Monsoon‐Induced Zonal Asymmetries in Moisture Transport Cause Anomalous Pacific Precipitation Minus Evaporation

Abstract

Basin-integrated precipitation minus evaporation (P − E) in the Pacific is near neutral while the Atlantic shows net evaporation. We link this P − E asymmetry to atmospheric moisture fluxes across the boundaries of the ocean drainage basins. Adopting an objective approach based on a comparison between actual fluxes and a zonally averaged circulation, we show that the asymmetry is dominated by moisture fluxes associated with the monthly mean flow at low latitudes rather than by differences in moisture fluxes into the Southern Ocean and Arctic catchments. In boreal summer, the eastward moisture flux, due to the Asian Summer Monsoon flow, opposes the zonal mean westward flux in the Trade Winds and results in more positive P − E over the Pacific than both the Atlantic and Indian Oceans, even in the annual mean. Our analysis reveals that moisture flux across Southeast Asia, rather than across Central America, is the dominant factor in the P − E asymmetry. The hypothesis that strong atmospheric moisture transport by the Trade Winds across Central America (Figure 2) is the cause of the P − E asymmetry between the Atlantic and Pacific appears often in the literature. Some authors specifically refer to the Isthmus of Panama (a very narrow strip of land) as the location of RESEARCH LETTER

A Lagrangian Perspective on the Atlantic and Pacific Precipitation‐Evaporation Asymmetry

“…Craig et al. (2020) suggested that the shapes of the southern hemisphere subtropical highs and the fractions of the ocean basins they occupy influences atmospheric moisture transport to and from the Southern Ocean basin. Partitioning Q n shows that northward $${Q}_{n}^{\mathit{Sou}}$$ along 35°S is weakest on the Atlantic sector by about 22% compared to the Indian and Pacific sectors resulting in a strong net southward Q n .…”

“…Starting from the North, the moisture import to the Arctic is dominated by the North Atlantic and North Pacific storm tracks (wedges of high Q n (x) across the widths of the basins in Figures 6d and 6f), demonstrating that Arctic precipitation is mostly associated with transient moisture fluxes (Craig et al, 2020;Gimeno et al, 2019;Vazquez et al, 2016). Across the Indian sector of the Arctic catchment boundary, northward Q n (x) is greatest around the Arabian peninsula corresponding with regions of high z BL and highly evaporative seas.…”

“…In this section the trajectories will be used to highlight the dominant dynamical processes affecting Q n and the locations where the strongest Q n are found with respect to the $$\overline{P-E}$$ asymmetry. This section focuses on Q n along the three meridional catchment boundaries since deviations from zonal‐mean zonal moisture flux along these boundaries are the most important for the existence of the $$\overline{P-E}$$ asymmetries (Craig et al., 2020).…”

“…Craig et al. (2020), however, showed that atmospheric moisture flux across Central America is close to the zonal mean zonal moisture flux: in that sense, it is not anomalous and cannot contribute to the zonal asymmetry between the Atlantic and Pacific. Instead, it is the westward Indian‐to‐Pacific moisture flux over South‐East Asia associated with the Asian Summer Monsoon that deviates substantially from the zonal mean and causes anomalous moisture transport into the Pacific basin, resulting in strong area‐averaged precipitation (Craig et al., 2017) and neutral $$\overline{P-E}$$.…”

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Atmospheric moisture transport between ocean basins plays an important role in maintaining the contrasting atmospheric moisture budgets (precipitation minus evaporation; P − E) between the Atlantic, Pacific and Indian Oceans. The Pacific has an approximately neutral budget (P − E ≈ 0) while the other two show strong net evaporation (Craig et al, 2017(Craig et al, , 2020. These characteristics have been linked to the asymmetries in sea surface salinity (SSS) and meridional overturning circulations (MOC).

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