Changes of extreme wave heights in Northern Hemisphere oceans and related atmospheric circulation regimes

This study assesses trends in seasonal extremes (90- and 99-percentiles) of Significant Wave Height (SWH) in the North Atlantic and the North Pacific, as simulated in a 40-yr global wave hindcast using the National Centers fo r Environmental Prediction-National Center for Atmospheric Research reanaly sis wind fields. For the last four decades, statistically significant chang es in the seasonal extremes of SWH in the North Atlantic (NA) are detected only for the winter (January-March) season. These changes are found to be i ntimately connected with the North Atlantic oscillation (NAO). To be specif ic, significant increases of SWH in the northeast NA, matched by significan t decreases in the subtropical NA, are found to be associated with an inten sified Azores high and a deepened Icelandic low. This is consistent with th e findings of previous studies based on different datasets. Changes in seas onal extremes of SWH in the North Pacific (NP) are found to be statisticall y significant for the winter and spring (April-June) seasons. Significant i ncreases in the extremes of SWH in the central NP are found to be connected with a deeper and eastward extended Aleutian low. For both oceans, no sign ificant trends of SWH are detected for the last century, though significant changes are found in the last four decades. However, multidecadal fluctuat ions are very noticeable, especially in the North Pacific.