, however, believe the recent fall in dollar/yen has been sparked by a rise in risk aversion which has boosted the Japanese currency as investors unwind carry trades, in which the purchase of riskier high yielding assets is funded by selling the low-yielding yen.

Indeed, the yen made sharp gains on Tuesday in response to rising risk aversion as global equity markets tumbled.

However, the yen gave back some of those gains on Wednesday, easing 0.2 per cent to Y116.30 against the dollar, 0.4 per cent to Y153.80 against the euro and 0.6 per cent to Y225.40 against sterling.

The high-yielding Australian and New Zealand dollars, which were particularly hard-hit on Tuesday, did even better, rising 1.1 per cent to Y91.50 and 0.8 per cent to Y80.25 respectively.

Marc Chandler, foreign exchange strategist at Brown Brothers Harriman, said he believed there were reasons to be sceptical that the recent rally in the yen meant that risk aversion had returned to the foreign exchange markets, given that the yen had failed to breach the highs hit on March 5.

“This is not to say that the move is necessarily over, and, while acknowledging the anxiety that the pace of the move creates, in terms of magnitude it has been quite modest.”

Mr Chandler added that fears that the problems in the US subprime mortgage market could destabilise the wider economy were overstated.

He said data clearly indicated that the recent rise in delinquent mortgages was contained to the subprime market, while the stress in the US mortgage market was not concentrated in places where economists have warned of real estate bubbles.

“The problems are most clear in places that the [US] housing boom passed by,” said Mr Chandler. “While there is a risk that the problems spread and become a more generalised liquidity issue, at this juncture it seems unwarranted that that is the greatest probability.”

However, Neil Mellor, currency strategist at Bank of New York, maintained that after a week-long improvement in sentiment for risky assets, the second phase of the deleveraging process had arrived.

According to Mr Mellor, the pound was likely to suffer since it has proved to be much more sensitive to the unwinding of carry trades than the euro, with the daily movement in euro/yen exceeding that of sterling/yen on only one occasion in the last two weeks.

“If, as we contend, the second phase of the deleveraging process is only just getting under way, this leaves euro/sterling well placed for a concerted move towards, and possibly beyond, the £0.70 level.

Indeed, sterling fell to an eight-month low of £0.6867 against the euro and dropped to $1.9214 early in Wednesday’s session as the pound fell against the yen.

However, as sterling rallied against the Japanese currency, it also made strong gains against the euro, finishing 0.1 per cent higher at £0.6836.