Abstract
Surface latent heat flux (LHF) is an important component in the heat exchange between the ocean and atmosphere over the tropical western North Pacific (WNP). The present study investigates the factors of seasonal mean LHF variations in boreal summer over the tropical WNP. Seasonal mean LHF is separated into two parts that are associated with low-frequency (> 90-day) and high-frequency (≤ 90-day) atmospheric variability, respectively. It is shown that low-frequency LHF variations are attributed to low-frequency surface wind and sea-air humidity difference, whereas high-frequency LHF variations are associated with both low-frequency surface wind speed and high-frequency wind intensity. A series of conceptual cases are constructed using different combinations of low- and high-frequency winds to inspect the respective effects of low-frequency wind and high-frequency wind amplitude to seasonal mean LHF variations. It is illustrated that high-frequency wind fluctuations contribute to seasonal high-frequency LHF only when their intensity exceeds the low-frequency wind speed under which there is seasonal accumulation of high-frequency LHF. When high-frequency wind intensity is smaller than the low-frequency wind speed, seasonal mean high-frequency LHF is negligible. Total seasonal mean LHF anomalies depend on relative contributions of low- and high-frequency atmospheric variations and have weak interannual variance over the tropical WNP due to cancellation of low- and high-frequency LHF anomalies.
Highlights
Sea surface temperature (SST) changes in the tropical western North Pacific (WNP) play an important role in the climate variability
The present study investigates interannual variations of seasonal mean low-frequency (> 90-day) and high-frequency (≤ 90-day) latent heat flux (LHF) in boreal summer over the tropical WNP, which are associated with low- and high-frequency atmospheric variations, respectively
The effects of low-frequency surface wind speed and high-frequency wind intensity on the seasonal LHF variations are separately examined through a series of conceptual one-dimensional cases with different combinations of low-frequency surface wind and highfrequency wind intensity
Summary
Sea surface temperature (SST) changes in the tropical western North Pacific (WNP) play an important role in the climate variability. Weak background surface wind is an important condition for the seasonal accumulation of high-frequency LHF and the interannual variations of low-frequency surface wind over the tropical WNP affect largely the upscale feedback (Wang et al 2020; Wu et al 2020b). Both intensity of high-frequency atmospheric variability and low-frequency surface wind are modulated by tropical SST variations, which is indicative of a close relationship between their year-to-year variations.
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